Soil Strength Parameters Research Articles

Simple Approaches for the Design of Shallow and Deep Foundations for Unsaturated Soils I: Theoretical and Experimental Studies

Bearing capacity is one of the key properties required in the design of shallow and deep foundations. In conventional engineering practice, simple approaches are widely used for determining the bearing carrying capacity of foundations based on the shear strength parameters of saturated soils. However, foundations are typically placed in part or fully in the soil zone above the natural ground water table, which is in a state of unsaturated condition. The shear strength of unsaturated soils is significantly influenced by the matric suction. The bearing capacity of foundations cannot be reliably determined by extending conventional soil mechanics principles for soils that are in a state of unsaturated condition. This Companion Paper I, introduces how the shear strength can be used as a tool in the interpretation and prediction of the bearing capacity of foundations in unsaturated soils. In addition, both theoretical and experimental studies related to the bearing capacity of unsaturated soils are succinctly summarized. Numerical techniques that can be used for predicting the stress versus settlement behavior used for the design of shallow and deep foundations are summarized in Companion Paper II. The succinctly summarized information in the companion papers are valuable for geotechnical engineers for understanding and implementing the mechanics of unsaturated soils in the design of shallow and deep foundations.

The effects of particle breakage and shape on the strength parameters of sandy soil

Many laboratory and full-scale studies found that pile foundation is a reliable structure that has a long-term durability. This aspect makes it favourable when construction is in an area like the coastal where granular materials are normally in great scale. Generally, method used for the installation of pilling such as the drop hammer method will involve high energy to drive a single pile into the ground. Hence, the soil particles may undergo serious physical changes that will affect the engineering properties of soil used in the design work. The main aim of this research is to know the impact of pile installation work on sand particles. To understand the impact of sand particle breakage to the soil strength, an actual soil breaking mechanism simulated in the laboratory by using an automated soil compactor where the sand samples were crushed using 500 and 1000 times of blows respectively. The behaviour of sand were then analysed using a series of test which are; sieve analysis, specific gravity, relative density and the shear box test in order to measure the engineering properties of the sand. Mackintosh probe test was conducted in-situ to identify the undrained shear strength of sand and to correlate the cohesion of the sand with laboratory testing. This research confirmed that particle breakage has a significant influence with sand shape and therefore its strength changes with crushing impact.

A generalized artificial intelligence model for estimating the friction angle of clays in evaluating slope stability using a deep neural network and Harris Hawks optimization algorithm

In landslide susceptibility mapping or evaluating slope stability, the shear strength parameters of rocks and soils and their effectiveness are undeniable. However, they have not been studied for all-natural materials, as well as different locations. Therefore, this paper proposes a novel generalized artificial intelligence model for estimating the friction angle of clays from different areas/locations for evaluating slope stability or landslide susceptibility mapping, including the datasets from the UK, New Zealand, Indonesia, Venezuela, USA, Japan, and Italy. The robustness and consistency of the model’s prediction were checked by testing with various datasets having different geological and geomorphological setups. Accordingly, 162 observations from different areas/locations were collected from the locations and regions above for this aim. Subsequently, deep learning techniques were applied to develop the multiple layer perceptron (MLP) neural network model (i.e., DMLP model) with the goal of error reduction of the MLP model. Next, Harris Hawks optimization (HHO) algorithm was applied to boost the optimization of the DMLP model for predicting friction angle of clays aiming to get a better accuracy than those of the DMLP model, called HHO–DMLP model. A DMLP neural network without optimization of the HHO algorithm and two other conventional models (i.e., SVM and RF) were also employed to compare with the proposed HHO–DMLP model. The results showed that the proposed HHO–DMLP model predicted the friction angle of clays better than those of the other models. It can reflect the friction angle of clays with acceptable accuracy from different locations and regions (i.e., MSE = 12.042; RMSE = 3.470; R2 = 0.796; MAPE = 0.182; and VAF = 78.806). The DMLP model without optimization of the HHO algorithm provided slightly lower accuracy (i.e., MSE = 15.151; RMSE = 3.892; R2 = 0.738; MAPE = 0.202; and VAF = 73.431). Besides, two other conventional models (i.e., SVM and RF) provided low reliability, especially over-fitting happened with the RF model, and it was not recommended to be used to predict the friction angle of clays (i.e., RMSE = 6.325 and R2 = 0.377 on the training dataset, but RMSE = 1.669 and R2 = 0.961 on the testing dataset).

