Undrained Shear Strength Values Research Articles

Physical model case study: treatment effect of soft ground by vacuum preloading combined with liquid bag pressurization method

Vacuum preloading and composite ground reinforcement are commonly used methods for reinforcing soft soil, but there is a lack of integrated design method for vacuum preloading combined with composite ground. This case study introduces an innovative approach that combines vacuum preloading with liquid bag pressurization to achieve the integrated design of consolidation drainage method and composite ground reinforcement, which is different from the reported air bag pressurization. To illustrate the effectiveness of this method. Model tests were carried out to analyze the variation of water discharge, pore water pressure, ground settlement, and average consolidation degree in the process of vacuum consolidation. The study investigated the water content, undrained shear strength, and ground bearing capacity of composite ground after ground treatment. A correlation between average undrained shear strength and characteristic value of ground-bearing capacity was established to evaluate and predict the treatment effect of composite ground. Research shows that compared with traditional vacuum preloading, the undrained shear strength can be increased by 13.78%–65.08%, and the characteristic value of bearing capacity for the composite ground can be enlarged by 2.3–4 times. These results indicate that the vacuum preloading combined with liquid bag pressurization can significantly improve reinforcement effect on soft ground.

Undrained shear strength of artificially flocculated clays

The flocculation and sedimentation of suspended clay particles in marine environments are an important process to form natural marine clays. In this study, the process of flocculation and sedimentation is simulated using appropriate flocculating agents in the laboratory. A series of laboratory sedimentation tests are carried out on various floc samples prepared with different concentrations of flocculating agents, and the size and density of flocs are measured and are found to serve as good parameters to characterise the development of flocculation. Multiple series of undrained triaxial compression tests under normal consolidation, as well as under over-consolidation, are then carried out on a slightly flocculated sample and a largely flocculated sample, as well as an ordinary remoulded sample. The values of the undrained shear strength of the flocculated samples are compared with those of the ordinary remoulded sample. The largely flocculated sample exhibits a higher undrained shear strength under normal consolidation as well as under over-consolidation. A comparison with past studies on natural marine clays also indicates that the highly variable values of the undrained shear strength of marine clays can be attributed to the different levels of development of flocculated structures in clays.

Diatom-induced impact on shear strength characteristics of fine-grained soils

Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom–kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37° and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils.

Bearing Capacity of Annulus Stone Column Double-Encapsulated with Geotextiles

The behavior of annulus stone columns in soft clays is significantly influenced by the outer-to-inner diameter ratio. For this reason, in the present study, a comprehensive experimental investigation program was undertaken in the laboratory on a soft clay from Warangal, India, by varying its water content for achieving different undrained shear strength values and to determine the load-carrying capacity of conventional and annulus stone columns with different outer-to-inner diameter ratios that were double-encapsulated with geotextile. Investigations were also extended to determine the optimum L/D ratio and spacing between the groups of annulus stone columns. Based on these studies, a theoretical framework is developed for interpreting and determining the load-carrying capacity of the stone columns. The studies presented in this paper offer a novel ground improvement technique to enhance the load-carrying capacity of soft soils.

Computer-Aided Slope Stability Analysis of a Landslide—A Case Study of Jhika Gali Landslide in Pakistan

The present research study has been undertaken to carry out slope stability evaluation of the Jhika Gali landslide in Pakistan using GeoStudio. For this purpose, the site geometry of the existing slope adjacent to the slided one was measured and samples were collected from the site. The in-situ moisture content was 14% and dry unit weight was 18.63 kN/m3. Unconfined compression tests and unconsolidated-undrained (UU) triaxial tests were performed on samples reconstituted at in-situ dry unit weight, standard Proctor and modified Proctor maximum dry unit weights. The test results show that the shear strength and deformation parameters, i.e., undrained shear strength, angle of internal friction and deformation modulus decreased from 200 kPa to 90 kPa, 23° to 12° and 51 MPa to 32 MPa, respectively, with an increase in the percentage of saturation from 35% to 95% at a specific dry unit weight. The slope was also modeled in GeoStudio for limit equilibrium analysis, and slope stability analysis was performed using the values of undrained shear strength and the angle of internal friction as determined in the laboratory at varying degrees of saturation. The limit equilibrium analysis showed that the factor of safety reduces from 1.854 to 0.866 as the saturation of material increases from 35% to 95%. The results also suggest that, as the percentage of saturation increases above 85%, the soil loses its shear strength significantly and gains in bulk unit weight, so at this stage the material starts sliding. Additionally, slope stability analysis was carried out by changing the slope geometry in three different ways, i.e., by reducing the height of the slope, adding a counterweight at the toe of the slope and by making benches on the slope. The results of GeoStudio analysis showed that the slope will be stable even above 85% degree of saturation.

