Unsaturated Shear Strength Parameters Research Articles

Evaluation of the Unsaturated Shear Strength Parameters of Compacted, Heaving Soil Using Geotechnical Properties

Abstract The shear strength is a fundamental property of soil material under structural loads. The determination of shear strength properties of unsaturated soils is challenging and time-consuming. Geotechnical predictive models can be utilized to assess the unsaturated shear strength of heaving soil. This study attempts to propose predictive models to evaluate the unsaturated shear strength parameters of compacted heaving soil. These parameters include the angle of internal friction associated with the net normal stress (ϕ'), angle indicating the rate of increase in shear strength with respect to a change in matric suction (ϕb), and effective cohesion (c'). The geotechnical properties of soils were assessed through laboratory tests such as particle size distribution, consistency limits, specific gravity, modified Proctor compaction test, swelling test, suction test, and advanced triaxial testing. Multivariate analysis was conducted using NCSS 12 software to design the models. The validation of models includes the determination coefficient, probability value, comparing experimental values with predicted values, and comparing the developed models with other model found in recent literature. The models engineered in this study can estimate the unsaturated shear strength parameters of compacted heaving soil with acceptable precision.

Soil database development with the application of machine learning methods in soil properties prediction

Excessive rainwater infiltration can be an important causal agent of both slope and whole tree uprooting failures. Early warnings or stabilization measures on high-risk slopes or trees are critically important. To identify the high-risk areas, it is necessary to conduct seepage, slope and tree stability analyses over a large region. Given the spatial variability of soil properties, a soil database is therefore required before performing distributed or Geographical Information System (GIS) -based water balance and stability analyses. Considering that the unsaturated soil properties could be very different from saturated soil properties, in this study, a soil database containing both saturated and unsaturated hydraulic and mechanical soil properties was developed for the first time. Machine learning methods were used to predict the unknown soil properties. Based on the predicted soil properties, spatial distributions of different saturated and unsaturated soil properties were generated using the ordinary kriging method. Then the soil database was developed with Singapore island being divided into 97 zones, with each zone having similar soil properties. In this study, the importance of different input variables in soil properties prediction was also investigated. In addition to soil plasticity (i.e., Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI)) and grain size distribution (i.e., gravel, sand, and fines fractions), location (i.e., longitude and latitude) was found to be of high importance as well and are recommended to be used as input variables to predict soil properties, especially when data volume is relatively limited. For those soil properties that cover a large range of values, model performance is better when logarithm values were used as the outputs. Moreover, given the possible correlation between some output parameters, the prediction of the Soil-water Characteristic Curve (SWCC) from a multi-output model is recommended after comparing its performance with a single output model. Furthermore, the performance of two commonly used machine learning methods (i.e., random forest regression and artificial neural network) in soil properties prediction were compared and the prediction error resulting from the random forest regression method was generally smaller. The developed database includes the mean values of saturated permeability, saturated and unsaturated shear strength parameters, and SWCC in each zone. The database can be applied in regional GIS-based water balance and slope stability analyses to account for the spatial heterogeneity instead of assuming constant soil properties.

Case Study: Unsaturated Embankment Failure on Soft Soils

This paper describes the application of unsaturated soil mechanics to an interstate connecting-ramp embankment that failed during construction. Specifically, matric suction is incorporated into the calculation of the tension crack (TC) depth induced by desiccation and strain incompatibility and the contribution of matric suction to embankment shear strength. The results are compared with field observations to assess the viability of unsaturated soil mechanics in modeling compacted embankments in stability analyses. Results from this study suggest that using unsaturated shear strength parameters while introducing a TC in the compacted fill yields a reasonable inverse analysis of this interstate embankment. This may be preferred in slope stability analyses to the current practice of using an undrained shear strength (i.e., cohesion) for the unsaturated compacted fill and including a TC to generate a reasonable factor of safety.