Prediction of limit support pressure for face stability of deep tunnels buried beneath sea/water

An upper bound analysis procedure was presented in this paper to investigate the face stability of a deep-buried tunnel excavated beneath water/sea. By constructing a kinematic velocity field, the kinematic solution of support pressure required for tunnel face stability was formulated, considering a uniform and a linearly increased soil cohesion profile. A uniform and a linearly increased distribution of support pressure was considered in this study. The consistency of the proposed procedure was well substantiated through comparing with FLAC3D and spatial discretization technique combined with the upper bound analysis. The required support pressure and critical failure mode were discussed, considering the effects of soil strength parameters and pore water pressure. For the ease of practical use, some design charts were presented in terms of normalised quantities, and an empirical formula was proposed to calculate the required support pressure. This study aids to provide a quick estimate of effective support pressure required for tunnel face stability.

Assessment of Stabilized Silt Clay Sand with Oil Palm Fibre Bunch (OPFB) Local Fibre for Slope Foundations: A Case of Coastal Soils of Mombasa, Kenya

Improvement of shear strength parameters is essential for designing the OPFB fiber mix with silt clay sand for slope stability. The objective of this study was to assess the stabilized silt clay with oil palm fibre bunch (OPFB) local fibre for slope foundation. Series of laboratory tests were conducted on various materials under study and the results revealed that, OPFB mix can be used as an additive to cement for purpose of improving engineering properties of the Silt Clay sand to cut down costs without compromising the set standards. It was established that, the shear strength parameters of the soil-fibre mixture (φ and C) can be improved significantly up to an optimum and reach a certain point where it starts to decline. The shear stress–strain curves obtained from the CU triaxial tests for reinforced sands with 30 mm fibre length together with those for unreinforced silty sand were compared; the result indicated that, fibre-reinforced specimen showed higher deviator stress at 0.25% fibre and reduces at 0.5% fibre. The strain corresponding to the peak deviator stress was increased by fibre content. Patterns of stress–strain curves for all reinforcedsamples indicated improvement in the deviator stress for all compositions and fibre content. Deviator stress of fibre-reinforced soil showed a slight increase with increasing pore pressure. The increase of the fibre content caused an increase in pore water pressure due to inclination of specimens to decrease the volume. Changes in the shear strength of fibre-reinforced soil indicated that soil strength parameters (internal friction angle φ’ and cohesion C’) increase as the internal friction surface increases between fibre and soil at certain point.

BEHAVIOR OF SALINE SOIL STABILIZED WITH POLYPROPYLENE FIBER AND CEMENT

This research deals with a problematic saline soil, which is widespread in many cities of the world, and try to obtain a solution for the reduction in its strength due to soaking by using more than one additives. In order to improve saline soil and to explain its effect, more than one additive were used in the research; polypropylene fiber is the major additive, which was used with cement in different percentages. Soil strength parameters were assessed by conducting direct shear test for both the natural and treated soil, which were tested under soaked condition after, submerged in water for 24 hour. The result of direct shear test showed that, for natural soil, there are large lose in the strength of soaked soil compared to that of dry soil due to the solubility of salt in soil. This decay in soil strength about (48.6%) for cohesion and (67.8%) from angel of internal friction. Meanwhile, with adding PPF to soil and under soaked state, the soil strength parameters grew significantly with the increase in the rate of fiber, where both the soil cohesion and angel of internal friction increase. Soils, treated with (1.5%) polypropylene fibers as well as cement, their strength parameters improved significantly as cement amount increases, where the soil cohesion and angel of internal friction increase. From experimental model which is built up to study and examine the laboratory result under complete submerged state, has a result for natural and treated soil showed improvement in bearing capacity of treated soil about 10 times compared to natural soil bearing capacity when tested under soaked condition.

Determination of stochastic shear strength parameters of a real landslide by back analysis

One of the most hazardous phenomena leading to enormous monetary loss and threatening human life is slope instability. The major contributors to such disasters are slope geometry, slope material strength, geohydrological condition, structural discontinuity, weathering, development of weak zones, lithological disturbance, and heavy rainfall. As the accuracy of parameters obtained from geotechnical investigations is vital for a practical understanding of the geotechnical project, the back analysis is a pragmatic approach to forecast and control landslide and slope instability. The current paper presents a stochastic back analysis of a recent landslide near a highway located in the south of Iran. Some background information has been gathered through air photos, field observations, and photographs indicating slope failure is pretty recent, and some boreholes were drilled to obtain the required geotechnical parameters of the soil media. Due to the uncertainties in these parameters, the stochastic back analysis approach was adopted. To this end, soil strength parameters have been calculated using the FEM program coded in MATLAB. Results that properly aligned with the findings of the post-event investigations showed a computationally more efficient back analysis approach. The improved knowledge of the geotechnical strength parameters gained through the stochastic back analysis better elucidated the slope failure mechanism, which provides a basis for a more rational selection of remedial measures.

