ENGM 272 - Deep Foundation and Earth Retaining Structures Assignment

Assignment Task

Background: Critical analysis of failure case records of foundations

Engineers can learn a lot from back-analysis of collapse of foundations. In this course work, you are required to reflect on collapses/failures of foundations. Please note that while solving a real problem is a challenge but you may appreciate that this will provide you extra insights on the issues related to design. Furthermore, you will be able to compare recommendations from different codes of practice and best practice guidelines. You will also be accustomed with real data (realistic ground profile, foundation dimensions etc) that you would encounter in practice .

We have used this approach over the past many years for teaching this subject and the students find this experience rewarding. The students know the answer to the questions of the coursework and the challenge is: Is your analysis able to explain ( hopefully convincingly ) why the foundations collapsed? Once you complete the coursework, you know what to avoid in design and construction.

A note on the study of case histories:

Foundation Engineering is challenging due to multi-disciplinary nature:

• You will need the loads from the structural engineer or in some cases (for example wind turbine foundation) from aerodynamic engineers and hydrodynamic
• The ground profile you would need may come from site investigation which is a mixture of various disciplines (Geology + Geophysics+ Lab testing).
• The process of construction will be decided by the resident engineer often without your consent!

Often in a design, the weakest link is the interface between two disciplines. This particular coursework will help you to appreciate the multi-disciplinary nature of the subject and the challenges faced by the profession.

Case study 1: Complete uprooting of pile-supported building

For the first time in March 2011, it was observed that following an earthquake and Tsunami, the pile- supported buildings were completely uprooted from the ground – see Figure 2 & 3 for details.

Currently, there are no guidelines in any codes of practice on how to design pile-supported buildings in coastal areas which may be subjected to Tsunami . Work in underway to develop codes of practice as these collapses took the pile designer by a big surprise as pile foundation is considered as all-safe solution.

Background of the problem : During the 2011 Tohoku earthquake, many coastal cities and towns were devastated completely. A catastrophic destruction occurred in Onagawa, Miyagi Prefecture, where very high and fast tsunami waves flowed in and inundated the low-lying part of the town entirely. Some of these structures had pile foundation, and it was observed that the piles were sheared off at some dept

Details of one such building:

Observed foundation failure includes a combination of sheared piles and uprooted piles. See the photographs and diagrams (below) for dimensions and data. For missing data, make reasonable assumptions and state them. You may use engineering judgement, scale the photographs or do some literature review. This coursework will help you to handle real data and issues faced by the engineers.is

Case study 2: Transcona Grain Elevator bearing failure in Winnipeg, Manitoba (Canada) in 1913.

Figure 10 shows the details of the collapse along with the section. This is well documented case study and you are welcome to search literature on this case study. Some links are given below:

Figure 10: Photograph of the collapse along with the cross section.

The RCC superstructure and the adjoining bin house was completed in September 1913. The structure that collapsed was the bin house and composed of 65 bins arranged in in 5 rows having 13 bins in each row. The bins were approximately 4.27 m in diameter with about 28.05 m high supported on a concrete raft of 0.61 m thick and 23.5m wide and 59.5 m long at a depth of about 3.66 m.

Filling with grain began soon after the construction in September 1913 and at lunchtime on 18 Oct after 875,000 bushels of grain had been evenly stored in the bins settlement and tilting of the bin house was observed. Within an hour, settlement of 0.30 m followed by tilting towards the west was observed. The earth on the west side started to heave as the structure moved towards it and the movement slowed down. The movement of the structure continued and it eventually came to rest after about 24 hours with a final angle of 26º 53' to the vertical.

The soil profile can be assumed as follows unless you obtain a better source:

Morley (1996) reports the following:

According to a 1913 article in Engineering News ("Failure" 1913), small plate load tests were made when excavation was completed that indicated that the soil was able to bear between 383 and 479 kPa. The bin house was designed to have a dead load of 18,140 metric tons or 125 kPa. Filled with grain, the total load would be a little more than 316 kPa. When the bin house failed, it was loaded with 875,000 bushels of wheat uniformly distributed, bringing the total load to only about 297 kPa .

