Undrained Shear Strength Of Soil Research Articles

A Novel Method for Estimating the Undrained Shear Strength of Marine Soil Based on CPTU Tests

The undrained shear strength is an essential parameter in the foundation design of marine structures. Due to the complex marine environment and technical limitations, it is difficult and costly to obtain offshore samples. Piezocone penetration tests (CPTU) are relatively low-cost compared to drilling and sampling methods. Therefore, based on the soil behavior type index (Ic) derived from CPTU results, a model for estimating cone factors (Nkt, Nke) is proposed to improve the accuracy of estimation of undrained shear strength. The result shows that the soil behavior type index (Ic) and cone factors take on a negatively correlated exponential relation. Incorporating a cone factor that varies with the soil behavior type index (Ic) significantly enhances the accuracy of undrained shear strength predictions compared to the conventional method of using a constant cone factor. This approach reduces the root mean square error (RMSE) for Nkt (Nke) from 0.124 (0.126) MPa to 0.056 (0.06) MPa, and the mean absolute error (MAE) from 0.0154 (0.016) MPa to 0.0032 (0.0036) MPa. The method was validated at an additional location and the predictions were in high agreement with the results of the consolidated quick direct shear test. The developed method can serve as an effective tool used in the design of foundations of marine structures.

Probabilistic design optimization of rectangular skirted mudmats under combined loading in spatially variable soil

Natural soil exhibits inherent spatial variability due to complex physical and chemical formation processes, making it necessary to analyze, evaluation and design foundations using probabilistic method. In this study, the bearing capacity of rectangular skirted mudmats in spatially variable soil is investigated by three-dimensional random finite element analysis (3D-RFEA). Given the high computational cost of 3D-RFEA, an active learning probabilistic method based on Kriging surrogate model is proposed to predict the mean and coefficient of variation of bearing capacity factor of mudmats. Then, considering the effects of the skirt depth ratio of mudmats and the variation coefficient and scales of fluctuation of the undrained shear strength of soil, a prediction model of probabilistic failure envelope is proposed by combining the normalized deterministic failure envelope and the cumulative distribution function (CDF) of the uniaxial bearing capacity factor. Based on the prediction model of probabilistic failure envelope, an example of probabilistic design optimization of mudmats is finally given. Compared with the normative method, the design method based on the prediction model can design mudmats directly with the permissible failure probability.

P-y curves from in-situ ROBOCONE tests: a similarity approach for laterally loaded piles in clay

p-y curves from in-situ ROBOCONE tests: a similarity approach for laterally loaded piles in clay

Evaluation of Stress History and Undrained Shear Strength of OC Clayey Soil in Northern Jakarta

It is difficult to determine the accurate undrained shear strength (su) of clay with complicated stress history. In addition to the laboratory tests such as Triaxial UU and Unconfined tests, the in-situ tests such as Vane Shear, SPT, and CPTu tests were conducted for undrained shear strength evaluation. Moreover, the overconsolidation ratio (OCR) by CPTu correlation presented the soil stress history profile along the depth and the Oedometer tests were used to evaluate the results. It was found that the clayey soil is overconsolidated up to -31.0 m below the surface in Northern Jakarta due to groundwater extraction. The Stress History and Normalized Soil Engineering Properties (SHANSEP) formula is widely used to model the relationship between undrained shear strength, OCR, and vertical shear stress. The su results from laboratory and in-situ tests showed good agreement with SHANSEP formula. Moreover, several empirical formulas for OC clayey soil were developed for future deep excavation projects in Jakarta area.

Large deformation finite element analyses of axially loaded helical piles in cohesive soil

Large deformation finite element (LDFE) analysis is suitable for research on soil-pile interaction. The objectives of this study were to develop an LDFE model for axially loaded double-helix piles and to examine the factors that control their failure modes. The model was implemented with the re-meshing and interpolation technique with a small strain approach. Three types of helical piles in clay modeled as a Tresca material were simulated in an axisymmetric setup. The model was calibrated against the results of centrifuge model tests. Analyses show that the failure mode changed gradually from a cylindrical shear mode (CSM) to a transitional failure mode and then an individual bearing mode (IBM) with increasing inter-helix spacing. Current results suggested that there might not be a unique inter-helix spacing ratio that distinguishes CSM from IBM. Further, the effects of helix embedment depth, inter-helix spacing, and undrained soil shear strength on the failure modes were assessed by parametric analyses. Finally, the failure modes and helix-bearing factors, shown to depend on these parameters, were summarized in several charts for potential practical applications. Results suggested that the lower helix's bearing factors remained unchanged, but the upper helix's bearing factors changed with the embedment depth and failure modes.

