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. First author funded by Commonwealth Scholarshi

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. </jats:p

Seismic Response of Pile Foundations in Soft Clays and Layered Soils

Pile foundations are widely used to support large engineering structures by transferring loads to deeper layers of soil. During earthquakes, in addition to the axial loads, pile foundations are subjected to inertial loads and kinematic loads due to the motion of the superstructure and the vibrations of surrounding soil, respectively. Due to the significant damage caused by earthquake induced soil liquefaction, studies in the past few decades have focused more on liquefaction induced effects on pile foundations embedded in sands. However, post-reconnaissance reports of several earthquakes have concluded that pile foundations in soft clays and layered soils with significant stiffness contrast have also undergone severe damage during earthquakes. In this research, three different series of centrifuge experiments were performed to investigate the dynamic behaviour of soft clays, two-layered soils with significant stiffness contrast and the dynamic response of pile foundations embedded in these soil types. The first series of centrifuge experiments were focused on studying the dynamic response of floating piles in soft clay, which in turn depends on the dynamic behaviour of the soft clay layer around the pile. It was found that the dynamic response of clay depends on the earthquake intensity as well as the shear strength and stiffness of the clay layer. The second and third series of centrifuge experiments were specifically designed to investigate the seismic kinematic and inertial loads acting on pile foundations embedded in two-layered soil models with soft clay underlain by dense sand. The results have shown that obtaining a reliable value for the kinematic pile bending moment using established methods in the literature required accurate assessment of the earthquake-induced shear strain at the interface between the two soil layers. Moreover, it was found that non-linearity effects in soil are significant and need to be accounted for. Further, the phase difference between the kinematic and inertial loads and its influence on pile accelerations, rotations and bending moments was evaluated. This research has revealed that the ratio of free-field soil natural frequency to the natural frequency of the embedded structure might not govern the phase relationship between the kinematic and inertial loads as reported in previous research. Instead, the phase relationship between the two loads agrees well with the conventional phase variation between the force and displacement of a viscously damped simple oscillator subjected to a harmonic force. Lastly, the pile displacements (y) and corresponding soil pressures (p) were determined from the experimentally measured pile bending moments to establish p-y curves and compared with the corresponding curves recommended by design standards. The drawbacks of adopting p-y curves developed for monotonic or cyclic loading to dynamic loading conditions were highlighted through this comparison. The influence of earthquake characteristics such as frequency and intensity, pile group effects and soil layering on soil stiffness and ultimate lateral resistance of p-y curves were discussed in detail. Eventually, the analysis and interpretation of the centrifuge tests provided a better insight into the previously unexplored aspects of seismic soil-pile-structure interaction in soft clays and layered soils with significant stiffness contrast. This research also highlighted the importance of considering soil non-linearity effects in seismic analysis and design of pile foundations

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

Role of Pile Spacing on Dynamic Behavior of Pile Groups in Layered Soils

This research investigates the influence of pile spacing on the dynamic behavior of pile groups by performing a series of specifically designed dynamic centrifuge experiments on pile foundations embedded in a two-layered soil profile. A single pile and two 3×1 row pile groups with different pile spacing were used as model pile foundations, and the soil models consisted of a soft clay underlain by dense sand. The influence of earthquake frequency on the dynamic behavior of two-layered soils is discussed using the centrifuge data and 1D site response analysis from DEEPSOIL. Further, the results of these centrifuge tests agreed with the conviction that the group effects will be diminished with the increase in pile-to-pile spacing in a pile group due to reduced pile-soil-pile interaction. However, these reduced pile group effects can lead to larger kinematic pile bending moments in the widely spaced pile group compared with a closely spaced pile group. Moreover, the single pile always has larger bending moments than both the tested pile groups - an exception to this is when there is a significant phase difference between the kinematic and inertial loads for a single pile but not for the widely spaced pile group. The influence of pile spacing on the shadowing effects and location of peak bending moments in the piles of a group are also discussed in this paper. Lastly, an attempt is made to evaluate the individual contribution of inertial and kinematic loads for the seismic design of pile foundations considering soil-pile-structure interaction effects

Seismic behaviour of soft clay and its influence on the response of friction pile foundations

In recent years, much of the research in geotechnical earthquake engineering has focused on liquefaction of loose, saturated sands and silts. However, the dynamic behaviour of soft, clayey soils and their interaction with pile foundations during the earthquakes have received relatively little attention. In this study, an attempt is made to investigate the dynamic behaviour of soft clay and its interaction with pile foundations during earthquakes using high gravity centrifuge testing. A model single pile and two sets of 3 × 1 row model pile groups with different pile spacing were embedded in soft kaolin clay and tested under the action of model earthquakes at 50 times the earth’s gravity. The strength and stiffness of clay were evaluated using a T-bar test and an air hammer device respectively. The focus of this research is to investigate the dynamic response of friction piles in soft clay. However, this depends on the dynamic response of the soft clay layer around the pile. To this end, one-dimensional ground response analysis was performed using DEEPSOIL software to emphasise the importance of non-linear analysis in characterising the seismic behaviour of soft clays. It will be shown that clay response depends both on the earthquake intensity and the shear strength and stiffness of the clay layer. This has a direct bearing on the response of single piles and pile groups, with larger amplification occurring for small intensity earthquakes and attenuation occurring for stronger earthquakes

