Installing sand columns in clays is a common ground improvement technique used to treat soft soils, and the inclusion of geotextile to encase the sand column can further improve the performance of the reinforced clay composite. In this work, the shear strength and consolidation characteristics of clay, clay with an ordinary sand column (OSC) and clay with a geotextile encased sand column (GESC) were investigated by carrying out laboratory direct shear and oedometer tests to study the response of the reinforced clay composite under both lateral and vertical loading conditions. It was found that the OSC and GESC significantly increased the shear strength of the clay, mainly by improving the friction angle, with little impact on the apparent cohesion. Furthermore, the OSC and GESC reduced the compression index, swell index, and recompression index of the clay to some extent. The secondary compression indexes obtained from the reloading stage were lower than those from the initial loading stage, indicating that over-consolidated clay has a lower secondary compression index than normally consolidated clay. The coefficients of consolidation showed an upward trend as the effective consolidation stress increased. In addition, the compressibility and hydraulic conductivity tended to decrease with increasing effective consolidation stress.

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It is essential to measure the shear strength of soils and interface parameters between soils and 16 geosynthetics for the safe design and stability analysis of geosynthetic-reinforced soil 17 structures. These parameters recommended for engineering projects are normally measured by 18 laboratory single-stage direct/interface shear and pull-out tests. The conventional single-stage 19 tests are carried out on at least three representative specimens under three different normal 20 stresses. However, a large quantity of specimens is required for large-scale tests, with tedious 21 sample preparation procedures, so that large-scale single-stage testing becomes very labour 22 intensive, time consuming and expensive. Given that the multi-stage testing method is able to 23 measure the shear strength parameters by testing only one representative specimen, this paper 24 investigates the feasibility, reliability and applicability of the multi-stage testing method in 25 large-scale direct/interface shear and pull-out tests. Two compacted soils and a geogrid were 26 tested using both single-stage and multi-stage tests. It was found that the shear strengths 27 obtained from the multi-stage tests were slightly lower that those obtained from the single-28 stage tests, and the inferred apparent cohesion and friction angle matched closely. In addition, 29 the limitations of the multi-stage testing method were highlighted. The measured direct shear 30 strength of the soils, the interface shear strength and pull-out shear strength between the soils 31 and the geogrid are also compared and discussed in this paper.