Contribution Of Matric Suction Research Articles

3-Dimensional Numerical Evaluation of Geosynthetic Encased Stone Columns in Unsaturated Soils

Geosynthetic encased stone columns are often designed using the conventional framework of saturated soils ignoring the influence of in-situ unsaturated soil conditions. This article evaluates the performance of stone columns with and without geosynthetic encasement extending the mechanics ofunsaturated soils. The focus of numerical simulations was directed towards understanding the influence of matric suction on the confining support offered by the surrounding soil to stone columns with and without geosynthetic encasement. Investigations were extended considering the stiffness and the length of the geosynthetic encasement. The numerical studies suggest the load-carrying capacity of stone columnincreased with an increase in the matric suction in the boundary effect and the primary transition zone. However, the contribution of matric suction towards load-carrying capacity starts reducing from the secondary transition zone. The information on boundary effect and transition zones can be derived fromthe soil-water characteristic curve, which is a relationship between the water content and soil suction. In addition, the effect of stiffness and length of encasing material in unsaturated soils was found to be in contrast with saturated soils. The results of the study are promising towards developing procedures thatcan be used in the rational design of stone columns in unsaturated soils.

Foundation bearing capacity estimation on unsaturated soil slope under transient flow condition using slip line method

In this paper, an analytical method is proposed for estimating the bearing capacity of soil within the framework of slip line theory. The method incorporates a closed-form solution taking account the transient unsaturated vertical flow conditions. The proposed framework is validated for both level and sloping ground scenarios considering saturated and unsaturated soil conditions. More specifically, the bearing capacity of a strip foundation located on an unsaturated soil slope under the influence of different rainfall infiltration conditions is evaluated. A comprehensive parametric study is also conducted taking account the influence of the rainfall influx, rainfall duration, slope angle, foundation setback distance, soil type, and the depth of the ground water table. In addition, the contribution of matric suction towards the bearing capacity under transient flow conditions is evaluated. Results of this study provide a rigorous understanding of the performance of foundations on sloping ground extending the mechanics of unsaturated soils.

Analytical and Experimental Study of Pile Capacity in Unsaturated Sandy Clay

The design of pile foundation in technical practice is based on saturated soil mechanic or empirical procedure that ignores the influence of matric suction. Understanding the pile behavior and predicting the capacity of pile in unsaturated soil are important topics in foundation design. An experimental model test has been conducted on a single concrete pile model with a diameter of 16 mm and L/d ratios of 6, 10, 15 and 20. Model pile has been driven into sandy clay in the test box and is subjected to compression and uplift loading. The tensiometer instrument has been used to measure matric suction and the SoilVision has been used to get an estimate of the entire Soil Water Characteristic Curve (SWCC). The shear strength of sandy clay has been examined using a series of unconsolidated undrained triaxial tests to identify the relationship between cohesion, internal friction angle, and matric suction. The result has shown that end bearing pile capacity, skin friction capacity and total capacity of single piles are significantly influenced by the contribution of matric suction. Based on the experimental result, end bearing capacity and skin friction capacity of pile in saturated soil equation have been modified to calculate total capacity of pile in unsaturated soils by considering the influence of matric suction.

Estimating the Contribution of Suction on the Resilient Modulus of Subgrade Soils Using Capillary Saturation

AbstractThe matric suction is well known to significantly affect the resilient modulus of unsaturated subgrade soils. However, the contribution of matric suction on the resilient modulus varies wit...

Compacted expansive elastic silt and tyre powder waste

Building on/with expansive soils with no treatment brings complications. Compacted expansive soils specifically fall short in satisfying the minimum requirements for transport embankment infrastructures, requiring the adoption of hauled virgin mineral aggregates or a sustainable alternative. Use of hauled aggregates comes at a high carbon and economical cost. On average, every 9m high embankment built with quarried/hauled soils cost 12600 MJ.m-2 Embodied Energy (EE). A prospect of using mixed cutting-arising expansive soils with industrial/domestic wastes can reduce the carbon cost and ease the pressure on landfills. The widespread use of recycled materials has been extensively limited due to concerns over their long-term performance, generally low shear strength and stiffness. In this contribution, hydromechanical properties of a waste tyre sand-sized rubber (a mixture of polybutadiene, polyisoprene, elastomers, and styrene-butadiene) and expansive silt is studied, allowing the short- and long-term behaviour of optimum compacted composites to be better established. The inclusion of tyre shred substantially decreased the swelling potential/pressure and modestly lowered the compression index. Silt-Tyre powder replacement lowered the bulk density, allowing construction of lighter reinforced earth structures. The shear strength and stiffness decreased on addition of tyre powder, yet the contribution of matric suction to the shear strength remained constant for tyre shred contents up to 20%. Reinforced soils adopted a ductile post-peak plastic behaviour with enhanced failure strain, offering the opportunity to build more flexible subgrades as recommended for expansive soils. Residual water content and tyre shred content are directly correlated; tyre-reinforced silt showed a greater capacity of water storage (than natural silts) and hence a sustainable solution to waterlogging and surficial flooding particularly in urban settings. Crushed fine tyre shred mixed with expansive silts/sands at 15 to 20 wt% appear to offer the maximum reduction in swelling-shrinking properties at minimum cracking, strength loss and enhanced compressibility expenses.

