Medmerry Realignment Scheme: Design and

Medmerry Realignment Scheme: Design and Construction of an Earth Embankment on Soft Clay Foundation Toru Higuchi † High-Point Rendel Ltd, London, UK Tony Bruggemann † Atkins Ltd, Epsom, UK Sunday Obeahon Jacobs UK Ltd, Croydon, UK John Gosden Jacobs UK Ltd, Reading, UK Alastair Elder Jacobs UK Ltd, Croydon, UK

Summary The Medmerry Realignment Scheme will provide increased protection against flooding for the communities near Selsey on the south coast of England by realigning sea defences up to 2km inland whilst creating a new intertidal area for improving the region’s wildlife habitats. The new flood defences comprise a 7km-long embankment constructed from layers of site-won cohesive fill with a height of up to 5m. Since parts of the earth embankment were founded on alluvial soft clay, an accurate prediction of pore water pressure dissipation in the foundation was the key to adoption of a safer and more economical construction method.

Introduction The Environment Agency (EA) is charged with maintaining the flood defence assets on the Medmerry coastline, which is located on the south coast of England approximately 10km to the south of Chichester, West Sussex (Figure 1). The Medmerry coastline extends from Bracklesham in the northwest to Selsey in the southeast, being one of the stretches of coastline most at risk of flooding in southern England, and being also threatened by sea level rise. The existing frontage comprises a shingle flood defence ridge which is at risk of breaching annually. Following a serious breach event in 2008, realignment of the sea defences inland was selected as the best solution for this stretch of coast. The Scheme will not only provide a much higher standard of protection for the local communities, but also allow once-lost wildlife habitats to be recreated in a new intertidal area seaward of these new defences, which themselves will help absorb the force of waves. One of the challenges in the design phase was the prediction of the excess pore water pressure developed in soft clay foundation by the embankment loading since the rate of embankment fill placement is limited by the rate of dissipation of the excess pore pressures. The length of the embankment at Medmerry enabled the development of a staged construction approach, which utilised the space available to keep earth fill operations progressing while allowing natural dissipation to take place in most areas. This approach limited the need for costly foundation treatment to a short length of embankment. In addition the observational method was utilised to validate the approach adopted. Some of the key aspects of the Scheme from the project development through to design and construction of the earth embankment will be presented in this paper.

Project Description The frontage at Medmerry has a long history of management intervention. Between 1976 and 1980 a recharge scheme was completed, placing 230,000m³ of shingle on the beach. In December 1989, the shingle bank breached and approximately 70% of the imported beach material was lost. Subsequently, regular and extensive beach management has been implemented by importing beach material and re-profiling the shingle banks to ensure a suitably high (5m above Ordnance Datum [AOD]) and wide (5m crest width) barrier each year at an annual cost of between £200k and £300k. †

formerly Jacobs UK Ltd, Croydon, UK

Notwithstanding this level of maintenance, the banks have breached 14 times since 1994. The most serious recent event occurred in March 2008, estimated to be a 1 in 20 year event, when a breach occurred during a major storm, severing Selsey’s only road link (B2145) and isolating the town’s 10,000 inhabitants from the ‘mainland’. This caused over £5M of damage to local businesses and required the evacuation of some areas due to the risk to life. Following extensive consultation with local residents and interest groups who have actively contributed to the design of the Scheme, the Pagham to East Head Coastal Defence Strategy, produced by the EA, was approved by Chichester and Arun District Councils in October 2008. The preferred strategic option selected for the Medmerry frontage was Managed Realignment of the existing shingle banks and to hold this new realigned defence line in the medium to long term. The Scheme is initially to provide a standard of protection in excess of a 1 in 1000 year event, falling to a 1 in 100 event over the next 100 years, including an allowance for climate change. The new defences are to be built up to 2km inland from the coast and comprise a 7km-long earth embankment (Figure 1), managing the risk of coastal flooding to 348 properties and also giving additional protection to key infrastructure in Selsey including the road link, a wastewater treatment works and electricity substations.

Figure 1 Location and general arrangement of Medmerry Realignment Scheme

Figure 2 Geological section along the line shown on the plan in Figure 1

The Scheme will also enable the formation of 183 hectares of new intertidal habitats and 80 hectares of new transitional grassland. This will offset losses of internationally designated intertidal habitats through coastal squeeze, enabling other coastal flood and erosion risk management schemes to go ahead across the Solent area. While the formation of the intertidal habitats (mudflat invertebrates and saltmarsh plants) could typically colonise within the first two years, the development of the new transitional grassland habitats will be ready after one season.

