Case Studies Archive - Hawkins Forensic Investigation

Revealing the Truth

Kirsten Beckwith — Wed, 28 May 2025 16:26:43 +0000

A global insurance company instructed Hawkins on a matter requiring digital forensics services. The insured had made a significant claim for the theft of luxury jewellery from their home. Luckily, the claimant was able to provide still images from their home security system digital video recorder (DVR) of the reported burglary, as well as mobile phone digital photographs of the stolen jewellery.

However, the insurer had sufficient reasons to suspect that the insured was falsifying not only the home burglary, but also the theft of the jewellery, and the subsequent insurance claim. These reasons included some of the common warning signs in fraudulent insurance claims, such as the insurance policy being setup only a few days prior to the reported theft, limited and mixed information provided in the insured’s claim statements, as well as minimal damage to the property. Due to these concerns, the insurer instructed Hawkins to forensically examine the insured’s devices to determine the metadata of the DVR still images and mobile phone digital photographs, produce a timeline of events and prepare a detailed expert witness report.

Digital Forensic Investigation

The insured only provided their Swann DVR and Apple iPhone for forensic examination by Hawkins after they were served a court order instructing them to do so. Our investigator was able to acquire a copy of both the live and deleted data stored on each device, having bypassed the password for the DVR in the process as the insured had ‘forgotten’ it; an all-too-common excuse in fraudulent cases.

Upon analysing the extracted data, including the metadata of the DVR still images, Exchangeable Image File Format (EXIF) data of the mobile phone digital photographs, and system logs, we identified that the dates and times of the devices had been manually changed to display false dates and times. Metadata is data that provides information about aspects of other data, such as the created, last modified, and last accessed dates and times. EXIF data is a standard that defines specific information relating to images captured by a digital camera, such as created date and time, device make and model, and GPS location. Therefore, Hawkins could evidence that the still images and digital photographs had in fact been taken after the date and time of the reported burglary. Furthermore, after viewing the video footage recovered from the DVR, and not just relying on the still images provided, it was clear that the so-called burglary had in fact been staged. This was evident in the way that the individual entered the property by calmly obtaining a door key from under the door mat, as well as the witness statement information not correlating with the events that took place.

Case Outcome

The digital forensic investigation, and subsequent expert witness report from Hawkins, was completed within 24 hours of receipt of the devices. This was made possible by a combination of the knowledge and experience of the Hawkins’ Investigator and the digital forensic software Hawkins has invested in, which can rapidly process and extract the relevant data. After the insurer had reviewed the expert witness report, they proceeded to reject the policyholder’s insurance claim, cancelled the insurance policy, and brought criminal proceedings against them.

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If You Cannot Confirm The Provenance, How Do You Expect Me To Provide Expert Testimony?

Lorraine — Wed, 19 Apr 2023 13:48:55 +0000

I started my career working at LGC Forensics, now known as Eurofins, specialising in the examination and reporting of trace evidence, i.e. fibres, paint, glass, and foam. I loved investigating crime scenes to establish a forensic link between people and incidents.

I was known as the “Foam Queen” due to my interest in polyurethane foam evidence found in car seats, which could transfer onto the trousers of car thieves. I discovered that foams have reasonable evidential value, because of their distinctive pigments, dyes and differing rates of degradation. There are truly very few people who have the emotional and physical stamina to stare down a microscope for 8 hours a day, searching for foam on clothing and tapings taken from car seats. They were fun and challenging times and the foundation of my career in forensics.

Fast forward to 2012 and due to a family relocation, I found myself in Hong Kong.

I wondered if I could get a job in the local crime laboratory but my lack of the local language, barred me from such government-related work. Eventually, I found myself working as a forensic engineer, doing accident investigation on behalf of the insurance market.

My undergraduate degree and PhD were both in materials engineering, which gave me the necessary skills to understand materials failure and corrosion, both of which were applicable to land and marine based investigative work. Having specialised in both the criminal and civil fields, I found it interesting to make comparisons between the two systems. Both have many unique features but the one that really stood out to me was the way in which evidence is handled in civil work.

One of the most critical and fundamental concepts in forensic science is the Chain of Custody. Put simply, this is chronological paper trail recording; where the item came from, who handled it, its current location, and the location of all the supporting documentation to back all of this up.

When conducting criminal investigations, the chain of custody was very clear on all my cases. If a chain of custody was unclear, I understood the item was not admissible in court. Therefore, the paperwork was, and still, is extremely important.

In civil accident investigation cases, I noticed that this system was less rigorous, and I had to explain to clients that if and when the case went to court, establishing the provenance was critical.

Marine based investigations are, by far, my favourite type of case work. I love nothing more than taking a launch out to sea, boarding a vessel, finding a broken or corroded object, taking samples back to the laboratory, and figuring out what happened and why the object failed.

Writing a court compliant report and then providing expert testimony is the ultimate stage, and one I enjoy enormously. When you are invited to stand before the court, as an expert witness, you have the opportunity to talk to a room about a subject that fascinates you, and the best part is that everyone is forced to listen. Sometimes the questions posed by opposing counsel are challenging, but that is part of the job.

I have had several experiences in marine based case work where the sampling methods and samples themselves were questionable, which could compromise the chain of custody.

One of my favourite examples involved travel to a dry dock in Asia to inspect the hull of a vessel, which was damaged, and to determine the nature and circumstances of the crack and hole observed by the crew, i.e. who or what did it, and how.

When I arrived at dry dock, I was given access to the vessel and the crack damage, and I proceeded to mark out an area of the fracture surface that I wanted to have sectioned for later examination in the laboratory.

Normally, microscopic examination of a fracture surface can indicate the failure mode by identifying distinctive features along the surfaces of the material. After marking up the relevant areas of the crack, I was told that it was too dangerous for me to witness the steel containing the crack being cut out using an oxy-acetylene torch.

An hour later, I was presented with a large piece of metal from the hull of the ship. I asked which part of the crack it originated from but was informed that the crew did not know.

It transpired that my marked-out areas had indeed been cut out but sadly, had been dropped into the sea, which was rather unfortunately, 13 metres deep.

To avoid disappointing me, the crew decided that providing me with another random piece of the hull, in no way associated with the fracture, might suffice.

After realising my evidence had been lost, I asked if I could be provided with a magnet and a rope to retrieve my evidence from the seabed. It turned out there were no magnets or spare rope in dry dock, and there was insufficient time to find and instruct divers to find the pieces.

I persisted and was miraculously given permission to witness another attempt at removing more sections of the crack, along the fracture surfaces. During this time, I was told again to keep my distance for safety reasons and watched a welder wearing flip flops, light a cigarette with the acetylene torch before removing the pieces.