Case study 3: This case study is often known as “Kissing Silos”

[See Figure 11] and this will help you understand a major issues in foundation design in urban areas where we may try to construct closely spaced tall buildings. Further details can be found in Bozozuk, M (1976).

It has been observed that foundation failures occur in clays when a silo is quickly loaded for the first time. There are different mechanisms and processes in terms of foundation-soil interaction. One of the important ones is the filling of the silo. As the filling process progresses, the loads are applied and carried by both the soil skeleton and the pore water within the clay. The pore water pressure will reduce the effective stress in the soil and hence decrease the shear strength of the soil which may lead to bearing capacity type failure. With time the excess pore-water pressure dissipates and the soil gains strength. With unloading of the silo, vertical stresses are reduced.

Figure 11 shows two silos that were built on Lake Agassiz clay in Red River Valley in Canada. Bozozuk (1976) reported that the silos were too close so that the “pressure bulbs” under the foundations overlapped. This caused larger stresses and, in turn, larger settlements under the parts of the ring foundations where they were closer to each other. The end result was tilting, touching, and eventual structural failure of the silos. If a group of bins are constructed together to form a cellular structure, there is a potential for the individual stress bulbs to overlap in the two orthogonal directions

Reflect: What are the important lessons one may learn in constructing buildings in a highly congested city?

Case History 4: Analysis of collapse of a pile supported building in China

On 27 ^th June 2009 at about 5:30am on June 27, a 13 storey building under construction at Lianhuanan Road (Minhang district of Shanghai city) collapsed killing one worker. Figure 11 shows the collapsed building together with two identical unaffected buildings. Details on the foundations and ground profile has been obtained in Ng and Hong (2012).

After the collapse, the building remained intact. However, the piles located close to the excavation were broken into number of pieces within depths of 4 to 6m below the ground surface. On the other hand, piles located nearby the fill slope were generally broken at the pile head connected to the ground beam.

Foundation details and ground profile:

Pile details : 33 m long pre-stressed, precast lightly reinforced concrete piles. Each pile had an outer diameters of 400mm and inner diameter of 300mm.

Ground profile: The top soil consists of 28m of thick clay stratum and then medium dense sand. Undrained shear strength profile of the clay was estimated from the field using in-situ vane shear. Table 1 shows the ground profile and may be assumed as a reasonable estimate for carrying out calculations. Figure 11(b) shows the collapse of the building from another view and Figure 14 shows exposed reinforcements in the pile.

Events leading to failure

The events are described by [1] [2] [3] and [4] as in Figure 14 based on a newspaper:

• An excavation was being carried out to a depth of 6m one week before the collapse of the building.
• The excavated earth is being piled up on the other side of the to a height of about 10m
• The building foundation experienced unbalanced lateral pressure
• This may have caused lateral pressure in the piles causing bending moment which it was not designed for which may have caused the building to topple in the other direction of the pile up

An analysis was carried out by Professor Fayun Liang at Tongji University of this case study. He provided me with the following data on Young’s Modulus of the Soil which he used.

Case History 5: Collapse of Sleipner A Condeep platform

You are advised to look into the following sites:

Summary: The first concrete base structure for Sleipner A sank under a controlled ballasting operation during preparation for deck mating in Gandsfjorden outside Stavanger, Norway on 23 August 1991. It is reported that the crash caused a seismic event registering 3.0 on the Richter scale. The total economic loss was about $700 million.

Sleipner You may also look into other literature on this failure. Issues to think about?

1. Can this be Can simple hand calculations or order of magnitude calculations have spotted the error?

2. Was the modelling in Finite Element and method of construction compatible? Did geometrical shaping of some finite elements in the global analysis played a role?

3. Does it say something about the use of excessive computer modelling? It is often tempting to use software to carry out all types of How do we verify and validate the results what the program generate?