Scheme Objectives The Scheme has the following key objectives, drawn up through consultation with stakeholders: • to encourage community participation during development of the Scheme; • to provide a sustainable flood and coastal erosion risk management scheme; and • to maximise the extent of Biodiversity Action Plan habitat, making use of existing topography.

Project Inception In January 2009 Jacobs was selected under the EA’s National Engineering and Environmental Consultancy Agreement to undertake the project appraisal and, subsequently, was commissioned for detailed design and site supervision roles in accordance with the framework procurement principles. Team Van Oord was selected as the construction contractor through the EA’s National Contractors’ Framework and initially collaborated with the consultant under an ‘early contractor involvement’ role during detailed design to ensure best value in terms of buildability. Design and construction contracts with Jacobs and Team Van Oord were both procured as NEC Option C (Target contract with activity schedule). The project value is approximately £20M.

Environmental and Land Ownership Constraints Field surveys confirmed the presence of protected species, including water voles, great crested newts, reptiles and badgers. A mitigation strategy developed for the protected species, in consultation with Natural England to obtain the necessary licences, included fencing and trapping to move water voles and reptiles away from the immediate construction areas, creating new setts for badgers and discouraging nesting birds within the construction areas in partnership with farmers and the RSPB. Within the intertidal area, parts of the ground were raised above the mean high water spring level to form bird islands (Figure 1). Approximately 500 hectares of agricultural land were purchased by the EA at a cost of £8.8M to enable scheme construction including creation of intertidal habitat.

Ground Conditions Ground Investigations Four phases of ground investigations for the Scheme were carried out between 2003 and 2011, including 34 trial pits, 26 boreholes and 58 cone penetration tests (CPT’s). Over 700 disturbed samples were used for soil/water chemical analyses and geotechnical physical property testing while 98 soil samples were used for geotechnical mechanical property testing to ascertain types of soils and assess the stability and the settlement characteristics of foundation soil layers. During construction, soil characterisation tests (natural moisture content, sieve analysis, Atterberg limit and compaction) were conducted on materials from borrow pits and perimeter drainage channels to confirm the suitability for embankment fills in accordance with the project’s earthworks specification.

Ground Models The geology on the Scheme site comprises Quaternary drift deposits (Alluvium) overlying the Middle Eocene Bracklesham Group Formation, which in turn is underlain by the Lower Eocene London Clay Formation. The London Clay was not encountered in any of the boreholes drilled to a maximum depth of 15m during the ground investigations. Figure 2 illustrates an inferred geological section along the line shown on the general arrangement in Figure 1. The uppermost layer of the Alluvium, 1m to 2m in thickness, is a firm to stiff crust made of desiccated clay/silt, which was formed mainly due to fluctuations in the groundwater level. The desiccated Alluvium is underlain by moist alluvial soils, up to 9m in thickness, comprising a complex interbedded

sequence of very soft to firm clay/silt and very loose to medium dense sand with the occasional presence of peat and organic clay. The Bracklesham Group encountered on the Scheme is subdivided into four sub-units with its stratigraphical sequence being the oldest at the north of the site and becoming younger towards the south as shown in Figure 2. The material is over-consolidated due to the erosion of later overlying deposits and generally classified as laminated/interbedded firm/stiff clay and dense sand. Parts of the earth embankment, approximately 2km in total, are situated on low-lying areas at ground levels typically 1.5m to 2.0m AOD. These areas are either in close proximity to the coastline or along existing streams/creeks; their foundations consist of topsoil (< 0.3m-thick) and a 5m to 9m-thick layer of Alluvium underlain by the Bracklesham Group with the groundwater strike recorded at a relatively shallow depth (1.7m to 2.5m below ground level [BGL]). The rest of the embankment, approximately 5km in total, is located on relatively higher ground, typically, above 2.5m AOD as marked in Figure 1. These areas are frequently used as farmland and their foundations generally consist of topsoil, directly underlain by the Bracklesham Group, with the groundwater level typically deeper than 6m BGL. At the locations where existing streams intersect the alignment of the embankment a 1m to 2m-thick layer of alluvium, comprising soft to firm clay/silt and medium dense sand, was also present between the topsoil and the Bracklesham Group.

Cut-off Trench A network of land drains running across farm fields was present at depths typically 0.5m below ground level. These were intersected by means of 1m-wide cut-off trenches excavated, along the centre line of the embankment, to 1m depth and backfilled with cohesive embankment fill materials as shown in Figures 3 and 4.

