Undrained Shear Strength Research Articles

Study on penetration resistance for thin-walled bucket foundation in muddy clay

In view of the problem that the resistance reduction effect of ring stiffeners is not considered in the existing calculation methods of penetration resistance, based on the field penetration date of thin-walled bucket foundation with ring stiffeners and combined with the field installation situation, the resistance reduction effect of the angle iron ring stiffeners on the skin friction resistance was explored, the calculation method of the penetration resistance considering the resistance reduction coefficient β was proposed. The results show that because the resistance reduction effect of ring stiffeners is not considered, the calculation results of the standard and theoretical equation method are larger than the measured resistance. The end resistance increased by ring stiffeners is about 55%–65% of the end resistance calculated by the standard and theoretical equation method, the end resistance increased by ring stiffeners should be considered in actual project. It is proposed that the calculation method considering the resistance reduction coefficient β can accurately predict the penetration resistance and the required differential pressure of bucket foundation with ring stiffeners, the recommended resistance reduction coefficient is 0.52 for the static penetration method, 0.33 for the undrained shear strength method, and 0.31 for the c and φ parameter method.

Geotechnical property development and micro-characteristic evolution of solidified sludge

Low-strength sludge takes up a lot of space in landfills, creating an environmental concern and a resource waste that needs to be treated right away. A novel mixed solidifying agent is introduced in this work to improve the geotechnical characteristics of sludge. The role of each component in the mixed agent is explored by using a series of different additive dosages in solidification. According to water content tests, shear wave velocity tests, unconfined compression strength tests, direct shear strength tests, X-ray diffraction and scanning electron microscopy, the strength development of solidified sludge is comprehensively analysed under different dosages and curing ages. The microanalysis results show that the geotechnical characteristics improved mostly due to hydration products such as ettringite, calcium silicate hydrate gel, aluminium gel and hydrogarnet. Moreover, the generation of hydration products consumes the water in sludge, fills soil particle pores and connects soil particles, which makes the structure of solidified sludge compacted, hence improving the strength of the solidified sludge. Furthermore, the relationship between the shear wave velocity and undrained shear strength parameters follows the power function and indicates that the shear wave velocity can characterise the strength level of the solidified sludge.

Influence of fine content on undrained monotonic behavior of carbonate sand

The behavior of carbonate sand is different from that of siliceous sand in terms of high compressibility and crushability, specific particle shape, and physical properties. Thus, investigating the behavior of these soils is of great importance because of their prevalence in coastal areas. Nevertheless, a few studies have considered the presence of fines in the mixture with carbonate sand. The present study explores the influence of non-plastic fines on the undrained monotonic behavior of Bushehr carbonate sand. For this purpose, 30 undrained monotonic triaxial tests were performed on carbonate sand with silt ranges of 0% to 40% with two different void ratios. According to the results, the undrained shear strength of carbonate sand diminished with the addition of silt percentage up to 20%, and increased dramatically at 30% silt content. Also, the results of 40% silty sand revealed that this sample is stronger than samples with 10% and 20% silt content. By surveying the location of the phase transformation point in samples, it was observed that silt particles reduce the dilative phase. Also, confining pressure intensified the contractive phase while relative density had the opposite effect. The results showed that the effect of particle breakage is negligible in this study.

Geotechnical Considerations for Foundation Design in Parts of Yenagoa, Bayelsa State, Niger Delta Nigeria

This research was to determine geotechnical properties of the subsoils in some part of Yenagoa and environs to obtain proper foundation design parameters, six towns in Yenagoa local government area, Bayelsa state was investigated. Six geotechnical boreholes was drilled and laboratory studies of soils samples were obtained from 0-20.25m deep. Subsurface soil profiles were delineated followed by determination of their index and mechanical properties, including Atterberg limits, particle sizes distribution, undrained shear strength, shear box test and consolidation coefficient. The general soil profile consists of (from top to bottom), , upper Silty clay horizon (0.0-5.25m thickness) soft to firm for Yenagoa study areas, Medium silty clay horizon (0.75 to 1.5m thickness) soft to firm Yenagoa study areas, low clayey sand horizon (0.75 to 1.5m thickness) soft Yenagoa study areas, peaty clay (1.0m thickness between 3.0-4.0m) soft Igbogene Yenagoa, upper sand horizon (3.0m thickness) silty sand Etegwe town Yenagoa, lower sand horizon (13.5 to 18.0m) silty sand to fine to medium and medium coarse appear in all the boreholes in Yenagoa. Yenagoa sub-soil show clay of high plasticity, silt of intermediate to high plasticity (CH, MI and MH) according to unified soil classification system from the results it shows that pad foundation is more economical in the study areas. Raft foundation is more economical in the six towns study areas of Yenagoa with Allowable bearing capacity of the upper clay layer ranges from 23-128KN/m2 In view of the significant variations in the stratification and engineering geological index properties of the soil in the six towns in Yenagoa while geotechnical data of one location cannot be used as a basis for design of foundation in a nearby land. Axial load carrying capacity for 305, 306mm, 356, 360mm, 406mm, 600and 610mm diameter for bore pile and tubular driven steel cased piles respectively were calculated for all the studies areas. Where high rise building is required in the various study areas. The study shows that the frequent causes of building collapse in Yenagoa are as a result of inadequate geotechnical investigations of the subsoil, poor quality materials, and poor work supervision.

