Undrained Shear Strength Profile Research Articles

Development of a new T-bar for the geotechnical centrifuge at TU Delft

An accurate estimation of undrained shear strength of clay seabed is important for interpreting lateral pile-soil interaction response. The cylindrical T-bar is a widely used site investigation tool for profiling the undrained strength (su) of soft soils. As such, a new miniature T-bar penetrometer is designed and fabricated at TU Delft for characterization of the undrained shear strength profile of clay layer in centrifuge models, and OCR profile can be then derived from undrained shear strength profile with known pre-consolidation stress level. The miniature T-bar penetrometer head is a cylinder of 5 mm in diameter and 20 mm in length and the miniature T-bar penetrometer head is connected with a rigid shaft (Figure 1(a)). The T-bar penetration resistance along the depth of clay sample can be obtained and further interpreted into the undrained shear strength profile.
 Rate of penetration is one of the key parameters that govern the drainage behaviour of soil response around the T-bar and the resulting penetration resistance. A very low rate of penetration leads to partial pore water dissipation and thus partial drainage condition. When a very large rate of penetration is applied, the fully undrained condition is achieved. However, the effect of T-bar penetration rate has not previously been fully examined for normally consolidated and over consolicated clay in the centrifuge. In this paper, the tip resistance profile of T-bar penetration tests under different rates of penetration is analyzed to obtain undrained shear strength profile of clay soils.
 A series of T-bar tests are conducted in both normally consolidated and slightly over consolidated clay samples at 100 g. The sample drainage state is tested by varying the rate of penetration from 0.01 mm/s to 5 mm/s. The results are interpreted by two methods: (i) the conventional method by converting the measured penetration resistance to soil strength using a single bearing factor, indicating a full-flow mechanism at failure [1]; and (ii) an approach considering soil buoyancy and a reduced bearing factor arising from the shallow failure mechanism, indicating the shallow correction procedure has a significant influence on the soil strength profile inferred from a T-bar penetrometer test [2]. The interpreted undrained shear strength profile provides soil property and OCR information for further monopile tests in the centrifuge, allowing a comprehensive study of the soil-structure interaction on soft soils.

Centrifuge Modeling of the Seismic Behavior of Soft Clay Slopes

This paper presents the experimental results and analysis from two centrifuge experiments that simulated the seismic response of a gentle slope in soft clay. The two models consisted of a three-degree and a six-degree slope in soft clay, respectively, which are representative of typical slopes found on marine seabeds on the continental margins. The models were built in a laminar container in order to reproduce infinite slope boundary conditions. In-flight characterization investigations consisting of T-bar tests and air hammer tests were performed to obtain undrained shear strength profiles and shear wave velocities at various depths, respectively. A suite of earthquakes was applied, including sinusoidal waves and scaled real motions, in order to observe the response of the models in terms of the propagation of shear waves and the generation of lateral displacements at various depths in the slopes. The results showed that the model preparation approach ensures the repeatability of the experiments, enabling the evaluation of the impact of the slope angle on the seismic response of the gentle slopes studied. On average, the permanent displacements measured at the surface of the six-degree slope were three times greater than those measured at the top of the three-degree slope. In these slopes, nonlinear effects were observed in terms of the peak ground acceleration (PGA) that depended both on the slope angle and the intensity of shaking.

A simplified model for predicting the accumulated displacement of monopile under horizontal cyclic loadings

Monopile-supported offshore wind turbines are continuously subjected to stochastic cyclic loadings during its lifetime of 25–30 years and it is challenging to accurately predict the accumulated displacement of monopile. In this paper, a simplified model with two cyclic loading factors is proposed to predict the accumulated displacement of monopile in soft clay. These two cyclic loading factors could consider the effects of the pile diameter, number of cycles, loading profile and undrained shear strength profile on the accumulated displacement. The model is validated by comparing with the centrifuge data and results obtained from the hyperplastic accelerated ratcheting model. Furthermore, the evolution of the accumulated displacements of monopile for the ultimate limit state (ULS) storm loading history and the long-term fatigue limit state (FLS) loading history are also investigated. This paper could provide a method for design codes requiring an accurate estimation of monopile displacements.