Fill Material Assessment The fill material for embankment construction, a total of 400,000m³, was sourced from within the scheme boundaries. The borrow pit areas were designed to provide optimum fill material recovery whilst maintaining adequate flow paths for drainage within the area of the finished scheme and optimising the extent of intertidal habitat as shown in Figure 1. During the ground investigations, undertaken at potential areas for the borrow pits, three types of materials were evident: wet, very soft/soft sandy clay (Alluvium); clayey/gravelly sand (Alluvium); and firm/stiff sandy clay (Bracklesham Group). It was considered that the wet, very soft/soft sandy clay was not suitable as embankment fill material due to its low strength. Also, granular soils were less favourable as fill material because of their higher permeability. Hence, the firm/stiff sandy clay was selected as the main fill material. Based on Proctor compaction tests carried out on 12 samples, the air void content at the optimum moisture content for the majority of samples fell between 5 and 10% with a corresponding range of unit weight between 19 and 20 kN/m³, regardless of the soil type. A set of 6 consolidated undrained triaxial compression tests with pore water pressure measurement was undertaken to determine a “cautious estimate” of the effective stress parameters for design of the embankment slopes.

In-situ Permeability of Alluvial Strata For sections of the embankment founded on a thick layer of cohesive soils, an assessment of the insitu permeability of the foundation was required to estimate the rate of dissipation of the pore water pressures induced by the embankment construction. Results from 14 oedometer tests on the Alluvium clay samples gave a range of values for the “vertical” coefficient of consolidation, cv, between 0.3 and 20 m²/year. However, it is known that the use of small-scale laboratory tests for determining the permeability may give values from tens up to several 1 2 3 hundred times smaller than field test values [RRL (1971) ; TRRL (1973) ; TRRL (1974) ]. These disparities between the field and laboratory values could result from the presence of natural drainage paths which are produced by vegetation, varying climatic conditions and other phenomena which occurred during the depositional history of the strata [TRRL (1973)]. Based on the CPT pore pressure dissipation test results, the “horizontal” coefficient of consolidation, 4 ch, was evaluated using the method proposed by Teh and Houlsby (1991) , giving a value of 10

m²/year for peat and a range of values for Alluvium clay between 45 and 500 m²/year. Taking a ratio of ch/cv as 1.5 from published data [TRRL (1973)] and applying it to the lowest value of ch, the design cv value for Alluvium clays was selected to be 30 m²/year. For peat and organic clays, cautious values of cv and ch were taken as 3 and 5 m²/year, respectively.

Geotechnical Design Parameters Based on results from the in-situ and the laboratory tests undertaken and study of published data, cautious estimates of the geotechnical design parameters for design of the embankment were derived as presented in Table 1.

Effective cohesion, c’ [kN/m²]

Angle of shearing resistance, φ’ [Deg.]

40

50

3

30

-

-

-

17

40

40

0

25

0.15

0.1

60

Alluvium (clay)

17

35

10

0

25

0.3

0.4

30

Alluvium (granular)

17

-

-

0

28

-

0.2

-

Alluvium (organic/peat)

12

90

2

2

18

2

2.5

3

Bracklesham Group (cohesive)

19

35

40

0

26

-

0.1

100

Bracklesham Group (granular)

20

-

-

0

35

-

0.025

-

Coef. of consolidation, cv [m²/year]

Undrained shear strength, cu [kN/m²]

19

Alluvium (desiccated)

Compression index, Cc

Plasticity index

Embankment Fill

Stratum

Unit weight, γ [kN/m³]

Coef. of volume compressibility, mv [m²/MN]

Table 1 Geotechnical design parameters

Embankment Design The new flood defence comprised an embankment constructed from site-won cohesive fill as well as some of the less cohesive material encountered in the borrow pits. The embankment had a height of up to 5m with a crest width of 4m. The crest level (5.2m AOD) of the embankment has been designed to withstand a 1 in 100 probability event, including an allowance for climate change, based on the following current tide levels: MLWS = -2.3m AOD; MHWS = 2.3m AOD; and HAT = 2.7m AOD. The inclination of the seaward slope of the embankment has been defined to be 1V:10H so that it will not erode by wave run-up on the high tides while that of the landward slope has been taken as 1V:3H to maintain its footprint to a minimum. In this section the method of slope stability analysis and prediction of post-construction settlement as well as the construction monitoring regime will be described.