Case study 6: Concrete Foundation collapse for wind turbines

Here are some educated guess on the problem and the following may be assumed: A nearshore wind farm is to be constructed on the east cost of the Chinese sea near the Dalian area and the area is pinned in Figure 19. The wind turbine generators used in this power plant can be assumed to be 1.5 MW. The loads on the foundation are provided in the paper.

Exact ground profile is not known but based on a hotel construction nearby the following can be assumed.

Existing ground geology is also complex, with a highly fractured rock profile underneath and a high ground water level.

Use engineering judgement for parameters you may need and state them.

Reflect on the following:

• Provide a brief discussion on how such failures can be prevented in the future?
• Is the foundation suitable for the application? What would you have done differently?

Case study 7: Builder fined after Extension foundations cause Neighbour’s house to collapse

Introduction

On the night of 23 ^rd February 2012, part of a house in Wolverhampton collapsed after a Contractor dug a trench for the extension of the neighbour’s property the previous day. The trench was dug 600mm lower than next door. The Contracting firm was an experienced Builder, Astbury Design and Build Ltd who had been employed to build the extension at a property in Blackburn Avenue.

Figure 20: Photos of Case Study 7: Neighbour’s house collapses due to extension construction

An investigation conducted by the Health and Safety Executive revealed that the Contractor breached the Party Wall Laws by failing to notify the owner of the adjoining property of its intention dig a trench and proposed methodology for executing same.

The owner of the adjoining property who hired its own Consultant prior to the collapse had warned the Contractor about possible implications with its construction methodology, which could have led to the undermining of the adjoining property’s foundation. After the collapse, the Contracting firm pleaded guilty in court for breaching Section 3(1) of the Health and Safety at Work etc Act 1974 and was ordered to pay £12,000. The Health and Safety Executive told reporters after the court hearing that the entire collapse could have been averted if the Contractor and owner had followed the procedure relating to party walls and detailed the work to the neighbour’s Consultant. There were no injuries since the property was unoccupied at the time of the incident and the house was since rebuilt.

Case study 8: Failure of a bridge due to scour of the foundation

The Malahide Viaduct is a viaduct, owned and operated by Iarnród Éireann (Irish Rail), which crosses the Broadmeadow tidal estuary situated about half a mile northof Malahide train station. It serves the Dublin to Belfast railway line and local commuter trains.

On 21 ^st August 2009, a little before 18.30 hrs, at peak commuting time, two spans ofthe viaduct fell into the sea. The collapse occurred very shortly after a packed train had passed over the viaduct.

A full investigation was carried out by the Railway Accident Investigation Unit (RAIU)which culminated in a report being issued in August 2010. The failure ultimately wasattributed to scour undermining the foundation at the base of the pier supporting the two spans. What is interesting about this case study is that a member of the local sea scouts had reported damage and it had been inspected and cleared by the areaengineer earlier on in the same week.

Scour is defined as “the erosion or removal of streambed or bank material from bridge foundations due to flowing water” (Kattell and Eriksson, 1998). This is a hugeissue all over the world as a large proportion of bridges cross water.

Information about scour:

Scour occurs in three main forms, (Prendergast and Gavin, 2014)

1. General scour – occurs naturally in rivers and involves the degradation of theriverbed due to changes in hydraulic

2. Contraction scour – occurs due to the increase in scour velocity caused bynarrowing of channels due to abutments and the like.

3. Local scour – localised erosion at piers caused by downward The belowimage shows the types of local scour that can occur.

Figure 22: Scour processes (Prendergast and Gavin, 2014). History of the Malahide Viaduct

History of the viaduct (all information on the history of the viaduct is from the RIAUreport (RAIU, 2010) and

an article in The Structural Engineer (Brady, 2013)).

The original viaduct was built in 1843, it was a timber structure with 11 equal spans of 15.85m. The spans were supported on ten timber piles in the bed of the estuary and the tidal movement of the water resulted in scouring around the piers. A few years later, large stones were installed around the piers, with more added along theline, forming a virtual permeable stone weir along the entire length. This helped by reducing the tidal action, preventing the scour.

