Thesis (M. Sc.)--University of Hong Kong, 2004. / Also available in print.

published_or_final_version / Civil Engineering / Master / Master of Philosophy

This dissertation includes the three major parts of the study: volume change, and lateral earth pressure due to suction change in expansive clay soils, and design of civil infrastructure drilled pier, retaining wall and pavement in expansive soils. The volume change model in expansive clay has been refined to reinforce realistic characteristics of swelling and shrinkage behavior of expansive clay soils. Refinements include more realistic design soil suction versus depth profiles and improved characterizations of the effects of soil cracking, overburden stress, and lateral earth pressure. The refined model also includes an algorithm of assigning suctionvolumetric water content curves and diffusivity through the soil. The typical lateral earth pressure distribution during wetting against a stationary wall is proposed. The proposed stationary retaining wall-soil system in expansive soils includes an upper movement active zone and a lower anchor zone. Mohrâ s circles and failure envelopes are used to define the effective horizontal stress and shear failure in an unsaturated soil. The prediction of the horizontal pressures due to suction change in a soil is compared with the in situ measurement of natural horizontal pressures and the measurements from the large scale tests. It is found that agreement between the measured and predicted horizontal pressures is satisfactory. Case studies of axial and bending of the pier are presented with both uniform and non-uniform wetting. The pier case study for axial behavior shows a good agreement with a heave at ground surface and uplift forces. Three case studies for bending behavior of the pier and retaining wall are presented based on suction change. Pavement design program has been refined to extend the design capabilities into both flexible and rigid pavements supported by pavement treatments. The comparative case studies using both current and new methods in pavement design show that the current method criterion of 1-inch is unnecessarily conservative. Furthermore, the current method does not provide a means of anticipating subgrade shrinkage that will result in longitudinal cracking along the edge of the pavement. The design calculations with both methods lead to the conclusion that neither the swelling movement, as in the current method, nor the total movement, as in the new method, is a reliable indicator of likely acceptable pavement performance. Instead, all of these case studies show that it is important to use the predicted history of the present serviceability index and the international roughness index as the proper design guideline for an acceptable treatment of the subgrade of an expansive soil.

Thesis (M. Phil.)--University of Hong Kong, 1976.

Thesis (M.S.)--Ohio University, August, 1998. / Title from PDF t.p.

Lateral earth pressures (LEP) that arise due to backfill on retaining structures are typically determined by extending the principles of saturated soil mechanics. However, there is evidence in the literature to highlight the LEP on retaining structures due to the influence of soil backfill in saturated and unsaturated conditions are significantly different. Some studies are reported in the literature to interpret the variation of LEP on the retaining structures assuming that the variation of matric suction in unsaturated backfill material is hydrostatic (i.e. matric suction is assumed to decrease linearly from the surface to a value of zero at the ground water table). Such an assumption however is not reliable when the backfill behind the retaining wall is an expansive soil, which is extremely sensitive to the changes in variation of water content values. Significant volume changes occur in expansive soils due to the influence of environmental factors such as the infiltration and evaporation. In addition to the volume changes, the swelling pressure of the expansive soils also varies with changes in water content and can significantly influence the LEPs behind the retaining wall. In this thesis, a framework for estimating the LEPs of unsaturated soils is proposed considering the variation of matric suction with respect to various water flow rates (i.e. infiltration and evaporation). The proposed approach is extended for expansive and non-expansive soils in this thesis taking into account of the influence of both the cracks and the lateral swelling pressure with changes in water content. A program code LEENES (Lateral pressure estimation on retaining walls taking account of Environmental factors for Expansive and Non-Expansive Soils) in MATLAB is written to predict the LEP. The program LEENES is valuable tool for geotechnical engineers to estimate the LEPs on retaining structures for various scenarios that are conventionally encountered in geotechnical engineering practice. The studies presented in this thesis are of interest to the practitioners who routinely design retaining walls with both expansive and non-expansive soils as backfill material.