I travelled back to the laboratory, with 20kg of metal on my back and had the samples further sectioned down for the necessary tests.

I learned a very good lesson – I should always be present during any cutting/removal work in order to make sure that the correct samples are being taken. Thankfully, due to my persistent and pedantic nature, I was able to find out what happened and correct it, in this case, and secure the relevant evidence.

In addition to being present and involved with sampling methodology and its subsequent execution, we as experts in a marine context need to ensure that the samples reach the laboratory safely.

I mentioned carrying heavy metal previously, which you can do if it is considered safe and not breaching any health and safety regulations.

However, if the case material is a metal ore cargo, such as nickel or aluminium, this is not practicable. Metal ores are shipped in bulk carrier vessels, usually in cargo holds that can hold un excess of 20,000 tonnes of material. Bulk carriers can experience liquefaction of the cargo, in which the bulk materials can enter a liquid state, causing instability and sometimes capsize. When the cargo is observed to be in the early stages of liquefaction, and forensic engineers can sample the material on-board, and test it establish whether the cargo was safe to load.

Due to the nature and size of the cargo stored in each cargo hold, sampling quantities can vary between hundreds of kilograms to several metric tonnes. To maintain the chain of custody, I need to ensure that the samples are bagged, tagged, and transported appropriately. I have, in certain circumstances, sat with the bags of nickel ore in a truck to ensure that they reach their target destination.

While marine investigation work is excellent fun, experts need to be robust when it comes to overseeing sampling of material/cargo and transportation to the laboratory. I find it is best to communicate the importance of chain of custody from the outset, making it clear to the Master of the Vessel and the Ship Owners in advance that I wish to sample the broken/corroded material or cargo.

When I am finished with my inspection on board, I request either the Chief Officer or the Master to sign the chain of custody documentation to indicate that they have released the item to me.

When I arrive at the laboratory, I request the exhibit handlers on site to sign that they have received the items, allowing me to commence preparing a court compliant report, knowing that I have secured the provenance.

Finally, all I need to worry about is the science, my ability to convey it appropriately to the court, and of course how to maintain good decorum on Zoom. In these truly novel times, you worry about your bandwidth as much as your appearance and demeanour.

About the Author

Sophie is the Head of Marine at Hawkins and is based in our London office. In addition to her casework, Sophie regularly presents to law firms, P&I Clubs and loss adjusting companies on materials failure analysis, corrosion, and cargo/liquefaction matters. She is available 24/7 to discuss any urgent matters.

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Full Numerical Analysis Part 3: Application to the Forensic Investigation of Geotechnical Incidents

Lorraine — Sun, 20 Mar 2022 20:16:15 +0000

Technical Content: High

Read Time: 15 minutes

With the rapid advancement of computer technology, full numerical modelling is now routinely carried out in geotechnical engineering. Producing a full numerical model is not straightforward; if the numerical analysis is not applied properly, the output can lead to defective design and ensuing failure. However, if used with skill and care, full numerical modelling has enormous potential to explain the engineering behaviour of geotechnical structures and produce optimal and safe design.

In Part 1 of this article I introduced the technique, and described both its advantages and disadvantages. In Part 2, I described two case studies to show how the technique can be applied to forensic investigation of geotechnical and other soil/structure failures.

In this final part, I will describe two more cases studies, exemplifying the power of the full numerical approach.

CASE STUDY 3: TIEBACK WALL FAILURE IN SOUTH KOREA [2]

The retaining wall consisted of an 18.3m high, cast-in place pile wall, supported by 9 layers of ground anchors, plus a further 21.7m high H-pile wall supported by numerous layers of rock bolts, with a 2m toe embedment, to retain a total height of 38m.

Based on the site investigation, a fracture zone was noted in the hard rock layer at depths below 30.4m.Sequential excavation proceeded until a catastrophic wall failure occurred after the final excavation stage.

After the collapse, a geotechnical expert performed a full numerical analysis to model the sequential behaviour and subsequent failure.

The response of the anchored retaining wall was simulated by two-dimensional, non-linear finite element analysis. The shear strength reduction method was used to predict the potential failure planes in the anchored wall structures. With the input of sequential excavation stages, the model successfully simulated the progression of the failure plane and captured the change in failure mechanism.

The predicted failure surface shape was a good representation of the real incident failure surface, showing that the numerical method accurately modelled the real life incident. The model confirmed the failure mechanism and failure to take the fracture zone into account. If the model had been used as part of the design, the incident could have been predicted and avoided.

CASE STUDY 4: BUILDING-OVERTURNING IN SHANGHAI, CHINA [4]

On 27 June 2009, a 13-storey building within a construction site (labelled in the images below as Building 7) abruptly fell over with little warning, killing one worker. Prior to the incident, a 4.6 m deep basement excavation was ongoing on the south side of both Building 6 and Building 7. The excavated soil was dumped on the north side of the two buildings and formed 10m high Stockpile 1. As recorded by the local weather station, about half an hour before the building toppled, there was an intense rainfall event lasting for 5 hours.

The preliminary conclusion from the Forensic Diagnosis Committee was that horizontal ground movement, caused by unequal lateral earth pressures on the two sides of Building 7, sheared off the underlying piles, causing the building to topple to the ground.

Considering the complexity of this failure, a three-dimensional full numerical analysis was conducted to verify the postulated failure mechanism.

The three-dimensional model included Building 6, Building 7, Stockpile 1 and the 4.6m deep excavation. The building superstructures, pile foundations and temporary earth retaining structures of the excavation were all modelled as per the actual site conditions. The simulated construction procedures duplicated the following on-site activities:

(1)Construction of Building 6 and Building 7

(2)Formation of Phase 1 of Stockpile 1 (4 m high) behind the buildings

(3)Consolidation of subgrade underneath the stockpile for six months

(4)Excavation of the 4.6 m deep basement and formation of Phase 2 of Stockpile 1 (10 m high)

Surprisingly, the simulated building displacements at the completion of excavation and stockpiling indicated a slight northward (instead of southward) inclination of the two buildings. The simulated northward inclinations of about 0.091% for Building 6 and 0.10% for Building 7 were unnoticeable. Moreover, the predicted maximum structural forces of the underlying piles were well within their design capacities and Stockpile 1 was still in a stable state.

These simulation results were consistent with the following two facts:

(1) No obvious sign of inclination was reported for either building before the abrupt failure

(2) The post-failure examination on the piles left in place showed that they had undergone either compressive or tensile failure, not shearing or bending failure, as preliminarily concluded by the Forensic Diagnosis Committee.

