Siang Huat Goh

RESPONSE OF SOFT CLAY STRATA AND CLAY-PILE-RAFT SYSTEMS TO SEISMIC SHAKING

The behavior of pile foundations under earthquake loading is an important factor affecting the performance of structures. Observations from past earthquakes have shown that piles in firm soils generally perform well, while the performance of piles in soft or liquefied ground can raise some questions. Centrifuge model tests were carried out at the National University of Singapore to investigate the response of pile-soil system under three different earthquake excitations. Some initial tests were done on kaolin clay beds to understand the pure clay behavior under repetitive earthquake shaking. Pile foundations comprising of solid steel, hollow steel and hollow steel pile filled with cement in-fill were then embedded in the kaolin clay beds to study the response of clay-pile system. Superstructural inertial loading on the foundation was modeled by fastening steel weight on top of the model raft. The model test results show that strain softening and stiffness degradation feature strongly in the behaviour of the clay. In uniform clay beds without piles, this is manifested as an increase in resonance periods of the surface response with level of shaking and with successive earthquakes. For the pile systems tested, the effect of the surrounding soft clay was primarily to impose an inertial loading onto the piles, thereby increasing the natural period of the piles over and above that of the pile foundation alone. There is also some evidence that the relative motion between piles and soil leads to aggravated softening of the soil around the pile, thereby lengthening its resonance period of the soil further. The centrifuge model tests were back-analyzed using the finite element code ABAQUS. The analysis shows that the simple non-linear hypoelastic soil model gave reasonably good agreement with the experimental observations. The engineering implication arising from this study so far is that, for the case of relatively short piles in soft clays, the ground surface motions may not be representative of the raft motion. Other than the very small earthquakes, the raft motion has a shorter resonance period than the surrounding soil.

Centrifuge Study of the Effects of Lattice Leg on Penetration Resistance and Bearing Behavior of Spudcan Foundations in NC Clay

This paper reports the primary results of a centrifuge model study into the possible effects of the lattice leg on the penetration resistance and vertical bearing capacity of spudcan foundations in normally consolidated clay. Up to now, the possible effects of the lattice leg has been largely ignored in both research and design of spudcan foundation. Centrifuge experimental results show that there is an increase in penetration resistance for spudcan footing equipped with lattice leg, in comparison with spudcan footing connected to slender circular column leg as widely used by current research. Larger excess pore water pressure was generated by the spudcan penetration with lattice leg in compare with that without. Moreover, the presence of the lattice leg is shown to affect the cavity depth formed around the penetrated spudcan footing, which is simply assumed completely back flow for spudcan penetration in soft clay by SNAME (2008). It was found that the bearing capacity of spudcan foundation has been further underestimated by SNAME (2008) due to the neglecting of lattice leg effect. Taken altogether, this implies that changes in penetration resistance arising from the lattice leg may be due to the effect of the latter on the backflow pattern. Apart from highlighting the effect of the sleeve with big openings, the results also highlight the possible future use of sleeves to enhance the bearing capacity and possibly reduce the penetration of spudcan foundations in various soil deposits.Copyright

Failure Modes of Tunnels with Improved Soil Surrounds

AbstractThis paper presents the study of possible failure mechanisms and stability of large-diameter tunnels in cement-treated soil surrounds. The failure modes and critical tunnel support pressure...

