Validation of methods for assessing tunnelling-induced settlements on piles

Validation of methods for assessing tunnelling-induced settlements on piles Mike Devriendt, Arup Michael Williamson, University of Cambridge & Arup technical note Abstract For tunnelling projects, settlements of structures supported on piles are often appraised by using simplistic empirical assumptions. These include assuming the greenfield tunnellinginduced ground movement a given distance down the pile represents the displacement at the pile head. This paper compares two commonly used empirical assumptions with an analytical method of analysis. The analytical method has the advantage that the mobilisation of shaft friction and axial loading down the pile can be calculated. Analysis using the analytical method takes very little time to carry out, is an improvement in sophistication relative to empirical methods and is less time consuming than carrying out more rigorous analysis using finite element or finite difference methods. A sample set of calculations is provided that illustrates several important features of the analytical method and provides a comparison with results from the empirical methods. To validate the use of the analytical method a back analysis is presented comparing results with those obtained from an instrumented pile test during tunnelling for the Channel Tunnel Rail Link in north London. 1. Introduction 1.1 General A requirement of tunnelling projects carried out in urban areas is to assess its impact on third party assets. The impact on masonry buildings founded on shallow footings has been discussed by many authors and methods for their assessment are provided by Burland (1995) and Cording et al (21). The impact on structures founded on piles, however, is covered less extensively. Examples of the effect of tunnelling on piles in recent years include studies from Loganathan et al (2) and Selemetas (25), which provide results from centrifuge modelling and field testing. Practising engineers carrying out impact assessments of structures founded on piles commonly resort to making empirical assumptions relating to the settlements that occur to piles and the overlying building as a consequence of tunnelling-induced displacements. Further details of these simplifying assumptions are provided and discussed in Section 2. Many low-rise masonry structures built in recent years are supported on individual piles with relatively low stiffness sub-structure linking the piles. In such instances a plausible method to assess the impact on the overlying structure is by calculating the pile head displacements and imposing these onto a simple beam type analysis proposed by Burland (1995). Given the relative lateral flexibility of the piles and limited restraint offered by the sub-structure, greenfield horizontal strains at sub-structure level arising from tunnelling-induced horizontal ground movements are sometimes conservatively used in these analyses. From the authors experience, the widespread use of empirical methods for calculating vertical displacements of the piles relates to a perception that more refined analysis takes much longer to carry out and requires the use of numerical methods such as finite element analysis. While this may be carried out where structures are considered to be at higher risk of damage, validation of the empirical assumptions for the less sophisticated analyses is not well documented. This paper describes an analytical method of assessment using existing published techniques that can be carried out quickly and provides the engineer with an understanding of the pile behaviour subjected to ground movements. The results presented in the paper also provide some evidence to support the use of the empirical techniques described in Section 2. Both the empirical and analytical methods could be used as part of a Stage 2 or 3 analysis when assessing impact on buildings as described in accordance with Burland (1995). More refined methods such as using finite element analysis may also be considered necessary to supplement the results from the empirical and analytical methods. 