Two-layered Soil Profile Research Articles

Site-specific response of a 5 MW offshore wind turbine for Gujarat Coast of India

In India, the coastline of Gujarat is known to have a high potential for offshore wind energy. However, the region is highly seismically active with large magnitude earthquakes experienced in the past. The aim of the work is to investigate the site-specific response of an offshore wind turbine (OWT) for assessment of suitability of installation of OWT along the Gujarat coast. Seismic hazard analyses of 44 sites are performed using deterministic approach and response spectrum matched acceleration time histories at all these sites are obtained. Monopile for the wind turbine is designed considering a two-layered soil profile. As per specifications of National Renewable Energy Laboratory (NREL), three-dimensional finite element model of 5 MW OWT along with the soil domain is developed in the software ABAQUS considering material nonlinearity. Earthquake time histories combined with wind and wave load are being employed for the analyses. It is observed that the sites lying between Kachchh and Saurashtra regions are most vulnerable whereas sites along the western coast of Saurashtra are found to be least vulnerable. Further, seismic fragility curves for the OWT are derived for different damage states in order to evaluate the probability of damage under various seismic scenarios.

Influence of phase difference between kinematic and inertial loads on seismic behaviour of pile foundations in layered soils

A series of dynamic centrifuge experiments was conducted on model pile foundations embedded in a two-layered soil profile consisting of soft-clay layer underlain by dense sand. These experiments were specifically designed to investigate the individual effect of kinematic and inertial loads on a single pile and a 3 × 1 row pile group during model earthquakes. It was observed that the ratio of free-field soil natural frequency to the natural frequency of structure might not govern the phase relationship between the kinematic and inertial loads for pile foundations as reported in some previous research. The phase relationship obtained in this study agrees well with the conventional phase variation between the force and displacement of a viscously damped simple oscillator subjected to a harmonic force. Further, as expected, the pile accelerations and bending moments can be smaller when the kinematic and inertial loads act against each other compared to the case when they act together on the pile foundations. This study also revealed that the peak kinematic pile bending moment will be at the interface of soil layers for both single pile and pile group. However, in the presence of both kinematic and inertial loads, the peak pile bending moment can occur either at the shallower depths or at the interface of soil layers depending on the pile cap rotational constraint.

Experimental investigation of kinematic pile bending in layered soils using dynamic centrifuge modelling

This research provides an insight into the previously unexplored aspects of kinematic pile bending, especially for large-intensity earthquakes where the soil behaviour is highly non-linear. In this study, a series of dynamic centrifuge experiments was conducted on pile foundations embedded in a two-layered soil profile to investigate the kinematic effects on pile foundations during model earthquakes. A single pile and a closely spaced 3 × 1 row pile group were used as model pile foundations, and the soil model consisted of a soft clay underlain by dense sand. It was observed that the peak kinematic pile bending moment occurs slightly beneath the interface of the soil layers and this depth is larger for the pile group compared to a single pile. Also, the piles in a group attract lower bending moments but carry larger residual kinematic pile bending moments compared to a single pile. Furthermore, the elastic solutions available in the literature for estimating the kinematic pile bending moments are shown to yield satisfactory results only for small-intensity earthquakes, but vastly underestimate for large-intensity earthquakes, if methods are applied injudiciously. The importance of considering soil non-linearity effects and accurate determination of shear strain at the interface of layered soils during large-intensity earthquakes for a reliable assessment of kinematic pile bending moment from methods in the literature is demonstrated using dynamic centrifuge test data.

A Simple Approach for Estimating the First Resonance Peak of Layered Soil Profiles

The first resonance peak, Gs1, represents the amplification ratio of seismic motion when resonance between input motion and the local site occurs. The Gs1is important for understanding amplification characteristics of local site, thus it has been adopted for evaluating site effects in the Japanese Seismic Code. Herein, a simple method for estimating the Gs1of layered soil profiles is proposed. By replacing a multi-layer soil profile on bedrock with an equivalent one-layer soil profile, the Gs1and fundamental period are easily obtained. To realize the one-layer profile, we develop a procedure to replace a two-layer soil profile on bedrock with an equivalent single-layer profile. This procedure is then applied successively to a multi-layer soil profile to obtain an equivalent single-layer soil profile. The validity of the proposed method is demonstrated by evaluating 67 representative sites. The results obtained using the proposed procedure agree well with those produced by the wave propagation method.