To further investigate the true causation and failure mechanism, the heavy rainfall event was simulated in the model by saturating the stockpile. Stockpile 1 underwent a base failure with significant ground heave, extending below the footing of Building 7, as was actually observed. The simulated stockpile behind Building 6 was still in a safe condition, evidenced by no substantial ground heave below Building 6. This analysis demonstrated that the intense rainfall was a decisive factor, which triggered a deep-seated slip failure of the stockpile and exerted an upward load underneath the north side of Building 7. Consequently, the building fell over suddenly in the southward direction.

CONCLUSION

The four case studies presented above (and in Part 2) only illustrate a few of the capabilities of full numerical analysis, and how it can assist with forensic investigation. If the analysis is performed properly, it can be applied successfully to various other types of cases as well.

With periodic updates to the analysis software, the process has become so user-friendly that even graduate engineers can perform the analyses with little guidance. This, however, might lead to the danger of misunderstanding either the way the software works or its limitations, as well as potentially using the wrong inputs or misinterpreting the results (i.e. the ‘rubbish in – rubbish out’ issue that effects all software).

Geotechnical engineering is part art, part science. The results of software analyses should be used to provide important evidence for judgments to be based on, and ‘well-winnowed’ experience should prevail.

ABOUT THE AUTHOR

Er. Zhuang Shimin is a Professional Engineer with a background in the design and construction of large-scale geotechnical solutions in Singapore and Hong Kong, including deep basements, retaining walls, reinforced soils, pipe jacking, deep shafts, deep foundations, pile-enhanced raft, segmental and cut & cover tunnels. He is experienced in both 2D & 3D geotechnical finite element analysis.

[1] https://www.midasgeotech.com/blog/influence-of-exc…

[2] S.S. Jeong, Y.H. Kim and M.M. Kim (2016) Developments in Geotechnical Engineering: Forensic Geotechnical Engineering, Chapter 27 Failure Case Study of Tieback Wall in Urban Area, Korea, Springer, India (ISBN: 978-81-322-2376-4)

[3] https://pilebuck.com/foundation/soldier-pile-laggi…

[4] Tan, Y., W. Jiang, H. Rui, Y. Lu, and D. Wang. 2020. “Forensic Geotechnical Analyses on the 2009 Building-Overturning Accident in Shanghai, China: Beyond Common Recognitions” J. Geotech. Geoenviron. Eng. 146

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Full Numerical Analysis Part 2: Application to Forensic Investigation

Lorraine — Mon, 07 Mar 2022 12:47:17 +0000

Technical Content: High

Read Time: 15 minutes

With the rapid advancement of computer technology, full numerical modelling is now routinely carried out in geotechnical engineering. However, producing a full numerical model is not straightforward; if the numerical analysis is not applied correctly, the output can lead to defective design and ensuing failure. If full numerical modelling is used with skill and care though, it has enormous potential to explain the engineering behaviour of geotechnical structures and produce both an optimal and safe design.

In the first part of this article, I introduced this technique and described both its advantages and disadvantages. In this second part of the article, I will describe two case studies to show how it can be applied to forensic investigation of geotechnical and other soil-structure failures. [A]

APPLICATION TO FORENSIC INVESTIGATION

For forensic investigations, the evidence obtained from both the desktop and site investigation must be reviewed and analysed to determine the cause of the failure. Failures seldom occur for a single reason, and especially for cases in which litigation is involved, the causes of failures are inevitably difficult to ascertain. Therefore, it is necessary to make various hypotheses regarding both why and how failures happen, as well as perform analyses to prove or disprove these hypotheses. Back‑analysis, which is often used in geotechnical engineering to estimate the material parameters on site, can be carried out using full numerical modelling to help establish failure scenarios, provide technical evidence, and assess the potential for future damage.

By using full numerical analysis, it is possible to quantify the sensitivity of predictions due to variations in different parameters and ground conditions. Based on the results of the analysis, the forensic engineer will then be able to offer opinions on either the relative responsibilities or the contributions to the ultimate failure [B].

In forensic investigations, the analytical tools adopted for full numerical analysis must be validated in accordance with a well-documented and stringent quality assurance program. For this reason, commercial software packages are preferred to in-house programs, because the former are usually well tested and improved based on the feedback from users [C].

CASE STUDY 1: RECLAMATION BUND FAILURE IN SINGAPORE [D]

Reclamation bunds are constructed to form a stable perimeter, within which the dredged soil can be laid (usually by pumping) to form the reclaimed area.

During the construction of a perimeter bund for land reclamation in the western part of Singapore, the bund suddenly failed at one part of the reclaimed land site. A geotechnical expert was instructed by the loss adjuster to identify the cause of the bund failure.

Using independently evaluated soil parameters, the expert checked the consultant’s original bund design and verified that the design had achieved the desired safety factor of safety of 1.57. Design factors of safety are often published in technical standards, and range from 1.3, where soil properties are highly reliable to 3-4, where there is a low confidence in the reliability of the soil parameters. With such a high safety factor of 1.57, the bund slope should not have failed even if certain parameters and situations become less favourable.

Parametric studies were therefore conducted using full numerical analysis to evaluate the probable causes of failure. The expert identified that the important factors affecting the bund slope stability included the thickness and strength of the marine clay, overfilling behind the slope, the bund slope gradient, ground water variation, and the distance between the bund edge and the sand compaction pile edge. This table shows a summary of the safety factor values against slope stability under various scenarios.

It can be seen above that a single factor alone probably could not trigger the bund failure, as the adopted safety factor was relatively high. Therefore, the cause of this bund failure must have been due to a combination of the scenarios.

To examine the mechanism of the slip and the extent of soil yielding, the expert carried out a full numerical analysis under a variety of configurations. This image shows the different extents of soil yielding for thicknesses of marine clay varying from 10 to 15m.

By evaluating the extent of the soil yielding, the analysis revealed that misalignment of the distance between the edges of the bund and the compaction pile foundation system would affect the bund stability the most. The predicted location of the greatest extent of the failure was indeed where the failure was observed.

CASE STUDY 2: LARGE LANDSLIDE IN QUEENSLAND, AUSTRALIA [F]

During bulk excavations to complete a road cutting, slumping and failure occurred along a section of the battered slope, known as Cut 3, following a period of significant rainfall.

To characterise the landslip mass, both the geometry of the landslip and the rate of movement related to groundwater levels were investigated using a large number of drill holes installed with the following instruments:

The inclinometer readings indicated the depth at which shear displacement was occurring at each of the drill hole locations. The initial results from the first inclinometers installed indicated that deep-seated movements were occurring in Cut 3.

Stability analysis for Cut 3 was carried out using two-dimensional full numerical modelling. The model below was set up to represent the conditions along cross sections oriented in the general direction of the maximum observed movement.