Kinematic and inertial seismic load effects on pile foundations in stratified soil

A series of centrifuge experiments were performed with pile foundations in stratified soil to investigate the kinematic and inertial load effects during earthquakes. A single aluminium model pile was embedded into a soft kaolin clay overlying the dense sand. Each experiment was carried out in two flights, to study the kinematic and inertial load effects separately. A few important findings of this study are: (i) the kinematic load induced pile bending is equally important as inertial load induced pile bending during the smaller intensity base excitations, (ii) the bending moment due to inertial loads is predominant in comparison to kinematic bending moment during the larger intensity base excitations, and (iii) the existing methods in literature for determining the pile kinematic bending moment at the interface of two-layered soils are underestimating the bending moment. However, the recently proposed methods are resulting in a better estimation than the old conventional methods

Seismic behaviour of soft clay and its influence on the response of friction pile foundations

In recent years, much of the research in geotechnical earthquake engineering has focused on liquefaction of loose, saturated sands and silts. However, the dynamic behaviour of soft, clayey soils and their interaction with pile foundations during the earthquakes have received relatively little attention. In this study, an attempt is made to investigate the dynamic behaviour of soft clay and its interaction with pile foundations during earthquakes using high gravity centrifuge testing. A model single pile and two sets of 3×1 row model pile groups with different pile spacing were embedded in soft kaolin clay and tested under the action of model earthquakes at 50 times the earth’s gravity. The strength and stiffness of clay were evaluated using a T-bar test and an air hammer device respectively. The focus of this research is to investigate the dynamic response of friction piles in soft clay. However, this depends on the dynamic response of the soft clay layer around the pile. To this end, one-dimensional ground response analysis was performed using DEEPSOIL software to emphasise the importance of non-linear analysis in characterising the seismic behaviour of soft clays. It will be shown that clay response depends both on the earthquake intensity and the shear strength and stiffness of the clay layer. This has a direct bearing on the response of single piles and pile groups, with larger amplification occurring for small intensity earthquakes and attenuation occurring for stronger earthquakes

Lightweight deflectometer for compaction quality control

Quality assessment and control (QA/QC) of compacted pavement layers involve regular monitoring of density and moisture content during compaction. In situ density is traditionally determined using sand-cone test method. However, many recent studies have indicated that stiffness- or strength-based quality measurements are easy to determine and more reliable than the density-based quality measurements. In this study, lightweight deflectometer (LWD) is used as a quality control device to assess the quality of compacted pavement layers. As a part of this study, an extensive LWD field testing program is undertaken on the expressway along the Outer Ring Road (ORR) located in Hyderabad, India, to determine the modulus of deformation (ELWD) of base and surface pavement layers. ELWD of compacted base and surface layers was found to commonly range from 37.6 to 58.6 and 89.3 to 125.7 MPa, respectively. In addition, a case study on a low-volume road is presented to demonstrate the relationship between the ELWD and in situ density obtained from the sand-cone test. LWD is found to be simple to operate and provides quick test results on any pavement layer. Hence, the frequency of quality control tests can be increased leading to an improvement in the overall quality of compacted pavement layers

Compaction Quality Control of Earth Fills Using Dynamic Cone Penetrometer

Quality control for compaction of earth fills is commonly performed by measuring the in situ density using the sand cone method. In situ density measurements from sand cone testing are highly operator-dependent; in addition, the test procedure is tedious and time-consuming. In this study, a dynamic cone penetrometer (DCP) was used to perform quality control (QC) of earthworks by measuring penetration resistance in compacted soil. DCP tests were performed on three test pads specially constructed using different soil types - clayey sand with gravel, clayey sand, and silty sand. The test results were expressed in terms of a dynamic penetration index (DPI), defined as the depth of penetration of the cone per hammer blow. Correlations are developed between DPI and compacted density for the three soil types considered. In order to meet the criterion of compacted density equal to or greater than 98% of the maximum density from a laboratory standard Proctor test, DPI values are found to range from 5 to 8 mm/blow, corresponding to 250 mm penetration of cone on tested soil types. The effect of the fall height of the hammer on the measured DPI is also studied by performing DCP tests for two fall heights, 575 and 450 mm. DPI values are found to increase by 11-26% when the height of the fall increases from 450 to 575 mm, for the highest energy level considered in the study. It is also found that DPI is very sensitive to the moisture content and in situ density of compacted layers. The DCP device provided quick test results and was simple to operate on any subgrade layer; hence, the frequency of QC tests can be increased, leading to an improvement in the overall quality of compaction of earthworks