Method of Analysis Since the crest level was constant throughout the length of the embankment, the critical sections of the embankment with the maximum height were inevitably located in the low-lying areas of weak foundations with the groundwater table at or near to the ground surface. With the view that these weak materials would consolidate through excess pore pressure dissipation after construction and gain in 5 6 7 strength [Gosden and Attewill (1985) ; Gosden (1986) ; Huat (1994) ], it was identified that the critical stability conditions including deep-seated slips were likely to occur during construction. Accordingly, the design analysis was focused on modelling of the short-term embankment behaviour with surcharge loading from a working dozer on the fill in order to define safe and economical embankment geometries and construction sequences. The rate of dissipation at each construction stage has been 8 modelled using the finite difference method developed by Gibson (1958) . The analysis comprised the following steps: • Selection of critical foundation sections for analysis • Prediction of the excess pore water pressure generated in the foundation by the fill placement • Prediction of the magnitude of the pore water pressure dissipation occurring during construction due to consolidation of the foundations • Slope stability analysis at the end of construction • Settlement analysis during/post construction

The Ultimate Limit State (ULS) stability of the embankment was analysed using limit equilibrium methods in conjunction with partial factors on soil strength and surcharge load in accordance with 9 Eurocode 7 [British Standards Institution (2004)] . The Serviceability Limit State (SLS) was investigated as a checking exercise for embankment sections where the factor of safety calculated by the Eurocode 7 method was close to unity. A SLS factor of safety of 1.3 at the end of construction was 10 taken as the acceptable factor of safety after U.S. Army Corps of Engineers (2000) .

Critical Foundation Sections For design analysis of the 7km-long embankment thirteen critical sections were selected as being representative of the foundations, based on their ground conditions and geometries. The length of each section varied between 50m and 3060m. Whilst the crest level, the crest width and the gradients of the slopes are constant, the height and the base width of the embankment are dependent on the existing ground levels. For a range of the ground levels 1.0m to 4.2m AOD the corresponding height and base width of the embankment had ranges of 4.2m to 1m and 59m to 17m, respectively. Two of the critical sections are illustrated in Figures 3 and 4 and described below. •

Section at Chainage 6700 (Figure 3) o Dimensions: Base width = 49.5m, 3.5m of fill to be placed in 3 stages o Ground conditions: 5m-thick very soft cohesive alluvial soils (cv = 30 m²/year), overlying interbedded sand/clay (Bracklesham Group)

•

Section at Chainage 5600 (Figure 4) o Dimensions: Base width = 59m, 4.0m of fill to be placed in 4 stages, incorporating a 3m-wide “set-back” terrace on the landward slope at the mid-height to relieve the loading in the foundation. o Ground conditions: 6m-thick very soft to soft cohesive alluvial soils including a 3.7mthick peat layer (cv = 3 m²/year; ch = 5 m²/year), overlying interbedded sand/clay (Bracklesham Group) o Prefabricated Vertical Drains (PVD’s) are installed in the foundation.

Figure 3 Embankment cross section at Chainage 6700

Figure 4 Embankment cross section at Chainage 5600

Prediction of Foundation Excess Pore Water Pressures The construction of the embankment was modelled by the application of increments of loading corresponding to a 1m to 4m-thick layer of earth fill per construction stage. Fill placement for each stage was carried out by compacting multiple layers of 200mm-thick fill. These increments were assumed to be added at the rate of 0.5m to 1m per week with an 8-week rest period between the construction stages, giving a range of total construction periods from one week to 32 weeks. The initial response of the foundation to placement of fill material was calculated in the form of two11 dimensional stress distributions using the method proposed by Das (1983) . It was assumed that an increase in load in cohesive soils can result in an increase of the pore water pressure, and hence, the amount of the excess pore pressure generated may initially equate to the degree of increased load.

Dissipation of Pore Water Pressure during Construction The rate of dissipation of the excess pore water pressure is governed by two factors: the distance to a drainage boundary and the coefficient of consolidation, cv. The ground investigations indicated that granular soil layers are present at the project site. The dissipation analysis assumed that these granular soil layers are free draining and will act as drainage boundaries to the cohesive foundation soil layers. At sections where a granular layer was not present, a 300mm-thick layer of granular fill was placed underneath the embankment core fill prior to fill placement. At a 100m-long section around Chainage 5600, where a 3.7m-thick layer of peat is present, a grid of PVD’s with the maximum spacing of 1.2m was incorporated in the foundation design to accelerate the rate of the excess pore pressure dissipation as illustrated in Figure 4. It was expected that a considerable amount of drainage would occur horizontally to the PVD’s in this particular section. A spreadsheet, based on the finite difference method incorporating the effect of the PVD’s, was developed to model the dissipation of the excess pore pressure with time in the cohesive foundation layers. Typical pore pressure dissipation prediction at Chainage 6700 is illustrated in Figure 5. In the granular foundation soil layers the excess pore pressure was assumed to be zero.