Inclinometer monitoring indicated that for Cut 3, the failure surface passed through a slickensided[1] zone beneath the trachyte[2] and near the surface of the claystone that underlies the cut. Stability modelling was constrained by explicitly modelling the measured geometry of the failure surface. Back-analyses were then carried out to assess the angle of internal friction of the material at the failure plane. The instrumentation data suggested that the rate of movement reduced to zero (i.e. Factor of Safety = 1.0) for an average groundwater level of RL[3] 85 m. Using this groundwater level, back-analysis indicated a friction angle of 8° for the material at the failure plane, which matched the laboratory test result undertaken on material recovered from the shear surface.

Forward-analyses, which aims to predict the ground behaviour with the calibrated model, were then carried out to assess factors of safety as a function of groundwater level, for the revised geometry following the remedial works. The results of these analyses helped the engineers to establish management protocols to allow measures such as road closures in the event of future groundwater level rises and predicted unacceptable reductions in factors of safety.

Hawkins regularly carries out causation investigation of geotechnical failures such as collapsed retaining walls and landslides, and where appropriate this can include full numerical back-analysis.

ABOUT THE AUTHOR

[A] https://blog.virtuosity.com/how-to-speed-up-calcul…

[B] Robert, W. Day (2011) Forensic Geotechnical and Foundation Engineering 2nd Edition, Chapter 3 The Investigation, McGraw-Hill, USA (ISBN: 978-0-07-176133-8)

[C] Richard N. Hwang (2016) Developments in Geotechnical Engineering: Forensic Geotechnical Engineering, Chapter 9 Back Analyses in Forensic Geotechnical Engineering, Springer, India (ISBN: 978-81-322-2376-4)

[D] Leung CF, Tan SA, Shen RF (2005) Predictions versus performance of land reclamation bund. In: Proceedings of 16th international conference on soil mechanics and geotechnical engineering, vol 5. Osaka, pp 3540–3541

[E] Web article from https://raajje.mv/20615

[F] D.C. Starr, J. Woodford and D.F. Marks (2016) Developments in Geotechnical Engineering: Forensic Geotechnical Engineering, Chapter 16 Characterisation of Failure at a Large Landslide in SE Queensland by Geological Mapping, Laboratory Testing, Instrumentation and Monitoring, Springer, India (ISBN: 978-81-322-2376-4)

[1] In geology, a slickenside is a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault.

[2] Trachyte is a type of light-coloured, very fine-grained extrusive igneous rock.

[3] These levels are referred to as “Reduced Levels” (RL) which means a height above (or below) a nominated datum.

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Investigations Into Steel Structure Failures Part II: Case Studies

Jodie — Mon, 31 Jan 2022 17:11:58 +0000

Hawkins has investigated many cases involving failures of steel structures. Our knowledgeable and experienced team of forensic investigators can establish the cause of a failure by either a detailed desktop study or site investigation, in addition to reviewing the relevant documentation and carrying out numerical back-analysis. Click here to read part one of this article if you would like more background information.

The Hawkins Built Environment team has found that although failures can be caused by either errors in the design, or errors at the construction and execution stage, most failures are often caused by a combination of both.

CASE STUDY 1: COLLAPSE OF A STEEL PORTAL FRAME

Following a storm that affected most parts of the UK, Hawkins was instructed to investigate the cause of the partial collapse of a steel portal framed structure that was under construction.

Photo above: Partially collapsed portal frame, which used to be part of a factory building

The portal frame comprised steel columns and pitched rafters, connected by moment-resisting welded connections. Hawkins carried out both a detailed inspection of the structure, and a review of the design of the structure, and found that the design was adequate. We then carried out a thorough review of the original construction sequence proposal and the sequence of actual construction, to establish whether there were any deviations from the intended sequencing of the works.

Witness accounts, site records, and the site investigation indicated that permanent cross bracings had not been fully installed before the construction of adjacent bays. Hawkins carried out calculations and analysis to assess the load carrying capacity of the structure without the full bracings. The results showed that the portal frame, as constructed, was weak and would fail in the event of high wind loading. In this case, the contractor also failed to use temporary bracings, which should have been provided in order for the structure to withstand wind gusts, as well as to stabilise the structure until permanent bracings were installed. Further investigation revealed that there was no qualified site engineer on site performing a supervisory role for the contractor.

This case highlights the importance of proper site supervision and risk assessment during the construction of steel structures. If proper site supervision and risk assessment had been carried out, it is likely the sequencing of the works would have been revised and temporary bracing would have been provided.

CASE STUDY 2: FAILURE OF A ROOF CANOPY

A large warehouse roof canopy failed after a period of heavy rain. Hawkins was instructed to establish the cause of the collapse, and in particular whether the failure was caused by defects in either design, construction, or a combination of the two.

Our investigator attended the site within 24 hours of the incident. The single-pitched roof canopy consists of rows of trusses made of thin, galvanised cold-rolled steel sections. The roof had deflected so much that it landed on a heavy goods vehicle. Our investigation revealed that the design of the canopy had numerous deficiencies, with the most significant ones being the insufficient capacity of the diagonal truss members, as well as insufficient slope of the roof. Our failure back-analysis showed that the diagonal members were inadequate given the intensity of the rain, causing the truss to sag overall. Because the roof pitch was too flat, a slight sag of the truss would have allowed water to accumulate on the roof, thus increasing the load. The increased load led to further deflection of the truss, eventually causing the failure.

Following on from our involvement, the client asked that we review the design of their assets worldwide.

CASE STUDY 3: BUILDING COLLAPSE

Hawkins was instructed to investigate the cause of a sudden collapse of an office building that was under construction. The building consisted of a four-storey steel frame structure. When Hawkins visited the scene, most of the remaining steel frame had already been brought down by the demolition contractor, and the main steel truss was half buried in the debris.

Hawkins examined all the joints of the truss at the Health & Safety Laboratory where the main truss had been transported to and temporarily stored. At one joint, the end of the diagonal member had been welded to the flange of a haunch. The fracture surfaces of the haunch joint indicated the mode of failure was purely ductile rupture caused by tensile overload.

Hawkins carried out an investigation using Finite Element Analysis (FEA) software to determine the likely tensile capacity of the haunch joint. The results showed that the web of the haunch was grossly under-designed and that its tensile capacity was much less than the forces it would be required to carry. Our investigation revealed that the collapse of the building was caused by the poorly designed haunch joint that was incapable of carrying the forces applied to it by the diagonal truss member.

CASE STUDY 4: POST-FIRE INSPECTION OF STEEL STRUCTURES

A fire broke out in a manufacturing plant that damaged five conveyors. Hawkins was instructed to carry out a post-fire structural assessment of the steel roof beams that had been located above the fire. We attended the site immediately, because a fast response was required to allow the repair and replacement of the conveyors to progress.