Soil Structure Interaction Using Silt Clay by Direct Shear Test: A Review Paper

Direct shear test is a type of laboratory test which is used to determine shear strength parameters such as cohesion c (kPa) and angle of internal friction φ (°). For performing this test various conditions should be considered. The conditions are such that there must have constant vertical pressure at the top of the sample and the volume of the sample must be constant to estimate shear strength parameters of loose and dense soils. A constant horizontal displacement rate is utilized to determine shear strength parameters. The researchers also found some factors which control these shear strength parameters. These controlling factors are surface roughness, soil composition, the magnitude of normal loading, and moisture content. In the direct shear test, the surface thickness of the sample influences shear strength parameters. The researchers have given a different report based on their analysis. They showed the different modes of shear strength parameters in different conditions of sand, clay, soil, etc. for the interaction of soil structure. By mixing geo-membrane, geotextile, etc. with soil, sand, clay greatly affects shear strength parameters. When horizontal displacement is applied, vertical deformation of the shear plane of soil increases up to the yielding point of sand. The direct shear box contains a gap between the upper and lower ring but it is not constant. But, there are some drawbacks to their research. They did not show how shear strength parameters behave in silty clay. In this paper, a relative review of soil-structure interaction using silty clay is shown. In the direct shear test, cohesion is increased due to an increase in moisture content. To determine shear strength parameters, the conditions of silty clay are unsaturated. For this investigation, a shear box is used. Different loading is applied to see the deflection in a specific time interval.

Estimation of shear strength parameters of a high plasticity clayey soil stabilized with lime at different curing temperatures using Response Surface Methodology (RSM)

Estimation of shear strength parameters of a high plasticity clayey soil stabilized with lime at different curing temperatures using Response Surface Methodology (RSM)

Selection of the test method and its influence on the obtained results of soil strength parameters

Selection of the test method and its influence on the obtained results of soil strength parameters

TO THE STATISTICAL ANALYSIS OF SOIL SHEAR TESTS RESULTS

The result of any shear tests, which are the most popular laboratory methods for determination the strength characteristics of soils, is a set of values of the ultimate shear resistance and the corresponding normal pressure along the shear section. The theoretical background for the analysis of the soil shear tests results is the theory of probability and mathematical statistics. At the same time, we have questions about the determination of the calculation values of soil strength parameters. So, during the processing of shear tests results according to a consolidated-drained scheme for sandy soils the calculation values of soil specific cohesion are often negative, which is incorrect. The article considers an example of processing of shear tests data of sandy loam for various normal pressures, and analyzes the results of statistical processing of soil tests. The normative methods for probabilistic assessment of soil strength characteristics are based on using of normal distribution law of a random variable. It is assumed that the valuesof a random variable can vary from -¥ to +¥, while the soil strength parameters can vary in a limited range, i.e. accept only positive valuesfrom 0 to +¥. This circumstance does not matter for the majority of clayed soils, which strength parameters are far enough from zero. However, it has a significant effect on the result for small sought-for values, particularly, in assessing of specific cohesion for sandy soils. Therefore, for such cases, the article presents a variant of the statistical processing of the shear tests results with using the reduced normal distribution law of a random variable.

Behaviour of fine-grained gypsum-rich soil under triaxial tests

This study presents the unconsolidated undrained (UU) and consolidated undrained (CU) behaviour of a compacted gypsum-rich sandy lean clay with a total soluble salt content of 35% and a gypsum content of 33%. The compaction curve of the soil exhibits a double peak indicating two maximum dry densities and two optimum moisture content (OMC) values for the standard American Association of State Highway and Transportation Officials (Aashto) compaction effort. For triaxial testing, soil specimens were compacted at the wet OMC. UU triaxial tests on both unsoaked and 120 d soaked specimens in fresh water indicate a significant decrease of soil cohesion and angle of internal friction due to long-term soaking. An increase of compaction effort from that of standard Aashto to that of modified Aashto improved both soil strength parameters. CU triaxial tests were carried out on specimens compacted at standard Aashto compaction in a manufactured mould to prepare the triaxial soil specimens. The test results revealed that the stress paths for the tested samples differ markedly from those displaying the behaviour of overconsolidated soils, as discussed in detail in this paper.