Failure potential of a circular tunnel face under steady-state unsaturated flow condition

In natural geotechnical structures, especially for those in the vicinity of water bodies or subjected to rainfalls, the soil properties are more likely governed by the unsaturated soil constitutive model rather than the fully saturated or dry one. In this contribution, the theory of unsaturated soil mechanics is introduced to tackle the stability problem of a circular tunnel face under steady-state unsaturated flow condition. By making reference to the existing findings, a generalized analytical solution for the vertical suction stress profile is discussed which permits to deal with unsaturated soil problems with the classical limit analysis method. An extended Mohr-Coulomb yield criterion is presented to characterize the tunnel face failure in unsaturated soil where the effect of suction stress on tunnel face stability is interpreted as the apparent cohesion. The increase in shear strength of unsaturated soil due to the existing of apparent cohesion is considered as a combined effect of n, α, ks and q, which respectively correspond to the pore size parameter, the inverse of air entry value (AEV), the saturated hydraulic conductivity and the flow rate. Within the framework of limit analysis, an advanced three-dimensional (3D) failure mechanism is used to include the unsaturated shear strength parameters into the work rate equation which eventually yields the critical retaining pressure for tunnel face stability. Parametric analysis is performed to show the influence of different unsaturated soil parameters on tunnel face stability. Four representative soil types are investigated to give some suggestions for tunnel excavations and designs in unsaturated soil.

Shear strength of an unsaturated loam soil as affected by vetiver and polyacrylamide

Shear strength is an important soil mechanical property that its improvement is one of the rehabilitation program for the reduction of soil erosion and degradation. The objective of this study was to investigate the effect of polyacrylamide (PAM) and the vetiver system as a cheap and long-term bioengineering method and their combination on unsaturated shear strength parameters (effective cohesion, c' (kPa), angle of effective internal friction, φ' (°) and angle of internal friction related to matric suction, φb (°)) of a loam soil. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water [0.2% (V0P2) and 0.4% (V0P4)], and simultaneous presence of vetiver and two concentrations of PAM (VP2 and VP4). Direct shear tests were performed at combinations of three normal stresses of 25, 50 and 100 kPa and four matric suctions of 0, 10, 30 and 50 kPa (12 tests per each treatment) to determine the shear strength parameters. It was found that vetiver and PAM decreased apparent angle of internal friction (φ) and φ', and increased c', total cohesion (c), φb, and as a result increased unsaturated shear strength. However, the positive effect of vetiver on shear strength was greater than that of PAM. It seems that PAM and vetiver enhanced the contact area, inter-particles bonds, aggregation, and inter-aggregate porosity, and as a consequence, increased the soil effective saturation, effective stress, cohesion and shear strength. Also, when matric suction increased the c and shear strength increased although the φ increased slightly. Simultaneous application of vetiver and 0.4% of PAM resulted in maximum shear strength, indicated although vetiver can increase the cohesive strength, it would be more pronounced when PAM was simultaneously applied. Simultaneous application of vetiver and PAM through increasing soil shear strength can be suggested as the rehabilitation program to reduce soil erosion and degradation.

Estimation of unsaturated shear strength parameters using easily-available soil properties

Unsaturated soil shear strength is an important parameter in soil erosion and management. Measurement of unsaturated shear strength at field scale is difficult, time-consuming, and very costly. This study was conducted to investigate the relationship between unsaturated shear strength parameters and soil properties, and to predict the unsaturated shear strength parameters (effective cohesion, c', angle of effective internal friction, φ' and angle of internal friction related to matric suction, φb) using multiple-linear regression (MLR). Direct shear tests were performed at combinations of three normal stresses of 25, 50 and 100 kPa, and four matric suctions of 0, 10, 30 and 50 kPa (i.e., 12 tests per each soil) to determine the shear strength parameters in 14 soils. Soil properties including particle size distribution (sand, silt, and clay percentages or geometric mean diameter, dg and geometric standard deviation, σg), organic matter content (OM), calcium carbonate content (CaCO3), compactness indices (bulk density, ρb and, relative bulk density, ρb-rel), and mean weight diameter of aggregates (MWDdry, MWDwet), structural stability indices (aggregate stability, AS, stability index, SI and index of crusting, Ic) were determined and used as predictors in MLR models. Strong negative correlations were found between c' and φ'. The c' positively correlated with clay content. Significant negative correlation was observed between c' and sand fractions and dg. Significant positive correlation was obtained between φ' and fractions of sand and dg. The φ' negatively correlated with clay, fine silt content (FSi), MWDdry, and AS. The φb had no significant correlation with soil properties, indicating that φb is independent of soil properties and basically is affected by matric suction. Clay, coarse sand (CS) and very fine sand (VFS) were applied in the model for predicting c'. Clay had a positive and, CS and VFS had negative effects on c'. Pedotransfer functions (PTFs) using the fine sand (FS) and VFS as predictor could estimate the φ' accurately, so that they entered to PTFs with positive signs. In addition, the φb was predicted by parameter Ic only, so that it had negative effect on φb. Overall, better prediction models were developed for φ' than for c' and φb.