Our investigator carried out a visual inspection of the scene from a mobile elevated work platform. The visual appearance of the roof steelworks did not indicate any visible distortion or deformation. No obvious change in colour and no obvious soot deposits were observed on the roof truss. To verify the post-fire yield strength, we carried out calibrated hardness tests at various locations, such as on the roof beams and around the joints. The test results indicated that the post-fire yield strength of the steel elements was still satisfactory. We found that there was no evidence to suggest that the structural roof steelwork had been adversely affected by the fire. Hawkins provided recommendations to clean the conveyors including rust removal and re-painting of steel elements. Hawkins’ fast response enabled the client to get the unit back into operation as quickly as possible and gave them the confidence to continue using the plant.

CONCLUSION

Failures of steel structures are often caused by errors in both the design and the installation. Hawkins’ Built Environment team has investigated many failures of steel structures. Often these investigations are multi‑disciplinary, involving engineers specialising in geotechnical and material science. Our team of experienced forensic investigators provide quick response to failures to assist with stabilisation of the debris and preservation of important evidence. We offer comprehensive failure investigation including root cause determination, assessment of the extent of the damage, and development of rectification proposals. We adopt state‑of‑the‑art technology such as laser scanning and aerial photography to reconstruct the failure scene to facilitate investigation and communication with our clients.

While the probability of structural failure is low, the consequences can be severe. We provide risk management services by carrying out reviews of installation methods and designs to prevent collapses occurring in the first place.

ABOUT THE AUTHOR

Yang Dang is a registered Professional Engineer in Singapore with a background in structural design. Before joining Hawkins, Yang has over 10 years’ experience in the structural design of steel, reinforced concrete, precast concrete building in Singapore, Malaysia, and UAE. Yang specialises in investigating building defects and collapse, façade and cladding failures, and review of structural design.

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High Water and the High Court: Hawkins Evidence Helps with Flood Claim

Jodie — Wed, 10 Jun 2020 16:22:00 +0000

In the case of Tayton and Tayton -v- (1) Warwickshire County Council and (2) Rugby Borough Council, the Taytons (the Claimants) purchased Brookside Cottage and a neighbouring field in 2013, with the intention of building a small housing development on the field. A watercourse flows in a southerly direction along the eastern boundary of the field, and thereafter through two highway culverts, as indicated in the plan below. The Taytons submitted a planning application to develop the field in 2016, but this was refused by Rugby Borough Council (the local planning authority) for several reasons; one of which was in relation to the risk of flooding. They submitted a second planning application but this was also refused and a subsequent appeal was dismissed.

The first of the two culverts is owned by Warwickshire County Council; although the Claimants alleged that Rugby Borough Council owned the second culvert, Rugby Borough Council disputed this. The Claimants alleged that the risk of flooding arose because both culverts had insufficient capacity, which they also claimed had caused the field to flood on several occasions. They brought a claim in nuisance against both councils; the claim included the cost of a flood management scheme and lost profit on the proposed development. They also sought an injunction to require the councils to enlarge the culverts (at a likely cost of more than £300,000 to Warwickshire County Council alone), based on established principles set out in Bybrook Barn Garden Centre -v- Kent County Council [2001]. Weightmans solicitors instructed Mr Richard Keightley of Hawkins to investigate the extent and cause of the flood risk to the Claimants’ property, on behalf of Warwickshire County Council’s insurer. The case was heard in a six day trial at the Technology and Construction division of the High Court in November 2019. Warwickshire County Council’s case was put by Mr Jonathan Mitchell of counsel, with evidence given by Expert Witnesses for all three parties.

The Claimants’ expert relied on a computer model of the watercourse, which had been developed previously to support the planning applications. The model suggested that the field was at risk of flooding from the watercourse overtopping its banks, and that enlarging the culverts would reduce the risk of flooding, while removing the culverts would remove that risk of flooding. The Claimants’ expert concluded that the model showed that the culverts were causing the watercourse to ‘back up’ and flood the Claimants’ property.

Mr Keightley found that the model was not accurately replicating the water levels in the watercourse and frequent flooding of the field, which were shown in the Claimants’ photographs. He explained that there were several reasons for this, including the methodology that had been used to model the catchment hydrology (which underestimated the flow in the watercourse when compared to other methods). Most crucially however, the parameters used in the model did not replicate the actual hydraulics of the culverts and watercourse.

Warwickshire County Council’s culvert; a photograph taken during Mr Keightley’s survey (L) and a photograph taken by the claimants during a flood event (R).

Mr Keightley used the Claimants’ photographs to demonstrate that the level of the watercourse downstream of the culverts during the flood events was higher than the model predicted. He also showed that the actual slope of the water as it passed through the culverts during the flood events was very shallow, and less than the slope of the base of the watercourse. Using ‘first principles’ hydraulics he explained to the Court that this showed there was a ‘hydraulic control’ downstream of the culverts, which was causing the watercourse to ‘back up’ and flood the claimants’ property. In other words, the culverts did have sufficient capacity and were not the cause of the flooding. If they had been, there would have been a significant drop in water level as the watercourse passed through the culverts.

Mr Keightley concluded that the culverts were not causing a risk of flooding to the Claimant’s property. He concluded that the risk of flooding arose primarily due to the field’s low-lying nature and limited permeability, and due to a hydraulic control in the watercourse downstream of the culverts (potentially overgrown vegetation and obstructions within the watercourse, or a smaller diameter culvert further downstream).

“I found Mr Keightley to be an impressive witness. Mr Keightley was rigorously cross-examined by Mr Darton and was criticised in his closing submissions for lack of independence. I disagree with that criticism. Mr Grime described Mr Keightley’s evidence as of ‘lecture quality’. Whilst Mr Keightley was not always ready simply to agree or disagree with questions put to him, my impression was that was because he was a careful expert witness using language with precision, who sometimes did not recognise the terminology used in the questions or who did not want to give an answer that he did not consider was strictly scientifically accurate”.

The claims against both local authorities were dismissed, with the Judge concluding that the cause of the flooding “was obstruction of the channel, not any inadequacy in the design or dimensions of the culverts. In particular, I accept the evidence of Mr Keightley that it is likely that any obstruction or hydraulic control to the flow of the water in the brook was downstream of both culverts”.

ABOUT THE AUTHOR

Richard Keightley is a Chartered Water and Environmental Manager and Chartered Environmentalist. He specialises in the investigation of flooding, water ingress and drainage issues, and has provided CPR-35 expert witness evidence in civil claims for both claimants and defendants. Richard joined Hawkins in 2017 as part of our civil engineering team, having previously worked at two major engineering consultancies, a water and sewerage company and New Zealand’s National Institute of Water and Atmospheric research

The post High Water and the High Court: Hawkins Evidence Helps with Flood Claim appeared first on Hawkins Forensic Investigation.