Effect of cement and fly-ash on the geotechnical properties of expansive clay soils

The study analyses the effects of two chemical agents - cement and fly ash - in changing the swelling potential and the strength parameters of an expansive clay soil collected from Currila area in the city of Durres, Albania. The results obtained show a significant reduction of the plasticity index, with the addition of 3% cement and the combination of 3.5% cement with 8% fly-ash for a curing period for 1, 24 and 48 hours. The agglomerations formed after the reaction with the additives, changed the particle size and plastic behaviour of the soil from clay to silt of high plasticity, and the compaction curve shifted from a bell-shaped one into a flat shape. The swelling potential decreased to zero, as observed for curing periods of 7 and 28 days, reducing the swelling pressure from 70, to 30, to 0 kPa and the change in volume due to settlement to a mere 0.15mm for a 20mm initial height. The unconfined compressive strength increased by a factor of 1.52 and 2.22 with the addition of 3.5C-8FA for a 7-day and 28-day curing period, respectively.

Strength and Deformation Characteristics of Reconstituted Sand under Different Stress Paths in True Triaxial Tests

To improve the geotechnical stress–strain analysis, the stress–strain behavior of geomaterial under general three-dimensional stress conditions prevailing in the field need to be captured. The true triaxial apparatus is an enhanced version of the conventional triaxial apparatus, which allows to simulate stresses by applying loadings independently in 3 orthogonal directions. This study evaluated the strength and deformation behavior of Bangka sand under true triaxial test conditions. The test specimens were prepared by means of the multi-sieve sand pluviation method. Various true triaxial test stress paths were applied under axial compression, lateral extension, axial extension, and lateral compression with the objective of understanding and developing the empirical correlation of coarse-grained soil strength parameters in axial compression stress paths related to other stress paths. The test results showed that an increase in the value of b, the parameter used to quantify the relative magnitude of the intermediate principal stress to the other principal stresses, resulted in an increase of the internal friction angle and a decrease of the peak stress ratio. In addition it was observed that the Lade-Duncan failure criterion fitted the results of this study better than other failure criteria, namely the extended von Mises, Mohr-Coulomb, and Matsuoka-Nakai failure criteria.

Seismic stability of three-dimensional slopes considering the nonlinearity of soils

Slopes are often unstable in adverse natural conditions, especially when suffering from seismic hazards. Due to the fact that conventional pseudostatic method fails to consider the dynamic properties of seismic waves, a pseudodynamic approach is adopted to characterize the seismic acceleration which varies with time and space. According to the upper bound theorem of limit analysis, an alternative strategy that slices the failure blocks in the horizontal direction to consider the spatial variations of seismic waves is proposed. Moreover, to consider the nonlinearity of the soil strength parameters, a tangential technique is used to capture the equivalent Mohr-Coulomb (M − C) parameters to obtain the least upper bound solution. The factor of safety (FOS) of the slope is implicitly restricted in the work balance equation established by the limit analysis upper-bound method with respect to a three-dimensional (3D) rotational mechanism. With the aid of the strength reduction technique, the least FOS can be eventually derived. By comparing to the pseudostatic method to the existing solutions, the validity and rationality of the proposed method is proven. A parametric study is carried out to reveal the influence of dynamic factors and nonlinear strength parameters on the safety of a slope.

In Situ Shear Test for Revealing the Mechanical Properties of the Gravelly Slip Zone Soil.

Slip zone soil is usually composed of clay or silty clay; in some special geological environments, it contains gravels, which make the properties of the slip zone soil more complex. Unfortunately, in many indoor shear tests, gravels are removed to meet the demands of apparatus size, and the in situ mechanical property of the gravelly slip zone soil is rarely studied. In this study, the shear mechanical property of the gravelly slip zone soil of Huangtupo landslide in the Three Gorges Reservoir area of China was investigated by the in situ shear test. The test results show that the shear deformation process of the gravelly slip zone soil includes an elastic deformation stage, elastic–plastic deformation stage, and plastic deformation stage. Four functions were introduced to express the shear constitutive model of the gravelly slip zone soil, and the asymmetric sigmoid function was demonstrated to be the optimum one to describe the relationship of the shear stress and shear displacement with a correlation coefficient of 0.986. The comparison between the in situ test and indoor direct shear test indicates that gravels increase the strength of the slip zone soil. Therefore, the shear strength parameters of the gravelly slip zone soil obtained by the in situ test are more preferable for evaluating the stability of the landslide and designing the anti-slide structures.

Influence of Natural Fibres in Strengthening of Black Cotton Soil

The expansive soil like black cotton soil spread over southern Indian terrain. As a fast-growing nation like India, the need for infrastructure activities quickly takes place. For better structural capacity the geotechnical features are important. The soil stabilization is a technique in the part of improving the soil engineering properties like California Bearing Ratio (CBR), shear strength and compaction factors. In the present study, the black cotton soil stabilized with fibres to improve the engineering properties of soil. The study carried with natural fibres like sisal, banana and jute fibres. The ratio of 0.2%, 0.4% and 0.6% added in the dry weight of soil fraction with the soil for the strength gaining. The addition fibres increase the strength in the reasonable value for CBR up to 21.55% in 0.4% of banana, 0.6% of sisal and 0.6% of jute fibre combination. Optimum Moisture Content (OMC) increased up to 20% for the same combination. The results provide the combined scattered fibre effect to increase the strength parameters of soil.