Seismic performance of strip foundations on liquefiable soils with a permeable crust

Modern seismic codes dictate that the use of shallow foundations on liquefiable soils may be considered only after appropriate ground improvement. No further instructions are given regarding the improvement depth, or what changes if the surface layer (crust) does not liquefy. Hence, it is standard practice to improve the entire liquefiable soil layer below the foundation, whatever its depth or thickness. This study examines the seismic performance of strip foundations laying on a two-layered soil profile, consisting of a bottom layer of liquefiable sand and a (manmade or natural) permeable crust on top. Regardless of its origin, this crust does not develop (significant) excess pore pressures during shaking and is stronger and stiffer than the underlying liquefied sand. The problem is investigated numerically, through fully coupled non-linear dynamic finite-difference analyses. Following the identification of the governing response parameters, a set of multi-variable relations is developed for the approximate assessment of the seismic footing settlements and the bearing capacity degradation due to liquefaction appearing below the permeable crust.

Vertical Soil Profiling Using a Galvanic Contact Resistivity Scanning Approach

Proximal sensing of soil electromagnetic properties is widely used to map spatial land heterogeneity. The mapping instruments use galvanic contact, capacitive coupling or electromagnetic induction. Regardless of the type of instrument, the geometrical configuration between signal transmitting and receiving elements typically defines the shape of the depth response function. To assess vertical soil profiles, many modern instruments use multiple transmitter-receiver pairs. Alternatively, vertical electrical sounding can be used to measure changes in apparent soil electrical conductivity with depth at a specific location. This paper examines the possibility for the assessment of soil profiles using a dynamic surface galvanic contact resistivity scanning approach, with transmitting and receiving electrodes configured in an equatorial dipole-dipole array. An automated scanner system was developed and tested in agricultural fields with different soil profiles. While operating in the field, the distance between current injecting and measuring pairs of rolling electrodes was varied continuously from 40 to 190 cm. The preliminary evaluation included a comparison of scan results from 20 locations to shallow (less than 1.2 m deep) soil profiles and to a two-layer soil profile model defined using an electromagnetic induction instrument.

Strain effects on kinematic pile bending in layered soil

The kinematic bending of single piles in two-layer soil is explored to account for soil stiffness degradation and associated damping increase with increasing levels of shear strain, a fundamental aspect of soil behaviour which is not incorporated in current simplified seismic design methodologies for pile foundations.A parametric study of a vertical cylindrical pile embedded in a two-layer soil profile to vertically-propagating S waves, carried out in the time domain by a pertinent beam-on-dynamic-Winkler-foundation (BDWF) model, is reported. Strain effects are treated by means of the equivalent-linear procedure which provides soil stiffness and damping ratio as function of shear strain level. Whereas the approach still represents a crude representation of the actual soil behaviour to dynamic loading, it is more realistic than elementary solutions based on linear visco-elasticity adopted in earlier studies.The paper highlights that soil nonlinearity may have either a detrimental or a beneficial effect on kinematic pile bending depending on the circumstances. The predictive equations for kinematic pile bending in visco-elastic soil recently developed by the Authors are extended to encompass strain effects. Numerical examples and comparisons against experimental data from case histories and shaking table tests are presented.

Numerical analysis of liquefaction-induced bearing capacity degradation of shallow foundations on a two-layered soil profile

Earthquake-induced excess pore pressure build-up and the associated shear strength degradation of liquefiable soils may result in bearing capacity degradation and seismic settlement accumulation of shallow foundations, two detrimental effects which need to be taken into account in order to ensure a viable performance-based design. This paper focuses on the first effect, in the case of strip and rectangle footings, resting on a deep liquefiable soil layer overlaid by a thinner non-liquefiable clay crust. A simplified analytical methodology is presented, based on the Meyerhof and Hanna [14] composite failure mechanism and the use of a reduced friction angle for the liquefied sand. The methodology is verified and evaluated against parametric numerical analyses with the Finite Difference Method, applying an advanced bounding surface constitutive model to account for the liquefied sand response. In addition, the existence of a critical clay crust thickness is explored, beyond which subsoil liquefaction does not affect the bearing capacity of the foundation.