Investigations Into Ground Related Failures Part 3 – Construction Stage Issues

Jodie — Fri, 16 Aug 2019 09:44:43 +0000

Geotechnical engineering is a sub-branch of civil engineering, focusing on the engineering behaviour of rocks and soils, often in relation to soil-structure interaction. Geotechnical engineers design and construct earthworks, foundations, slopes, retaining walls and other underground structures. The soil-structure interaction aspect requires detailed knowledge and experience of soil and rock mechanics along with engineering geology and structural engineering.

Civil engineering and building failures are often caused by geotechnical issues and these can be grouped by errors / omissions at the different stages of the project: Ground Investigation, Design, and finally construction, which will be covered in this article, the third and final part of this series on Ground Related Failures.

The Construction Stage

The Hawkins Civil Engineering team has found that many failures are caused by errors at the construction/installation stage. Research into what may be affecting the construction stage is ongoing, but initial observations show that there are fewer technically experienced engineers on site in either the contractor or supervisor role; many ‘engineers’ on site are actually contracts or programme managers, who have little to no technical knowledge or experience. The lack of supervision (by the traditional ‘Resident Engineer’) is of particular concern, and may be due to new types of contracts currently being used.

The result of the lack of technically competent engineers on site is that poor engineering decisions are sometimes made.

Partial Collapse of a Hospital

This case involved the collapse of the central part of a Victorian four-storey brick building during conversion (see Figure 1).

I analysed hundreds of contemporaneous site photographs and interviewed the contractors involved, and found that the beams and buttress walls between two main cross walls had been removed, without providing any temporary support to replace the lateral restraint that was lost. However, this would only have the effect of weakening the structure rather than causing the failure to occur.

I carried out a watching brief during the removal of the collapse debris, which enabled me to identify the original location of each of the debris parts in the building pre-collapse (see Figure 2).

After excavation, I was able to identify the initial point of failure as being the footings of one of the walls, while the collapse mechanism (see Figure 3) was a bearing failure in the ground (see Figure 4).

Further investigation revealed that an excavation had been carried out erroneously against the footings of this wall, which explained the mechanism of failure. I discovered that while structural engineers had been instructed on the project to carry out a temporary works design, they had no full-time supervisory role under the contract; as a result there was no suitably qualified or experienced engineer on site acting for the contractor or groundworks sub-contractor.

This case also highlights the importance of carrying out risk assessments for different stages of the design and construction, as required by the Construction, Design and Management (CDM) regulations. If a risk assessment had been carried out, it is likely that the sequencing of the works would have been improved to remove support only after temporary support was added, and after barriers were set up, in order to limit the extent of any excavation in the vicinity of the footings.

About The Author

Andrew Reeves is a Chartered Civil Engineer with a background in the design and construction of large-scale geotechnical solutions in UK, Germany and Hong Kong, including deep basements, large diameter bored pile foundations, retaining walls and tunnels.

More recently Andrew has carried out structural design in timber, masonry, steel and reinforced concrete as well as fire risk assessments for domestic and small-scale buildings in the UK.

Since joining Hawkins in October 2013, Andrew has investigated almost two hundred building-related failures including the collapse of structures, flooding of buildings and land, failure of basement waterproofing, failure of retaining walls and serviceability issues caused by subsidence.

The post Investigations Into Ground Related Failures Part 3 – Construction Stage Issues appeared first on Hawkins Forensic Investigation.

Pit Happens!

Lorraine — Wed, 14 Aug 2019 19:08:19 +0000

During 2012, a large five-star hotel and conference centre began experiencing what started out initially as relatively minor escapes of water, which were dealt with easily by the hotel’s in‑house maintenance team. However, during the next two years, the frequency and severity of the leaks intensified. This culminated in a significant escape of water that occurred during a live televised awards ceremony, when the escaping water entered the main electricity supply and distribution equipment, and caused a site-wide blackout.

At the time of this incident, the hotel was seven years old. The water supply to the hotel was derived from an onsite borehole. Water analysis results from when the borehole was commissioned indicated that the water was scale-forming, or ‘hard’, meaning that the water tended to leave scaly mineral deposits on the internal surfaces of the pipework.

The system functioned without issue for approximately five years, after which, the hotel operators unsurprisingly started experiencing problems with scale formation in the domestic water distribution pipework. In order to address this, the hotel staff procured a water softening system, which was installed and commissioned by its vendor. The escapes of water commenced approximately six months after the commissioning of the water softening system.

The hotel engaged the consultant that had originally designed and overseen the installation of the plumbing system to investigate the cause of the escapes of water. Perhaps understandably, the consultant suspected that the installation of the water softening system was at the root of the problem. The vendor of the water softening system was also engaged to sample the water, and he established that the softened water was ‘slightly corrosive’. A bypass that allowed a small proportion of borehole water directly into the main storage tank was installed on the consultant’s recommendation, and it eliminated the corrosiveness. However, the escapes of water not only continued, but also affected pipework that was replaced after the installation of the bypass.

Frustrated by the lack of progress, the relationship between the hotel operator and the vendor of the water softening system became strained, broke down and was heading rapidly towards litigation when the escape of water that caused the blackout occurred. In response to this, the insurer of the hotel instructed Hawkins to investigate.

Having considered the available documentation, I conducted a scene examination and discussed the circumstances of the incident with both the consultant and the hotel operator. Most helpfully, the in-house maintenance manager had maintained a meticulous record of the individual leaks, and even had the foresight to retain a selection of the failed pipework.

When analysing the available data, it was immediately apparent that all of the documented escapes of water involved pin-hole leaks in the return pipework on the ring-main recirculating domestic hot water system; however, this common thread was not identified by any of the previous investigations.

The general configuration of the domestic hot water system was typical of that used in hotels and other large buildings with multiple occupancy. The water was heated indirectly by a bank of boilers and stored in two, large insulated calorifiers, from which it was circulated around the building. Nothing about the configuration of the system had changed, with the exception of the addition and modification of the water softening system.

Each of the calorifiers was fitted with an electric-motor-driven circulation pump that was configured to produce a maximum water flow rate of 2 litres per second. Two 54 mm diameter flow pipes connected the cylinders to a single 108 mm diameter copper hot water distribution pipe. The corresponding ‘return’ pipe comprised a 28 mm diameter copper pipe, something that immediately gave me cause for concern.

I was given a selection of the failed pipework that had been removed and replaced by the in-house maintenance team over the course of the preceding two years. It was noticeable that most of the pin-holes from which the water escaped had occurred immediately downstream of soldered elbow, or “T” connections.