1.2. Constraints of paper The examples used in this paper are limited to reviewing settlement characteristics of single piles located above tunnels. When appraising settlements, consideration is also given to the axial stress in the pile and mobilisation of shaft friction. Only piles with toes in soil that do not derive a significant amount of their capacity from the toe of the pile are considered. The paper does not consider pile groups or lateral deflections of piles caused by tunnelling works. Further assessments would be required to assess these scenarios and the resulting impact on the pile(s) and overlying structure above. It is also realised that the stiffness of buildings and sub-structure founded on piles (for instance those supported by piled rafts) will have a significant effect on the resulting pile displacement and building distortion. 2. Analysis methods 2.1 Commonly used empirical methods 2.1.1 Assumed depth down pile approach (2/3 depth approach) Ground movements are commonly calculated at a given distance down the pile length (1/2, 2/3, 3/4 or pile toe depth). The displacement calculated at this given depth is assumed to also occur at the head of the pile. Generally for tunnelling induced ground movements, for pile toes located above the tunnel, the closer to the toe the displacement is calculated, the larger the assumed pile movement will be and the more conservative the approach. The authors do not know the Key to notation 2.1.2 Neutral axis approach The position of the neutral axis (NA) of the pile is calculated (ie where the pile shaft moves from being in compression to tension). Various empirical or analytical approaches can be used to calculate the position of the neutral axis. The tunnellingα = adhesion factor between soil and pile cu = undrained soil shear strength E = soil Young s modulus. Represents both fine and coarse grained soils (undrained & drained response), see Table 1 E concrete = Young s modulus of pile concrete Eu = Undrained soil Young s modulus k = tunnelling settlement trough width parameter at surface kn = kilo Newton L = length of pile m = metres MN = mega Newton mod = metres Ordnance Datum TBM = tunnel boring machine z = depth below ground level or point of reference basis for these assumptions. However, they believe engineers are extrapolating theories relating to the loading of pile groups and assumptions that settlements can be calculated by assuming an equivalent raft 2/3 down the pile (see Tomlinson, 1994). For the remainder of this paper, the settlement 2/3 down the pile will be used for comparisons with the other methods. A perceived drawback with the approach is that settlements will vary depending upon the location of the pile relative to the tunnel. Buildings generally have many piles that will vary in position relative to the tunnel. Therefore, making a constant assumption on the depth to assume ground settlement represent pile head displacement appears conceptually to have drawbacks. ground engineering march 211 25

technical note induced settlement at this position is assumed to represent the pile head displacement. The disadvantage with this approach is that the pile may not have a neutral axis position if it is situated to one side of the tunnel (ie only the top section of a pile is affected by settlement, or the soil adjacent to the lower section of the pile settles less than the soil adjacent to the upper section of the pile). Under these circumstances the pile may be under compression along the entire shaft length. 2.2 Analytical method The method discussed in this paper was developed by Poulos and Mattes (196, 1969) and Mattes and Poulos (1969) and is based upon linear elastic theory. An individual pile is discretised into a number of elements. Loads may be applied down the pile and displacements of the soil adjacent to the pile specified. A single Young s modulus is specified for the soil adjacent to the pile and a Young s modulus specified for the soil below the base of the pile. A Empirical assumed depth down pile approach 2/3L A B Poisson ratio is specified for the soil. A rigid boundary is set at an appropriate distance below the pile base. A Young s modulus is also specified for the pile shaft and the pile is assumed to behave elastically. Limiting values of shaft skin friction are specified at each element position and slip between the soil and pile shaft occurs once this value has been reached. The elastic analysis described above is modified to take account of this. First the displacements are calculated on the assumption that all elements are elastic. From the displacements, the shear stresses calculated are compared with the specified limiting stresses. The extra displacement caused by the out of balance force is calculated and added to the previous elastic solution. The shear stresses are then calculated again based on the modified displacements. This procedure is repeated until all shear