A parameterized model for local infiltration in two-layered soils with a more permeable upper layer

Summary A simple conceptual model for local infiltration into a two-layered soil profile with the upper layer much more permeable than the subsoil is proposed. It is a reformulation of a conceptual model of Corradini et al. (2000) [J. Hydrol. 273, 58–73] involving coupled ordinary differential equations that was developed for infiltration and soil water content distribution in a two-layered soil under any rainfall pattern. The simpler model framework proposed here is restricted to the estimate of infiltration rate alone in cases where the upper layer is more permeable than the lower one. First, the maximum value, K 1 h , of the saturated hydraulic conductivity of the upper soil, K 1 s , that yields the time to ponding while the dynamic wetting front is entirely within this homogeneous layer is defined by a conceptual approach. Then, parameterized forms for the time when the wetting front reaches the interface, for time to ponding associated with K 1 s ⩾ K 1 h , and for the suction head at the interface as a function of time are determined. With these functional forms, the current model utilizes algebraic equations for ponding time and soil infiltration capacity, thus providing substantial savings in computational effort for describing infiltration in two-layered soils. When compared to the detailed conceptual model, the proposed model represents the infiltration rate accurately, with less than 10% average error on the dynamic cumulative infiltration depth. This simplified model offers potential for upscaling to field-scale infiltration over layered soils exhibiting spatial heterogeneity.

Nonlinear Analysis of Torsionally Loaded Piles in a Two-Layer Soil Profile

This paper presents a method for predicting the nonlinear response of torsionally loaded piles in a two-layer soil profile, such as a clay or sand layer underlain by rock. The shear modulus of the upper soil is assumed to vary linearly with depth and the shear modulus of the lower soil is assumed to vary linearly with depth and then stay constant below the pile tip. The method uses the variational principle to derive the governing differential equations of a pile in a two-layer continuum and the elastic response of the pile is then determined by solving the derived differential equations. To consider the effect of soil yielding on the behavior of piles, the soil is assumed to behave linearly elastically at small strain levels and yield when the shear stress on the pile-soil interface exceeds the corresponding maximum shear resistance. To determine the maximum pile-soil interface shear resistance, methods that are available in the literature can be used. The proposed method is verified by comparing its res...

Experimental p-y loops for estimating seismic soil-pile interaction

Seismic soil-pile interaction is evaluated in this study based on back-calculated p-y loops constructed from sampled data of pile bending moments. Fundamental properties of p-y loops are implemented to derive distributed springs and dashpots, thereby quantifying soil-pile interaction in the realm of a Beam on Dynamic Winkler Foundation modeling. The procedure is validated by means of well-documented centrifuge tests of a single pile supported structure founded on a two-layer soil profile that comprises of soft clay overlying dense sand. Two shaking levels of a real earthquake motion applied at the base of the soil profile were examined and the generated seismic p-y loops were compared to cyclic p-y curves commonly used in pile design practice. The results demonstrate the strong influence of intensity of the input motion on seismic p-y loops while cyclic p-y curves established for soft clays tend to overestimate soil stiffness under strong excitation. Typical sets of recorded and computed structural response are presented, denoting the ability of the BDWF model related to p-y loops in reproducing adequately fundamental aspects of seismic soil-pile interaction.