The accuracy of acoustic measurement for seabed detection and the appraisal of density criteria associated with the nautical bottom in the Patimban port development area, north of West Java, Indonesia

The implementation of nautical bottom in port areas characterised by muddy seabed has been discussed within the past tens of years. The nautical bottom is defined as a critical depth limit beyond which contact with a ship's keel causes either damage or unacceptable effects on controllability and manoeuvrability. In many cases, it is described in terms of sediment density criteria representing a specific area, obtained from either on-site physical measurement using mechanical instruments or laboratory tests of sampled sediments. However, the definition of nautical bottom still lacks comparison with the widely used acoustic measurement through echo-sounding. In this paper, we discuss results from geotechnical drilling and free-fall cone penetrometer probing to estimate the nautical bottom and its density criteria, as well as a hydrographic survey by means of echo-sounding. The study area is a low gradient, shallow, and mud-dominated seabed in the Patimban port development area, north of West Java, Indonesia. Samples of seabed from geotechnical drilling facilitate the development of a calibration curve to empirically relate the sampled sediments' undrained shear strength and density obtained from laboratory tests. The calibration curve is used to estimate density profiles from undrained shear strength profiles obtained from the free-fall cone penetrometer. Undrained shear strength profiles from 46 stations are converted into density profiles and their vertical gradient to detect the boundary between the water and the top-most seabed layer, i.e., the nautical bottom. Along with the deployment of the free-fall cone penetrometer, a dual-frequency echosounder survey is launched. The detected nautical bottom in each station is compared to the depths obtained from the dual-frequency echo-sounding. It is advised that the nautical bottom in our study area shall be associated with sediment density criteria of 1.29–1.33 t/m3. The nautical bottom typically lies at about the upper one-fourth of the approximately 0.4 m separation of acoustic depths observed from the high- and low-frequency returns from the dual-frequency echosounder. This finding confirms that high-frequency echo-sounding tend to underestimate the nautical bottom in muddy areas, while the low-frequency tend to overestimate it. The agreement between acoustic and nautical depth seems to be influenced by seabed sediment density. In an area with higher sediment density (1.31–1.33 t/m3), the coefficients of determination, r2, between nautical and acoustic depths are 0.94 and 0.87 for the high- and low-frequency returns, respectively. On the other hand, in an area with lower sediment density (1.29–1.31 t/m3), the coefficients are reduced to 0.15 and 0.12 for the high- and low-frequency returns, respectively. The work presented in this paper offers a supporting procedure to cope with the uncertainty of seabed detection performed by hydrographic surveys across a muddy and shallow coastal environment. Seabed surveys through echo-sounding in areas equivalent to where this study is carried out should be accompanied by physical seabed probing to assure the accuracy of bottom detection.

A Novel Sensor for Undrained Shear Strength Measurement in Very Soft to Soft Marine Sediments Based on Optical Frequency Domain Reflectometry Technology.

Due to the short supply of conventional fill materials, such as sand, land reclamation using dredged marine deposits has recently been proposed, in which marine deposits with high water content are blow-filled into reclaiming areas. The strength development of the filled marine soils is of great importance during the sedimentation and consolidation to guide the filling process and construction of reclamation. In this study, a novel sensor based on optical frequency domain reflectometry (OFDR) technology with a simple design was developed for undrained shear strength measurement. The novel sensor consists of an optical fiber and a series of polyoxymethylene coins. Owing to the merits of OFDR technology on high resolution, fully distributed sensing, and immunity to electromagnetic interference, the novel sensor can be used to determine undrained shear strength profiles of very soft to soft marine sediments/soils with good accuracy. The sensor was calibrated in remolded marine deposits with different water contents. The good feasibility and performance of the novel sensor for undrained shear strength measurement were well validated in two physical model tests on marine deposits treated by horizontal drains with vacuum preloading.

