Discussion of "Vacuum and surcharge combined one-dimensional consolidation of clay soils"

Abstract

770 The authors are to be congratulated for developing a vacuum consolidation testing apparatus. The apparatus can (i) generate a vacuum pressure, surcharge pressure, or simultaneously combined vacuum and surcharge pressure in a soil specimen at a predetermined magnitude; and (ii) monitor the soil pore-water pressure, settlement, and volume change during the test at predetermined time intervals. Four sets of tests were conducted on clay soils using the apparatus to examine the vacuum and surcharge induced one-dimensional consolidation model proposed in the paper. Based on the interpretation of the test data, the authors concluded that the nature of the consolidation pressure, either surcharge, vacuum, or a combination, has no bearing on the soil consolidation and compression behaviour and on the soil parameters during the consolidation process, including the coefficient of consolidation and the hydraulic conductivity. It was then concluded from the close agreements between the model predictions and the measured data that the proposed one-dimensional model of vacuum and vacuum–surcharge consolidation describes the tested consolidation behaviour of soils well. Seemingly, the well-assembled and instrumented testing apparatus was designed for the one-dimensional consolidation test, as free drainage is allowed only at the top of the soil specimen. This is true for the soil specimen under the surcharge condition, but not under the vacuum condition. Under the vertical surcharge condition, the soil specimen would have the tendency to expand laterally, but this is restrained by the steel ring. Application of the surcharge pressure to the top of the specimen will inevitably result in an increase of the lateral pressure both in the soil specimen and on the inner surface of the steel ring. The increment of the lateral pressure is dependent on the soil’s constitutive behaviour, and it is not necessarily equal to the applied surcharge pressure. Under such a stress condition, pore water is expelled from the soil specimen, resulting in settlement of the specimen. As the lateral compression strain is restrained, the vertical strain interpreted based on the measured settlement represents exactly the volumetric strain of the specimen. Therefore, the stress and strain conditions of the soil specimen under the surcharge condition are consistent with the one-dimensional consolidation theory. Conversely, under the vacuum condition, application of the vacuum pressure will obviously generate suction in the soil specimen, leading to a negative pore-water pressure and an increase in the effective stress between soil particles. The magnitude of the increment of the effective stress equals the applied vacuum pressure. Pore water was then sucked out from the voids of the soil specimen. Compared with the unidirectional surcharge pressure, the vacuum pressure applied and the subsequently generated negative pore-water pressure in the voids between the soil particles are multidirectional. Therefore, under the vacuum condition, contraction of the soil skeleton would be expected all around the soil specimen. Since the steel ring is rigid, contraction of the soil skeleton on the horizontal plane is not restrained, and the lateral stress on the inner surface of the steel ring would decrease. As a consequence, the vertical strain interpreted based on the measured settlement represents only a part of the volumetric strain of the specimen. Most importantly, under the vacuum condition, consolidation of the soil specimen is in nature a three-dimensional problem rather than a one-dimensional one, though the drainage boundary conditions of the designed apparatus are the same under both the vacuum and the surcharge conditions. Therefore, the appropriateness of the simple extension of Terzaghi’s one-dimensional model to the vacuum consolidation is questionable. Based on the aforementioned analysis, the same settlement would be expected for the soil specimen consolidated by the same vacuum and surcharge pressure, as illustrated likely by the test results in Fig. 5. From the point of view of the soil skeleton, however, greater volume change may be expected for the soil specimen under the vacuum condition than under the surcharge condition. This is because the negative pore-water pressure would result in an all-around con-