Pore Pressure Generation and Dissipation Effects on Spudcan Fixity in Normally Consolidated Clay

Spudcan rotational fixity under combined vertical, horizontal and moment loading is often assumed to be invariant with time. In reality, the actual rotational fixity of spudcan footing is likely to change with time as excess pore pressure builds up and dissipates. This paper describes a series of centrifuge experimental tests conducted at 100-g acceleration using a small spudcan model and specimens of normally consolidated reconstituted kaolin clay. Using a servo-motor, belt-driven actuator system, loading episodes comprising one thousand cycles of combined loading were applied to model foundation. The PPTs are installed in soil specimens to measure the excess pore pressure and degree of saturation of soil. One small-rotation test is conducted just after spudcan’s penetration; while another one small-rotation is conducted when the excess pore pressure is fully dissipated after the spudcan’s penetration. The results show bending moments at four locations along the spudcan shaft, which indicates that the lattice confers a significant lateral soil resistance, and the presence of the lattice will also cause the location of maximum bending moment to be up-shifted along the leg, towards to the soil surface. The rotational fixity of the spudcan shows distinct changes over time, which is attributed to consolidation and settlement effects. Comparison of fixity of spudcan with and without lattice leg indicates that the lattice leg can lead to a large increase in lateral resistance of deeply penetrated spudcans. This lattice leg effect has been largely ignored in both academic study and industrial design.Copyright

Effective-Stress Finite Element Analysis of Spudcan Penetration

The numerical modeling of spudcan penetration involves technical challenges posed by large soil deformation coupled with significant material non-linearity. The Lagrangian approach commonly used for solid stress analysis often does not work well with large deformations, resulting in premature termination of the analysis. Recently, the Arbitrary Langrangian Eulerian (ALE) and the Eulerian methods have been used in spudcan analysis to overcome problems caused by the soil flow and large deformation. However, most of the reported studies are based on total stress analysis and therefore shed no light on the excess pore pressures generated during spudcan installation. As a result, much remains unknown about the long-term behaviour of spudcans in the ground, which is affected by the dissipation of excess pore pressures. This paper reports an effective-stress finite element analysis of spudcan installation in an over-consolidated (OC) soft clay. The Eulerian analysis was conducted using ABAQUS/ Explicit, with the effective stress constitutive models coded via the material subroutine VUMAT. The results demonstrated the feasibility of conducting effective-stress finite element analysis for undrained spudcan penetration in OC clays. The paper discusses the flow mechanism, stable cavity depths and bearing capacity factors when spudcan installation occurs in various OC soils. It was found that the pore pressure build-up concentrates in a bulb-shaped zone surrounding the spudcan. The size of the pore pressure bulb increases with increasing penetration. The maximum excess pore pressure, which is generated near the spudcan tip, is predominantly controlled by the undrained shear strength at the tip level.Copyright

Special Issue “Information Technology for Geotechnical Engineering”

The application of information technology to geotechnical engineering is not new. The storage of borehole information in a simple electronic form has evolved rapidly over the years to take advantage of numerous emerging technologies such as the World Wide Web and the geographical information system. There are also increasing attempts to integrate geoinformation from all available sources such as geophysical, geological, and remote sensing databases for surface and subsurface modeling as well as with data from downstream applications such as monitoring data from construction management of large projects or monitoring data for hazard/disaster management. The usefulness of electronic databases is also augmented by advancement in sensor technology (related to upstream quality and quantity of data) and decision support systems (related to downstream interpretation of data). The special issue contains nine invited papers. The papers are arranged according to the last name of the first author. Eight of them were presented at the International Symposium on Advances in Ground Technology and Geo-Information (IS-AGTG), which was held in Singapore between 1 and 2 December 2011. The guest editors are grateful to Professor Vassilis Marinos and his co-workers for contributing an additional paper to this special issue. Among the nine invited papers, two papers discussed the exploitation of information systems for tunneling projects (Chmelina et al. and Marinos et al.). Chmelina et al. described the application of an information system that integrates site data for monitoring and early warning at four tunneling projects, namely Metro Thessaloniki, Metro Budapest, Cityringen Copenhagen and Crossrail London. Marinos et al. described the development of a database named Tunnel Information and Analysis System (TIAS) based on geological, geotechnical, and structural information associated with 62 road tunnels in northern Greece. Two papers highlighted the potential to improve project management in a data rich environment for groundwater remediation (Miyata and Hata) and deep excavation (Rackwitz et al.). Miyata and Hata discussed the usefulness of information technology for monitoring contaminants and groundwater level, transferring data between the remediation well and the remote remediation center, and hosting a fuzzy decision support system for controlling the remediation well. Rackwitz et al. discussed the development of a web-based software platform DoMaMoS to support routine construction operations and to handle exceptional situations that can lead to disasters. The platform is applied to the ‘‘Spreedreieck’’ excavation project in Berlin to showcase its project/quality management and risk management capabilities. Two papers (Inoue et al. and Todo et al.) discussed the application of databases for subsurface mapping. K.-K. Phoon (&) S. H. Goh National University of Singapore, Singapore, Singapore e-mail: kkphoon@nus.edu.sg

