Influence of deep excavation induced ground movements on adjacent piles

A series of centrifuge model tests has been conducted to investigate the behavior of a single pile behind a retaining wall that eventually fails due to soil excavation in front of the wall. All the piles are located at 3 m behind the wall where the soil experiences large shear strain (>2%). The induced bending moment and deflection on the pile as well as the soil and wall movements are monitored at regular intervals throughout the tests. It is found that the pile performance depends greatly on the degree of wall instability. After a critical excavation depth, active wedge slip plane and tension cracks developed in the vicinity of the pile. The limiting soil pressure profile deduced from the measured maximum induced pile bending moment profile is established to be much lower than that of a conventional laterally loaded pile. Using the measured soil movements at the pile location as the input data, the calculated pile bending moment obtained using an existing numerical model generally show fair agreement with the measured values when the back-analyzed limiting soil pressures acting on the pile are employed in the back-analysis. The practical implications of the findings are discussed in the paper.

Pile Behavior Due to Excavation-Induced Soil Movement in Clay. II: Collapsed Wall

An underground passage is constructed in the Bund, Shanghai, which intersects an existing shield tunnel, the East Yan'an road tunnel. This research reports a series of centrifuge model tests conducted to investigate the effect of deep excavation above an existing tunnel. The results reveal that significant tunnel heave can be observed, induced by excavation above. For the test conducted to model the East Yan'an road tunnel, the influenced range of the tunnel extends to about 2·5 times the excavation width away from the boundary of the excavation. Considerable additional bending moments are observed at the crown and invert in the lining of the tunnel below the centre of the excavation, but the additional moments rapidly decrease to be negligible when the tunnel cross-section is 1·5 times the excavation depth away from the centre of the excavation. It is also found that the longitudinal deflection of the tunnel due to excavation above it can be modelled by a Gaussian curve, based on which the location of th...

Centrifuge modelling of deep excavation over existing tunnels

http://mcadbg.com/wp-content/uploads/2015/10/Numerical-investigation-of-the-failure-of-a-building-in-Shanghai-China.pdf

Numerical investigation of the failure of a building in Shanghai, China

A series of centrifuge model tests was conducted to investigate the behavior of pile groups of various sizes and configurations behind a retaining wall in very soft clay. With a 1.2-m excavation in front of the wall, which may simulate the initial stage of an excavation prior to strutting, the test results reveal that the induced bending moment on an individual pile in a free-head pile group is always smaller than that on a corresponding single pile located at the same distance behind the wall. This is attributed to the shadowing and reinforcing effects of other piles within the group. The degree of shadowing experienced by a pile depends on its relative position in the pile group. With a capped-head pile group, the individual piles are forced to interact in unison though subjected to different magnitudes of soil movement. Thus, despite being subjected to a larger soil movement, the induced bending moment on the front piles is moderated by the rear piles through the pile cap. A finite element program developed at the National University of Singapore is employed to back-analyze the centrifuge test data. The program gives a reasonably good prediction of the induced pile bending moments provided an appropriate modification factor is applied for the free-field soil movement and the amount of restraint provided by the pile cap is properly accounted for. The modification factor applied to the free-field soil movement accounts the reinforcing effect of the piles on the soil movement.

Behavior of Pile Groups Subject to Excavation-Induced Soil Movement in Very Soft Clay

A series of centrifuge model tests has been conducted to investigate the behavior of a single pile subjected to excavation-induced soil movements behind a stable retaining wall in clay. The results reveal that after the completion of soil excavation, the wall and the soil continue to move and such movement induces further bending moment and deflection on an adjacent pile. For a pile located within 3 m behind the wall where the soil experiences large shear strain (>2%) due to stress relief as a result of the excavation, the induced pile bending moment and deflection reach their maximum values sometime after soil excavation and thereafter decrease slightly with time. For a pile located 3 m beyond the wall, the induced pile bending moment and deflection continue to increase slightly with time after excavation until the end of the test. A numerical model developed at the National University of Singapore is used to back-analyze the centrifuge test data. The method gives a reasonably good prediction of the induced bending moment and deflection on a pile located at 3 m or beyond the wall. For a pile located at 1 m behind the wall where the soil experiences large shear strain (>2%) due to stress relief resulting from the excavation, the calculated pile response is in good agreement with the measured data if the correct soil shear strength obtained from postexcavation is used in the analysis. However, if the original soil shear strength prior to excavation is used in the analysis, this leads to an overestimation of the maximum bending moment of about 25%. The practical implications of the findings are also discussed in this paper.

C. E. Leung

Papers

Pile Behavior Due to Excavation-Induced Soil Movement in Clay. I: Stable Wall