In 1860, the viaduct was replaced by iron lattice girder beams supported on 11 masonry piers, which in turn were supported on two 300mm deep stone foundationcourses built on the existing weir. This incarnation had 12 spans in total, 8 spans of15.85m and 4 of 12.9m and the new piers were installed at the centre of the old spans.

In 1965 the viaduct was replaced again. Due to the need for a quick construction, increased load capacity required and ballasted deck, the new viaduct kept the original piers and pre-cast post-tensioned concrete beams replaced the lattice girders. Pre-cast concrete bedstones were added to the top of the piers. The photolog in Appendix A includes photo 1, which shows the bridge prior to failure.

As the history shows, the weir was subject to scour action from the beginning soworks to stabilise the weir were part of ongoing maintenance.

Between 1967 to 1972 a grout apron was installed around the weir by injecting concrete between the stones to create grouted rock armour to stabilise the weir. Thedepth of this was about 2m at the piers and 1.5m at the mid spans. Additionally, new stones were dropped into the estuary periodically over the years with the last discharge being in 1996.

Failure mechanism:

The driver of a Dublin to Balbriggan train travelling towards Dublin, noticed water splashing over the viaduct as the train approached. As the train was passing over at 18.22 hrs, he noticed some subsidence of the track. Using excellent judgement and quick thinking, he switched the train to coast and once the train was safely across, immediately issuing the alarm. This meant that all further traffic was stopped from crossing over. On inspection a short time later, it was discovered that pier 4 had collapsed and spans 4 and 5 had fallen into the seawater below. Photos 2-4 in Appendix A show the collapsed viaduct.

Earlier that week on August 17 ^th , a sea scout leader had been out canoeing and noticed that some of the stones at the pier base had been washed away. He rangIarnród Éireann to alert them. He later told the NCE that members of the sea scoutsregularly use the estuary to canoe and had been noticing a channel developing inthe weir since July that year (NCE, 2009).

Acting on this, Iarnród Éireann sent an area engineer to inspect the pier on August 18 ^th , just three days prior to the collapse. The engineer, who it later transpired, didnot know that the weir was load-bearing, noted some stones missing from the masonry pier and assumed this had been the damage referred to. He reported backthat there was no problem and the bridge was fine (RAIU, 2010).

Findings of the investigation:

In order to determine exactly what the mechanism of collapse was, the RAUI set upphysical 2D and 3D models in conjunction with University College Cork. These models were used to gain an understanding of the hydraulics of the weir. The models concluded that over time with lack of maintenance, the flow of the water through the viaduct at pier 4 had changed considerably (RAIU, 2010). Figure 23 illustrates the changes to the weir over a long period, caused by the changing of the flows.

First the weir started to elongate on the east side, with material leaving the top and being deposited on one side. This erosion continued to occur and eventually the grouting operation was carried out, but the elongation of the weir was not corrected.The grout layer helped but ultimately the erosion kept happening and once the erosion was so far down the side of it, the grout began to break away too, makingthe whole situation worse.

Figure 23: Progression of the damage to the weir over time (RAIU, 2010)Top – original situation, Middle – weir profile elongating to the east, Bottom – grout apron built and severe erosion to the weir around it.

Erosion of the grout was also happening mid span, where the depth of grout was already shallowest, this meant the faster flows between the piers were acting closerto pier 4. These eventually worked their way underneath the pier in an action knownas piping (a pipe like tunnel underneath a foundation – see Figure 24). Piping can be a common issue in the undermining of foundations due to scour.

Figure 24: Illustration of piping action under pier 4, plan at top, section below(RAIU, 2010).

The RAIU report concluded that “The immediate cause of the collapse of Pier 4 was as a result of the undermining of the remaining grout apron that surrounds and supports Pier 4 through the action of piping (scour)”. It also noted contributory factorsof long-term gradual elongation of the weir, partial removal of the grout at mid spanand short term propagation of the hydraulic jump (RAIU, 2010).

