Evaluation of Undrained Shear Strength Using Full-Flow Penetrometers

Full-flow penetrometers (the T-bar and ball) are increasingly used on sites with thick deposits of soft clays, particularly prevalent offshore. Full-flow penetration tests were performed at five international well-characterized soft clay test sites to assess the use of full-flow penetrometers to estimate undrained shear strength. Field vane shear data were used as the reference undrained strength. Statistical analyses of strength factors indicates that full-flow penetrometers provide an estimate of undrained shear strength at a similar level of reliability compared to the piezocone. Relationships for estimating the strength factor and soil sensitivity using only full-flow penetrometer data obtained during initial penetration and extraction are developed. A strong dependence of the strength factor on sensitivity was identified and can be used for the estimation of undrained strength. The effectiveness and use of the developed correlations are demonstrated through their application at an additional test site.     

Strength Measurement for Near-Seabed Surface Soft Soil Using Manually Operated Miniature Full-Flow Penetrometer

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles.     

Nondestructive Sample Quality Assessment of a Soft Clay Using Shear Wave Velocity

While improvements in equipment and sampling methods have enabled collection of better quality samples of soft clays for more reliable engineering design and performance prediction, current sample quality assessment methods typically require destructive laboratory testing performed long after samples are taken. This paper describes a nondestructive technique for sample quality assessment of soft clays using shear wave velocity. A portable bender element device was used to measure shear wave velocity (Vvh) in the field immediately following collection of Sherbrooke block, tube, and split spoon samples of Boston blue clay. Vvh values were compared to in situ values from seismic piezocone (VSCPTU) tests. The ratio Vvh/VSCPTU was compared with results from a conventional, laboratory-based assessment method. Results indicate a consistent correlation between laboratory-based methods and the Vvh/VSCPTU ratio, which ranges from Vvh/VSCPTU = 0.77 for the block samples to 0.28 for split spoon samples. The portable bender element device and nondestructive assessment technique offer the potential for field quality assessment and allow for real time adjustments to sampling techniques and/or more effective selection of samples for laboratory testing.     

Experimental Evaluation of Engineering Behavior of Soft Bangkok Clay under Elevated Temperature

This paper presents the results of a systematic well designed experimental investigation carried out to study the engineering properties of the soft Bangkok clay heated up to 90°C from room temperature (25°C). Details of modified oedometer and triaxial test apparatus that can handle temperatures up to 100°C are also presented. In the range of temperatures investigated, soft Bangkok clay exhibited temperature induced volume changes that depend mainly on the stress history, reduction in the conventional elastic zone, stiffening, and increased hydraulic permeability with increasing temperature as well as apparent overconsolidation state after subjecting the normally consolidated specimen to heating/cooling cycle. The results of this study provide additional data that can enhance the understanding of the thermohydromechanical behavior concepts of saturated clays.     

Effect of Microfabric on Shear Behavior of Kaolin Clay

The influence of geometric arrangement of platelets (microfabric) on the mechanical behavior of kaolin clay is investigated using lubricated end triaxial testing on solid cylindrical specimens. A series of compression and extension tests under drained and undrained conditions were performed on clay specimens with different microfabric for overconsolidated ratio values of 1 and 10. The solid cylindrical specimens with dispersed and flocculated microfabric were produced in the laboratory using slurry consolidation technique under K0 condition. Based on the experimental observations, it is evident that microfabric strongly affects the mechanical behavior of kaolin clay, such as its stress–strain relationship, effective stress ratio, shear strength, excess pore-pressure evolution, and volumetric response. The influence of confining pressure on clay specimens with dispersed and flocculated microfabric is also studied in this research. This study shows that the microfabric can change the basic nature of clay. For example the normally consolidated kaolin clay shows its dilative nature during shearing for dispersed microfabric and contractive nature for flocculated microfabric.     

Undrained Shear Strength of Pleistocene Clay in Osaka Bay

This study presents the undrained shear characteristics of Holocene and Pleistocene clay samples from depths of 20–200 m under the seabed in Osaka Bay. Automated triaxial K0 consolidation tests and anisotropically consolidated-undrained triaxial compression and extension tests are conducted using the recompression method. The average undrained strength ratio (su/σv0′) is 0.33 (SD = 0.03) when the extension strength is defined as the peak strength or the strength at an axial strain of 15%, while su/σv0′ is 0.29 (SD = 0.04) when the extension strength is defined as the shear stress at the axial strain corresponding to the peak compression strength. Circular arc stability analyses are carried out with the modified Fellenius and Bishop methods for the design cross section of the seawall structure of the Kansai International Airport to study the effects of different definitions of shear strength. The seawall is founded on 19 m of soft Holocene clay and 10 m of Pleistocene sand overlying the Pleistocene clay. The stability analyses show that the factor of safety and depth of the critical circle (i.e., above versus below the sand layer) are sharply affected by both the value of su/σv0′ (0.33 versus 0.29) and the method of slices (Fellenius versus Bishop). The marginal stability calls for careful monitoring of construction with field instrumentation.     

