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Furthermore, in consideration of a future computer environment for large-scale, wide-area and high-resolution computation using supercomputers and general-purpose graphics-processing units, it would become advantageous to solve the boundary value problem separately as much as possible, since an integrated system such as the Integrated Earthquake System e. Hori et al. This would facilitate the use of reliable existing simulation codes for each problem that have been well customised to be suitable to parallelised computing systems.

Considering such a change in the computer environment, an attempt to solve the soil and water coupling problem by separating the deformation problem and the seepage problem was made by Takeyama et al. Figure 4 Theoretical structure of the soil—water—air coupling problem. In the stress and strain expression, the effective stress can be described using the elastic component of strain as.

This model can seamlessly describe the mechanical behaviour of soils from the unsaturated to the fully saturated state.

In the fully saturated state, it is automatically reduced to the type of Cam-Clay model for saturated soils. In this sense, it is an extension of the Cam-Clay model for the unsaturated state. A comparison between experimental data and computed results by this model is shown in Figures 5 a and 5 b. The experiment for unsaturated soils was performed using the triaxial apparatus by Kato , in which suction and confining stress are applied to a compacted specimen, as shown in Figure 5 c.

Figure 5 Performance of model — comparison between computed and experimental results. This constitutive model can be easily extended to a dynamic problem under seismic loading conditions by introducing the subloading surface and rotational hardening concepts proposed by Hashiguchi , The extended yield function is described as.

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Castro , Tatsuoka and others reported that the hardening of a sandy soil is caused not only by plastic volumetric change, but also by plastic shear distortion. The evaluation law of the hardening parameter H is given as. M d is the parameter which defines the boundary between the hardening and softening. In mathematical modelling to solve the boundary value problem, the SWRC model proposed by Kawai et al.

Water repellent soils: the case for unsaturated soil mechanics

In this model, the drying and the wetting processes in the SWRC are separately formulated using logistic curves in the space of the degree of saturation and suction to describe the hysteresis loops with suction change. The permeability of pore water in the unsaturated state is modelled to decrease with desaturation after the paper by Mualem Wojtasik and Jez reported that absorption by trees caused differential settlement of the ground surface and the development of cracks in an adjacent concrete apartment building.

Figure 6 shows the development of cracks in an adjacent concrete building next to a grouping of trees. Cracks gradually appeared in the building, which was built in After the trees were cut down in , crack progression halted. According to the report, the soil foundation originally had a high moisture content, but the water content was diminished when a soil investigation was carried out in , as shown in Figure 6.

It was concluded that absorption by trees had caused a decrease in the water content of the foundation, resulting in differential settlement of the ground.

Figure 6 Differential settlement caused by plants. RC, reinforced concrete.

A numerical simulation was carried out to realise quantitatively the effect of plants on differential settlement Kawai et al. Figure 7 represents the finite-element mesh used in the computation. To investigate the effects of these trees, numerical simulation modelled the annual underground root growth by extending the finite-element region, as shown in Figure 7 , in which the growth of root system of trees under the ground is expressed by contour.

In this, the pore water in the root area is drained to correspond to the amount of transpiration from trees, estimated from climate conditions at the site by the Penman model Penman, The average amount of annual precipitation at the site and the annual evaporation, also estimated by Penman model, are imposed on the ground surface as hydraulic boundary conditions.


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The estimated values from to are shown in Figure 9. The initial suction distribution in the unsaturated region above the groundwater table is assumed to be as shown in Figure Here, since the distribution of the degree of saturation above the groundwater table can be estimated from the distribution of water content see Figure 6 under the assumption that the void ratio is constant, the initial suction distribution can be estimated from the SWRC the average value of wetting and drying curves in Figure 8 was used.

For reference, the influence of initial and boundary conditions in the unsaturated ground on its mechanical behaviour is numerically examined by Tanaka et al.

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UNSATURATED SOIL MECHANICS

Three cases are compared: the case where precipitation, transpiration through vegetation and evaporation from the ground surface are considered case A ; the case where precipitation and evaporation are considered, but transpiration is not taken into account case B ; and the case where precipitation, transpiration and evaporation are not considered at all case C. Figure 11 shows the computed surface settlement and change in the degree of saturation with time at the right corner of the building. In case A, the degree of saturation decreases with year, which is consistent with the occurrence of the settlement.

The degree of saturation was found to drop suddenly in , likely because tree roots reached under the building around Figure 12 indicates the surface settlement profile under the building in These findings showed that trees greatly influenced the occurrence of differential settlement, as seen in comparison to the initial settlement due to the self-weight of the building. There is no record of monitoring the settlement in this site to compare with the computed results, but at least it can be said that computed result considering the influence due to growth of trees case A well explains the development of differential settlement and is consistent with the occurrence of cracks in the apartment building, which was observed in the site.

Figure 11 Computed surface settlement and change in the degree of saturation over time at the right corner of the building: a computed surface settlement; b computed degree of saturation. Figure 12 Surface settlement profile under the building in the year For reference, the differential settlement of an unsaturated compacted soil due to rainfall is also discussed by Kawai et al. In this section, a trial to apply unsaturated soil mechanics to the salt damage problem is presented.

In this section, its theoretical framework is reviewed and its application to the salt damage problem is presented. The mass change due to dissolution of salt is explained in Figure Therefore, the continuity equation considering the dissolution of the salt is derived from the mass conservation law as. The seepage equation in Figure 4 is replaced by Equation An application case of this problem follows. When evaporation and rainfall repeatedly act on the ground surface, the transfer of salinity to the unsaturated soil above the groundwater level is computed.

Herein, based on the climate data in the north-eastern part of Thailand, the monthly average evapotranspiration from the ground surface estimated from the Penman model Penman, and the monthly average rainfall Figure 15 are repeatedly input on the ground surface for 8 years as boundary conditions. The computation results are presented in Figure Salinity concentrations rise towards the ground surface year by year Figure 17 a. The monthly total water head distributions in the ground are shown in Figure 17 b.

An upward hydraulic gradient appears in the dry season, and in the opposite direction, a downward hydraulic gradient develops in the rainy season. Throughout a year, the upward hydraulic gradient seems dominant to the downward hydraulic gradient, resulting in salt deposition on the ground surface within 3 years, as shown in Figure 17 c. Here, the value on the vertical axis in Figure 17 c indicates the ratio of the solute salt in this case exceeding the saturated dissolution amount against the dissolved solute in the saturated dissolution.

Figure 17 Computed results of salt damage: a development of salt concentration in the ground; b change of total water head distribution; c accumulation of salt crystals near the ground surface. GWL, groundwater level. To inhibit such salt damage, a mulching technique is examined where gravel is placed on the ground surface, presenting a capillary barrier. In this, rainfall and evaporation inputs to the ground surface are the same as in Figure For reference, the solute transfer during consolidation of the ground based on the solid—fluid—solute coupling model is discussed by Nomura et al.

This section considers incorporating this phenomenon into the soil, water and air coupling problem. Cited By.

Mechanics of unsaturated soils : simple approaches for routine engineering practice - Strathprints

Cited By Recommended. Journal of Geotechnical and Geoenvironmental Engineering March Journal of Geotechnical and Geoenvironmental Engineering April Author Delwyn G. Close mobile search navigation Article navigation. Volume 13, Number 1. Previous Article Next Article. Book Review January 01, Unsaturated Soil Mechanics A. Frances Ackerman A. Frances Ackerman. Google Scholar. Environmental and Engineering Geoscience 13 1 : Article history first online:. You do not currently have access to this article.

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