Infiltration analysis to evaluate the surficial stability of two-layered slopes considering rainfall characteristics

Shallow slope failures in residual soil during periods of prolonged infiltration are common throughout the world. Using a one-dimensional infiltration model and an infinite slope analysis, this study examines an approximate method of determining how infiltration influences the surficial stability of two-layered slopes. The method extends Moore's infiltration model, which is based on the Green–Ampt model, to cover more general situations, including those where water moves upward from a perched water table in decreasingly permeable soil. The method has also been used to evaluate the likelihood of a shallow slope failure being induced by a particular rainfall event. In making this evaluation, the method takes into accounts the rainfall intensity and duration of various return periods in a two-layered soil profile. A comparison of the results of the infiltration model with the results of numerical analyses shows that, with the use of properly estimated input parameters, the proposed model compares reasonably well with other model that rely on more rigorous finite element method.

Landslide Risk Analysis using a New Constitutive Relationship for Granular Flow

AbstractIn this study, landslide potential is investigated, using a new constitutive relationship for granular flow in a numerical model. Unique to this study is an original relationship between soil moisture and the inertial number for soil particles. This numerical model can be applied to arbitrary soil slab profile configurations and to the analysis of natural disasters, such as mudslides, glacier creeping, avalanches, landslips, and other pyroclastic flows. Here the focus is on mudslides.The authors examine the effects of bed slope and soil slab thickness, soil layered profile configuration, soil moisture content, basal sliding, and the growth of vegetation, and show that increased soil moisture enhances instability primarily by decreasing soil strength, together with increasing loading. Moreover, clay soils generally require a smaller relative saturation than sandy soils for sliding to commence. For a stable configuration, such as a small slope and/or dry soil, the basal sliding is absorbed if the perturbation magnitude is small. However, large perturbations can trigger significant-scale mudslides by liquefying the soil slab.The role of vegetation depends on the wet soil thickness and the spacing between vegetation roots. The thinner the saturated soil layer, the slower the flow, giving the vegetation additional time to extract soil moisture and slow down the flow. By analyzing the effect of the root system on the stress distribution, it is shown that closer tree spacing increases the drag effects on the velocity field, provided that the root system is deeper than the shearing zone.Finally, the authors investigated a two-layer soil profile, namely, sand above clay. A significant stress jump occurs at the interface of the two media.

A Simple Limit Equilibrium Approach for Calculation of Ultimate Bearing Capacity of Shallow Foundations on Two-Layered Granular Soils

The bearing capacity of shallow foundations in a non-homogeneous soil profile has been a challenging task in geotechnical engineering. In this paper, a limit equilibrium method is used for calculating bearing capacity factors of shallow foundations constructed on a two-layered granular soil profile. The main objective has been to determine the ultimate bearing capacity computed from equivalent bearing capacity factors Nq and Nγ and comparing that with numerical analysis using finite element methods. It will be shown that the data obtained form the developed method are well comparable with those obtained from FE approach, specially when the difference between shear strength parameters of layers is low which is a practical case for sedimentary soil profiles and also for artificially compacted soils. A computer program has been developed to investigate the influence of various parameters on bearing capacity factors.

Pile and penetrometer end bearing resistance in two-layered soil profiles

The paper examines the influence of soil strength and stiffness on the end resistance mobilised by a cone or displacement pile in a two-layered soil profile (i.e. strong soil overlying or underlying weak soil). The investigation involved a series of numerical analyses that idealise pile/cone penetration assuming the spherical cavity expansion analogue. It is shown how the end resistance mobilised by a penetrometer in a dense sand (i.e. strong soil) varies with the number of penetrometer diameters (D) from a weak soil (assumed here to be either a loose sand or soft clay), and depends on the ratio of the steady-state penetration resistances of the strong and weak layers. The penetrometer resistance in the weak soil is not affected by an adjacent strong layer at penetrations greater than 2D. A design procedure based on a numerical parametric study is proposed, and this is subsequently shown to compare well with centrifuge test data and with previous numerical research.