Photos taken by sea scout leaders of members canoeing, taken over the years wereprovided to the investigation and they showed the flow conditions around the weir. Reviewing the photos showed that in March 2009, there was no evidence of scour,but by July they clearly showed deep channels had formed under spans 4 and 5 (Meagher, 2018). The investigation also reviewed in depth the Iarnród Éireann inspection protocols,records. They spoke to current and past members of staff, members ofthe public.They highlighted recommendations they found that had been given to Iarnród Éireann as a result of previous inspections or reviews.

It is an important part of the report, without going into too much detail about thistopic, it was concluded that there were a number of failings (RAIU, 2010):

• Company policy recommendations on the frequency and type of scourinspections were not taken on
• Inspections of the viaduct were
• Inspections that were carried out, were incomplete or carried out
• Inspections were carried out by engineers without a complete knowledge ofthe structure or training in recognising scour
• Corporate memory loss - knowledge of the weir and its propensity to experience scour was lost due to changing of systems and moving of staffbetween

Repairing process:

The viaduct was successfully repaired and reopened for traffic within three months.The works consisted of repairing the hole left by the piping, repairing the weir in general, regrouting it and also reinforcing the piers with piles.

Conclusions and recommendations:

Although the report concluded that the immediate cause of failure had been the scour piping damage, it was emphatic that there were other contributing factors andfailures that had led to the point of collapse.

These included the long-termdegradation of the weir and the documentation,inspection and maintenance issues highlighted above.

It had been known since the mid-1840s that the weir was needed and that it was often replenished with stone over the years. Iarnrod Eireann, the owner of the asset managed to lose this knowledge from the correct department to their great cost. Therepairs took 3 months to complete, during which time they had to lay on replacementbus services. In addition to this, the repairs cost in the region of five million Euro (RTE, 2009)

The lack of a coherent inspection and documentation system with inadequately trained inspectors meant that the damage happened beneath the surface of the water and no one in the company was aware of it. This meant that vital maintenancewas not carried out and a disaster was avoided only by luck.

The repair that was carried out will go some way to making sure that the scour problem has been mitigated. As a result of the report recommendations, Iarnrod Eireann put in place a comprehensive inspection plan for all of its assets around the

country. It is unlikely that scour will again be a forgotten issue among the railway bridges of Ireland. The repair went one step further though in installing piles. Thismeans that the effect of future scour will not be as important in the integrity of thebridge.

Traditional means of minimising scour damage have been centred around threemethods (Bonilla-Gris, 2010).

1. The protection of the base using rock, concrete, mattresses or

2. The installation of elements nearby which will help to guide and modify the flow so it moves away from the

3. Supporting the foundation at a level deeper than the scour

When designing a bridge where the foundations are supported in the water, it isimperative that the hydraulics of the water are considered. If these can be considered, then appropriate protection can be provided to prevent scour from occurring. At the design stage it is possible to minimise the likelihood and mitigate the severity of scour by sensible design. For example, larger bridge openings and streamlined pier geometry can make sure velocity of the water is not excessive (May, Ackers andKirby, 2002).

As this case study showed, inspection and maintenance are key in preventing scour. Inspections must be carried out regularly and be carried out by divers who can get under water and really find out what is happening.

There is a growing field of monitoring instruments which would help with identifying the effects caused by scour. However they are often expensive, liable to flooding damage and the data can be difficult to interpret (Prendergast and Gavin, 2014). So for the moment, good old- fashioned attention, inspection and maintenance are key.

Case study 9: Leaning Tower of Pisa

In the 1990s the tower of Pisa was inclined southward. Reading the material illustrated below, investigate, and summarise the studies about the failure of Pisa tower.