I sectioned the pipes in the laboratory to enable the internal surfaces to be examined. There was evidence of corrosion surrounding the areas in which each of the pin-holes had formed. The corroded surfaces were shiny and pitted with “horse-shoe” shaped marks surrounding the pits. The wall thickness of the tubes was also reduced significantly.

There was a thin layer of scale present on the non-corroded internal surfaces of the sections of pipework that had formed part of the original installation. However, there was no scale on one pipe that had been installed after the water softening system was introduced, which had then subsequently failed.

The patterns of corrosion damage were very distinctive and typical of patterns that occur as a result of a process known as erosion-corrosion.

Erosion-corrosion is a type of corrosion that is normally caused by a combination of high water velocities, turbulent flows and corrosive water. The high velocity turbulent flow strips away any protective scales and oxides from the inner surface of the pipework, which exposes the copper and allows it to be eroded and corroded by the water. The products of these processes are then swept away, exposing the underlying copper to further erosion and corrosion.

In general, British Standard design requirements require water velocities to be kept below 3 metres/second. However, in order to avoid erosion-corrosion related issues, water velocities in ring main hot water systems should be kept below 1.5 metres/second; although some published research suggests that problems are unlikely to occur at flow velocities below 2.5 metres/second. Notwithstanding this, there is also a requirement to maintain water velocities above 0.5 metres/second, in order to avoid other unrelated problems.

Copper pipes are available in a range of standard sizes based on the external diameter of the pipe. It has been standard practice in the plumbing industry for many years, when sizing pipework in ring main hot water circuits, to select a return pipe size that is “two sizes” smaller than the flow pipe, in order to achieve acceptable flow rates in both pipes.

In this instance, the flow of water through the system was maintained by two electric-motor-driven circulating pumps which acted in parallel and were capable of producing a combined maximum flow rate of 4 litres/second in the 108 mm diameter flow pipe. This would result in a flow velocity of just over 0.4 metres/second in the main flow pipe, which though just below the minimum recommended flow rate, was probably acceptable.

Nevertheless, at times of low demand (e.g. during the night) most of, or possibly all of, the water flowing through the flow pipe would have been returned to the calorifiers via the single 28 mm diameter return pipe, on which the pin-holes were forming. The reduction in diameter from 108 mm to 28 mm would have caused an acceleration of the flow by a factor of approximately 16, producing flow velocities in the region of 6.4 metres/second in the return pipe, which is well in excess of the threshold above which erosion-corrosion is likely to occur.

For a system with a flow pipe diameter of 108 mm, the appropriate return pipe diameter should have been 66.7 mm (i.e. two sizes smaller than the flow pipe); a 28 mm diameter pipe is “six sizes” smaller than the flow pipe. This was clearly not in accordance with industry practice, and was always likely to have caused problems. Therefore, it was clear to me that the issues had almost certainly occurred due to the excessive flow speeds caused by the erroneous sizing of the pipework. Nevertheless, the reasons for the problems only manifesting after the installation of the water softening system still required an explanation.

Both the original tests results and the available witness information indicated that the water from the borehole was scale-forming. Indeed it was the formation of scale within the pipework that had prompted the installation of the water softening system. Ironically, the formation of the scale appears to have been sufficient to protect the pipework from the potential adverse effects of the unduly high water velocity in the return pipework. The non-corrosive nature of scale-forming water would also have helped. However, following the introduction of the water softening system, the water became slightly corrosive, and this would have resulted in the removal of the protective layer of scale from the pipework, particularly in areas with high-velocity turbulent flow, leaving them vulnerable to damage by erosion-corrosion.

Whilst it seemed to both the operators of the hotel and their consultant that the softening of the water was the key factor that initiated the failure, the fact that the properties of the softened water were well within prescribed limits and the evidence that the leaks were only occurring in areas with excessive flow velocities indicated that this position was not valid.

In this case, considerable time and money were effectively wasted on pursuing what transpired to be the wrong party. An incomplete investigation was carried out by a party that not only had a vested interest in the outcome, but also made an erroneous interpretation of the evidence. This case was somewhat unusual, because the maintenance manager was very proactive in documenting and preserving the physical evidence; however, this is often not the case. It can be both useful and cost effective to instruct Hawkins at the earliest opportunity, so that a sound and impartial technical assessment of the issues can be made, and the appropriate actions can be taken.

ABOUT THE AUTHOR

John Holland is a Principal Associate and Chartered Mechanical Engineer based in our Glasgow Office. Since joining Hawkins, John has amassed considerable experience in the investigation of issues affecting both commercial and domestic plumbing systems.

John specialises in the investigation of fires, explosions, escapes of fluid, engineering failures, personal injuries – particularly those involving plant and machinery, building damage resulting from water ingress, and road traffic accidents.

The post Pit Happens! appeared first on Hawkins Forensic Investigation.

Calcium Oxide and its Exothermic Reactions

Lorraine — Thu, 18 Apr 2019 18:57:57 +0000

Hawkins is routinely appointed to investigate the cause of fires occurring at farms. This particular case study discusses some unusual aspects that caused an exothermic reaction, and relates to the use of calcium oxide (also known as quicklime), as an animal-friendly sanitization product. Calcium oxide is most often used in the form of a white powder, made from the thermal decomposition of materials like limestone and seashells. It can be used to disinfect and sanitize, because it creates a dry environment where bacteria will not thrive. It is also often used in agricultural settings as a fertilizer and soil conditioner. Quicklime has previously been used as a weapon of war, it is thought to have been one of the components of Greek Fire. During the reign of Henry III, the English Navy destroyed an invading French fleet by blinding them with clouds of quicklime. In fiction, Bernard Cornwell’s 19^th Century hero, Richard Sharpe used quicklime that he made by burning oyster shells, to stave off the French army (Sharpe’s Siege).

BACKGROUND INFORMATION AND DISCOVERY OF THE FIRE

Before Hawkins carried out an examination of the building, the circumstances of the incident were discussed with the two workers who discovered the fire:

The fire occurred in a wooden building, which housed laying hens, and had an open-air veranda on one side.

At approximately 14:30, the two workers cleared the birds from the veranda, and then spread a bag of calcium oxide over the floor of the veranda to disinfect it. They then put fresh straw over the floor before letting the birds back onto the veranda. Finally, the workers left the building to carry out work elsewhere.

At 17:00, one of the workers noticed smoke coming from the veranda of the building. He went to investigate and saw smoke and flames at the north-western corner of the veranda.

Though there were electric lights in the veranda, these were switched off at the time of the fire. The workers said there had neither been recent work on, nor problems with, the fixed wiring in the building.

One of the workers was a smoker, but he stated that he did not smoke inside the buildings.

INVESTIGATION

Damage attributable to direct fire attack was limited to the veranda.

I noted several piles (approximately 20 cm in diameter) of a white powder amongst the straw in the veranda. I did not find any smoker’s materials in the area of fire damage, nor did I find any around the other farm buildings.