Effect of Low Temperature on the Undrained Shear Strength of Deep-Sea Clay by Mini-Ball Penetration Tests

The technology for in situ testing of the undrained shear strength of deep-sea clay is underdeveloped. Indoor tests remain necessary, and there is a large temperature difference between in situ and laboratory tests. To analyse the effect of temperature on undrained shear strength, in this study the physical characteristics of marine clay samples from the South China Sea were determined, followed by penetration tests by the mini-ball method under low (4 °C) and room (20 °C) temperatures. The results indicated that the clay strength increased by 14.1–30.0% as the temperature decreased from 20 °C to 4 °C, and the strength of the bound water and the viscosity of the free water in the clay sample increased as the temperature decreased, which was the root cause of the increase in the clay strength. Based on the research, it is possible to correct the undrained shear strength values measured in laboratory tests and provide more reasonable parameters for ocean engineering.

Consolidation effect of prefabricated radiant drain vacuum preloading technology: A physical model case study

The large amount of dredged soil produced annually by rivers, lakes, seas, and oceans affects the ecological environment. Dredged material such as soft clay, which has high fluidity and high water content, is often utilized for building ground in engineering practice. Conventional vacuum preloading can effectively treat such soil but has a deprived consolidation effect. To improve the treatment effect, in this study, prefabricated radiant drain (PRD) vacuum preloading technology is proposed innovatively by adding prefabricated horizontal drains (PHDs) based on a prefabricated vertical drain (PVD). And a four-way connector is designed to connect PVD and PHDs, PHDs are attached to PVD to transmit vacuum pressure. A series of laboratory tests are run to compare and analyze the differences between conventional PVD and PRD vacuum preloading. The water discharge, average surface settlement, water content, and undrained shear strength values are analyzed. Then the parametric analysis of PHD spacings and length-width ratios (characterizing PHD length) are investigated. Overall, with the new PRD method, the consolidation effect of soft clay ground with a water content of 80% is improved by approximately 15%.

Undrained shear strength correlation analysis based on vane tests in the Jacarepaguá Lowlands, Brazil

The test sites analyzed here consist of clay deposits located in the Jacarepaguá Lowlands in Rio de Janeiro, characterized by high plasticity, high compressibility and low undrained shear strength. The deposits are made up of lightly overconsolidated aged clays, montmorillonite being the predominant clay mineral. Soft clay deposits are usually superficial, with thicknesses generally varying between 6 m and 17 m and geologically recent and originated from marine regressions and transgressions, that occurred between 6000 and 3500 years ago. The objective of this study is to analyze a large database of undrained shear strength measurements obtained by 461 vane tests performed at 15 different sites. In general, most of the data correspond to very soft clays, with undrained shear strength values lower than 25 kPa. The undrained shear strength measurements are correlated with plasticity index and with maximum excess pore pressure, measured with piezocone tests. The method for estimating the undrained shear strength su(DT) of soil from the excess pore pressure generated during piezocone dissipation tests proposed by Mantaras et al. (2015) was validated against the vane test database.

The Influence of Soil Saturation on the Geotechnical Test Results

The consistency limits are important properties of soil. The fall cone test method has a good repeatability, and the operator’s judgement has little effect on the results. This paper presents a new sample preparation method, in which the sedimentation method and consolidation test are combined, to prepare the saturated specimens for fall cone test for determining the consistency limits, and the sample prepared by this new method can be considered completely saturated. For the specimens prepared by the previous sample preparation method, the decrease in the water content leads to a decrease in the degree of saturation of soil. The test results show that the specimens prepared by this new method can reach a low water content, and the minimum penetration depths are as low as 2 mm, and the plastic limit could be obtained directly. In addition, the van shear test results show that the unsaturated specimens have larger values of undrained shear strength than the saturated specimens at the same water content, which could explain the difference of consistency limits obtained from different specimen preparation methods.