Analysis of bucket foundation installation in clay considering soil large deformation

Bucket foundations are increasingly used to support offshore wind turbines. Its bearing performance has been extensively investigated, but its installation process has been addressed in a few investigations. This study investigates the penetration resistance during the installation of bucket foundations in clay through finite element models that consider large soil deformations. The proper finite element modeling scheme is assessed first, then a validated numerical model is employed to investigate the effects of length-diameter ratio and soil properties on the soil plug and penetration resistance. The results indicate that a minimum element dimension and penetration velocity should be set to 1/6th of skirt thickness and 0.3 m/s to ensure a good balance between computational accuracy and efficiency. An increase in the bucket length-to-diameter ratio increases the height of soil heave, while soil nonuniformity reduces it. In addition, the height of soil heave is slightly influenced by the friction coefficient and undrained shear strength of the soil. On the other hand, the penetration resistance is strongly affected by the soil undrained shear strength and bucket length-diameter ratio, but slightly affected by the friction coefficient.

Horizontal bearing capacity of monopile in three-dimensional spatially varying soils with linearly increasing mean strength

This paper aims to study the effects of the non-stationary soil property on the horizontal bearing capacity of three-dimensional monopile in spatially variable soils. The soil undrained shear strength is assumed to obey lognormal distribution and is simulated as non-stationary random fields. The mean value of the undrained shear strength linearly increases with depth, while the standard deviation keeps constant. The random finite-element method is applied to analyze the reliability of the bearing capacity. The influence of the correlations and non-stationary property on the mean and coefficient of variation of the bearing capacity are discussed. It is found that the correlation distance has no obvious effect on the bearing capacity and the bearing capacity increases with the increase of non-stationary coefficient. The results can guide the reliability-based design of horizontally loaded piles embedded in spatially variable soil.

Evaluating the Undrained Shear Strength of Kalimati Soil Using Field Vane and Unconfined Compression Test; a Study on Applicability of Bjerrum’s Correction Factor

Rapid urbanization and geological challenges in Kathmandu have made it crucial to understand the geotechnical properties of the soil in the region. This study here focuses on the undrained shear strength of silty clay in Kalimati, Kathmandu, and investigates the applicability of Vane Shear tests and its parameters over other shear strength tests from other methods. Geotechnical investigations were carried out at three locations within the Kathmandu Engineering College premises and in-situ and laboratory tests were conducted on undisturbed and disturbed samples. Then the study compared the field vane shear strength, undrained strength from unconfined compression tests, and laboratory vane shear strength of the samples. For the field vane shear test the correction factor as suggested by Bjerrum was used and the results indicate that this method overestimates the shear strength in Kalimati soil, indicating no adjustment is necessary. The findings of this study have important implications for the design and construction of structures in the region, and contribute to a better understanding of the geotechnical properties of Kathmandu soil.

Prediction of Undrained Bearing Capacity of Skirted Foundation in Spatially Variable Soils Based on Convolutional Neural Network

Skirted foundations are widely used in offshore and subsea engineering. Previous studies have shown that soil undrained shear strength variability has a notable impact on probabilistic analyses of skirted foundation bearing capacity. This study proposes an efficient machine-learning method to predict the uniaxial bearing capacity factors of skirted foundations under pure horizontal and moment loads, without relying on traditional time-consuming random finite element methods. A two-dimensional convolutional neural network is adopted to capture the potential correlation between soil random fields and bearing capacity factors. The proposed CNN-based model exhibits satisfactory prediction performance with regard to coefficients of variation and scale of fluctuations in two directions. Specifically, coefficient of determination (R2) values exceed 0.97, while root mean square error (RMSE) values remain below 0.13 for the surrogate model. In addition, more than 96% of the predictions are associated with a relative error of 5% or less, providing evidence of the proposed 2D-CNN model’s satisfactory prediction performance.