stresses do not exceed the specified limiting shear stress. The pressure at the base of the pile is calculated from elastic theory and considering the displacement at Empirical neutral axis approach Neutral axis A Axial load in pile B the base of the pile. This assumption and those related to the tunnellinginduced soil displacement assumed at the pile/soil interface will be discussed further in Section 3.1. Figure 1 summarises some of the analysis assumptions in Sections 2.1 and 2.2 for the empirical and analytical methods. The analytical method is described in the commonly used text book by Fleming et al (1992). The method is also used in the OASYS program PILE. 3.Sample calculations 3.1 General To provide a comparison between the methods described in Section 2, a series of calculations have been carried out for the scenario defined in Figure 2. The input parameters for the calculations are also given. The Mair et al (1993) method is used to calculate the sub-surface settlement adjacent to the pile shaft. The analytical method described in Section 2 relies on relative displacements between pile shaft and soil being specified along the pile length. If all of the pile is in the settlement trough, the pile head and toe displacement is considered prior to the analysis. The lesser settlement of these two is noted and this settlement is subtracted from all of the settlements calculated down the pile shaft to form the input relative pile shaft/soil displacement. Following completion of the analysis, the lesser settlement calculated prior to the analysis is added to the results of all settlements of the pile to provide the overall pile settlement. The analytical method also calculates a settlement from initially imposing the 1MN load prior to applying the tunnelling-induced soil displacements. A load of 1MN corresponds with a factor of safety of approximately 3.5 for the soil parameters and pile defined in Figure 2. This initial loading settlement is subtracted from the results presented in the paper. For Pile A, initial calculations using the PILE program indicated that a pressure in the base of the pile was developing despite the soil adjacent to the toe settling more than the pile toe. In such instances it would be reasonable to expect the pile toe to generate zero pressure. The error arises from PILE assuming that the displacement of the soil or rock beneath the toe to be zero and calculating a pressure using elastic theory from the pile base displacement. The error may be overcome by specifying a low stiffness for the soil beneath the base. This does not introduce subsequent errors on other parts of the calculation. Where the pile toe settles more than the adjacent ground, it is not necessary to make this correction. This may be the case if a pile is located a given lateral distance from the tunnel and the toe is not influenced by the tunnelling induced settlement. 3.2 Results Figures 3 and 4 present results from the analysis for Pile A located above the centre line of the tunnel and Pile B located 1m from the centre line respectively. A sequence of graphs is shown in these figures to provide Greenfield displacement from tunnel at B assumed to be pile head displacement at A Greenfield displacement from tunnel at B assumed to be pile head displacement at A Pile B 2m length,.6m diameter pile Pile 1m from tunnel centreline Applied load of 1MN 1m Pile A 2m length,.6m diameter pile Pile directly over tunnel centreline Applied load of 1MN Ground level at +4mOD Analytical approach 25m depth to tunnel centreline Specify soil displacement and limiting shaft friction at nodal positions Rigid boundary Model requires following inputs: Poisson ratio of soil Young s modulus of pile Young s modulus of soil adjacent to shaft Young s modulus of soil beneath base Skin friction limit down shaft to achieve mobilisation Soil displacement adjacent to pile shaft Pile load Depth to rigid boundary Figure 1: Empirical and analytical approaches 7.m diameter 1% volume loss Mair et al (1993) settlement calc Analytical modelling assumptions α =.5 cu = 5 + 1z (z depth (m) below surface) Eusoil adjacent to shaft = 6,kN/m 2 Eusoil below toe = 1,kN/m 2 (for pile loading only for other analyses please refer to Section 3.1 for modelling assumptions) Poisson ratio of soil =.5 Rigid boundary 4m below ground level E concrete = 2 x 1 6 kn/m 2 Not to scale Figure 2: Example scenario for comparison calculations 26 ground engineering march 211