An experimental investigation on strength characteristics of fiber-reinforced clayey soil treated with lime or cement

This paper presents an experimental investigation on the strength properties of fiber-reinforced clayey soil stabilized with lime or cement at lower content (5% by weight) cured for 28 days. A series of consolidated undrained (CU) triaxial compression tests and unconfined compressive (UC) strength tests were conducted on compacted untreated, lime-treated and cement-treated clayey soil specimens reinforced with different polyester fiber content (i.e., 0%, 0.05%, 0.1%, 0.2% by weight) to evaluate the effect of fiber content on the strength behavior of tested soil. Test results indicated that the addition of lime or cement gave a significant increase in strength and strength parameters at a certain fiber content, where the improvement effect of cement is better than that of lime. Strain-softening curves were obtained for lime-treated or cement-treated fiber-reinforced specimens compared with the strain-hardening for fiber-reinforced ones. The residual strength is about 75% on average the peak strength for fiber and lime-treated specimens compared to about 50% decrease of peak strength for fiber and cement-treated specimens. The stress and strain curve for the specimen with higher fiber content lies above on that with lower fiber content. Fiber-reinforced plain soil showed a ductile failure mode compared with brittle failure mode for lime or cement-treated specimen. The values of undrained shear strength, unconfined compressive strength, cohesion and internal friction angle of plain soil and lime-treated soil followed the same increase trend when the polyester fiber content increased from 0% to 0.2%. While for lime-treated soil specimens, peak values can be observed when the fiber content was 0.1%. It is recommended to use the optimum fiber content to achieve a better improvement effect in the engineering applications.

Reliability analysis of a cohesive slope in a spreadsheet using FOSM, FORM and Monte-Carlo Simulation

In geotechnical engineering uncertainties arises from loads, soil characteristics and their properties, calculation models etc. To minimize these uncertainties in geotechnical problem, various reliability based and (or probabilistic based) approaches have been developed. This abstract presents a MS-Excel spreadsheet environment based practical framework for estimating the reliability index and failure probability of a cohesive finite slope using First-order second moment method, First-order reliability method and Monte Carlo Simulation. The height and the angle of slope is considered to be 5 m and 45º respectively. The inclination of the slope is 2H:1V and the hard stratum is assumed to be present at 15 m below the soil. The values of saturated unit weight and undrained shear strength are assumed to be 18 kN/m3 and 20 kPa respectively. The stability analysis is carried out using Swedish slice method under undrained condition. The excel spreadsheet developed in the study is mainly divided into two forms i.e. deterministic model worksheet for calculating the factor of safety and uncertainty model for generating the random variables of uncertain parameters. The undrained shear strength is considered as an uncertain parameter. The nominal factor of safety value is found out to be 1.248 and the critical slip circle has coordinate (2.6, 8.8), having radius of 16 m. The reliability index is found out using FOSM, FORM and MCS.

Effect of clay fraction and mineralogy on fall cone results of clay–sand mixtures

In this study, the relationships among plasticity ratio, clay fraction and liquidity index of clay-sand mixtures were investigated. Two types of clays, namely, kaolin (K) and calcium-montmorillonite (CM) were used to prepare clay-sand mixtures of different grain size distributions. Clays were mixed with clean sands of two different grain size distributions (well graded sand-SW and poorly graded sand-SP) at clay contents ranging between 0% to 100% by increments of 10%, in terms of dry weight. Fall cone, Casagrande, plastic limit and laboratory vane shear tests were performed on various clay- sand mixtures with different clay contents. Experimental results revealed that, above 100% water content, undrained shear strength values of CMSW and CMSP mixtures were determined to be around 50 kPa. On the other hand, undrained shear strengths of KSW and KSP mixtures are around 30 kPa, for a water content of 30%. Average undrained shear strengths of CMS and KS mixtures at their plastic limits are obtained as 210.68 kPa and 119.62 kPa, respectively. Average undrained shear strengths of these mixtures at their liquid limits are determined to be 2.40 kPa and 1.88 kPa, respectively. It is concluded that various mechanisms control liquid and plastic limits of clay-sand mixtures which are influenced by gradation of sand, clay content as well as origin of clay. As a result, several relationships among parameters including water content, liquidity index, undrained shear strength, cone penetration depth, fall cone liquid limit and Casagrande liquid limit were presented.

Correlation of Undrained Shear Strength and Liquidity Index of Fine-Grained Soils in West Java, Indonesia

The undrained shear strength of fine-grained soils is an important property in geotechnical engineering. However, the process of determining undrained shear strength can be challenging. Hence, the correlation of undrained shear strength with other soil properties needs to be established. Several studies have estimated undrained shear strength using the liquidity index. In the current study, we aim to find the best correlation of undrained shear strength and liquidity index of fine-grained soils in West Java, Indonesia at various water contents using the form of correlation of undrained shear strength and liquidity index and also factors influencing the undrained shear strength. The values of undrained shear strength and liquidity index are obtained by laboratory test and data collection from previous studies of soils in West Java. Results show a good linear relationship between the logarithmic value of undrained shear strength and liquidity index for 173 variations of soil samples. We also can conclude that the undrained shear strength of fine-grained soils is controlled by the proportion of finer grains presence and also the plasticity of the soils.