A comparative field study of free-fall cone and sphere penetrometers in soft sediment

This paper describes the field performance of two instrumented free-fall penetrometers (FFPs), which offer the possibility of low cost rapid strength profiling without requiring the equipment associated with conventional CPTs or drilling and sampling. The devices considered are a free-fall cone penetrometer (FFcP) and an instrumented free-fall sphere (IFFS). The performance of the FFPs in assessing soil undrained shear strength was investigated and compared through field trials in the soft sediments of the Swan River, Perth, Western Australia. The penetrometers achieved impact velocities of 1.4–5.8 m/s from drop heights of 0.7–3.0 m above the riverbed, and embedded to depths of up to 1.6 times the FFP length (1.45 m) and 4.5 times the IFFS diameter (0.89 m). The FFP data provided the shear strength indirectly using the acceleration measured from an onboard inertial measurement unit. The accuracy depends on the assumptions made regarding the magnitude of dynamic bearing and shaft frictional resistance. Uncertainty associated with dynamic bearing resistance causes a ±10% and ±13% variation in the calculated strength for the IFFS and the FFcP, respectively. However, the equivalent uncertainty in dynamic shaft resistance can cause a variation in the calculated strength of up to ±76% for the FFcP. The FFcP has higher embedment potential than the IFFS, but the IFFS has the advantage of a simpler interpretation without needing to account for dynamic shaft resistance.

Sustainable technique to improve undrained shear strength of soil by using Bacillus Subtilis bacteria

As population grows, more land areas are used up for accommodation, industrial needs as well as infrastructure. This results in diminishing availability of soil with good characteristics that can be used. This limitation forced future construction to be built on soils with poor characteristics. These types of soil require soil improvement in order to make them viable for construction. A relatively new and sustainable improvement method is bio-grouting techniques. The effectiveness of bio-grouting to improve organic sandy silt is studied in this paper. Cylindrical samples with 3.5 cm diameter and 7 cm height are improved by spraying bacteria solution at different solution-to-sample mass ratio and varying proliferation period. The solution-to-sample mass ratio varied from 0.2% - 1%, while the proliferation period is varied to three durations, i.e., 5 days, 13 days, and 21 days. The results showed that the method applied to soil samples could form calcite and obtain an increase in strength. For 5 days and 13 days proliferation period, the optimum solution-to-sample mass ratio is 0.6%, while for 21 days proliferation period, the optimum solution-to-sample ratio is 0,8%. However, for 21 days proliferation period, samples improved with 0.4% and 0.6% shows comparable degree of improvement as 0.8%.

Developing of neuro-swarm system to estimate the undrained shear strength of soil by CPT data

In this study, a novel hybrid metaheuristic model was developed to forecast the undrained soil shear (USS) property from cone penetration test (CPT) data (data from bore log sample from 70 different sites in Louisiana). This algorithm produced with the integration of grey wolf optimization (GWO) and multilayer perceptron neural network (MLP), named GWO - MLP, where different numbers of hidden layers were tested (1 to 4). The duty of optimization algorithm was to determine the optimal number of neurons in each hidden layer. To this objective, the system comprised five inputs entitled sleeve friction, cone tip persistence, liquid limit, plastic limitation, too much weight, and USS as outcome. The developed models for forecasting the USS of soil show the proposed best models have R2 at 0.9134 and 0.9236 in the training and predicting stage. Although the total ranking score of GWO-MLP2 and GWO-MLP4 is equal, the OBJ value shows that GWO-MLP4 has better performance than GWO-MLP2. In this case, considering the time of model running and a greater number of hidden layers suggests that GWO-MLP2 could be most appropriate. Therefore, the GWO-MLP3 model outperforms other GWO-MLP networks in the training and testing phase.