Figure 5 Typical % pore water pressure dissipation predicted [Example: two weeks after placing the second layer of fill at Chainage 6700]

Slope Stability Analysis Slope stability analysis was carried out under both undrained and drained conditions based on the Janbu method for circular slip surfaces using the Geosolve software SLOPE (Ver.12R.01). It was considered that circular slip surfaces are the most likely mode of failure for cohesive earth fill embankments founded on relatively thick deposits of soft ground and constructed. Since the construction of the embankment was planned to utilise a multi-staged fill placing approach, allowing the foundation to gain some strength in-between stages, the undrained slope stability analysis tended to underestimate the stability of the embankment. Hence, analysis was carried out using the effective stress soil parameters with the excess pore water pressures in the foundation, based on predictions of the dissipation at each construction stage. The pore water pressures in the fill were expressed in terms of the pore pressure ratio, ru, which is the ratio of the pore pressure to the total overburden pressure, and the value assumed for ru in the fill was 0.2.

Design rates of fill placement were determined by calculating the maximum allowable increase in the embankment height per stage, which maintained adequate slope stability. In general a design layer thickness that can achieve >60% dissipation of the excess pore pressure over the eight-week rest period was selected for each construction stage.

Settlement Analysis Due to the mass of embankment fill that is to be constructed over thick deposits of soft clay, large settlement is expected to occur. The analysis has rationalised the compressibility by using consolidation coefficients, Cc, in preference to the modulus of volume compressibility, mv, as the latter relates to a specific stress range. Based on the analysis, the predicted maximum total settlement generally ranged from 250mm to 750mm. However, over the section where PVDs were required, the amount of settlement was estimated to be in excess of 1,000mm. Most of this settlement will take place during construction with the balance taking place after completion of construction. The post construction settlement was estimated to range from 50mm to 200mm and can be accommodated by overfilling the embankment.

Geotechnical Instrumentation Since there were limited data to assess the dissipation of the excess pore water pressures in the foundation on which the stability analysis largely relied, the observational method was proposed to control construction at key locations. Instrumentation was aimed primarily at measuring pore pressures in the foundations to ensure that the predicted levels are not exceeded and to serve as an early warning system and, secondarily, at allowing the designer to revise the rate of fill placement and to adopt more economical construction method if measurements showed this were acceptable. The settlement behaviour of the embankment was also monitored to calibrate the prediction of the postconstruction settlement. The instruments installed comprised the following: • Eighteen vibrating wire piezometers with data loggers at three depths per location at six locations as illustrated in Figures 3 and 4. • Five settlement profile tubes installed at 1m below the fill/foundation interface at five locations as illustrated in Figures 3 and 4. • Two sets of four settlement pins at two locations where a pre-loading method was used to accelerate consolidation prior to construction of drainage outfall structures.

Details of Construction In this section some of the challenges encountered during construction and their impacts on the programme will be described. Also, the behaviour of the embankment observed will be discussed.

Programme Construction commenced in September 2011 and was originally programmed for 22 months. However, the weather conditions in 2012, being documented as the second wettest year on record in the UK, had a significant impact on the progress of earthworks despite the efforts made by the project team, including trying different construction techniques and equipment. The wet ground surface made it very difficult for construction equipment to move around the site and saturated materials taken from the borrow pits needed to be dried out before being placed as embankment fill. Also, additional works on archaeology and ecological surveys have contributed to delays. As a consequence, the construction programme has been extended and is due to complete by the end of 2013. Completion of the majority of the earthworks will be marked by a breach of a 100m-long section of the existing shingle bank (shown in Figure 1). The breach is to be created by excavation of material from the existing shingle bank to a level of -1.1m AOD, which represents the MLWN tide level. It is likely that the breach will now take place in September 2013.

Borrow Pit Design During the outline design phase the borrow pit locations and shapes were determined primarily by 12 hydrodynamic modelling results undertaken by ABPmer (2010) so that they are to function as dendritic channels allowing the in- and out-flow of the tide on each tidal cycle, creating further intertidal habitat. A further review carried out during the detailed design/construction phase modified the borrow pit areas, based on the geology and information taken from additional ground investigations.