Shear Strength in Preexisting Landslides

Drained residual shear strength is used for the analysis of slopes containing preexisting shear surfaces. Some recent research suggests that preexisting shear surfaces in prior landslides can gain strength with time. Torsional ring and direct shear tests performed during this study show that the recovered shear strength measured in the laboratory is only noticeably greater than the drained residual strength at effective normal stress of 100 kPa or less. The test results also show that the recovered strength even at effective normal stresses of 100 kPa or less is lost after a small shear displacement, i.e., slope movement. An effective normal stress of 100 kPa corresponds to a shallow depth so the observed strength gain has little, if any, impact on the analysis of deep landslides. This paper describes the laboratory strength recovery testing and the results for soils with different plasticities at various rest periods and effective normal stresses.     

Centrifugal and Numerical Modeling of a Reinforced Lime-Stabilized Soil Embankment on Soft Clay with Wick Drains

A centrifuge test was performed on a reinforced embankment backfilled with lime-stabilized soil on soft clay installed with wick drains. The finite-element program PLAXIS was employed to simulate the centrifuge test on the basis of the test dimensions. Verifications of the numerical model were taken by comparing the numerical results with the measurements obtained from the centrifuge test, and it was found that both were in good agreement. Three cases of unreinforced, one-layer reinforced, and two-layer reinforced embankments were simulated and compared on the basis of the numerical model. A centrifuge similarity drainage relationship was also proposed in this paper on the basis of the equal degree of consolidation between the model and prototype drains.     

Initial Shear Modulus of Remolded Sand-Clay Mixtures

This paper presents the dynamic properties of undisturbed and remolded clays and sand-clay mixtures based on the results of undrained hollow cylindrical torsional cyclic simple shear tests. The clays were obtained from different sites having a wide range of plasticity, covering fluvial and marine sediments. The main focus of this investigation is the dynamic properties of sand-clay mixtures whose properties are intermediate between those of clays and sands. The initial shear moduli are examined in terms of fines content, and conventional plasticity index. An equivalent plasticity index has been defined in the present investigation and a better correlation is obtained between the initial shear modulus of sand-clay mixtures and this parameter than with the conventional plasticity index. Further, a simple method has been suggested to determine the equivalent plasticity index based on the conventional consistency limit tests and grain size analysis. An empirical correlation for predicting the initial shear modulus is also proposed.     

Computational Techniques and Shear Band Development for Cylindrical and Spherical Penetrometers in Strain-Softening Clay

This paper addresses numerical simulation of deep penetration of full-flow penetrometers in strain-softening, rate-dependent, cohesive soil, and the observed phenomenon of periodic shear bands. The analysis was conducted using a large deformation finite element approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain-softening, with strain-rate dependency being incorporated in order to avoid spurious mesh dependency. Parametric analyses were carried out varying the strain-softening parameters (hence the relative brittleness of the soil), the rigidity index of the soil, and the strain-rate parameter. Increased brittleness of the soil led to reduction in the penetration resistance, but also to increasingly significant oscillations in the resistance–penetration responses. The oscillation was found to result from periodic shear bands evolving cyclically ahead of the advancing cylindrical and spherical penetrometers. Analyses with different values of rigidity index confirmed further that the periodic shear bands were a real material phenomenon, rather than due to errors in numerical simulation. Similar phenomena have been observed for continuous flow problems in granular materials. However, rising strain-rate dependency tended to suppress the oscillations.     

Comparison in Undrained Shear Strength between Undisturbed and Remolded Ariake Clays

Triaxial consolidation undrained shear tests are performed on both undisturbed and remolded Ariake clays to investigate the undrained shear strength behavior. When the applied confining stress is larger than the triaxial consolidation yield stress, the strength envelopes expressed in the plot of undrained shear strength versus confining stress of both the undisturbed and the remolded Ariake clays are straight lines through the origin. The strength envelope of the remolded Ariake clays lies above that of the undisturbed Ariake clays when the applied confining stress is larger than the consolidation yield stress. This difference is caused by the difference in water content between undisturbed and remolded states. When the data obtained from triaxial consolidation undrained shear tests of both the undisturbed and the remolded Ariake clays are plotted in the plot of undrained shear strength versus water content, it is found that the undrained shear strength decreases uniquely with the increase in water content.     