Finite Element Analysis of Two Nearby Interfering Strip Footings Embedded in Saturated Cohesive Soils

The issue of interaction between nearby footings is of paramount practical significance. The interference effect should be taken into account since the footing may really be separated from or bounded by other footings on one or both sides. In this regard, this paper studies the effect of two nearby interfering strip footings embedded in saturated cohesive soils, which will help to provide a better understanding of the impact of footing depth on the interference effect. A numerical study is carried out using the finite element program (Midas GTS-NX), and the behavior of closely placed strip footings embedded in the saturated cohesive soils is investigated under the influence of different factors such as the spacing between footings, the depth of footings, soil undrained shear strength, and the groundwater table. It was concluded that the soil cohesion and the footing depth ratio have a notable influence on the interference of closely spaced footings. For all cohesion values, it has been observed that the spacing needed for interference to vanish decreases with an increase in the depth of the footing and water table. In addition, as the S/B ratio increases, the ultimate bearing capacity (UBC) of interfering footings decreases until it reaches the same value as an isolated footing at greater spacing. The UBC is approximately 10% higher at S/B = 1 compared to the isolated footing. However, at S/B = 1, the UBC of two footings achieves a value equal to that of an isolated footing and does not change when the S/B ratio increases. With increasing footing depth, there is an increase in UBC. Finally, the highest values of x were obtained in all cases when Cu = 40 kPa. This indicates that the interaction between footings is greater when the soil is softer. Doi: 10.28991/CEJ-2023-09-03-017 Full Text: PDF

Probabilistic approach to the stability analysis of caisson foundations under combined loading in spatially variable soil

Caisson foundations are increasingly used in offshore engineering and are usually subjected to combined vertical-horizontal-moment (VHM) loading caused by self-weight, wind, waves, and currents. The bearing capacities of foundations under combined loading are represented by the VHM failure envelopes. In the literatures, failure envelopes for caisson foundations were usually established using a deterministic model of soil properties, ignoring the actual uncertainty and spatial variability of marine soil. The present study focuses on the investigation of the bearing capacities for caissons in spatially variable undrained soil under both uniaxial loading and VHM combined loading. Random finite element method combined with Kriging metamodeling technique and Monte Carlo simulation was applied to evaluate the uniaxial bearing capacities and to define the probabilistic VHM failure envelopes. The soil's undrained shear strength was considered to be increasing with depth, while its coefficient of variation was depth-independent. The effects of the coefficient of variation and scale of fluctuation of undrained shear strength on the uniaxial bearing capacity, on both size and shape of VHM failure envelopes for caisson foundations were discussed. It is shown that spatial variability of soil not only causes a change in the size of the failure envelope but also affects its shape when compared with the deterministic case. Therefore, it is necessary to consider the spatial variability of soil when designing caisson foundations and the present results provide some guidance for the design.

Effect of Natural Pozzolana on the Unconsolidated Undrained Shear Strength of a Lime-Stabilized Clay Soil

Effect of Natural Pozzolana on the Unconsolidated Undrained Shear Strength of a Lime-Stabilized Clay Soil

Evolution of breakout force with operation period for deeply embedded spudcan in clay

The extraction of spudcan foundation in clay can often be difficult and time-consuming considering the large pull-out force required to overcome the breakout resistance. This is particularly an issue for long operation periods, during which the dissipation of excess pore pressure in the soil will increase the base suction and undrained shear strength of soil. This paper investigates the spudcan breakout force in clay deposits through a coupled pore fluid-effective stress large deformation finite element approach. The complete process of spudcan installation, operation and removal is simulated. The numerical approach is verified against existing centrifuge test results. A good agreement can be obtained between the numerical and experimental results in terms of normalised breakout force and operation period. A series of parametric study is then carried out to explore the effects of installation depth, operational load and soil parameters. Based on the numerical results, a unified relationship can be established between the dimensionless forms of breakout force and operation period, for which a simple algebraic expression is proposed for the convenient assessment of breakout force recovery.

Modeling the Undrained Shear Strength with Soil Index Properties for Niger Delta Soft Clays

The aim of this study was to model the Undrained Shear Strength (USS) of soil found in the coastal region of the Niger Delta in Nigeria with some soil properties. The undrained shear strength (USS) is a key parameter needed for most geotechnical/structural designs. Accurate determination of the USS of soft clays can be challenging to obtain in the laboratory due to the difficulty in remoulding the clay to its in-situ conditions before testing and more accurate test such as Cone Penetration test (CPT) can be quite expensive. This study was carried out at Escravos site which is located in Delta state, Nigeria. Three Boreholes were drilled and soil samples were collected at 0.75 m intervals up to a depth of 45 m. Laboratory tests were used to obtain the moisture content, bulk unit weight, liquid and plastic limit, while CPT was used in obtaining the undrained shear strength. Classification of the soil samples was done by adopting the Unified Soil Classification System and various models relating the USS with the soil properties were developed. The result showed that most of the soils at Escravos site were predominately inorganic clay of high plasticity which are problematic due to the expansion and shrinking nature of this type of soil. The model developed showed that the soil properties that gave the best fit with the USS were the moisture content and effective stress of the soil. The coefficient of determination (R2) and the root mean square error (RMSE) obtained for this model were 0.805 and 6.37 KN/m2, respectively.