This thesis presents a unique finite element investigation of the seismic behaviour of 2 retaining wall types – a rigid retaining wall and a cantilever retaining wall. The commercial finite element program PLAXIS2D was used to develop the numerical simulation models. The research includes: (1) validating the finite element model with the results of 3 previously existing centrifuge tests taken from literature; (2) investigating the seismic response of rigid and cantilever retaining walls including studying the effects of contribution of wall displacement, wall and backfill seismic inertia and stiffness of the foundation soil; (3) developing analytical methods to concrete the findings of the numerical models. Based on the results of the seismic response of a rigid retaining wall, a unique relationship between the seismic earth pressure and wall displacement has been developed for the active and passive modes of failure. The seismic active earth pressure has been found to be not dependent on the wall displacement while the seismic passive earth pressure has been found to be highly affected by the wall displacement. The maximum seismic passive earth pressure force and relative horizontal displacement are predicted when the ground earthquake acceleration is applied with maximum amplitude and minimum frequency content. The seismic response of the wall was not affected by the ratio of the frequency content of the earthquake to the natural frequency of the wall-soil system. For the cantilever retaining wall detailed structural integrity and global analyses have been carried out. It has been observed that the seismic earth pressure, computed at the stem and along a vertical virtual plane are found to be out of phase with each other during the entire duration of the earthquake, and hence, the structural integrity and global stability should be evaluated and assessed individually. A critical case for the structural integrity is observed when the earthquake acceleration is applied towards the backfill soil and has frequency content close to the natural frequency of the retaining wall, while, for the global stability, the critical case is observed when the earthquake acceleration has maximum amplitude and is applied towards the backfill soil with minimum frequency content. The structural integrity is also found to be highly dependent on the ratio between the frequency content of earthquake acceleration to the natural frequency of the cantilever retaining wall. The relative horizontal displacement of a rigid and cantilever retaining wall is found to be highly affected by the duration of the earthquake in contrast to what has been observed for the seismic earth pressure force. The structural integrity of a rigid and cantilever retaining wall reduces when the backfill soil has a higher relative density, while the global stability increases when the backfill soil has a high relative density during an earthquake. The results obtained from the analytical methods reveal that the wall seismic inertia force has a significant effect on the structural integrity only for the top of the stem while the base of the stem does not get affected significantly. The modified Newmark sliding block method provided a more reasonable estimation of the relative horizontal displacement of a rigid retaining wall and a cantilever retaining wall compared with the classic Newmark sliding block method.

Triaxial shear test is the most accurate test for deciding the undrained shear strength of clay. In every test the ratio between the horizontal and vertical stresses, the coefficient of earth pressure at rest (K0′), has to be decided. It’s widely believed that the choice of this parameter will influence the results, but it’s not known to what extent. In this thesis 20 consolidated undrained active triaxial shear tests on clay has been con- ducted with a K0′ at 0.6 and 0.8. The clay was collected with a 54 mm piston sampler at the Norwegian Geo-Test Site in Trondheim, Norway, on depth of 3.0 to 7.8 meters. Besides the triaxial testing, index tests and oedometer tests was conducted on every cylinder to do a gen- eral classification of the clay. The clay has a sensitivity of 9-20, a water content of 35-51 %, a plasticity index of 27-65 % and an over consolidation ratio of 2.6-6.8. The consolidation phase of the triaxial test was conducted in five loading steps with a rest time in-between to monitor the amount of pore water expelled at each stress level. The loading steps was 50 %, 75 % and 100 % of maximum cell pressure and thereafter at 50 % and 100 % of the maximum deviator stress. The shear phase was done at a speed of 1.5 % per hour to a total of 10 % axial strain. It is not possible to reach an unambiguous conclusion from the results, but the maximum shear strength of tests with a K0′ at 0.8 is 17 % higher, while the total amount of pore water extortion is equal between the two values. The amount of creep in the latest steps is on the other hand smaller for a K0′ at 0.8. This indicates that the soil is handling the stress level better than with a K0′ at 0.6.

No description available.

This dissertation is concerned with controlling the motion of a semisubmersible structure induced by high-frequency waves. The structure consists of a rigid platform and a flexible superstructure. Motion of a structure in fluids generates forces depending on the motion itself. The added mass and damping terms stemming from this motion depend on the frequency of motion. It is well known that for a given wave height, the wave energy is distributed according to a Rayleigh distribution. Because mass and damping terms vary with the frequency of the wave motion, there is an infinite number of sets of dynamical equations, one for each frequency in the Rayleigh distribution. Practical considerations make it necessary to discretize the frequency spectrum, so that there are as many dynamical equations as frequency increments. The center frequency in each increment is computed by equipartitioning of the wave energy distribution represented by a Bretschneider spectrum. The excitation forces are estimated for each increment and the design of optimal control is carried out by the Independent Modal-Space Control (IMSC) method. The net control forces can be found by summing the forces associated with each increment. The technique is demonstrated by means of a numerical example in which the wave-induced vibration of a cylindrical platform with a flexible cantilever beam is suppressed. / Ph. D. / incomplete_metadata