Probabilistic site characterisation using monitored spudcan installation data

Predicting spudcan penetration is essential for reducing risk during the installation of an offshore mobile jack-up platform. However, predictions made prior to installation often deviate from those recorded in practice, mainly because inherent uncertainties in seabed soils are not adequately considered. In this paper, the characteristics of the seabed are quantified using monitored spudcan installation data by developing a Bayesian framework coupled with Metropolis algorithm-based Markov chain Monte Carlo (MCMC) simulation. Applied to sand overlying clay conditions multiple readings from the load–penetration curve are used to derive statistical interpretations of seabed parameters such as sand thickness, sand friction angle and the undrained shear strength profile of clay. Illustrative examples demonstrate the proposed method using data from geotechnical centrifuge tests and a monitored offshore jack-up. The accuracy of the method is also retrospectively examined against a database of 66 centrifuge tests. The results show that the proposed method can provide consistent soil parameters with reduced uncertainty regardless of the influence of the updating sequence, number of MCMC chains, scale factors and the prior distributions applied. This method offers the offshore industry a new methodology to statistically interpret seabed conditions by incorporating monitored data. Potentially it could be combined with spatial interpolation methods to infer ground conditions and more accurately predict future spudcan installations.

Geotechnical characterization and stability analysis of subaqueous slopes in Lake Lucerne (Switzerland)

Tsunamis occur not only in marine settings but also in lacustrine environments. Most of the lacustrine tsunamis are caused by seismically- or aseismically-triggered mass movements. Therefore, an assessment of the stability of subaqueous slopes is crucial for tsunami hazard assessment in a lake. We selected Lake Lucerne (Switzerland) as a natural laboratory to perform an in-depth geotechnical characterization of its subaqueous slopes. This lake experienced documented tsunamis in 1601 and 1687. Some of its slopes still bear sediment volumes with a potential for tsunamigenic failure. To identify such slopes, we interpreted available reflection seismic data and analyzed the bathymetric map. Then, we performed 152 dynamic Cone Penetration Tests with pore pressure measurement (CPTu) and retrieved 49 sediment cores at different locations in the lake. These data were used to characterize the failure-prone sediments and to evaluate the present-day static stability of subaqueous slopes. Obtained results allowed the definition of three classes of slopes in terms of static stability: unstable slopes, stable slopes close to the unstable state, and stable areas. Non-deltaic slopes with thicker unconsolidated fine-grained sediment drape and moderate-to-high slope gradients (> 5–10°) have the lowest Factor of Safety. In agreement with previous studies, the failure plane for the non-deltaic slopes is embedded within the fine-grained glaciolacustrine sediments. Deltaic slopes with prevailing coarse-grained sediments mostly appear statically stable. Finally, we generalized the measured undrained shear strength profiles {s}_{u}(z) into the depth-dependent power-law models. These models define the {s}_{u} of Lake Lucerne’s sediments and can be applied to other lakes with similar sedimentation history.

Physical modeling of thermal improvement in normally consolidated and overconsolidated soil

Knowledge of the soil-temperature relation and its peculiarities have been widely discussed in Geotechnics. Previous studies show that heated fine-grained soil induces pore pressure, and as a result, the soil exhibits a consolidation effect similar to that of conventional consolidation when pore pressure is dissipated. The outcome of this process depends on soil characteristics such as the overconsolidation ratio and plasticity. Consolidated soil typically induces positive pore pressure while overconsolidated one induces negative pore pressure when heated. The phenomenon described was explored in this study aimed at the thermal consolidation product regarding the decrease in void ratio and consequent increase in undrained shear strength (Su) of a soil submitted to heating. These aspects were investigated using physical models at 1g gravity field, built on a laboratory scale. The heat source was placed inside a driven torpedo pile model in a medium consisting of a kaolin and metakaolin mixture. The undrained shear strength profile was defined by T-Bar tests involving different effective stresses and consolidation temperatures. The results show that for both mediums (normally consolidated and overconsolidated), undrained shear strength tends to grow proportionally to the temperature variation and permanent volume change is reached after a heating-cooling cycle. On the other hand, undrained shear strength indicates less significant variations for higher overconsolidation ratios.