Relationships between undrained shear strength, liquidity index, and water content ratio of clays

Clays are very important in environmental geotechnical applications because of their characteristics. This paper considered over 500 data from literature and from laboratory. Undrained shear strength (remolded and undisturbed), liquidity index (IL), and water content ratio (WCR) have been compared among each other. Because natural clays have different physical and mineralogical properties, no unique relationship exists. The relationship between remolded and undisturbed undrained shear strength and liquidity index (IL) is scattered. On the contrary, if only remolded undrained shear strength values are plotted against the liquidity index, a good tendency is observed considering also the ratio of plastic limit to liquid limit in the range of IL between 0.8 and 1.3.

On determining the undrained bearing capacity coefficients of variation for foundations embedded on spatially variable soil

Abstract This paper presents an efficient method and its usage for the three-dimensional random bearing capacity evaluation for square and rectangular footings. One of the objectives of the study is to deliver graphs that can be used to easily estimate the approximated values of coefficients of variations of undrained bearing capacity. The numerical calculations were based on the proposed method that connects three-dimensional failure mechanism, simulated annealing optimization scheme and spatial averaging. The random field is used for describing the spatial variability of undrained shear strength. The proposed approach is in accordance with a constant covariance matrix concept, that results in a highly efficient tool for estimating the probabilistic characteristics of bearing capacity. As a result, numerous three-dimensional simulations were performed to create the graphs. The considered covariance matrix is a result of Vanmarcke’s spatial averaging discretization of a random field in the dissipation regions to the single random variables. The matrix describes mutual correlation between each dissipation region (or between those random variables). However, in the presented approach, the matrix was obtained for the expected value of undrained shear strength and keep constant during Monte Carlo simulations. The graphs were established in dimensionless coordinates that vary in the observable in practice ranges of parameters (i.e., values of fluctuation scales, foundation sizes and shapes). Examples of usage were given in the study to illustrate the application possibility of the graphs. Moreover, the comparison with the approach that uses individually determined covariance matrix is shown.

Yoğrulmuş Killerde Dayanım ve Deformasyon Parametrelerinin Düşen Koni ve Veyn Deneyleri ile Belirlenmesi

Yoğrulmuş ve örselenmemiş killi zeminlerin drenajsız kayma mukavemeti pratik olarak veyn deneyi ile belirlenebilmektedir. Diğer bir pratik deney olan düşen koni deneyi ile de yoğrulmuş kohezyonlu zeminlerin drenajsız kayma mukavemetine geçiş yapılabilmektedir. Ancak bu geçiş sırasında bir düşen koni faktörüne (K) ihtiyaç duyulmaktadır. Deneysel verilerle kalibre edilen koni faktörü, farklı su muhtevaları için hesaplanan drenajsız kayma mukavemetleri ve kritik durumdaki boşluk oranları kullanılarak drenajlı kayma mukavemeti açısına geçiş yapılmaktadır. Bu çalışmada, yoğrularak hazırlanan farklı endeks özelliklere sahip kohezyonlu zemin örnekleri üzerinde, literatürde önerilen yöntemlerden faydalanılarak sıkışabilirlik ve mukavemet parametreleri pratik olarak belirlenmiştir. Aynı zeminler için ödometre ve üç eksenli basınç deneylerinden ile elde edilen sıkışabilirlik ve mukavemet parametreleri ile karşılaştırılarak uygulanan yöntemin kullanılabilirliği değerlendirilmiştir.

Near‐shore geophysical and geotechnical investigations in support of the Trieste Marine Terminal extension