Void Ratio Estimation of Soft Soils Using Electrical Resistivity Cone Probe

Electrical properties of soils have commonly been used to estimate geotechnical properties. This paper introduces a new device, an electrical resistivity cone probe (ERCP), to determine the electrical resistivity of seashore soft soils and estimate void ratio in the field. The probe consists of inner and outer electrodes with a coaxial structure. The probe tip is conical to minimize disturbance during penetration. A four-terminal pair configuration is used to prevent electrical interference. The electrical resistance is measured during a consolidation test, penetration tests in a large-scale calibration chamber, and at two field sites. With the resistivity of soils and electrical resistivity of pore water extracted from undisturbed soils, the void ratio is estimated using Archie’s law. The void ratio estimated by the ERCP in an oedometer cell is almost the same as the volumetric void ratio of sand determined from consolidation tests. In addition, the void ratio profile obtained by the ERCP agrees well with the volume-based void ratio in a sand-clay mixture prepared in the calibration chamber. The void ratio profile estimated in the field is inversely proportional to the standard penetration testing N-value and the cone-tip resistance of the cone penetration test. This paper demonstrates that the ERCP may be an effective device for the estimation of the void ratio of seashore soft soils.     

Evaluation of a Dike Damaged by Pile Driving in Soft Clay

This paper presents a case study detailing a riverbank dike damaged by pile driving in very soft clay in Shanghai, P.?R. China. Driven piles were designed to support the existing dike to be raised to a higher elevation. The subsoil mainly consisted of very soft clay with its natural moisture content greater than its liquid limit. This paper describes the phenomena of dike movement and crack development during pile driving based on field observations and instrumentation data. Cone penetration tests and vane shear tests were conducted after pile driving to investigate the slip surfaces. Degradation of soil strength was identified as the main cause for the failure of this dike. Slope stability analysis was conducted to back-calculate the degraded undrained shear strength of the clay. The results indicate that the soil strength in the disturbed area due to pile driving approached the level of its remolded strength.     

Influence of Osmotic Suction on the Soil-Water Characteristic Curves of Compacted Expansive Clay

Unsaturated clays are subject to osmotic suction gradients in geoenvironmental engineering applications and it therefore becomes important to understand the effect of these chemical concentration gradients on soil-water characteristic curves (SWCCs). This paper brings out the influence of induced osmotic suction gradient on the wetting SWCCs of compacted clay specimens inundated with sodium chloride solutions/distilled water at vertical stress of 6.25 kPa in oedometer cells. The experimental results illustrate that variations in initial osmotic suction difference induce different magnitudes of osmotic induced consolidation and osmotic consolidation strains thereby impacting the wetting SWCCs and equilibrium water contents of identically compacted clay specimens. Osmotic suction induced by chemical concentration gradients between reservoir salt solution and soil-water can be treated as an equivalent net stress component, (pπ) that decreases the swelling strains of unsaturated specimens from reduction in microstructural and macrostructural swelling components. The direction of osmotic flow affects the matric SWCCs. Unsaturated specimens experiencing osmotic induced consolidation and osmotic consolidation develop lower equilibrium water content than specimens experiencing osmotic swelling during the wetting path. The findings of the study illustrate the need to incorporate the influence of osmotic suction in determination of the matric SWCCs.     

Evaluation of Cyclic Shear Strength of Two Cemented Calcareous Soils

The mechanism controlling the cyclic shear strength of cemented calcareous soils was investigated based on the results obtained from monotonic and cyclic triaxial tests on two different types of calcareous soil. Undrained cyclic triaxial tests performed on artificially cemented calcareous soils with different loading combinations showed that the effective stress path moved towards or away from the origin, due to the generation or dissipation of pore pressure with progressive cycles. Previous investigations have shown that the Peak Strength Envelope or the State Boundary Surface or the Critical State Line forms a boundary beyond which effective stress paths during cyclic loading cannot exist. However, in this study it was observed that the maximum stress ratio (ηmax) obtained from monotonic tests defined the boundary for the cyclic tests. Based on the information obtained from this study, an approach for evaluating the cyclic shear strength is proposed. It was observed that the modified normalized cyclic shear strength had a strong linear relationship with the logarithm of the number of cyclic to failure irrespective of confining pressure, type of consolidation and stress reversal.     

Staged Construction and Settlement of a Dam Founded on Soft Clay

Alibey Dam is located near Istanbul in Turkey on the Alibey Stream, 4.5?km north of its point of confluence with Golden Horn, an ancient submerged river mouth. It was constructed as an earthfill dam over 30-m-thick soft valley sediments. Before the construction of the dam, field and laboratory tests were performed to determine the geotechnical characteristics of the foundation soils. During the construction and many years after the construction of the earthfill embankments, including the cofferdams and the intermediate fills, the response of the foundation soils was monitored by extensive field instrumentation generating a unique long-term (over 25 years) database. With proper instrumentation and careful monitoring of the collected data, field construction rates could be adjusted and the earth dam was safely constructed on the thick soft deposits. Approaches to settlement prediction were evaluated in a historical context, starting with the simplified one-dimensional approach available at the time of construction to more sophisticated analyses including the employment of modern numerical methods, in terms of the recorded data. Standard subsurface exploration and field testing supplemented with conventional laboratory testing provided the relevant material parameters that were used in the finite element method. The only exception to this was the overall hydraulic conductivity of the deposit, which controlled the rate of consolidation. Early field observations were used to assign the appropriate hydraulic conductivity. An elastoplastic soil model in a coupled analysis of consolidation was employed in the analysis that yielded realistic predictions of field behavior in response to the complex construction history. The accurate prediction and monitoring of the behavior of soft and thick soil layers subjected to staged construction, as in the case of Alibey Dam, is very important for planning of the construction as well as the expected behavior after construction.     