Measurement of unsaturated shear strength parameters of silty sand and its correlation with unconfined compressive strength

Abstract Compressive strength and direct shear testing parameters on unsaturated soil are often used in analysis and design of several geotechnical infrastructures. Rarely, any attempt has been made to quantify shear strength of unsaturated soil under different net normal stress conditions. The objectives of this study are to quantify shear strength parameters of unsaturated silty sand and to establish a simple correlation of it with unconfined compressive strength. Consolidated drained direct shear tests and unconfined compressive strength tests were performed to determine shear strength of unsaturated completely decomposed granite (CDG) soil. To interpret this, a series of typical soil water characteristic curves (SWCC) of CDG soil under different net normal stresses were also measured. Experimental results show that the unsaturated shear strength significantly increases with increase in net normal stress and matric suction. A new model for prediction of unconfined compression strength of unsaturated soil from shear strength and initial suction was proposed. The predictions from newly proposed model were found to be consistent with the measured unconfined compressive strength. This correlation can be useful in economical and rapid design/analysis of geotechnical infrastructure.

A <i>Ø</i><sup>b</sup> Model for Predicting Shear Strength of Unsaturated Soils

Generally the shear strength of saturated soils has been predicted by the Mohr-Coulomb theory. However, field data accumulated in the past have evidently shown that saturated conditions are actually never reached in arid and semi-arid regions. An analytical model is developed to predict shear strength of an unsaturated soil with respect with matric suction. The formulation takes into account the influence of the mean stress on soil water retention curves. The results of the model developed for predicting the shear strength are compared with available experimental results. The good agreement between the estimates and the experimental data indicates that the proposed equation is promising and reliable for estimating preliminary unsaturated shear strength parameters.

Effects of Dry Density and Grain Size Distribution on Soil-Water Characteristic Curves of Sandy Soils

The unsaturated soil mechanics is receiving increasing attention from researchers and as well as from practicing engineers. However, the requirement of sophisticated devices to measure unsaturated soil properties and time consumption have made the geotechnical engineers keep away from implication of the unsaturated soil mechanics for solving practical geotechnical problems. The application of the conventional laboratory devices with some modifications to measure unsaturated soil properties can promote the application of unsaturated soil mechanics into engineering practice. Therefore, in the present study, a conventional direct shear device was modified to measure unsaturated shear strength parameters at low suction. Specially, for the analysis of rain-induced slope failures, it is important to measure unsaturated shear strength parameters at low suction where slopes become unstable. The modified device was used to measure unsaturated shear strength of two silty soils at low suction values (0 ~ 50 kPa) that were achieved by following drying path and wetting path of soil-water characteristic curves (SWCCs) of soils. The results revealed that the internal friction angle of soil was not significantly affected by the suction and as well as the drying-wetting SWCCs of soils. The apparent cohesion of soil increased with a decreasing rate as the suction increased. Further, the apparent cohesion obtained from soil in wetting was greater than that obtained from soil in drying. Shear stress-shear displacement curves obtained from soil specimens subjected to the same net normal stress and different suction values showed a higher initial stiffness and a greater peak stress as the suction increased. In addition, it was observed that soil became more dilative with the increase of suction. A soil in wetting exhibited slightly higher peak shear stress and more contractive volume change behaviour than that of in drying at the same net normal stress and the suction.