ABSTRACTThe Port of Trieste is an international hub for land and sea trade with the dynamic markets of central and eastern Europe. Thanks to its deep natural draft (about 18 m), the modern high‐capacity vessels can moor to the piers. In view of the foreseen increase in maritime traffic, this harbour is undergoing modernization in order to improve the commercial traffic capability. In this expansion plan, the container Trieste Marine Terminal, Pier VII, is seeking an extension by about 200 m. In support of this feasibility study, multidisciplinary data acquisition was conducted in order to characterize the seabed, the sub‐bottom sediments and the bedrock (flysch formation) in front of the Trieste Marine Terminal. The acquisition of high‐resolution swath bathymetry, side‐scan sonar and magnetometer data allowed a detailed analysis of the seabed conditions from an environmental and safety perspective. High‐resolution seismic reflection data enabled us to characterize the Plio‐Quaternary soft sediments and the underlying bedrock. A static underwater refraction survey was performed using hydrophone array deployed on the sea bottom to obtain seismic velocities and to achieve a reliable time‐to‐depth conversion of reflection seismic data by first‐arrival tomographic inversion. In addition to geophysical investigations, 11 offshore boreholes were drilled for detailed logging. In situ standard penetration tests were performed on core samples with the use of a pocket penetrometer and pocket vane in order to obtain uniaxial compressive strength, undrained shear strength and undrained cohesion values, and assess the cohesive soils. During drilling, 17 undisturbed samples and 12 semi‐disturbed samples were extracted to perform laboratory tests for the identification of the principal geotechnical parameters. The goal was to obtain a reliable geological/geotechnical model in front of the Trieste Marine Terminal – from the seabed to the bedrock. Below the seafloor, a sequence of about 20–30 m thickness, containing Plio‐Quaternary soft sediments, overlies the flysch, which locally presents alteration with rocks of reduced quality. The geophysical–geotechnical integrated approach allowed us to identify and map the top of the bedrock and provided valuable information for planning the pier extension project.

Influence of Grain Sizes on Undrained Strength of Sand Mixtures

Sand is known as the main material in land reclamation works to develop and widen an area. It is important for the Geotechnical Engineer to ensure the sand used can accommodate the burden imposed from the structures to be built on it. Previous researchers have conducted studies on the strength of sand, whether focusing on the sand itself or with the presence of fines. However, the study of sand grain size effects in sand mixtures in affecting sand behavior is extremely limited. The sizes and angularity of the sand particle are believed to contribute to the behavior of sand mixtures soil. Hence, the study to investigate the effect of sand grain size on sand mixtures in term of undrained strength is being carried out. The sand was sieved to coarse sand, medium sand and fine sand then each size was mixed with kaolin at 0 %, 20 % and 40 %. The undrained strength was obtained from triaxial test on undrained condition. As a result, it is found that the maximum deviator stress, qmax increases with the decrease of fines (kaolin) content. On the other hand, at the same fines content, increased of sand size leads to the increased of qmax, which means the increased of undrained shear strength. In addition, the angular shape of sand particle was thought to contribute to the high value of undrained shear strength for the sand mixtures.

Undrained Shear Strength and Monotonic Behavior of Different Nonplastic Silts: Sand-Like or Clay-Like?

The common assumption in geotechnical engineering research and practice has been that nonplastic silts behave sand-like, whereas, as silts become more plastic, they start to behave clay-like. To date, there are practically no studies focusing solely on various nonplastic silts and their monotonic response, perhaps because of the solid belief that nonplastic silts behave sand-like in all aspects. A series of constant volume direct simple shear tests were conducted on three different nonplastic silts. Because the specimens were reconstituted to have loose grain structures, volumetrically contractive tendency was observed for all the silts with relatively small effective stress friction angles. It was observed that Ko compression lines and critical state lines are parallel to each other for all three silts, which is considered to be a clay-like behavior in literature. It was shown that undrained shear strengths can be normalized with the effective vertical consolidation stress, which is also considered to be a clay-like behavior in literature. Normalized values are compared with the ones obtained for plastic silts compiled from previous studies. Normalized undrained shear strength values for plastic silts in literature were seen to be typically greater. Possible reasons such as density effect and strength anisotropy were discussed.

Effect of Grain Size and Shape on Undrained Behaviour of Sands

The stress–strain and stress path characteristics of sands are influenced by their grain size, shape, and packing. Morphological characteristics and size of particles play important role on the undrained shear strength of sands. Often, effects of these parameters are complex and cannot be easily distinguished. This study advances the knowledge of the role of particle size and shape on the undrained shear strength of sands. To eliminate the consequence of morphological characteristics, two sands with different particle sizes but similar angularity, and another sand with different roundness were selected for the study. These morphological characteristics for all three sands were determined from the analysis of scanning electron microscope images. F131 sand with higher median grain size and lower shape factors (rr and rs) had highest undrained peak shear strength and phase transformation value. Undrained strength (qpt) and effective principal stress (P′pt) in phase transformation point had direct relationship with grain median grain size (D50) and inversely effect of shape factor (rr and rs). F131 and F161 sands represented highest peak and ultimate steady-state strengths, respectively. Flow potential appeared to be directly proportional with (rr and rs) and inversely with D50. The peak index decreased with increasing shape factors (rr and rs).