Modelling tunnel behaviour under seismic actions: An integrated approach

This paper intends to describe the integration of physical and numerical modelling, focusing on tunnels under seismic actions. It shows how numerical calculations can be used in association with centrifuge testing to model different aspects of tunnel behaviour during earthquakes. The scope of the paper has been limited to a few aspects, mainly concerning the change of internal forces in the tunnel lining during shaking and the effect of soil liquefaction. The interaction between a tunnel and a building in a soil layer undergoing liquefaction has also been taken into account

Effect of Liquefaction on Pile Shaft Friction Capacity

Piled foundations are commonly used worldwide, and observed failures of these foundations during earthquakes has led to active research in this area. However, the way in which piles support axial loads during earthquakes is still not fully understood. In this paper, the results from centrifuge tests are presented which consider how axial loads are carried by piles during earthquake loading. It will be shown that the piles in dry soils mobilise additional shaft friction to carry the seismically induced axial loading. However, in the case of a pile group passing through a liquefiable soil layer and founded in a dense sand layer, the pile group suffered large settlements as it loses the shaft friction in the liquefied layer and attempted to mobilise additional end bearing capacity. Further, with the post-seismic dissipation of pore pressures and the consequent settlement of the soil, the piles register significant down drag forces. This resulted in a reduction of the loads being supported as shaft friction and required further end bearing capacity to be mobilised

Physical Modelling of Earthquake-induced Liquefaction on Uniform Soil Deposit and Earth Structures Settlement

Earthquake-induced liquefaction has been a complex and challenging topic in the field of geotechnical engineering due to its ability to cause catastrophic damage to the surrounding area. The manifestation of earthquake-induced liquefaction as observed from the effect of its past occurrence is damages on the ground and structures such as buildings, earth structures, and important lifelines structures. Liquefaction is caused by the loss of strength and stiffness of the cohesionless saturated soils due to the rapid dynamic loads from the earthquake. However, its complexity and uncertainty make the problems as one of the challenging problems in geotechnical engineering. One of the method to analyse the phenomena is through Physical modelling. Model subjected to the geotechnical centrifuge is required to analyse and observed the earthquake-induced liquefaction phenomena and this study aimed to understand the liquefaction phenomena, mechanism, and consequences through physical modelling by centrifuge and laboratory tests. This involved the physical modelling of the embankment which lies on a liquefiable foundation ground and subjection to earthquake motion of the 2011 Tohoku Earthquake retrieved from K-Net Mito stations. Moreover, geotechnical centrifuge test with 50 g of centrifugal acceleration was conducted to create the conditions of the actual field and the behaviour of the model related to acceleration, pore pressure, and displacement was observed using sensors. The liquefaction manifestation was observed in the model with the occurrence of lateral spreading, remnants of the sand boils, and deformation of the embankment. Furthermore, excess pore water pressure was rapidly developed and the pore pressure ratio (ru) higher than 1 was found to have indicated the occurrence of liquefaction while the embankment settle was estimated at 0.43 m

Seismic behaviour of structures with basements in liquefiable soil

Earthquake induced liquefaction can cause significant damage in the built environment. Structures on shallow foundations can suffer large settlement and rotation, and light under- ground structures can uplift. The performance of structures with basements, which intuitively combines these two problems, is not understood. In this thesis, the seismic behaviour of structures with basements in liquefiable soil has been investigated. A series of highly instrumented dynamic centrifuge tests showed that the presence of a basement can reduce the liquefaction induced settlement of a structure whilst maintaining the natural isolation provided by liquefied soil. The generation of positive excess pore pressures during shaking increased the buoyancy force provided by the basement. When the ratio of uplift to total weight during liquefaction was controlled, this buoyancy reduced settlement compared to structures with shallow foundations without basements. The centrifuge test data showed that structures with basements were susceptible to suffer a large accumulation of rotation during earthquake induced liquefaction. Compared to structures without basements, resistance to rotation provided by the soil decreased because the buoyancy provided by the basement reduced the vertical effective stresses below the structure. Rotation was exacerbated by an increase in the moment loading imposed by the structure due to the presence of the basement. An increase in the plan area of the basement was found to reduce residual rotation. A mechanical model for displacement and rotation in liquefiable soil was developed using data from the centrifuge test series. It was based on a traditional mass-spring-damper model, with the addition of slider elements to incorporate accumulation of displacement and rotation. The model was able to replicate the centrifuge test results when liquefaction occurred. Parametric studies using the model found that residual rotation was highly sensitive to the basement width and height of the centre of gravity of the structure. In summary, liquefaction induced settlement and rotation, and seismic demand of a structure can be minimised by including a basement and controlling the basement geometry, mass distribution, and total weight of the structure. The increase in usable space that a basement provides in a building is anticipated to make this mitigation method an attractive option compared to conventional alternatives such as soil improvement

Design and Performance of a Single Axis Shake Table and a Laminar Soil Container

Correct evaluation of shear modulus and damping characteristics in soils under dynamic loading is one of the most important topics in geotechnical engineering. Shaking tables are used for physical modelling in earthquake geotechnical engineering and is key to the fundamental understanding and practical application of soil behaviour. The shaking table test is realistic and clear when the response of geotechnical problems such as liquefaction, post-earthquake settlement, foundation response and soil-structure interaction and lateral earth pressure problems, during an earthquake is discussed. This paper describes various components of the uniaxial shaking table at university of Guilan, Iran. Also, the construction of the laminar shear box is described. A laminar shear box is a flexible container that can be placed on a shaking table to simulate vertical shear-wave propagation during earthquakes through a soil layer of finite thickness. Typical model tests on sandy soil conducted on the shaking table and the results obtained are also presented. Appropriate evaluation of shear modulus and damping characteristics of soils subjected to dynamic loading is key to accurate seismic response analysis and soil modelling programs. The estimated modulus reduction and damping ratio were compared to with Seed and Idriss’s benchmark curves