An Expedite Method to Predict the Shear Strength of Unsaturated Soils

An expedite method to predict the shear strength of unsaturated soils is proposed and tested against many soils of different origins and subjected to different test types. The method uses an empirical hyperbolic function that has been successfully used to fit experimental data. The parameters of this function are obtained considering effective shear strength parameters from saturated soil and from test results of air dried samples tested without the need of suction control. Air dried samples can be, alternatively, replaced by test results of samples tested at suction larger than the maximum suction expected in the problem under analysis. The good agreement between the estimate functions and the experimental data shown by both alternatives makes them promising and reliable to estimate unsaturated shear strength parameters for preliminary purposes.

Engineering properties of gold tailings

ABSTRACT A comprehensive laboratory testing program has been conducted to establish the engineering properties of gold tailings from Kidston Gold Mines in North Queensland, Australia. The engineering properties of the tailings have been categorised into textural, hydraulic, and mechanical properties. The textural properties include the particle size distribution and the specific gravity. The hydraulic properties include the water characteristic curve, and the saturated and unsaturated hydraulic conductivity. Various models that describe the tailings water characteristic curve and hydraulic conductivity function have been presented. The mechanical properties include the bulk modulus, Poisson's ratio, the friction angle, the dilation angle, and the unsaturated shear strength parameters. The engineering properties of the tailings are vital to the management of the tailings both during the mine's operation, and long after its de-commissioning.

Unsaturated strength behaviour of compacted lateritic soils

This technical note presents the unsaturated shear strength parameters from two theories for two different compacted lateritic soils. The two theories are: (1) a shear strength equation for unsaturated soils, proposed by Fredlund et al. (1978), using two independent stress state variables, the net normal stress and the matrix suction; and (2) Toll's (1990) proposed framework for unsaturated soil behaviour also incorporating volume change. Tests were conducted on two lateritic soils, developed from basalt and sandstone, which have been used for road pavements in south east Queensland, Australia. The shear strength properties were studied on 200mm high, 100mm diameter compacted specimens. Their classification and compaction properties are tabulated. Values obtained for Fredlund's parameter, for the two soils tested, vary with suction. Values obtained for Toll's parameter, for these soils, are in the same range as his results for Kenya gravel. Suction does not affect the shear strength of an unsaturated soil below degree of saturation 53%-55%.

P55 Designing a registration system for the clinical research database system

Excessive rainwater infiltration can be an important causal agent of both slope and whole tree uprooting failures. Early warnings or stabilization measures on high-risk slopes or trees are critically important. To identify the high-risk areas, it is necessary to conduct seepage, slope and tree stability analyses over a large region. Given the spatial variability of soil properties, a soil database is therefore required before performing distributed or Geographical Information System (GIS) -based water balance and stability analyses. Considering that the unsaturated soil properties could be very different from saturated soil properties, in this study, a soil database containing both saturated and unsaturated hydraulic and mechanical soil properties was developed for the first time. Machine learning methods were used to predict the unknown soil properties. Based on the predicted soil properties, spatial distributions of different saturated and unsaturated soil properties were generated using the ordinary kriging method. Then the soil database was developed with Singapore island being divided into 97 zones, with each zone having similar soil properties. In this study, the importance of different input variables in soil properties prediction was also investigated. In addition to soil plasticity (i.e., Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI)) and grain size distribution (i.e., gravel, sand, and fines fractions), location (i.e., longitude and latitude) was found to be of high importance as well and are recommended to be used as input variables to predict soil properties, especially when data volume is relatively limited. For those soil properties that cover a large range of values, model performance is better when logarithm values were used as the outputs. Moreover, given the possible correlation between some output parameters, the prediction of the Soil-water Characteristic Curve (SWCC) from a multi-output model is recommended after comparing its performance with a single output model. Furthermore, the performance of two commonly used machine learning methods (i.e., random forest regression and artificial neural network) in soil properties prediction were compared and the prediction error resulting from the random forest regression method was generally smaller. The developed database includes the mean values of saturated permeability, saturated and unsaturated shear strength parameters, and SWCC in each zone. The database can be applied in regional GIS-based water balance and slope stability analyses to account for the spatial heterogeneity instead of assuming constant soil properties.