High positive pore pressures in subgrade soils can be expected to contribute to reduction in soil strength and stiffness. Measurement of elevated pore water pressures in the subgrades of instrumented sections of Specific Pavement Studies conducted in Ohio over the past several years have raised concerns about the long-term stability of these test sections. The objective of this study was to identify the effects of moisture content and pore water pressure on the resilient modulus (Mr) of unsaturated and saturated cohesive soils. Test results conducted on unsaturated cohesive soils at three different moisture contents (optimum, 2 to 4% dry of optimum, and 1 1/2 to 3% wet of optimum) showed that the resilient modulus and the effect of confining stress decreased with increasing moisture content. Laboratory tests on fully saturated cohesive soils showed that the resilient modulus of saturated soils decreased to less than half that of soil specimens tested at optimum moisture content. Residual pore water pressure increased with an increase in the deviator stress, and a decrease in the loading period. The time to dissipate residual pore water pressure, which was large in comparison to the load rate, increased with increasing deviator stress. The resilient modulus of fully saturated cohesive soil was much less under faster (1 second per cycle) cyclic loading than slower (8 seconds per cycle) cyclic loading.

This study presents the results of one of the first attempts to characterize the pore water pressure response of soils subjected to traffic loading under saturated and unsaturated conditions. It is widely known that pore water pressure develops within the soil pores as a response to external stimulus. Also, it has been recognized that the development of pores water pressure contributes to the degradation of the resilient modulus of unbound materials. In the last decades several efforts have been directed to model the effect of air and water pore pressures upon resilient modulus. However, none of them consider dynamic variations in pressures but rather are based on equilibrium values corresponding to initial conditions. The measurement of this response is challenging especially in soils under unsaturated conditions. Models are needed not only to overcome testing limitations but also to understand the dynamic behavior of internal pore pressures that under critical conditions may even lead to failure. A testing program was conducted to characterize the pore water pressure response of a low plasticity fine clayey sand subjected to dynamic loading. The bulk stress, initial matric suction and dwelling time parameters were controlled and their effects were analyzed. The results were used to attempt models capable of predicting the accumulated excess pore pressure at any given time during the traffic loading and unloading phases. Important findings regarding the influence of the controlled variables challenge common beliefs. The accumulated excess pore water pressure was found to be higher for unsaturated soil specimens than for saturated soil specimens. The maximum pore water pressure always increased when the high bulk stress level was applied. Higher dwelling time was found to decelerate the accumulation of pore water pressure. In addition, it was found that the higher the dwelling time, the lower the maximum pore water pressure. It was concluded that upon further research, the proposed models may become a powerful tool not only to overcome testing limitations but also to enhance current design practices and to prevent soil failure due to excessive development of pore water pressure.

This dissertation, "Failure of Saturated Sandy Soils Due to Increase in Pore Water Pressure" by Sainulabdeen Mohamed, Junaideen, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled FAILURE OF SATURATED SANDY SOILS DUE TO INCREASE IN PORE WATER PRESSURE Submitted by Sainulabdeen Mohamed Junaideen for the degree of Doctor of Philosophy at The University of Hong Kong in January 2005 Most slope failures are attributable to an increase in pore water pressure during or following periods of intense rainfall. These failures are induced by reduction of effective confining stress whilst shear stress remains nearly constant, as opposed to the failures caused by increasing shear stress in standard laboratory tests. Although the significance of the different stress paths has been recognized, studies of soil behaviour in constant shear stress path, which closely represents conditions during a rise in pore water pressure, have been scarce. Furthermore, past studies suggest that the steady state line is not unique and the volume change tendency of soils is stress path-dependent. These findings make it difficult to predict the behaviour of soils during a rise in pore water pressure. Nevertheless, there is an urgent need to study such behaviour as rainfall-induced slope failures occur in various types of soils and occasionally turn into debris flows, posing a serious risk to both life and property. The principal objectives of this study were to characterize the soil behaviour during a rise in pore water pressure and to examine existing theories in respect of the instability of soils. A comprehensive testing program, which included load-controlled constant shear stress path tests and standard strain rate-controlled undrained tests, was conducted on completely decomposed granites. Specimens prepared at various densities were used for the tests. A fast data-acquisition system was utilized so that readings during rapid deformations could also be recorded. The study's findings showed that the steady state line was reasonably unique for a given soil and that the volume change tendency was not path-dependent. The steady state concept could be used to explain soil behaviour during a rise in pore water pressure; and the difference between the current void ratio and the void ratio at the steady state at the current stress level could be used to characterize soil behaviour. Instability of the soils occurs at higher stress ratios in the constant shear stress path than in the undrained loading stress path. To define the instability state due to a rise in pore water pressure, the post-peak portion of undrained effective stress paths is of very limited use for this type of soil. Conversely, using the constant shear test results, the instability state can be adequately defined for soils of various densities. Furthermore, the concept of collapse surface can be extended to define the instability state of soils of various densities due to undrained loading. DOI: 10.5353/th_b3070854 Subjects: Shear strength of soils Slopes (Soil mechanics) Sandy soils - Testing

During the last decades completely new technologies for high speed railway vehicles have been developed. The primary goals have been to increase traction, axle load, and travelling speed, and to guarantee the safety of the passengers. However, new developments have revealed new limitations: settlement and destruction of the ballast and the subgrade lead to deterioration of the track; irregular wear of the wheels causes an increase in overall load and deterioration in passenger comfort; and damage of the running surfaces of the rail and the wheel is becoming more frequent. These problems have been investigated in the Priority Programme SPP 1015 supported by the Deutsche Forschungsgemeinschaft (DFG), with the goal of better understanding of the dynamic interaction of vehicle and track, and the long-term behavior of the components of the system. The book contains the scientific results of the programme as presented at the concluding colloquium held at University of Stuttgart, Germany, 2002.