Excess Pore Pressures Around Underground Structures Following Earthquake Induced Liquefaction

Excess Pore Pressures Around Underground Structures Following Earthquake Induced Liquefaction

Siau Chen Chian (University of Cambridge, UK) and Santana Phani Gopal Madabhushi (University of Cambridge, UK)
Copyright: © 2012 |Pages: 17
DOI: 10.4018/jgee.2012070103


Underground structures located in liquefiable soil deposits are susceptible to floatation following an earthquake event due to their lower unit weight relative to the surrounding saturated soil. This inherent buoyancy may cause lightweight structures to float when the soil liquefies. Centrifuge tests have been carried out to study the excess pore pressure generation and dissipation in liquefiable soils. In these tests, near full liquefaction conditions were attained within a few cycles of the earthquake loading. In the case of high hydraulic conductivity sands, significant dissipation could take place even during the earthquake loading which inhibits full liquefaction from occurring. In the case of excess pore pressure generation and dissipation around a floating structure, the cyclic response of the structure may lead to the reduction in excess pore pressure near the face of the structure as compared to the far field. This reduction in excess pore pressure is due to shear-induced dilation and suction pressures arising from extensile stresses at the soil-structure interface. Given the lower excess pore pressure around the structure; the soil around the structure retains a portion of this shear strength which in turn can discourage significant uplift of the underground structure.
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Reduction In Excess Pore Pressure Due To Soil-Structure Interaction

In the analysis of pipelines and tunnels buried in liquefiable soil deposits, reduction of excess pore pressure around the structure was observed by Ling et al. (2003), Stringer and Madabhushi (2007), Chian and Madabhushi (2011), and Chou et al. (2011) in centrifuge and 1-g shaking table tests. The reduction in pore pressure as compared to the far-field was observed to be most significant at the invert and crown of the pipe. Ling et al. (2003) suggested that the reduction in pore pressure around the structure was due to implied flow along the vicinity of the pipe. Stringer and Madabhushi (2007) postulated that suction beneath the pipe exist and causes the soil under the pipe to retain its full shear strength, thereby inhibiting the pipe from floatation. Chou et al. (2011) indicated that the hydraulic gradient towards the middle of the base of the structure draws water and soil at the invert of the structure to fill the space vacated by the uplifting structure. This can be inferred by the soil displacement vectors pointing towards the pipe invert in Figure 1.

Figure 1.

Cumulative displacement vectors of soil and structure in a centrifuge test (Chian and Madabhushi, 2010)

Figure 2.

Force components acting on an underground structure in static condition

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