Torsional Piles in Two-Layered Nonhomogeneous Soil

Response of a pile subjected to torsional loading is significantly affected by soil profile. To investigate this characteristic, pile-soil interaction may be modeled by a series of independent springs distributed along the pile shaft (load transfer model). With this modeling, in this paper, analytical solutions are developed for the pile embedded in a two-layered soil profile. The soil stiffness of each layer is assumed to increase as a simple power law of depth, starting with any value at ground level. The solutions are expressed in terms of Bessel functions of noninteger order; and also presented in simplified expressions for some special cases. The solutions are verified using a spreadsheet, numerical program GASPILE, which is based on a nonlinear load transfer model. They are further collaborated using more rigorous numerical approaches available for a two-layered homogeneous soil. With the solutions, extensive parametric study was conducted and presented in nondimensional charts, in order to show consistency among these solutions, significant influence of stiffness profile on torsional response, and to identify conditions of utilizing these solutions. A case study is provided to illustrate the use of the current solutions, in particular for deducing soil modulus from torsional test results.

Numerical Solution for Laterally Loaded Piles in a Two-Layer Soil Profile

Piles are often embedded in a layered soil profile, such as sand or clay layer underlain by rock. Several existing solutions are available for laterally loaded piles in a layered soil system. However, these solutions are only applicable to constant soil stiffness for each layer. In this paper, a variational approach is employed to numerically solve the problem of laterally loaded piles in layered soils using beam on an elastic foundation model. The soil stiffness can be either constant with depth or linearly varying with depth. The numerical solution is validated against an existing solution for linearly varying soil stiffness in a single soil layer system and an existing solution for a two-layer soil system with constant soil stiffness. Case studies using the proposed solution for field lateral load tests on full size drilled shafts embedded in weak rock with an overlying sand layer are presented. The simplicity and the relative ease of using the solution make it a good alternative approach for estimating the deflection and moment responses of a laterally loaded pile in a two-layer soil profile.

Transit-Time Design for Diffusion through Composite Liners

Transit-time design methods are presented in this paper for determining the design thickness for composite liners consisting of a geomembrane and a compacted soil liner or geosynthetic clay liner. The design methods are based on a closed-form analytical solution for transient solute diffusion of volatile organic compounds in a composite liner and results from a numerical model. An analytical solution for diffusion in a two-layer soil profile, which is useful for transit-time design of composite liners, is also presented. The analytical solutions are used to develop graphical solution charts that can be used to design composite liners for which the effluent concentration and contaminant flux are less than a specified value. Design examples are included for a composite liner having a compacted soil liner and a composite liner having a geosynthetic clay liner. The method is relatively simple to apply and can be used for preliminary design of composite liners, evaluating experimental results, and verifying more complex numerical models.

Modeling local infiltration for a two-layered soil under complex rainfall patterns

A general model for local infiltration–redistribution–reinfiltration in a two-layered soil profile is proposed as an extension and generalization of a formulation we developed earlier for infiltration and redistribution in crusted soils. It is of analytical/conceptual type and involves a Runge–Kutta–Verner numerical solution of a system of two ordinary differential equations. Given the basic hydraulic properties of soils, there are practically no parameters to estimate by calibration. The wetting profile during infiltration is represented by a single similarity curve, while in the reinfiltration stage, which occurs for rewetting after a period of redistribution, the profile of soil water content in each horizontal layer may be composed of two similarity profiles which evolve in time. The model is tested by comparison with numerical solution of Richards’ equation, carried out for a variety of synthetic experiments upon the combination of three soil types of contrasting hydraulic characteristics. It simulates particularly well the entire cycle infiltration–redistribution–reinfiltration. In all cases, where either layer may be less permeable, the water content at the surface and the interface as well as the infiltration rates are very accurately estimated. The usefulness of compound profiles of water content in the reinfiltration stage is also shown by comparison with the simple approach, which relies on a single similarity curve.

EVAPORATION FROM SHALLOW WATER TABLE THROUGH LAYERED SOIL PROFILES

ABSTRACT The purpose of this study is to estimate the steady state evaporation rates from layered soils in the presence of high water table under isothermal conditions. A finite difference numerical scheme based upon the one-dimensional Richards equation has been employed to estimate the evaporation rates from a two-layered soil profile overlying a shallow water for appropriate initial and boundary conditions. The method takes into account the relevant atmospheric factors and soil moisture characteristics of the two layers. The effects of sequence and thickness of the soil layers and water table depth on the evaporation rates have been examined.