Seismic Centrifuge Modeling of a Gentle Slope of Layered Clay, Including a Weak Layer

Abstract This article presents a model preparation methodology for simulating the seismic behavior of a gentle slope in clay with the presence of a soft, weak layer employing centrifuge testing. The model consisted of a three-layered slope of relatively soft clay with a 3° inclination, representative of Brazilian marine subsoils. In-flight characterization of the undrained shear strength and shear wave velocity profiles were achieved through T-bar penetrometer and air hammer tests. The model was subjected to a series of earthquake simulations at different amplitudes, and the response was tracked with accelerometers and displacement transducers. Additional data were obtained using a particle image velocimetry (PIV) methodology also described in this work. The results show that the proposed model preparation methodology enables the simulation of the strength contrast between the weak and relatively stronger surrounding layers using a laminar container. The additional displacement and acceleration data obtained from the PIV were in good agreement with the corresponding displacement transducer and accelerometer measurements. From the spectral analysis, a shift in the fundamental period was observed as the strain amplitude was increased, suggesting that strain rate effects mobilize higher stresses and a strength rate correction should be considered for the calibration of numerical models and comparison with existing methods for calculation of dynamic displacements in slopes.

Undrained Bearing Capacity of Strip Footing near Slopes Considering the Orientation of Strength Increase

Abstract The effect of undrained shear strength profile is an important practical consideration for estimating the bearing capacity of a strip footing placed near slopes or excavations. Previous st...

Three dimensional undrained bearing capacity analysis of laterally loaded pile in heterogeneous marine deposits

In this study, the ultimate undrained bearing capacity of a laterally loaded pile, embedded in heterogeneous marine deposits exhibiting linear and bi-linear undrained shear strength profiles was calculated by putting into practice the three-dimensional finite element lower bound limit analysis in conjunction with the second-order cone programming method. The lower bound finite element bearing capacity of a laterally loaded single pile can be found by formulating the element equilibrium, discontinuity equilibrium, boundary conditions, yield function, and optimization of the objective function through these constraints by second-order cone programming method. A parametric study is also conducted to evaluate the most effective factors including the dimensionless pile embedded length ratio, soil strength parameter, heterogeneity factors, transformation depth and soil–pile interface adhesion. The findings of the current study have been compared and validated with the results reported in the literature and presented in the form of tables and figures. The results show that that the effects of heterogeneity, linear and bi-linear variations of undrained shear strength with depth are more meaningful for soft marine clay in comparison to stiff clay and the curves of heterogeneous to homogeneous bearing capacity ratio reach to the unit value by increasing the dimensionless cohesion factor.

Simplified analysis of curvilinear penetration of free fall sphere projectile in soft soil

Free fall projectiles have been used in offshore engineering to determine in-situ undrained shear strength profiles of near-surface soft soils from accelerations recorded during penetrating. In doing so, however, only the equation of motion in the vertical direction has been formulated with the vertical acceleration by assuming vertical impact and penetration. In this study, the governing equations of curvilinear penetration of a sphere projectile are established with consideration of an angle of impact with soil surface. By treating the penetration process as an initial value problem, an algorithm is proposed for computing the trajectory, velocity and linear acceleration of penetration. The effects of impact angle, impact velocity and soil strength non-homogeneity on the penetration behavior are analyzed using a set of parameters reported in literature. The computed results of the final embedment depth and linear acceleration of sphere are shown to be in reasonably good agreement with published field data. Moreover, a new explicit equation is derived for determining the undrained shear strength of soil based on both vertical and horizontal linear accelerations. The accuracy of the undrained shear strength estimated by assuming vertical penetration is assessed. The impact angle is shown to have important influence on soil strength determination.

Forensic study on the collapse of a high-rise building in Shanghai: 3D centrifuge and numerical modelling

