Model tests were conducted to analyse the sliding stability of a retaining wall constructed by soilbags. The aim was to obtain an equation calculating the active resultant earth pressure of sand acting on the wall in the ultimate state. Additionally, shear tests on multi-layers of vertically-stacked soilbags were designed to investigate how the interlayer friction resistance varied with the height of the wall. The results show that the active earth pressure acting on the soilbag-constructed retaining wall in the ultimate state is non-linear, but it can be calculated from force equilibrium of a differential element. The interlayer friction resistance of soilbags is found to be related to the shape of the sliding surface. Based on the obtained equation and the unique shear tests results, the sliding stability of the retaining wall constructed by soilbags could be appropriately analysed.

The measurement of the shear strength of unsaturated soils in terms of two independent stress state variables is usually difficult, expensive, and time-consuming. This paper presents a proposal to combine a filter paper method and a conventional direct shear test to obtain this measurement. The feasibility of this approach is illustrated through tests on an expansive soil. First, the filter paper method is used to establish the soil-water characteristic curve of the soil, and a series of conventional direct shear tests is subsequently conducted to measure the shear strength of the soil. The matric suction of the soil at failure is estimated from the soil-water characteristic curve based on the water content of the soil tested. The test results show that the failure envelopes of the expansive soil are nonlinear on the shear strength versus the matric suction plane for different net normal stresses. The unsaturated shear strength parameter ϕb is equal to the effective friction angle ϕ′ when the soil is close to being saturated, and ϕb decreases as the soil becomes drier. The combined method proposed in this paper may be a practical technique for analyzing the shear strength of unsaturated soils in terms of two independent stress state variables because it can be conducted in most geotechnical laboratories.

The Shear characteristics of stacked soilbags are related to their interlayer arrangements and properties of the materials with which the bags(geosynthetics) are filled. To study the effects of those factors on the shear strength and failure mode of stacked soilbags, a series of shear tests were conducted. The results show that although the shear failure surface occurred at the horizontal interface between soilbags when they were arranged vertically, it was ladder-like when the soilbags were arranged in a staggered manner. The angle of insertion was found to govern the shape of the shear failure surface, and, thus, the final shear strength of soilbags arranged in a staggered manner. Two shear failure modes of the stacked soilbags were observed with different infilled materials. When the frictional resistance of the contact interface was smaller than the shear strength of the materials with which the bags had been filled, only interlayer sliding failure occurred. Otherwise, the simple shear failure of materials filling the bags occurred first, followed by interlayer sliding failure.

Soilbags are three-dimensional soil-confining units used in road foundations. This paper uses vertical repeated loading tests to investigate the performance of soilbags as reinforcement in road subgrades as well as the influence of such factors as the frequency and amplitude of loading, the number of reinforcement layers, and the buried depth of the soilbag reinforcement. The results show that soilbags as reinforcement are effective at reducing permanent and resilient deformations of the road subgrade as well as vibrations caused by traffic loads. The soilbags as reinforcement perform better when the number of soilbag-reinforced layers is increased. More than two layers of soilbags are needed to reinforce the road subgrade to make efficient use of the effect of interlayer insertion between soilbags. Furthermore, a thin surface soil cover is desirable to spread the applied pressure more effectively over the soilbags and level the subgrade.

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