Case study 10: Carsington Dam

The Carsington Reservoir project involves pumped storage of water from the River Derwent near Hognaston, Derbyshire, UK. A 1200m long and 38.5m high above foundation level embankment dam was constructed to form a 36000 Ml capacity reservoir. A location plan together with geological map of the site is shown in

• The construction of the dam commenced in 1980. The embankment was constructed with clays and weathered mudrocks which were obtained locally from borrow areas within the reservoir area. Clays were utilized to construct the embankment core with an upstream boot shaped extension and mudrocks to form the dam shoulders. Horizontal drainage layers of crushed limestone at vertical interval of about four metres were incorporated in the embankment construction. A cut-off grout curtain was provided on the upstream of the embankment. Schematic details of the embankment are shown in Figure 7.2. The embankment was failed in June 1984 when it was nearly complete.

The owner of the dam commissioned an investigation which concluded in 1986 that reconstruction of the dam can be carried out at the site with modified design. A further investigation was carryout in 1987. The reconstruction commenced in 1989 and completed in 1992 ([2], cited various references).

• Site Geology
• The site is underlain by a thick sequence of mudstones part of Namurian Series of Late Carboniferous age. To the immediate north is the sequence of the Early Carboniferous Limestones of the Peak District. The bedrock strata have been subjected to subaerial weathering, probably during the Tertiary Period, and therefore weathered to some extent. Bedrock is overlain by a thin layer of solifluction deposits of colluvium or ‘head’ deposits, consisting of yellow and grey slightly gravelly clay
• Failure Investigation
• The failure mainly developed in the upstream shoulder and crest over a length of circa 500m, with the crest dropping some 10m (see Figure 7.3 & 4).
• Figure 25: upstream slip failure surface of the Carsington earth embankment dam (taken from Sachpazis 2013)
• The post failure investigations indicated the initial failure occurred from shearing through the core and along a layer of yellow clay (colluvial/head deposits) beneath the embankment foundations [3]. Multiple shear surfaces were also noted in the clay core which were caused by rutting from moving plant. Yellow clay layer was found to contain solifluction shears which are pre-existing shear planes developed within colluvial deposits when moving downhill during their erosion/transportation/deposition process over the geological
• In UK practice the traditional design for this type of dam was a 1:3 on upstream and 1:2 or 2.5 slopes downstream with a central watertight clay core taken down into foundation as a cut-off [4]. In the downstream construction of the dam this tradition was followed, and it was completely stable. But on the upstream, the mysterious boot shaped extension of the clay core did not bring any benefits, perhaps the dam designers were anxious to prolong the drainage path under the dam but did not realize its detrimental effect on the stability. It is therefore considered that the stability would have not been endanger if the traditional design of vertical core extended down to form a cut-off would have been adopted. This proposition suggests that as the downstream embankment has also been founded on yellow clay layer, but it is stable therefore a traditional design would have produced stable dam even with both shoulders founded on yellow clay. However, during failure investigation almost every engineer asked this question that why it had not been stripped off in the preparation of the embankment foundation. The removal of this layer would have removed the pre-existing slip surfaces and would have given additional stability for a traditionally designed dam. The reason for the large ‘boot’ the clay core extension to upstream has never been satisfactorily explained by the original dam design team.

Failure slip plane

• Based on instruments measurement which included the measured settlement of rows of 5 pegs placed across the unfinished crest over a period between October 1983 to April 1984 and blockage of a piezometer were clear indications of excessive strains and developing a slip plane. Measured pore pressures indicated an increase in pore water pressure during pause periods i.e. when no fill was being placed, therefore, there should have been a careful consideration stability before starting filling at the beginning f 1984
• In his discussion on [4], Dr Cox (University of Westminster) comments that the slip at Carsington can be analyzed in context of a large number of existing roads, rail and dam embankments where Carboniferous mudstone has been utilized as fill material in both the embankment foundation and embankment slopes. These have side slopes typically between 1:2.5 (22°) and 1:1.5 (34°), with a frequent value of 1:2 (27°). Compared with the dam failure at Carsington where embankment slope was 1:3 (18), these surviving embankments are on much steeper slopes. A study by Parsons & Perry (1985) of some 300km motorway embankments showed that the slopes constructed with Carboniferous Mudstone fill are the most stable slopes with 2?ilure rate at 27° and less than 1% at 18° with mainly shallow
• In Dr Cox opinion one of the reasons of the original embankment failure was insufficient compaction during placement of the dry mudstone fill, which had a low density with high air voids. Premature flooding of the upstream embankment caused inundation settlement (collapse compression) of the newly placed dry

Under these conditions’ embankment would have caused some element of earth tremble resulting in temporary liquefaction of the loose flooded base layer which would have contributed in facilitating to initiate failure plane.