Effect of silica fume on the ultrasonic pulse velocity of cemented sand

One of the geotechnical engineering strategies for dealing with weak soils in a construction site is to modify their mechanical properties with the aid of soil improvement techniques. Chemical stabilization of soil using cement is one of these effective approaches that can considerably enhance workability and shear strength parameters of soil. However, very limited investigations have been carried out to reveal the effect of silica fume on the ultrasonic pulse velocity (UPV) and the ultrasonic stiffness (Eu) of cement-treated sandy soil. For this purpose, in the present research, a series of UPV tests were performed on sand-cement-silica fume specimens. The cement percentages were 3, 5 and 7% and silica fume contents were 0, 0.25, 0.5 and 1% by weight of dry sand. The cylindrical samples were prepared and cured for 3, 7, 14, 28, 42 and 56 days and tested. The results show that inclusion of silica fume to the cemented sand increases UPV and Eu which this enhancement is more pronounced at longer ages.

Evaluation of Reservoir Slope Stability Considering Friction Angle Uncertainty and Earthquakes

Approximately 76% of the reservoirs built in Korea are facilities that are more than 50 years old. The risk of disasters has increased because of reservoir aging and the deepening polarization of drought and floods caused by abnormal weather conditions. In addition, the number of earthquakes has been gradually increasing in Korea, and anxiety has increased significantly because of the recent series of large earthquakes since the earthquakes in Pohang and Gyeongju. Accordingly, risk assessment and preparation are required through the stability evaluation of earthquakes in old reservoirs. The stability evaluation method, based on the limit equilibrium method, depends highly on the soil strength parameters; hence, it is essential to identify the soil strength parameters accurately. However, even though the soil strength parameters are of the same layer, they have different values because of the heterogeneity of the ground. It is difficult to select the representative value of the soil strength parameters because of the time and economic constraints of the ground survey. Therefore, in this study, the probability of failure of an old reservoir owing to seismic events was calculated, and the stability was evaluated. The water level and the uncertainty of the strength constant of the reservoir were considered. The analysis showed that the probability of collapse has increased because of the increase in the water level and the coefficient of variation of the friction angle, and the stability of the reservoir, considering the seismic events, was found to not satisfy the minimum factor of safety for slope activities.

Relationship between the Shear Strength and Microscopic Pore Parameters of Saline Soil with Different Freeze-Thaw Cycles and Salinities

Saline soil is a widely distributed special soil with poor engineering properties. In seasonally frozen regions, the poor properties of saline soil will cause many types of engineering damage such as road boiling, melt sinking, and subgrade instability. These engineering failures are closely related to the shear strength of saline soil. However, there are relatively few studies on saline soil in cold regions. The strength of the soil is always determined by its microstructure; therefore, the study aims to investigate the relationship between the shear strength and microscopic pore structure of saline soil with different freeze–thaw cycles and salinities. The shear strength characteristics of saline soil with different salinities subjected to different freeze–thaw cycles were obtained by triaxial tests. In addition, the microstructure of the soil samples was investigated by scanning electron microscopy (SEM) tests, and the microscopic pore parameters of the soil samples, including porosity (N), average pore diameter (D¯), average shape coefficient (K), surface fluctuation fractal dimension (F), and orienting probability entropy (Hm), were obtained by image processing software quantitatively. Based on the experimental results, the influence of freeze–thaw cycles and salinity on the shear strength characteristics and microstructure of the soil samples were analyzed. Besides that, in order to effectively eliminate the collinearity between independent variables and obtain a stable and reasonable regression model, principal component regression (PCR) analysis was adopted to establish the relationship between the microscopic pore parameters and the failure strength of the soil samples. The fitting results demonstrated that the failure strength of saline soil is mainly related to the size and direction of the pores in the soil, and it has little correlation with pore shape. The failure strength of the soil was negatively correlated with the average pore diameter (D¯) and porosity (N), and it was positively correlated with the orienting probability entropy of the pores (Hm). This study may provide a quantitative basis for explaining the variation mechanism of the mechanical properties of saline soil from a microscopic perspective and provide references for the symmetry between the changes of the macroscopic properties and microscopic pore structure of the saline soil in cold regions.