Quicklime is not typically considered a fire hazard, but calcium oxide will react violently with moisture and give off heat (an exothermic reaction). It was initially unclear, however, whether sufficient heat could be generated by such an exothermic reaction to ignite the straw.

I conducted tests to assess if the exothermic reaction of calcium oxide and water could ignite straw. A layer of clean, dry straw was placed on a metal tray. A pile of 500g of calcium oxide was placed on the straw, and then 500ml of water was added. A layer of straw was placed on top of this; there were four temperature sensors (Channels 1 – 4) inside the top layer of straw.

Figure 1 shows the temperatures recorded during this test. The temperature of Channel 1, Channel 3 and Channel 4 increased rapidly once water was added to the calcium oxide. Channel 3 and Channel 4 reached a maximum of approximately 100°C and then plateaued, while Channel 1 continued to rise until it reached approximately 370°C, about nine minutes after water was added. Initially steam came from the straw, but this soon subsided.

Approximately eighteen minutes after the water was added “crackling” was heard coming from the straw, and smoke began to appear. I removed the top layer of straw and saw that the straw underneath it was blackened. However, moving the top layer of straw allowed air to reach the blackened straw, and caused it to glow. A flaming fire quickly developed.

CONCLUSIONS

The tests showed that calcium oxide, when mixed with water, reacts exothermically, and represents a viable ignition source for the straw bedding material on the veranda.

When I reviewed the instructions provided with the calcium oxide, I noticed that they failed to specifically mention the risk of fire. However, the instructions did stipulate that the product must be spread evenly, and that the ground should be wetted prior to treatment.

If the calcium oxide had been spread evenly onto a pre-wetted surface, the exothermic reaction would have occurred rapidly, and heat would have been lost to the surroundings. The farm workers’ failure to spread the calcium oxide in a way that would ensure a rapid reaction was then compounded by the placement of straw on top of the calcium oxide. This reduced the amount of heat lost to the surroundings, and instead provided a readily ignitable material.

Hawkins has prepared a letter for the manufacturers, which detailed these findings and suggested that manufacturers of quick lime review the advice provided with the product–highlighting the need to spread the calcium oxide evenly to reduce the risk of fire.

ABOUT THE AUTHOR

Dr Ian Tatner is a Chartered Engineer (CEng) with a PhD in Corrosion Fatigue of Stainless Steel. Though his career began in the offshore oil and gas pipeline installation industry, Ian joined Hawkins in early 2009, and has been investigating fires and explosions, as well as pedestrian falls, ever since. He has seen a broad range of cases from fires in domestic properties, to more complex vehicle fires and factory explosions. Ian has also investigated numerous pedestrian falls in both public and private buildings.

The post Calcium Oxide and its Exothermic Reactions appeared first on Hawkins Forensic Investigation.

Investigations into Ground Related Failures Part 2 – Design Stage Issues

Lorraine — Wed, 09 Jan 2019 14:10:18 +0000

Civil engineering and building failures are often caused by geotechnical issues and these can be grouped by errors / omissions at the different stages of the project: ground investigation, design, and construction, often with a combination of interacting issues. In the first of a series of articles on ground related failures, issues with ground investigation and the false economy of skipping this step were discussed. In this article the discussion continues with a closer look at failures mainly caused by poor design.

The Design Stage of a Construction Project

Once the ground investigation (GI) and testing has been carried out and the results issued in a Factual Report, usually written by the GI contractor, the data require careful analysis and consideration by an experienced geotechnical engineer, who will write an Interpretative Geotechnical Report.

The Interpretative Geotechnical Report is often not carried out to save money, but is extremely important since it is the link between the GI and the design. It is imperative that conservative soil parameters are chosen as the basis of the design and this is not straightforward when test results can be widely scattered. The ‘moderately conservative’ approach should be taken rather than using the average or the ‘worst case’ design parameters.

Designing structures in or on the ground requires detailed knowledge of not only the soil in the undisturbed state, but also the effect of the construction itself on the soil parameters. Installation of piles is a good example, where bored piles disturb and so weaken the soil much more than driven piles or even continuous flight auger (CFA) piles.

The worst case loading situations should also be adopted, which means for retaining walls, for example, that accidental high ground water should be assumed (e.g. from a burst water main) and that excavation in front of the wall may take place in the future (e.g. to replace services).

Case Study: Sheet Pile Retaining Wall

This case involved the failure of a temporary sheet pile retaining wall installed at the boundary of a new homes’ building site to allow the installation of a permanent reinforced concrete (RC) retaining wall.

Immediately behind the sheet pile on the retained side was a series of brick boundary walls and an existing Victorian brick house. The sheet pile retaining wall failed by forward rotation during the excavation for the permanent wall, allowing the ground behind to move forward, causing the failure of the brick walls and half of the house, which later required full demolition (see Figure 1).

As part of my investigation into the cause of the failure, I designed, contracted and supervised a site investigation (SI) to understand the soil parameters at the site, including carrying out standard penetration tests (SPT), cone penetration tests (CPT) and window sampling, and then carried out a review of the original design.

It was clear that the original design significantly underestimated the ground water level and the depth of the excavation in front of the wall, as well as the surcharge load from the house on the retained side. In addition, the design overestimated the strength of the soil using an ‘average’ design line rather than a ‘moderately conservative’ approach and did not specify what strength the backfill material was to be (the sheet-piles were installed in a slip-trench, which was then backfilled).

Further, the design only used the stronger short-term undrained soil parameters, rather than the weaker long-term drained soil parameters. It is reasonable to use short term parameters for temporary works design, but only if the structure has a short lifespan and if drained conditions are not likely to be encountered during this timescale.

I then carried out a back-analysis of the wall, including modelling the construction sequence (see Figures 2a to 2c), using the specialist geotechnical software FREW (Flexible REtaining Wall analysis) by OASYS. My back-analysis, using more reasonable, moderately conservative parameters, showed that the deflections in the undrained state were three times that expected in the original design and once the parameters were switched to drained, full failure occurred as took place in reality.

In this example poor design led to the failure of the sheet pile and consequential damage to the adjacent property.

Read part III here: Construction Stage Ground Failures: Case Study

About The Author

More recently Andrew has carried out structural design in timber, masonry, steel and reinforced concrete as well as fire risk assessments for domestic and small-scale buildings in the UK.

Since joining Hawkins in October 2013, Andrew has investigated many building-related failures including the collapse of structures, flooding of buildings and land, failure of basement waterproofing, failure of retaining walls and serviceability issues caused by subsidence.

The post Investigations into Ground Related Failures Part 2 – Design Stage Issues appeared first on Hawkins Forensic Investigation.