Geotechnical Properties of Foundation Subsoils in Parts of Port Harcourt City, Obio/Akpor and Ikwerre Local Government Area, Rivers State, Nigeria

The study was carried out with the objectives of determining the stratification, geotechnical index properties of the soils in part of Port Harcourt city and Obio/Akpor local Government Area, Rivers state. Field Exploration and laboratory studies of soils samples were obtained from 0-20.25m deep. Subsurface soil profiles were delineated followed by determination of their index and mechanical properties, including Atterberg limits, particle sizes distribution, undrained shear strength, shear box test and consolidation coefficient. Results reveal an overlying light brown sandy clay, soft to firm consistency, clay thickness vary from 9.0 to 13.5m. Beneath this overburden lie yellowish brown to light grey fine to medium to coarse grained sand, loose to medium dense to dense consistency and poorly graded sand, thickness vary from 6.75 to 11.25m. The clays show low to intermediate clay plasticity (CL-CI). The shear strength parameters of these c- soil gave values range of 40-60KN/m2 . Allowable bearing capacities for the shallow foundation and bored pile foundation analysis was also carried out for the soil profiles with diameter 306, 406, 460 and 600mm for the deep foundation for various study areas were calculated.

Prediction model of strength properties of marine gas-bearing sediments based on compressional wave velocity

The strength properties of marine sediments are closely related to the spudcan penetration and jacket installation of offshore oil platforms. However, gas affects the properties of sediments to a certain extent and poses certain hidden dangers to the operation. Therefore, rapid and accurate determination of the strength properties of gas-bearing sediments has become an important issue. In this study, a series of acoustic and mechanical experiments were conducted in the laboratory by replicating marine gas-bearing sediments. Based on the two acoustic wave propagation theories, the calculated compressional wave velocity was compared with the measured wave velocity, and it was found that the result of the porous media model (P-M) is consistent with the actual situation. Laboratory mechanical property tests showed that cohesion and internal friction angle were positively correlated with density and negatively correlated with water content. The bubble fractional volume affects the undrained shear strength of the sediment, and the multi-parameter equation of the strength properties and physical properties of the marine sediment is fitted by the multivariate nonlinear regression method. On this basis, a single-parameter strength property prediction model of marine gas-bearing sediments based on compressional wave velocity was established with physical properties as a bridge, and the strength properties of gas-bearing sediments were accurately predicted. This prediction model avoids the disturbance of sediments measured by in-situ sampling and improves the accuracy of the strength properties of gas-bearing sediments in marine areas that cannot be sampled.

Static Design for Laterally Loaded Rigid Monopiles in Cohesive Soil

Rigid monopiles with small slenderness ratios (i.e., ratio of monopile embedded length to outer diameter) are widely used as foundations to resist lateral load and moment transferred from superstructures, e.g., large diameter steel pipes used by offshore wind turbines and piers in electric utility industry or sound barriers. A design model for laterally loaded rigid monopiles in cohesive soil is presented in this paper. The proposed design model assumes a constant depth of rotation point as well as a trilinear distribution model of soil lateral reaction along the embedded length of the monopile, and introduces a mobilization coefficient of soil reaction to quantify the magnitude of soil reaction mobilized under a certain load applied at the monopile head. The relationship between the mobilization coefficient and monopile head rotation is established by back-analyzing test results measured from series of laterally loaded pile tests, and then a general design procedure for a laterally loaded rigid monopile in cohesive soil is recommended. The feasibility and reliability of the proposed design model is validated against three cases of numerical simulations on laterally loaded piles in cohesive soils. It shows that this study’s proposed design model produces a relatively satisfactory prediction of the nonlinear load-deformation response, and can be used for laterally loaded monopile design in the sites with undrained shear strength being uniform or increasing linearly with depth.