results from some of the calculations carried out using the analytical approach. The top left graph shows the greenfield ground movement from Mair et al (1993) and those calculated for the pile from the proposed analytical approach. The pile settlement has been calculated from the method described in Section 3.1. The top right graph provides the displacements calculated from the analytical method for various analyses. The series in the graphs provide the displacement from loading the pile with 1MN, settlement from applying the relative settlement on the pile from the tunnelling induced settlement (referred to as downdrag in the graph series title) and the combination of both settlement and pile loading. The bottom left and bottom right graphs show the shaft friction mobilised down the pile and the axial stress within the pile shaft. A comparison is given at the bottom of each figure describing the pile head settlement calculated from the empirical and analytical approaches. For the pile located above the centreline the analytical and 2/3 depth approach calculate very similar pile head settlements (approx 19mm), while the neutral axis approach calculates a marginally smaller settlement (17.2mm). For the pile located 1m from the centreline, the analytical and 2/3 depth approach calculate the same pile head settlement (approx 9mm). There is no neutral axis position in the pile, therefore, the neutral axis approach yields no result. Figure 5 (overleaf) shows further results varying the position of Pile B and shortening the pile to 1m in length for providing further comparison of the pile head displacement against the empirical methods. The graph illustrates the limitations of the NA approach identifying that it can only be used where the pile being assessed is close to the centreline of the tunnel. The analytical (PILE) analysis and 2/3 depth approach produce similar results for varying the position of the pile relative to the tunnel centreline and varying the pile depth. The analytical approach has the further advantage that an understanding of the pile behaviour can be gained by review of the mobilisation of shaft friction and axial loading in the pile. An appreciation of the axial loading can be used for further calculations where the structural capacity of the pile is checked. 4. Comparison with case study displacements 4.1 General The study described in Section 3 identifies comparisons and drawbacks with the empirical and Ground and pile displacement 4 36 2 Shaft friction 4 36 1MN 1 MN load, load, downdrag No No pile pile load, downdrag 2-15 Ground displacement (Mair et al) Pile displacement 1 2 3-1 -5 5 1 15 2 2/3 depth down pile approach Take settlement at 26.66mOD from Mair et al (1993) Settlement = 19.2mm Neutral axis position approach NA (ie tension to comp point) = 29.6mOD Settlement from Mair et al (1993) = 17.2mm Analytical approach (from PILE) Pile head settlement = 1.9mm Ground and pile displacement 4 36 2 Ground displacement (Mair et al) Pile displacement Shaft friction (kn/m 2 ) 2 4 6 1 Shaft friction 4 36 11 MN MN load, downdrag No -5 5 1 15 Shaft friction (kn/m 2 ) 2/3 depth down pile approach Take settlement at 26.66mOD from Mair et al (1993) Settlement = 9.1mm Neutral axis position approach NA (ie tension to comp point) = not present Analytical approach (from PILE) Pile head settlement = 9.1mm Pile displacement from relative pile/soil settlement 4 36 11MN load, downdrag 2 Figure 3: Results from Pile A above centreline of tunnel Axial stress in pile -4, 2 4 6 1 1MN load, no downdrag 1MN load, downdrag -2, 2, 4, Tunnel geometry 7.m diameter 1% volume loss axis level = 25m below ground level Pile geometry.6m diameter 2m length Pile displacement from relative pile/soil settlement 4 36 1 MN load, downdrag 2 Figure 4 - Results from Pile B, 1m from tunnel centreline Axial stress in pile 1MN load, no downdrag 1 MN load, downdrag 2 4 6 1, 2, 3, 4, Tunnel geometry 7.m diameter 1% volume loss axis level = 25m below ground level Pile geometry.6m diameter 2m length ground engineering march 211 27