EXPERIMENTAL STUDY OF MODEL PILED RAFT FOUNDATION EMBEDDED WITHIN PARTIALLY SATURATED COHESIONLESS SOILS

This paper presents an experimental study conducted on model piled rafts in partially saturated sandy soil. The aim of the experimental program is to study the effect of matric suction on the load carrying capacity of piled rafts embedded within partially saturated sandy soil .The influence of number of piles are presented and discussed in this study. The piled raft is arranged in different configurations of piles (single, double and triple piled raft with spacing 3.5D c/c) with the same area ratio (raft area to the cross section area of piles) to avoid different contact pressure area and to show the effect of different piles number and its group action. The influence of matric suction (i.e., capillary stresses) in partially saturated zone is typically not taken into account in the conventional design of both shallow and deep foundations so that the present research study the determination and contribution of matric suction towards the load carrying capacity of piled raft. The experimental work consist of 3 models of footing single piled raft (8.3 x 5) cm, double piled (16.6 x 5) cm and triple piled raft (25 x 5) cm. All these models are loaded and tested under both of fully saturated condition (i.e., matric suction equals to 0kPa) and unsaturated conditions (i.e., matric suction value equals to 6kPa , 8kPa and 10kPa), which are achieved by predetermined lowering of water table. The relationship between matric suction and depth of ground water table was measured in suction profile set by using three Tensiometers (IRROMETER). The soil, water characteristic curve (SWCC) estimated by applying fitting methods through the program (SoilVision). The results of experimental work demonstrate that the load carrying capacity of piled raft increases with increasing all values of matric suction as the number of piles supporting the raft increases. And the matric suction has a significant influence on the load carrying capacity of all tested models. The increasing value of the ultimate bearing capacity for single, double and triple piled raft under unsaturated conditions is approximately (2.1-4.47), (2-4.44) and (1.5-3.54) times higher than that at saturated condition respectively

Numerical evaluation of reinforced slopes with various backfill-reinforcement-drainage systems subject to rainfall infiltration

This paper presents a numerical study investigating the hydraulic response and stability of geosynthetic-reinforced soil slopes subject to rainfall. A series of numerical simulations of unsaturated slopes with various backfill–reinforcement–drainage systems subject to rainfall infiltration was performed by comprehensively considering the combined effect of backfill (i.e., sand, silt, and silty clay), reinforcement type (i.e., geogrid or nonwoven geotextile), and rainfall intensity (350 and 500 mm/day). The backfills were modeled using three soil–water characteristic curves (SWCCs) representing the general suction range associated with sand, silt, and clay. The influence of sand cushions in improving the stability of reinforced clay slopes was also assessed. The numerical results reveal that the loss of matric suction and development of a capillary barrier effect within clay backfills could have adverse impacts on both the global and local stabilities of the reinforced clay slopes. The contribution of matric suction in enhancing slope stability was initially high for reinforced clay slopes; however, the global stability of the reinforced clay slope substantially decreased due to the loss of matric suction as the rainfall infiltration proceeded. The local instability of the geotextile-reinforced clay slope occurred due to the capillary barrier effect at the geotextile–clay interface. The reinforced marginal soil slopes cannot effectively drain the infiltrating water under torrential rainfall. Free drainage conditions may not be assumed for these slopes if the drainage is not properly considered. Both the global and local factors of safety (FS) of the reinforced sand slope were minimally influenced by the loss of matric suction induced by rainfall infiltration. The required reinforcement tensile strengths for the reinforced silt and clay slopes to maintain FS = 1.3 were, respectively, approximately 3 and 4 times larger than that for reinforced sand slopes. Numerical results also indicated that the inclusion of sand cushions, which provide both strength and drainage functions, can effectively enhance the slope stability. An optimal sand cushion thickness of 15 cm (replacing 20% of marginal backfill with sand) was determined in this study.