New Structure-Based Model for Estimating Undrained Shear Strength

This paper presents an experimental study of the strength in anisotropic clays by means of centrifuge model, cone penetration, and vane shear tests. To understand the effects of void ratio, overconsolidation ratio, and testing rate on the undrained shear strength (Su) of anisotropic Speswhite clay, a new centrifugal testing technique is designed to obtain constant overconsolidation ratio (OCR) profiles with varying void ratios (e), called the “descending gravity test.” The parameters controlling the generation of peak shear strength are quantified. As a result of this function, a new material and rate-dependent surface is defined in the e-OCR-Su space, which is identified as a “structural state capacity surface” since it relates the anisotropic structure to structure inherent capacity and properties. A new function for the estimation of excess pore pressure (uex) generated by cone penetration is found. By combining the strength and pore pressure functions a new model is proposed, called the “CU model.” The CU model is a structure-based model that provides reliable estimates of shear strength for in situ saturated clays using the knowledge of void and overconsolidation ratios. Finally, by combining Su-e-OCR and uex-e-OCR relationships, it estimates the void ratio and OCR profiles of anisotropic clays from piezocone penetration test results.     

Quality of Conventional Fixed Piston Samples of Norwegian Soft Clay

It is well accepted that the quality of soft clay samples obtained using standard fixed piston samplers can be relatively poor and that block samples are necessary to yield very high quality samples. However, for many practical projects it is not economically viable or physically practical to obtain block samples. In this project, the quality of standard 54?mm composite piston samples of soft clay is examined by comparing six separate sets of 54?mm samples to parallel block sampling. Sampling and laboratory testing was carried out by three different organizations at a well characterized highly uniform soft clay site in Norway. As expected, the work showed that the block samples behaved significantly differently from those obtained using the 54?mm sampler and were of higher quality. Block sample-derived parameters were considerably different from those obtained from the 54?mm sample tests. However, significant differences were also found between the different sets of 54?mm samples. Although the differences are less than when compared with block samples, the consequences of poor quality 54?mm sampling will be significant in engineering design. It is concluded that the differences are due to small details in the sampling operation such as the need to keep the piston effectively stationary at all times, to avoid overcoring and to handle the recovered sample carefully. If a well trained driller follows good quality practice, then relatively good samples can be obtained by the fixed piston sampler, which are suitable for analysis and design of routine engineering works.     

Bearing Capacity of Piled Rafts on Soft Clay Soils

The conventional design of a piled foundation is based on a bearing capacity approach, and neglects the contribution of the raft. As a consequence, piled foundations are usually designed by overconservative criteria. With respect to the conventional approach, a more rational and economical solution could be obtained by accounting for the contribution of the raft toward the overall bearing capacity, but this potential is not exploited due to the lack of theoretical and experimental research on the behavior of piled rafts at failure. Based on both experimental evidence and three-dimensional finite element analyses, a simple criterion is proposed to evaluate the ultimate vertical load of a piled raft as a function of its component capacities, which can be simply evaluated by the conventional bearing capacity theories. The results presented in the paper thus provide a guide to assess the safety factor of a vertically loaded piled raft.     

Effect of Pile Driving on Static and Dynamic Properties of Soft Clay

A full-scale closed-ended pile was driven into a deep deposit of soft clay that was instrumented with inclinometers and pore pressure transducers at three radial locations and three depths. This paper presents the results and interpretation of both field measurements of shear-wave velocity and the laboratory testing program performed on pre-pile and post-pile “undisturbed” specimens. A companion paper provides full details of the site investigation, field measurements of excess pore pressure, and the deformation field around the pile. Shear-wave velocity profiles at four radial distances were obtained as a function of time following pile driving using the suspension logging method. Compressibility characteristics for this soil were determined through one-dimensional constant rate of consolidation tests carried to very high stresses. Shear strength testing included anisotropically consolidated undrained triaxial tests performed on specimens at two confinement levels to study the effect of fabric and evolving anisotropy. Direct simple shear testing was performed on specimens in their normal vertical orientation, and rotated 90° to observe changes in structure/fabric orientation after pile installation.