• In his discussion on [4], Professor Bromhead (Kingston University) comments that he was approached in late October 1983 to undertake a series of stability analysis on the Carsington Dam embankment, which was approaching to its completion. There were already concerns on stability for this reason, a berm was constructed on upstream toe of the dam. Analysis were carried out on one of the sections which was considered that highest section. The problem in the analysis was selection of soil properties. At the time additional tests had been undertaken and the engineer’s original parameters had been refined. However, the number of tests were in sufficient compare to the scale of the project. Another uncertainty was the pore water pressure data to be used in the
• The failure of Carsington Dam is extensively studied case study within UK geotechnical practice and there are numerous publications on both failure and the reconstruction of the dam. The failure incident provides extensive learning materials on slope stability and
• It is likely that causes of failure on such a large scale (over 500m length of the structure) would probably involve a number of factors rather than pinpoint a single factor. However, the failure of Carsington dam highlights the requirement of close interaction between design engineers and
• The design engineers must have identified the soft yellow clay layer and removed/stripped during ground preparation for embankment foundation. Lesson learnt from this incident must be used in routine UK Author (during current employment) has worked on number of projects where similar ground conditions were present, particular on large shed as well as road projects where enabling works require significant earthworks. Preparation of formation level prior to commence upfilling is vital. Any residual soft layers can cause both instability as well as settlement issues.
• Similarly, compaction of materials should strictly follow earthworks specification and a high-quality management system. Investigation studies indicated on Carsington dam that it was evident the materials were either compacted dry or perhaps in some instances over-compaction which resulted in rutting in the clay fill and hence prone to create slip surfaces, which eventually resulted in the dam
• In summary, it is a vital lesson for geotechnical engineers and whilst working on a project involving slope stability issues, they must ensure to:
• Undertake thorough detailed ground investigation;
• Establish correct geotechnical parameters;
• realistic pore water assumptions should be made and must be confirmed by field investigations;
• A realistic factor of safety should be adopted;
• Rigorous stability analysis should be carried out to assess the critical slip surfaces and global stability issues

Case study 11: Segmental Retaining Wall Failure in Chung-Nam, Kore a

The wall was constructed for a newly developed factory complex in Chung-Nam Province, approximately 200 km south of Seoul, Korea. At one location, a 150-m-long retaining wall was required, ranging in height from 1 to 7.4 m, to retain an embankment for use as an approach road to the factory complex. The wall was situated on a slightly sloping ground, immediately next to a 2-m-wide waterway located approximately 4 m away from the wall face.

The collapse occurred in late July 2003 after a heavy rainfall in this region. As seen in the photos taken after the collapse in Fig. 30, the collapsed portion of the wall extended approximately to 60 m, resulting in a total slid volume of soil over 1,200 m ³ .

Including the observed failure pattern, numerous evidences led to a conclusion that the wall failure was somewhat closely related to the global slope and external instabilities. A further field investigation revealed that the failure pattern followed the circular type and was similar to the calculated one from the limit equilibrium-based slope stability analysis, passing the sloping toe and the retained zone as depicted in Fig. 31.

Case study 12: Find yourself one failure of Geotechnical Structures/Foundations/Earth Retaining Walls or linked with geostructures from literature

Please select any cases study from the geotechnically related failures (for example, this could be excavations, tunnels, in-ground barriers, buildings, wind turbines, landfills, etc.). If required, carry out either a simplified (hand calculations or using excel spreadsheet) or standard (industrial level software and methods) or advanced calculation (FEM software) to justify your discussion and conclusion.

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