Mechanical characteristics of non-saturated tailings and dam stability

The failure of tailings dams has repeatedly occurred in China, which seriously threats eco-environment and social economy. The mechanical properties of non-saturated tailings are a matter of great concern, yet remain poorly understood. In this paper, engineering properties of non-saturated tailings are measured and complemented by analysis and interpretation from multiple perspectives. The results indicate that the soil–water characteristic curve, influenced by dry density, can be described with an appropriate model through parameter analysis. In order to calculate the stability of a typical tailings slope under different conditions, a new approach that considers the contribution of matric suction is elaborated in a sound theoretical framework.

Governing failure mode of unsaturated soil slopes under rainwater infiltration

Due to rainwater infiltration and loss of matric suction above the wetting font, both translational and rotational slips may occur in an unsaturated soil slope. It is useful to know which failure mode is more critical. The governing failure mode is likely to be affected by soil properties, slope geometry, depth of wetting front and contribution of matric suction to shear strength, which were investigated in this study. Specifically, upper bound limit analysis was adopted to develop stability charts based on rotational failure mechanism, and infinite slope model was used to develop stability charts based on translational failure mechanism. For a slope, the failure mechanism which gives the lower factor of safety is the governing failure mode. It was found that the failure mode of an unsaturated soil slope under rainfall is determined by parameter group c ′/γ H tanϕ ′ for a given slope angle, depth of wetting front and contribution of matric suction to shear strength. With the increase in depth of wetting front and greater contribution of matric suction to shear strength, a slope is more prone to translational failure. Slopes with angle β = 45° are most vulnerable to translational failure, while gentler slopes (e.g. β = 15°) and steeper slopes (e.g. β = 75°) are more susceptible to rotational failure.

Bearing capacity of shallow foundations in saturated and unsaturated sands from SPT–CPT correlations

The in situ bearing capacity of sandy soils is determined or estimated using plate load tests (PLTs), cone penetration tests (CPTs), or standard penetration tests (SPTs). The contribution of matric suction toward the bearing capacity of unsaturated sands is, however, not estimated from these tests. In this paper, SPTs and PLTs were conducted in situ on sandy soil in Ottawa, Canada under saturated and unsaturated conditions to demonstrate the contribution of matric suction on the bearing capacity results. In addition, relationships are proposed to estimate the bearing capacity of sands under both saturated and unsaturated conditions from the SPTs results. Comparisons are provided between the measured and estimated bearing capacity values for three different sands from the literature using the proposed relationships. The results of the study suggest that the proposed relationships can be used in the estimation of the bearing capacity of both saturated and unsaturated sands.

Shear behaviour of rock joints with unsaturated infill

Behaviour of soil-infilled rock joints has significant importance with respect to the strength of fractured rock mass. The presence of even a small amount of fine-grained infill material within a joint can reduce its shear strength considerably, depending on the degree of saturation of infill. Therefore, it is crucial to examine how the infill material can adversely affect the joint shear strength. Previous studies of infilled joints have mainly been focused on idealised regular joint patterns owing to the simplicity and reproducibility in laboratory testing. Current literature on infilled rock joints has also neglected the effect of the degree of saturation of infill on the shear behaviour. In most instances, fully saturated infill has been used or assumed, and the contribution of matric suction on the shear strength of joints having unsaturated infill has not been studied. In this study, a series of triaxial tests on natural joint profiles having joint roughness coefficient (JRC) of 10–12 is carried out at constant moisture content. A semi-empirical model is proposed and validated on the basis of laboratory data.

Bearing Capacity of Desiccated Tailings

The development of matric suctions in soils contributes to their shear strength, resulting in an enhanced factor of safety against bearing-capacity failure. In this paper, matric suction profiles of desiccated mine tailings are predicted from a steady-state solution for evaporative conditions, and from an isothermal mathematical model that simulates liquid and vapor water flow through soils. The shear-strength envelope with respect to matric suction is established by testing reconstituted tailings samples in a modified triaxial cell, in which matric suction can be controlled. The contribution of matric suction to the shear strength is interpreted as an additional apparent cohesion for use in bearing-capacity calculations. Because of the nonlinearity of the shear-strength profile, a numerical method of analysis is adopted to predict the ultimate bearing capacity of the desiccated tailings. A subsequent decrease in bearing capacity following 2D water infiltration into a partially capped tailings deposit and accompanying suction loss is investigated.