Discussion of ‘Effect of organic matter content on Atterberg limits and undrained shear strength of river sediment’ by Gang Wang, Xia Bian, Ye-Jiao Wang, Yu-Jun Cui and Ling-Ling Zeng

Discussion of ‘Effect of organic matter content on Atterberg limits and undrained shear strength of river sediment’ by Gang Wang, Xia Bian, Ye-Jiao Wang, Yu-Jun Cui and Ling-Ling Zeng

Experimental study of ultra-high backpressure influence on the monotonic mechanical characteristic of silty sand

In this paper, a series of monotonic triaxial tests were carried out at a relatively wide range of backpressure conditions (from 100 kPa to 29 MPa) to investigate its influence on the shear properties of silt sand.Backpressure effect on the shear strength, excess pore pressure, and volumetric strain behavior were analyzed,and the underlying influencing mechanism was preliminarily discussed. According to test results, the application of higher backpressure conditions on quasi-saturated samples resulted in a significant buildup of negative excess pore water pressure, which led to an increase in shear strength. The trend became less pronounced after the backpressure exceeded 5 MPa. Nevertheless, within this range, the shear strength increased by approximately 40 kPa for every 100 kPa increase in backpressure. Furthermore, the consolidated-drained (CD) test results revealed that the value of backpressure (at least within 29 MPa) had no detectable influence on the shear properties of saturated samples. It was indirectly proved that the undrained shear strength was influenced by the change of effective stress caused by excess pore water pressure in the shear dilation stage of quasi-saturated samples. The applicability of the effective stress principle to predict soil strength even in ultra-high backpressure conditions was confirmed.

Three-dimensional undrained face stability of circular tunnels in non-homogeneous and anisotropic clays

The advanced 3D finite element limit analysis is employed to examine three-dimensional (3D) face stability of circular tunnels in undrained clays with the impact of strength non-homogeneity and anisotropy. The soil behaviour for all analyses is governed by the anisotropic undrained shear (AUS) strength model, which requires three anisotropic undrained shear strengths that are simply evaluated from conventional laboratory tests. The upper and lower bound solutions of 3D face stability of circular tunnels are accurately analyzed by considering four dimensionless parameters, including the cover-to-depth ratio, the overburden stress factor, the strength gradient ratio, and the anisotropic strength ratio. The significance of four dimensionless parameters on the stability load factor of the 3D face stability of circular tunnels, and the predicted failure mechanisms due to the influence of cover depth ratios, are also illustrated. A nonlinear regression analysis is employed to the computed data, which enables a new design equation of the factor of safety incorporating the conventional stability number. Thus, the new tunnel stability factors are proposed for the first time in the literature. A comprehensive application of the proposed safety factor equation is also presented and compared. The 3D effects of the tunnel face configuration between 3D geometry and 2D plane strain heading on the tunnel stability factors are also discussed. The new proposed design equation of the factor of safety provides an accurate and practical tool in assessing face stability of circular tunnels in non-homogeneous and anisotropic clays.

Probability of unsatisfactory performance of monopile foundations for offshore wind turbines considering variations in seabed properties

Safe design of monopiles for Offshore Wind Turbines (OWT) requires an accurate estimation of the foundation soil material properties. Uncertainty in soils is mainly due to their inherent variability which involves various spatial and temporal processes they have been exposed to over their natural geological process. The limited amount of information from geotechnical investigations in the seabed, considering the size of OWT farms, adds to the uncertainty. Commonly used deterministic design approaches idealize soil layers by assigning average values to the mechanical soil properties and make use of empirical correlations that can significantly differ from offshore conditions. Probabilistic design approaches have been suggested in the literature to account for the effect of uncertainties on the foundation’s performance by applying a coefficient of variation (COV) to soil properties. Such methods include theoretical investigations by applying selected COV values to predetermined mean CPT profiles for sands and undrained shear strength for clays. This study calculates the probability of unsatisfactory performance, Pfsys, of monopiles in sands by implementing variation in friction angle and unit weight using current API and DNV-GL design guidelines. Monte Carlo Simulation is employed to generate random fields using lognormal, normal, and uniform distributions. Pfsys is presented for a case study found in the literature and a design exercise for which local COV for the Belgian coast is also proposed.

