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SUMMARY:Ground movements induced by deep excavation: Physical and analytic
 al  models - Sidney Lam\, CUED
DTSTART:20091127T163000Z
DTEND:20091127T173000Z
UID:TALK19405@talks.cam.ac.uk
CONTACT:Zelda Stuck
DESCRIPTION:Very deep excavation pits have become increasingly common in m
 odern underground construction projects. They often serve as an effective 
 means to accommodate massive underground complex and efficient mass transi
 t systems. However\, the excavation process could lead to undesirable grou
 nd movements\, which might damage buildings and lifeline systems nearby. I
 n this regard\, designers often use multi-strutted structural systems as a
 n economical way to limit ground movements. Although the general soil resp
 onse to excavation is understood in principle\, designers still lack simpl
 e and practical methods for calculating undrained ground movements. Judgme
 nts are generally empirical\, and serious accidents are common.\n\nThis se
 minar presents a new apparatus for the centrifuge model testing of deep ex
 cavations in soft clay\, which emulates the field construction sequence of
  a multi-propped retaining wall during centrifuge flight. A comparison is 
 given between the new technique and the previously used method of draining
  heavy fluid to highlight the importance of correct modeling of passive so
 il stress in soil excavation process. Moreover\, digital images taken of a
  cross-section during the test are analyzed using Particle Image Velocimet
 ry (PIV) to illustrate ground deformation and soil-structure interaction m
 echanisms.\n\nA decision-making tool based on an extended Mobilizable Stre
 ngth Design (MSD) method which permits the designer an extremely simple me
 thod of predicting ground displacements during construction is validated b
 y a highly non-linear finite Element Analysis (FEA) of deep excavations. T
 his newly extended MSD approach accommodates a number of issues which are 
 important in underground construction between in-situ walls\, including: a
 lternative base heave mechanisms suitable for different excavation geometr
 y\; the influence of support system stiffness in relation to the sequence 
 of propping of the wall\; and the capability of dealing with stratified gr
 ound. In addition\, a simplified MSD framework is proposed for analyzing a
  database of 155 deep excavation case histories worldwide. The approach ex
 amines the governing factors controlling deformation in deep excavations a
 nd offers simple guidelines for designing support structures for deep exca
 vations. These developments should make it possible for a design engineer 
 to take informed decisions on the relationship between ground movements an
 d the influence of wall stiffness\, or on the need for and influence of a 
 jet-grouted base slab prior to conducting project-specific FEA.\n
LOCATION:Engineering Department - LR6
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