Probabilistic Assessment on 3D Stability and Failure Mechanism of Undrained Slopes Based on the Kinematic Approach of Limit Analysis

Most slope collapses have three-dimensional (3D) characteristics due to the uniform soil. The traditional plane-strain analyses neglect the 3D effects and could yield an overestimation or underestimation of the slope stability. The present study analyzes the stability of 3D slopes characterized by spatially variable undrained shear strength. A 3D slope reliability analysis approach that can fully consider the spatial variability of soil properties is proposed based on the plastic limit analysis upper-bound theorem. Within the framework of Monte Carlo simulation, the effects of strength gradient, the coefficient of variation, and the scale of fluctuation of soil undrained shear strength on the slope reliability index are examined. The length of the 3D failure surface is highlighted as an essential characteristic in 3D slope analysis. The slope failure mechanisms are classified, and the occurrence probability of each type is quantified. The obtained results indicate that the degree of nonstationarity and the coefficient of variation of undrained shear strength have significant impacts on the slope failure width, and the variation of failure mechanism highly depends on the combined effect of slope inclination and soil strength. The findings could guide slope reinforcement and facilitate a more efficient and accurate risk evaluation of slopes.

Assessment of the Undrained Shear Strength and Settlement of Organic Soils under Embankment Loading Using Artificial Neural Networks

In engineering practice, due to the high compressibility and very low shear strength of organic soils, it is difficult to construct an embankment on organic subsoil. High variability and significant change in geotechnical parameters cause difficulties in predicting the behavior of organic soils under embankment loading. The aim of the paper was to develop empirical relationships used in the preliminary design for evaluating the settlement and undrained shear strength of organic subsoil loaded by embankment based on data obtained from four test sites. Statistical multiple regression models were developed for evaluating the settlement in time and undrained shear strength in time individually for peat and gyttja. Neural networks to predict the settlement and undrained shear strength in time for peat and gyttja simultaneously as double-layer subsoils as well as a separate neural network for peat and a separate neural network for gyttja as single-layer subsoils were also developed. The vertical stress, thickness, water content, initial undrained shear strength of peat and gyttja, and time were used as the independent variables. Artificial neural networks are characterized by greater prediction accuracy than statistical multiple regression models. Multiple regression models predict dependent variables with maximum relative errors of about 35% to about 60%, and neural networks predict output variables with maximum relative errors of about 25% to about 30%.

Estimation of undrained shear strength in rate-dependent and strain-softening surficial marine clay using ball penetrometer

The spherical penetrometer (known as ball) has been widely used in laboratory tests and in-situ investigation to determine the undrained shear strength of fine-grained sediments in offshore geotechnical engineering. The conventional interpretation of the ball penetration tests suffers from either neglecting the effect of the strain softening and strain rate for steady state penetration or inaccurate accounting for the evolving bearing factor during the shallow failure stage of soil. In this study, the continuous penetration of the ball is simulated using the “Remeshing and Interpolation Technique with Small Strain” (RITSS) with a strain softening and rate-dependent soil model. Three stages from shallow failure mechanism to deep flow-round mechanism are clarified. The strength distribution of ball penetration into softening and rate-dependent soil is also revealed. A parametric study is performed to investigate the effect of strain rate and softening parameters on the failure mechanism and the evolved bearing factor profile. The effects of the dimensionless soil strength sum /γ′D and the frictional condition of the soil-ball interface are explored. Empirical formulas are developed to estimate the critical depths of the ball penetration test. A practical interpretation framework, incorporating the effects of strain rate and softening, is developed to assess the soil undrained shear strength using the ball penetrometer.