Motion characteristics of vertically loaded anchor during drag embedment in layered clay

As a widely used taut-wire mooring system for deepwater platforms, the Vertically Loaded Anchor (VLA) has better performance in bearing capacity, angle adaptability, and deepwater installation than other systems. However, the installation process of the VLA and its motion characteristics are significantly impacted by multi-layered seabed soil. In this paper, the coupled Eulerian‒Lagrangian (CEL) large deformation finite element analysis method has been applied to analyse the continuous penetration of a VLA in nonuniform clay with an interbedded stiff layer. A detailed parametric study has been carried out to explore the trajectory, drag angle, movement direction and drag force of the VLA in layered clay with different embedded depths, thicknesses and undrained shear strength of the stiff layer. The CEL numerical analysis results have been validated by comparison with the analytical solutions from the inverse catenary equation. Excellent agreement has been obtained between the results from the CEL analyses and the analytical solutions. The stiff layer leads to concave and convex shapes on the trend lines of the movement direction angle and drag forces, respectively. The embedded depth of the stiff layer determines where the concave and convex shapes appear on the trend lines, while the thickness affects the sizes of the openings of the shapes. The most decisive parameter, an abrupt variation in the undrained shear strength, causes predominant rotation at the interface of layered clay. It diminishes the final embedment depth and ultimate stable drag force, meaning that the bearing capacity of the VLA severely declined in layered clay.

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%.

Dynamic installation of torpedo anchors in slightly overconsolidated clay-overlaying-sand deposit

Torpedo anchor installation involves uncertain design and construction parameters, especially in layered soil deposits. Large deformation finite element and coupled Eulerian–Lagrangian (LDFE/CEL) analyses are performed in this study to elucidate the dynamic installation of torpedo anchors in an over consolidated clay-overlaying-sand deposit. The parametric analysis is conducted after model verification to explore the effect of upper clay layer thickness, anchor impact velocity, anchor submerged weight, soil properties and anchor geometric dimensions. The soil failure patterns and dynamic installation behaviour in the clay-overlaying-sand soil deposit are found to differ substantially from those in single-layer clay and sand deposits, which exhibit a squeezing flow pattern at the clay-sand soil layer interface. As the demarcation of the two velocity phases during the anchor penetration process (acceleration and deceleration phases) falls below the interface of two soil layers, the contribution of the internal friction angle of sand to the anchor final penetration depth exceeds that of the clay undrained shear strength. Based on intensive modelling data, an empirical method for predicting the anchor penetration depth in single and layered soil deposits is derived for engineering applications.

Engineering Geological Properties of Subsurface Soils for Foundation Purposes in Parts of Bayelsa State, Niger Delta

The geotechnical properties of soil samples obtained from eight (8) boreholes from some parts of Bayelsa State were determined to assess their suitability as foundation materials. The geotechnical characteristics of the soils were determined from the laboratory and field works. The Atterberg limit results reveal that the liquid limit ranges from 46.5% to 98.3%, the plastic limit ranges from 23.5% to 56.3% while the plasticity Index values range from 17.1% to 51.3%. The clays are highly plastic (CH) in the USCS designation. The natural moisture content ranges from 35.9% to 91.3%, the moisture content is relatively high, this could be attributed to the wet season period of sampling. The particle size analysis disclosed that the cohesionless samples are predominantly fine to medium and medium dense sands. The triaxial test results shows that values of Cohesion (C) ranges between 16 – 40KN/m2 and the friction angle (ϕ) ranges between 3 – 6o. The result of the undrained shear strength of the clay ranges between 20Kpa and 24Kpa. The coefficient of consolidation (Cv) of the clay soil samples varies between 1.13 m2/yr and 2.89 m2/yr, the coefficient of volume compressibility, (Mv) for these same materials varies between 0.215 m2/MN and 6.338 m2/MN. Generally, indicating clay layers of high to very high compressibility. The calculated values for ultimate bearing capacity (qu) and allowable bearing capacity (qa) varies between 100.29KN/m2 to 151.49 KN/m2 and 33.42 KN/m2 to 50.50 KN/m2 respectively. The analyses showed that the values of ultimate bearing capacity increases with depth. The calculated settlement ranges between 69mm to 653mm with the thickness of clay of 7.5m and 30.0m respectively. Raft foundation is best suited for these weak, soft foundation materials for light structural loads but for heavy structural load, a pile foundation is recommended.