technical note 4 X 16 2 2m pile: Analytical (PILE) Empirical (2/3 depth) Empirial (NA) Distance from tunnel centreline (m) 5 1 15 2 1m pile: Analytical (PILE) Empirical (2/3 depth) X Empirial (NA) Figure 5: Pile head settlements for 1 and 2m piles Pile FO,.456m diameter. Load of approximately kn applied to pile 13m Not to scale Extensometer E3 6m 7m Made ground Alluvium Terrace Gravels London Clay Depth below ground level (m) 3 7.5 11.2 Ground water level (4m) Down line tunnel,.15m diameter Axis level 1.9m below ground level Crown of tunnel 1.25m below toe of pile FO Figure 6: CTRL Down Line tunnel, Pile FO and Extensometer E3 general arrangement Figure 7: Panoramic view of the CTRL test site showing the test set up for Pile FO and three other piles tested analytical approaches. A further study has been carried out to back analyse case study observations of a pile subjected to tunnelling induced settlements. Results of pile settlements as a consequence of tunnelling have been obtained from Selemetas (25). A summary of the work may be found in Selemetas () and Selemetas et al (26). The references include results from the Channel Tunnel Rail Link (CTRL) London tunnels construction beneath a series of test piles installed ahead of the tunnel drives. The tunnels are.15m in diameter with tunnel axis level 1.9m below ground level. This paper will consider settlements of Pile FO detailed in the references during the passage of the down line tunnel. Figures 6 and 7 provide an illustration of the tunnels and pile test general arrangement. The ground and groundwater conditions are also summarised in Figure 6. Pile FO was located approximately 7m from the Down Line centre line. The pile was installed to a depth of 13m and had a diameter of.456m. The pile was installed by driving a steel tube into the ground, removing the soil within the tube, then installing grout within the tube. The tube was then removed and the instrumented reinforcement cage lowered into position within the grouted hole. Instrumentation comprising inclinometers, extensometers and piezometers were also installed. This paper will consider the results from extensometer E3 alongside the results from Pile FO. Extensometer E3 was located 6m from the centreline of the down line tunnel. Back analysing the data, Selemetas () reported a volume loss of.5% and a trough width parameter, k of.42 during the drive of the Down Line tunnel beneath Pile FO. At surface level this corresponds with a distance, i from the maximum settlement to point of inflection on the settlement curve of approximately m. 4.2 Back analyses To appraise whether the analytical method reproduces results similar to the case study, the following back analyses have been carried out: a) Comparison of the load/settlement and axial stress response of the pile during loading of the pile with a load of kn; b) Assessing the settlement and axial load in the pile caused by the tunnel when the face of the tunnel had progressed 34m past the pile position. This corresponds with approximately five days after the tunnel face passed the pile position; c) Assessing the settlement and axial load in the pile caused by the tunnel 22 days after the face of the tunnel had progressed past the pile position. For assessing b and c, the displacements from Extensometer E3 have been applied to the analytical PILE analysis of the pile. A 1m lateral difference between the offset from tunnel centre line of the extensometer and the pile is acknowledged, but this is considered to only have a relatively small effect on the accuracy of the back analysis. By carrying out calculations using the method proposed by Mair et al (1993), the difference is likely to equate to applying displacements that are approximately 1mm larger than the soil displacements adjacent to the pile. 4.3 Results 4.3.1 Initial loading response The load/settlement response and load distribution in the pile observed during loading of pile FO and from the PILE back analysis is shown in Figure. Reasonable agreement is achieved between the observed load/displacement response and stress down the pile. Table 1 provides the input parameters used for the PILE analysis. The following should be noted when considering the input parameters: n The stiffness of the ground adjacent to shaft and beneath the base was set at a high value relative to what may be used for conventional analysis. Such high values are plausible given the very low strains that the ground adjacent to the pile experienced. Another possible reason for the high stiffness used in the back analysis may be due to the partial driving pile construction method. For information, the stiffness beneath the base would approximately correspond to Eu = 2cu; n The top of pile shaft skin friction in made ground was set at a relatively high value. Test results for the pressure down pile suggest this to be appropriate. To achieve similarity between observed and back analysed load/ settlement behaviour, the input parameters were amended in accordance with above noted observations. 4.3.2 Pile response when tunnel face is 34m past pile and 22 days following passage of tunnel The settlement response of the head of Pile FO with time caused by the tunnel construction is shown in Figure 9. The position of the face of the TBM is also shown on this graph. This figure has been taken from ground engineering march 211