Revised Rate of Fill Placement Heavy continuous rainfall encountered on the Scheme site in 2012 forced to make a number of changes to the rate of embankment fill placement. These included the following: • In most of the low-lying areas removal of the topsoil was reduced to just comprise the near surface layer containing substantial quantities of vegetation roots since the high groundwater level prevented dozers from placing the first lifts of fill once the crust made of the topsoil and the desiccated Alluvium was punctured; • In most of the low-lying areas the rest period was reduced in exchange for reducing the inclination of the embankment side slope from 1V:3H to 1V:5H and installing additional sets of piezometers to more closely monitor the behaviour of the slopes; • The core of the embankment was placed at a range of moisture contents subject to achieving an average air voids ratio of 5% and a maximum air voids ratio ≤10% by compaction with a minimum shear vane strength of 40 kN/m² controlling the fill strength; and • Trials were conducted using various methods of compaction to establish the optimum thickness of each compaction layer and resulted in an increase of the layer thickness from 200mm to 300mm, thus, improving the rate of filling and still maintaining the desired density.

Behaviour of the Embankment Figures 6 and 7 show observed excess pore pressure and settlement against time at Chainage 170 and 6700, respectively. Piezometer readings at both locations, where the thickness of clay layer is in excess of 5m, indicate that the pore pressures in the foundation rise immediately with fill placement. While the excess pore pressure generated by the placement of the final layer at Chainage 170 dropped off to the hydrostatic pressure level over a period of two months, that at Chainage 6700 only dropped by 40% after four months. This indicates that some magnitude of settlement is yet to take place as the excess pore pressure continues to dissipate at Chainage 6700. Though a thick clay foundation layer was encountered at both the Chainage sections, the presence of sand beds within the clay layer at Chainage 170 Figure 6 Observed pore pressures and settlement at Chainage 170 accelerated its dissipation rate. Settlement gauges have revealed varying amounts and rates of settlement due to consolidation of the underlying strata. For both the Chainage sections a predicted value of the total settlement was 600mm. The trend of the settlement behaviour presented in Figures 6 and 7 indicates the actual total settlement is unlikely to exceed the predicted value.

Figure 7 Observed pore pressures and settlement at Chainage 6700

Diversion of Existing Services New drainage solution was designed to work with the existing drainage system of rifes as well as making use of the local topography. Four drainage outfall structures were constructed along the length of the new embankment. The 11kV power cable, which runs across a large section of the site, has been diverted by re-routing around the perimeter of the new embankment.

Archaeological Investigation The fill materials for the embankment were dug from a number of large shallow borrow pits and perimeter drainage channels within the Scheme. Prior to excavation, trial trenches (40m long, 3m wide and 0.5m deep) were carried out for every 1000m² of area excavated. Upon discovery of signs of archaeology from those trenches extensive archaeological fieldwork was undertaken and discovered a variety of nationally and regionally important archaeological remains and artefacts, dated from the Neolithic Age (4000 BC to 2500 BC) to World War II, which were submitted to a local museum 13 [Environment Agency (2012)] . The archaeological investigation, however, forced changes to borrow pit design due to the requirement for in-situ preservation of the remains and the amount of fieldwork carried out was far greater than the level anticipated during the detailed design, which resulted in a significant impact on the construction programme and, consequently, the cost.

Conclusions This case study described a new concept for sustainable flood and coastal erosion risk management undertaken at Medmerry, presenting challenges at both design and construction stages covering the environment, archaeology, ground conditions and weather delays. One of the challenges in the design analysis was the modelling of the embankment behaviour, i.e., prediction of the excess pore water pressure dissipation, using a simple finite difference method based on a spreadsheet, which is cost-effective compared with finite element methods. The instrumentation allowed the designer to revise the rate of fill placement and the duration of the rest periods, ensuring the adoption of a safer and more economical construction method. Under exceptionally adverse weather conditions encountered during works, the opportunities for modifying work sequences and methods were severely limited for earth works on very soft soil foundations. Use of specialised low ground pressure plant or hard core haul roads were considered to be prohibitively expensive. At times much of the site was inundated and work was suspended.

Acknowledgements The authors would like to acknowledge the co-operation and assistance of Mr Colin Maplesden (Project Manager, Environment Agency) and Mr Howard Hibbard (Supervisor, Jacobs UK Ltd).

References 1

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9 10 11 12

13

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