Predicting and controlling ground movements around deep excavations

Deep excavations frequently cause problems, and sometimes trigger catastrophic collapses, especially in soft clay. In principle, these problems are well understood, but designers may fall between the two stools of naive empiricism and overelaborate finite element analysis (FEA). A new approach, Mobilizable Strength Design (MSD), has been developed to bridge this gap. MSD specifies deformation mechanisms tailored to each stage of construction. Each stage is analysed for energy balance, with incremental subsidence creating a drop of potential energy which must equal the work done deforming the soil and the support system. Incremental deformations are summed, while soil non-linearity is allowed for. The non-linear response of a representative shear stress-strain test is required, but estimates can be based on routine soil characterisation. It is demonstrated that MSD back-analyses not only fit FEA results for soft clay within ± 30%, but also fit the soil-structure deformation data of 110 field studies within a factor of 1.4. Finally, a new set of dimensionless groups is defined to characterise deep excavations in clay without the need for any analysis at all. These are used to chart the maximum wall displacements taken from the field database, and an elementary formula is proposed which predicts these 110 maximum displacements within a factor of 2.9. Guidelines are deduced for designers. In particular, it is shown that wall stiffness within the typical range of sheet-piles, secant piles and diaphragm walls has little or no effect on wall deformations.

[1]  Jonathan D. Bray,et al.  Effect of Pile Driving on Static and Dynamic Properties of Soft Clay , 2002 .

[2]  A. S. Osman,et al.  A new design method for retaining walls in clay , 2004 .

[3]  Bengt B. Broms,et al.  Lateral Wall Deflections of Braced Excavations in Clay , 1989 .

[4]  Richard J. Finno,et al.  Stress‐Strain‐Strength Responses of Compressible Chicago Glacial Clays , 1992 .

[5]  Bolton,et al.  Supporting excavations in clay - From analysis to decision-making , 2009 .

[6]  Toshiyuki Mitachi,et al.  Small Strain Shear Modulus of Clay Sedimentation in a State of Normal Consolidation , 1994 .

[7]  Theodor Krauthammer,et al.  Reinforced Concrete Slabs , 2001 .

[8]  William Powrie,et al.  Comparison of measured and calculated temporary-prop loads at Canada Water Station , 2000 .

[9]  Malcolm D. Bolton,et al.  Energy Conservation as a Principle Underlying Mobilizable Strength Design for Deep Excavations , 2011 .

[10]  F. E. Richart,et al.  EFFECTS OF STRATINING ON SHEAR MODULUS OF CLAYS , 1976 .

[11]  澁谷 啓,et al.  SMALL STRAIN SHEAR MODULUS OF CLAY SEDIMENTATION IN A STATE OF NORMAL CONSOLIDATION , 1994 .

[12]  S. Leroueil,et al.  Yielding of Mexico City clay and other natural clays , 1992 .

[13]  Lucy C. Jen,et al.  The design and performance of deep excavations in clay , 1997 .

[14]  B P Williams,et al.  The Design and Construction of Sheet-Piled Cofferdams , 1993 .

[15]  Giulia Viggiani,et al.  Geotechnical Aspects of Underground Construction in Soft Ground , 2012 .

[16]  John Burland,et al.  SETTLEMENT OF BUILDINGS AND ASSOCIATED DAMAGE , 1975 .

[17]  William Powrie,et al.  BEHAVIOUR OF DIAPHRAGM WALLS IN CLAY PRIOR TO COLLAPSE , 1988 .

[18]  M. A. Stroud,et al.  The standard penetration test in insensitive clays and soft rocks , 1975 .

[19]  William Powrie,et al.  Embedded Retaining Walls - Guidance for Economic Design , 2003 .

[20]  田中 勉,et al.  International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground (軟弱地盤中の地下建設の諸問題に関する国際シンポジウム) に参加して , 1997 .

[21]  T D O'rourke,et al.  BASE STABILITY AND GROUND MOVEMENT PREDICTION FOR EXCAVATIONS IN SOFT CLAY , 1993 .

[22]  L. Bjerrum,et al.  Direct Simple-Shear Tests on a Norwegian Quick Clay , 1966 .

[23]  Thiam-Soon Tan,et al.  Deep excavations in singapore marine clay , 2006 .

[24]  Bengt B. Broms,et al.  Behaviour of foundations and structures , 1977 .

[25]  Andrew J. Whittle,et al.  Evaluation of a constitutive model for overconsolidated clays , 1993 .

[26]  Horn-Da Lin,et al.  STRESS-STRAIN-TIME FUNCTION OF CLAY , 1998 .

[27]  W. Powrie,et al.  Installation effects of a bored pile wall in overconsolidated clay , 2006 .

[28]  A. S. Osman,et al.  Ground Movement Predictions for Braced Excavations in Undrained Clay , 2006 .

[29]  M. Novak,et al.  Dynamic properties of some cohesive soils of Ontario , 1981 .

[30]  M. Vucetic,et al.  INFLUENCE OF SOIL TYPE ON THE EFFECT OF STRAIN RATE ON SMALL-STRAIN CYCLIC SHEAR MODULUS , 2003 .