Depth (m) Pile head settlement (mm) Axial stress response during loading of pile FO Analytical (PILE) analysis 2 Pile FO measured 4 6 1 14 2 4 6 1, 1,2 1,4 1,6 Load/settlement response during loading of pile FO Analytical (PILE) analysis.1 Pile FO measured.2.3.4.5.6.7 5 1 15 2 25 Applied pile head load (kn) Figure : Measured and back analysed load/settlement and axial stress distribution of Pile FO during loading Table 1: Parameters used in analytical (PILE) back analysis of Pile FO Depth below Limiting friction Material ground level (m) (kn/m 2 ) Made ground 1 23 Made ground 2 46.1 Made ground 3 1 Alluvium 4 1 Alluvium 5 1 Alluvium 6 1 Alluvium 7 1 Alluvium 44.1 Terrace gravels 9 4.9 Terrace gravels 1 53. Terrace gravels 11 5.6 Terrace gravels 4. London Clay 13 46. London Clay Additional parameters used: E concrete = 23.4 x 1 6 kn/m 2 Rigid boundary set at 3m below ground level E shaft = 9 x 1 3 kn/m 2, E below base = 15 x 1 3 kn/m 2 Load applied to pile = kn, Poisson ratio of ground =.25 Pile axial stress response Analytical (PILE) analysis: 2 TBM face 34m past Pile FO 4 22 days after TBM has passed 6 Pile FO measured after tunnelling - approx. days after tunnel face passed pile 1 14 5 1, 1,5 2, 2,5 Pile settlement response 2 4 6 1 14 2 4 6 1 14 16 Analytical (PILE) analysis: Extensometer E3 displacement: Depth (m) Depth (m) -3-1 1 3 5 7 9 11 13 Pile FO measured: -1 2 4 6 1 14 Days after 22 March 23 Pile FO head settlement TBM progress Pile head settlement: TBM face 34m past Pile FO TBM 22 days past Pile FO Figure 9: Pile FO settlement/time response from passage of TBM 2/3 depth extensometer E3 displacement: Figure 1: Tunnelling-induced Pile FO response 3 25 2 15 1 5-5 Position of TBM face relative to Pile FO Selemetas (25) and extrapolation has been carried out (see vertical and horizontal lines on graph) to establish the settlement of the pile when the tunnel face was 34m past the pile position and 22 days after the passage of the tunnel face. A comparison between observed and back analysed settlement and pressure distribution down the pile shaft is shown in Figure 1. For the back analysis the same input parameters as given in Table 1 were used for the analysis. The settlement obtained if the pile head displacement was assumed to be the greenfield displacement at an elevation of 2/3 down the pile at Extensometer E3 is also shown in Figure 1. Reasonable agreement is seen between the observed and back analysed pile head displacement using either the analytical or 2/3 down the pile assumption. It is noticeable that the observed displacements are marginally less than the calculated displacements. This may be attributed to the larger displacements taken from Extensometer E3 assumed in the back analysis relative to those likely to have occurred adjacent to Pile FO. Using the analytical method, the stress down the pile is slightly larger than observed from the case study. The greatest difference is seen at approximately 4m depth. The larger displacements taken from Extensometer E3 assumed in the back analysis relative to those likely to have occurred adjacent to Pile FO may partly explain why the back analysis over predicts the axial stress. It should be noted that the back analysis has assumed a low soil stiffness at the toe of the pile because it has been inferred from the case study data that the soil beneath the pile toe is likely to have settled more or about the same magnitude as the pile. Re-running the analysis with a higher soil stiffness at the toe would yield larger stresses in the pile. 5. Conclusions The back analysis and comparative studies contained in this paper provide a reference for understanding the applicability of empirical and analytical methods to represent the behaviour of settlements arising from tunnelled excavations on single piles. The empirical method of assuming the neutral axis (zero axial force) position in a pile corresponds with the head displacement shows ground engineering march 211 29