This paper reports a forensic investigation on the unexpected collapse of a 13-storey high-rise residential building in Shanghai soft clay. Prior to the building collapse, a shallow excavation was carried out on one side of the building while a slope was filled on the opposite side. Despite the observed breakage of all the piles in the field, a preliminary forensic numerical analysis adopting a typical undrained shear strength (cu) profile in Shanghai suggests that the piles are very unlikely to break. The surprising contrast has led to this detailed forensic study on the underlying mechanisms for the building collapse, through a site-specific characterisation of the cu profile, three-dimensional centrifuge modelling and numerical back-analysis. Although the typical cu profile in Shanghai increases almost linearly with depth, the clay in this site consists of two major layers with distinctive strength – that is, a relatively soft clay layer (average cu = 23 kPa, thickness = 13·5 m) underlain by a relatively stiff clay layer (average cu = 58 kPa, thickness = 18·2 m). By reproducing this cu profile in the centrifuge test and numerical analysis, the resulting direction of the building collapse and the depths of pile breakage were broadly consistent with those observed in the field. It was found that the surcharge of the fill slope caused the bearing capacity to be exceeded in the upper soft clay layer. This led to the development of a sliding wedge under the surcharge, which extended horizontally (at a depth of 11 m) across the pile foundations to reach the excavation side, and therefore the breakage of the piles. If a typical strength profile in Shanghai (with cu linearly increasing with depth) were adopted, the failure mechanism could be significantly altered from the aforementioned deep slip failure to a shallow passive failure, causing much smaller bending moment in each pile.

Numerical investigation of spudcan-footprint interaction in non-uniform clays

The re-installation of jack-up rigs near an existing footprint left by the previous installation can be problematic due to the development of horizontal and moment loads on the spudcan footing and hence on the rigs. This paper reports large deformation finite element results of spudcan-footprint interaction in non-uniform clay deposits. The performance of the numerical model was verified by simulations of existing centrifuge tests. A detailed parametric study was carried out varying influential factors including the initial spudcan penetration depth to form the footprint, spudcan diameters, soil sensitivity, undrained shear strength profile and the offset distance between spudcan and footprint centres. The effects of these factors on the development of the vertical, horizontal and moment loads are quantified, and the peak magnitudes of the induced horizontal and moment loads at different offset distances are then compared. It is found through the numerical analyses that the critical offset distance, which maximises the peak horizontal and moment loads on the spudcan, ranges between 0.5 and 0.75 times the diameter of the spudcan used for the formation of the footprint. By contrast, an offset distance of 1.5–2.0 times the diameter of the larger spudcan of those used for initial penetration and re-installation is able to result in a minimal influence of the footprint on the re-installation.

Measured and calculated spudcan penetration profiles for case histories in sand-over-clay

This paper reports five case histories of jack-up rig installation in layered soil profiles where a dominate feature was a stronger sand layer overlaying a weaker clay layer. In all cases a relatively continuous load-penetration profile was measured during installation of each of the three spudcan foundations. Summary site-investigation data is provided and consisted of mainly torvane, minature vane, unconsolidated undrained triaxial and pocket penetrometer tests for determining undrained shear strength of the clays and blow counts for deriving the relative density of sand. A statistical averaging method recommended in the InSafeJIP guidelines was used to provide the best fit of the undrained shear strength profile in the clay as this then allowed for spudcan load-penetration profiles to be estimated without introduction of user interpretation or bias. Sand properties were taken as provided in the original site-investigation report. Comparisons between load-penetration profiles calculated using the industry-standard ISO guideline, more recently proposed mechanism-based calculation method and three-dimensional large deformation finite element simulations are made with the measured data, leading to valuable insights for practitioners for estimating behaviour of jack-up installations in problematic sand-over-clay soil profiles.

In situ undrained shear strength characterization using data from free fall ball penetrometer tests

Large-deformation finite element analyses (FEAs) employing coupled Eulerian-Lagrangian technique are performed to simulate penetration of a spherical free fall penetrometer (FFP) in clay. FEA results are analyzed following the principle of energy conservation. Although final depth of penetration dp varies with penetrometer mass, diameter and impact velocity, bearing capacity factor at depth dp is a unique function of dp normalized with penetrometer diameter. A simple methodology is proposed for predicting in situ undrained shear strength profile using FFP test data. Shear strength values predicted using the proposed methodology are in excellent agreement with instrumented test data reported in literature.