Analysis of Internal Forces and Deformation for a Single Pile in Layered Soil Based on the p-y Curve Method

The p-y curve method can be used to describe the stress–strain relationship of soil under elastic, elastoplastic, and plastic states, and is more accurate than other methods in dealing with the nonlinearity of pile–soil interaction. However, the p-y curve method also has several limitations. First, when constructing the p-y curve, undrained shear strength Cu is taken to be constant, and the diversity of soil layers around the pile side are also not considered. Second, in the analysis of the internal forces and deformation of the pile, the influence of the vertical load from the top of the pile and the self-weight of the pile, are both ignored. Third, in the analysis of internal forces and deformation, the pile side soil is set equivalent to a homogeneous soil layer, and the layering of the soil is not considered at all. In order to study the nonlinear problem of internal forces and deformation of a single pile in layered soil in greater detail, this paper analyzes a calculation model based on Wang’s calculation model, and compares several commonly used p-y curve calculation models. An internal force and deformation analysis model for a laterally loaded single pile, that explicitly considers the second-order effect is then established, by considering the differences between p-y curves of different soil types, as well as the change in Cu with depth. The differential equation of pile deflection for a single pile in layered soil is also presented, together with the corresponding finite differential solution algorithm program. This model was validated using a horizontal load test of a pile, and comparison of the calculated results with the measured results shows that the method outperforms existing p-y curve methods for deformation and internal force analysis of horizontally loaded piles.

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.

Effect of Specimens’ Height to Diameter Ratio on Unconfined Compressive Strength of Cohesive Soil

Abstract The undrained shear strength (Su) and cohesion (Cu) of cohesive soils are frequently determined using an unconfined compression test. However, the test results are heavily dependent on specimen size. This causes uncertainty in geotechnical analyses, constitutive models, and designs by overestimating or underestimating the shear strength of cohesive soils. Therefore, the study aims to assess the effect of the height-to-diameter ratio on the unconfined compressive strength (UCS) of cohesive soil. The soil specimen was tested on a compacted cylindrical specimen at the maximum dry density and optimum moisture content with a height to diameter (H/D) ratio of 1–3 for 38, 50, and 100 mm specimen diameters. Disturbed sample specimens were considered for the laboratory program. Accordingly, the standard Proctor compaction test determines soil classification and compaction characteristics. The unconfined compression test was performed for undisturbed and compacted remolded states of various diameters of cohesive soil specimens to investigate the strength variation with the specimen variation in H/D ratio. The laboratory test results revealed that cohesive soil's unconfined compression strength value drops rapidly with height-to-diameter ratios and the soil specimens’ diameter increases. However, the UCS value was stable at H/D ratio from 1.75 to 2.25. As the specimens’ diameter and H/D ratio increased, the peak UCS value axial strain decreased. Similarly, the gap between the axial strains of peak UCS value for the smallest and the most significant H/D ratio decreased with increase in the specimens’ diameter.

Estimating the Bearing Capacity of a Square Footing on a Geogrid-Stabilised Granular Layer over Clay Using the Finite Element Method

The bearing capacity of a square footing on an unstabilised and geogrid-stabilised granular layer over clay is estimated using finite element techniques in ABAQUS (a commercial finite element software). The model incorporates a range of soil parameters and layer thicknesses. The two-layered soil system in the model assumes a strong upper granular layer and a weak lower clay layer. The parametric results of the finite element analyses are used to develop design curves with dimensionless parameters in terms of linear gradient and the ratio of the undrained shear strength of clay to the effective vertical stress of the granular layer. The design curves are compared with the published results in the literature and found to be promising. The developed design curves are applied to estimate the minimum thickness of the granular layer and are compared with other existing methods. This application illustrates the simplicity of the proposed technique and reveals that the proposed approach yields a reasonable prediction for the bearing capacity when compared with other established methods.

A new method for estimation of undrained shear strength of cohesive soil for design use from CPTU

電気式コーン貫入試験（CPTU）結果のみからなる，新たな粘性土の設計用非排水せん断強さ（本論文ではsu(mob)と称す）の評価手法を提案する。この手法は，CPTU から得られるコーン貫入時の過剰間隙水圧から粘性土の正規圧密状態における強度増加率を推定し，得られた値と実効コーン貫入抵抗の一次的関係から算定されるコーン係数を介して su(mob)を評価するものであり，国内外 25 地域 31 地点で得られたデータに基づいている。本手法で求めた su(mob)は，別途室内試験および原位置試験から求めた su(mob)と比較され，その信頼性と適用性を実証している。提案する手法によれば，人的誤差を伴う，ボーリング～乱れの少ない試料採取～室内せん断試験という過程なしに，当地の粘性土の su(mob)を評価できる。