technical note reasonable correlation with the 2/3 depth assumption and analytical method. The neutral axis method cannot be applied where no neutral axis occurs down the pile. This is found to be the case where a pile is located to the side of a tunnel. The analytical and 2/3 depth method showed good agreement with pile head displacements observed from the CTRL case study. The use of the analytical method also provides information about the mobilisation of shaft stresses and axial force down the pile as an output to the analysis. For back analysing the CTRL case study, the analytical method calculated pile stresses that showed good agreement with the measured data for loading the pile. The analytical method over-predicted pile stresses when back analysing the effects of tunnelling on the pile, however, this may partly be attributed to assumptions made for the back analysis relating to the tunnelling induced soil displacements adjacent to the pile. The analytical method appears to be appropriate for carrying out analysis in a time efficient manner prior to the use of more sophisticated methods such as finite element analysis. The use of more sophisticated methods would also allow other aspects to be considered such as the pile cap, sub-structure and horizontal displacements. Further comparisons between relatively simple analytical models described in this paper and case study displacements should be carried out to supplement these studies. Future underground construction projects present an opportunity to obtain further data to carry out comparative studies. In addition, the analytical method may be of use for comparing with numerical models or providing a simple model to compare centrifuge modelling test results against. Acknowledgments The authors would like to thank Professor Robert Mair of Cambridge University, Alice Berry of Arup and David Harris for their constructive comments on the paper and discussion on the subjects contained in the paper. This work is funded by the EPSRC and Arup, and has been carried out as part of a PhD the second author is undertaking at University of Cambridge. References Burland J B (1995). Assessment of Risk of Damage to Buildings Due to Tunnelling and Excavation. Proceedings of 1st International Conference on Earthquake Geotechnical Engineering, IS- Tokyo. Cording E J; Long J H; Son M; and Laefer D F (21). Modelling and Analysis of Excavation-Induced Building Distortion and Damage Using a Strain-based Damage Criterion. CIRIA SP21, Responses of Buildings to Excavation-Induced Ground Movements, Proceedings of the International Conference, Imperial College, London, pp 5-256. Fleming W G K; Weltman A J; Randolph M F and Elson W K (1992). Piling Engineering 2nd edition. E & FN Spon. Loganathan N; Poulos H G; Stewart D P (2). Centrifuge Model Testing of Tunnelling-Induced Ground and Pile Deformations. Géotechnique 5 No. 3, pp3 294. Mair R J; Taylor R N; Bracegirdle A (1993). Subsurface Settlement Profi les Above Clay in Tunnels. Géotechnique 43 No. 2, pp315-. Mattes N S; Poulos H G (1969). Settlement of Single Compressible Pile. Journal of the Soil Mechanics and Foundation Division, Proc. of ASCE, Vol. 95, No. SM1, Jan 1969, pp19-26. Poulos H G; Mattes N S (196). The Settlement Behaviour of Single Axially Loaded Incompressible Piles and Piers. Géotechnique 1, pp351-371. Poulos H G; Mattes N S (1969). The Behaviour of Axially Loaded Endbearing Piles. Géotechnique, Vol. 19, No 2, pp5-3. Selemetas D (). Pile Settlement Due to Tunnelling in London Clay: A Case Study. Ground Engineering, June, pp29-. Selemetas D (25). The Response of Full-scale Piles and Piled Structures to Tunnelling. University of Cambridge. Selemetas D; Standing J R; Mair R J (26). The Response of Full-scale Piles to Tunnelling. Geotechnical Aspects of Underground Construction in Soft Ground, pp763 769. Tomlinson M J (1994). Foundation Design and Construction, 6th edition. Longman. their ChALLeNGe CAre Construction Challenge, 2 July 211, Staffordshire Take on teams from across the construction industry YOUr ChALLeNGe Photo: CARE / Amy Vitale Challenge Yourself: As part of a team of four, you will cover a marathon on foot, bike and canoe. After the challenge, enjoy a well-deserved meal and party. Challenge Poverty: Funds raised will support CARE International s fight against poverty. We provide relief to families affected by humanitarian emergencies and work with communities to help them find routes out of poverty. This includes providing shelter to people whose lives have been devastated by natural disaster or war, and training and supporting communities in safer and longer-lasting construction techniques. www.carechallenge.org.uk/constructionchallenge Email: challenge@careinternational.org Tel: 2 7934 947 3 ground engineering march 211