A two-phase coupled Eulerian-Lagrangian FEM for offshore spudcan analysis

This paper reports a development of a two-phase coupled Eulerian-Lagrangian FEM for analyzing the short-and long-term response of offshore spudcan footing. The proposed method is composed of three integral components which are effective stress Eulerian analysis for the shrot-term installation behavior of spudcan, mesh-to-mesh solution variable mapping and coupled-flow Lagrangian analysis for the post-installation, long-term working behaviour of spudcan. By combining the advantages of Eulerian and Lagrangian finite element methods, this coupled Eulerian-Lagrangian FEM is tailored to solve offshore foundation problems which often see rapid and large-scale undrained soil flow during installation followed by much longer periods of embedment in the ground during operation.

Solving the wave propagation in fully saturated porous media using dual mesh method

In this paper, Biot’s u-U wave propagation formulation for saturated porous media is implemented on commercial FE codes. The new method couples the solid and pore fluid phases using two FE meshes, one for the solid and the other for the fluid phase. The fluid-solid coupling is implemented using dashpots and user-defined material subroutine. The proposed method is applied to published example and consistent results are obtained. The wave propagations from dry to saturated media are studied. The two dilatational waves are clearly observed in the vicinity of fluid solid interface. In the end, effects of the permeability and loading frequency on the wave profiles are also studied.

Effective-stress finite element analysis of spudcan penetration with lattice leg in clay

An undrained effective-stress method was adopted to examine the pore pressure response by spudcan penetration with and without lattice leg. Coupled Eulerian-Lagrangian analysis technique was conducted using ABAQUS/Explicit v6.11. For spudcan penetration with lattice leg, the computed excess pore pressure at spudcan base shows good agreement with centrifuge experimental results. The comparison result verifies and extends the feasibility of this effective-stress finite element method, in which lattice leg is taken into account. Larger excess pore pressure is found to be developed by the involvement of lattice leg. The soil strength is also shown to influence the pore pressure response.

SOIL-PILE INTERACTION DURING LIQUEFACTION-INDUCED LATERAL SPREAD

This paper describes a numerical model for the analysis of foundation piles subjected to large lateral ground deformation triggered by liquefaction. The model involves the use of p-y curves, but avoids the empiricism associated with the selection of degradation coefficients or reduction factors. To obtain a proper p-y characterization of the reaction between laterally deformed liquefied soil and an embedded pile, triaxial extension is recognized as the most appropriate analogue for the loading conditions. A suite of undrained triaxial extension tests was carried out using Nevada sand to establish the relevant strength and deformation parameters. Using the material parameters obtained from these tests, 2D FLAC analyses were performed to develop strain-softening p-y curves. Application of these p-y curves to the analyses of centrifuge experiments involving lateral spread effects on piles yields good agreement between the computed and measured responses. The strain-softening model provides excellent predictions of the measured peak and residual moments. Furthermore, the computed soil pressure diagrams agree well with the recommendations made by the Japan Road Association, which were calibrated using case histories from the 1995 Kobe earthquake. The same p-y curves were applied to the analysis of a field problem involving an actual pile foundation subjected to lateral spread effects during the 1964 Niigata earthquake.