FULL-SCALE FAILURE TEST ON A STAGE-CONSTRUCTED TEST FILL ON ORGANIC SOIL

A test fill has been built on top of 8 metres of very soft organic and calcareous soils. The fill was first built in stages and the increase in shear strength due to consolidation in the different stages has successfully been utilized in the construction of the subsequent stages. In three initial stages the deformations, the pore pressures and the increases in shear strength were carefully monitored over three and a half years. The fill was then brought to failure by successively increasing the height of the fill. The final failure test was completed in a week and can be considered as an undrained failure test. In the early stages, it was shown that by constructing the fill in stages, it was possible to safely construct a fill twice as thick as the original shear strength in the ground would have permitted. After another two years of consolidation even this load could be doubled to an 8 m thick fill before failure occurred. Failure occurred along a non-circular failure zone which clearly could be divided into zones of active shear, direct simple shear and passive shear. The shape and location of the failure zone and the ultimate load were governed by the existence of a weak layer in the middle of the compressible and low permeable soil. Due to the limited degree of consolidation in this layer, it had a much lower shear strength than the soil above and below with shorter drainage paths. With more time for consolidation or by speeding up the process with effective vertical drains, the final fill could have been made even higher. The increase in shear strength due to consolidation was found to be confined to the soil directly below the fill. It was largest and about uniform under the crest area, decreasing very moderately out to the midpoints of the slopes and then rapidly decreasing to cease almost completely under the toe of the slope. This generally confirmed the simple model that the shear strength increase is confined to the soil below the area within the midpoints of the slopes. Evaluation of the stability of the fill became increasingly complex as the construction progressed.