Estimation of soil strength in fine-grained soils by instrumented free-fall sphere tests

The dynamic response of a sphere in soft clay is considered through field tests in which a 0·25 m dia. steel sphere was allowed to free-fall in water and dynamically penetrate the underlying soft soil. The test data, collected in a lake and a sea environment, relate to sphere velocities of up to 8 m/s, reaching sphere invert embedments close to ten diameters. An inertial measurement unit located within the sphere measured the motion response of the sphere during free-fall and penetration in soil. The resulting acceleration data were used within a simple framework that accounts for both geotechnical shearing resistance and fluid mechanics drag resistance, but cast in terms of a single capacity factor that can be expressed in terms of the non-Newtonian Reynolds number. The merit of the framework is demonstrated by using it as a forward model in a series of inverse analyses that calculate the undrained shear strength profile from acceleration data measured in free-fall sphere tests. The good match between these profiles and those obtained from ‘push-in’ piezoball penetrometer tests points to the potential for an instrumented free-fall sphere to be used as a tool for characterising the near-surface strength of soft seabeds.

Centrifuge modelling of an instrumented free-fall sphere for measurement of undrained strength in fine-grained soils

This paper describes centrifuge tests in which a model free-fall sphere was allowed to free fall in water before dynamically embedding within reconstituted samples of kaolin clay and two offshore natural clays. Instrumentation within the sphere measured accelerations along three orthogonal axes. The resultant acceleration was used to calculate sphere velocities and displacements. This allowed the penetration resistance acting on the sphere to be expressed in terms of a single capacity factor that captures soil resistance from both shearing and drag, and varies uniquely with the non-Newtonian Reynolds number. Undrained shear strength profiles obtained from a simple inverse analysis of the acceleration data show good agreement with those obtained using conventional push-in penetrometer tests.

Finite element modeling of partially embedded pipelines in clay seabed using Coupled Eulerian–Lagrangian method

Vertical seabed penetration and lateral movement of deep-water offshore pipelines are simulated using the Coupled Eulerian–Lagrangian (CEL) approach in Abaqus finite element (FE) software. Abaqus CEL has been used in some previous studies to simulate large-deformation behavior of offshore pipelines; however, the effects of strain rate and strain-softening on undrained shear strength (su) have not been considered. In this study, the effects of these factors are critically examined. The available built-in models in Abaqus CEL cannot account for these factors directly, especially the strain rate; therefore, the development of user subroutines is required. In the present study, a simple but realistic soil constitutive model (published by Zhou and Randolph in 2007) that considers the effects of strain rate and strain-softening on su is implemented in Abaqus CEL. The effects of FE mesh size and shear band formation on penetration resistance are discussed based on a comprehensive FE simulation. Lateral analyses are performed for “light” and “heavy” pipes in clay seabed having a linearly increasing undrained shear strength profile for smooth and rough pipe–soil interface conditions. The FE results are compared with previous theoretical, numerical, and centrifuge test results. Based on the present FE analyses, it is shown that, similar to the remeshing and interpolation techniques with small strain (RITSS) technique developed at the The University of Western Australia, the Abaqus CEL can successfully simulate the response of partially embedded pipelines in deep-water clay seabed, provided strain rate and softening dependent clay models are implemented. A methodology to implement such a model using Abaqus user subroutine is also presented.

Long-term axial capacity of deepwater jetted piles

Jetting is a technique commonly used to install well conductors, which is faster than the conventional drill and cement method. Conductors are designed to support the buoyed weight of the first string of casings (typically, 3–5 days after jetting) and later to provide the lateral stability for the well system against cyclic loading from environmental elements. The axial bearing capacity of a jetted conductor increases with time due to consolidation and thixotropy effects; however, the field data for set-up only extend to about 10 days. Installation of jetted conductors as piles and anchors may be an economic solution for offshore developments. The long-term axial capacity of jetted conductors was investigated through a series of centrifuge modelling and laboratory thixotropy testing. The centrifuge tests simulated jetting installation of four model conductors and measured capacities at set-up times ranging between 10 and 1000 days in a kaolin clay seabed with an undrained shear strength profile similar to that encountered in deepwater Gulf of Mexico. The centrifuge test results were compared to the field data in the Gulf of Mexico. The implications of the centrifuge test results for deepwater Gulf of Mexico conditions are discussed. A method is presented for estimating the long-term axial capacity of jetted conductors in Gulf of Mexico seabed conditions with recommendations for additional studies.