Dilip Kumar Baidya

Nonlinear Dynamic Response of Piles under Horizontal Excitation

This paper presents test results from cast-in situ reinforced concrete single and group piles subjected to strong horizontal excitation. The tests were conducted for different eccentric moments simulating different excitation levels to obtain the frequency-amplitude response of the pile. Moderate nonlinear behavior is observed in both horizontal and rocking components of vibration. The experimental results were compared with dynamic interaction factor approach using nonlinear solutions. The accuracy of the nonlinear analysis in predicting the dynamic response depends on the choice of parameters that best characterize the response of boundary zone around the pile and the realistic length of pile separation. It is shown in this study that by allowing for boundary zone and separation between pile and soil, close agreement between theoretical predictions and measured response curves can be achieved.

Effect of Presence of Rigid Base within the Soil on the Dynamic Response of Rigid Surface Foundation

The present experimental investigations study the effect of presence of rigid base at any depth within the soil mass on the dynamic response of foundation under vertical mode of vibration. Model block vibration tests on rigid surface footing are conducted on a different soil layer of finite thicknesses underlain by a rigid base. A concrete block of size 400 × 400 × 100 mm is used as the model block, and a Lazan type mechanical oscillator is used for inducing vibration in vertical direction. The finite soil layers of different thicknesses are prepared in a pit at the bottom of which a massive concrete layer of 300 mm thick was cast to represent it as a rigid base. In the investigation two different soils, namely, local in situ soil and sand are used. In total 72 tests are conducted in different loading combinations and soil types, and several important observations are reported. Two different methods are proposed to analyze the dynamic response of a foundation resting on a finite layer underlain by a rigid layer. Finally, the experimental results are compared with the results that were obtained by the proposed method. A Mass-Spring-Dashpot (MSD) model with proper consideration of damping factor is found to provide reasonably accurate results. Elastic Half Space Theory (EHST) with equivalent soil properties is found to underestimate the displacement amplitude.

Reliability-Based Analysis of Cantilever Sheet Pile Walls Backfilled with Different Soil Types Using the Finite-Element Approach

AbstractThe paper presents an analytical study of a cantilever sheet pile wall considering the effects of uncertainty of soil properties. The failure probability (Pf) of the sheet pile wall is combined with the sensitivity (S) of geotechnical random variables on the failure mode, and a new factor, the probabilistic risk factor (Rf), is proposed for each random variable corresponding to different variations of the random variables. The cantilever sheet pile wall that is driven into a cohesionless soil layer and backfilled by (1) cohesionless soil and (2) cohesive soil is considered separately for analysis against the rotational failure mode. Pf is obtained by developing the response surface, based on finite element models. The sensitivity of each random variable is obtained by F-test analysis. It is observed that the cohesion of foundation soil (c2) and water table positions are the important factors that influence the stability of the cantilever sheet pile walls to a great extent. Finally, design guidelin...

Dynamic vertical response of model piles — experimental and analytical investigations

Abstract Dynamic response characteristics of reinforced concrete model piles were investigated in the field under varying levels of vertical harmonic excitation. In the investigations single piles and 2 × 2 group piles with length to diameter ratios 10, 15 and 20. For group piles, spacing to diameter ratios of 2, 3 and 4 for each length to diameter ratio were used. In both the cases, two different conditions of pile cap — embedded into soil and above the ground surface, were considered in this investigation. The measured response was compared with the response obtained by different analytical methods. The stiffness and damping of piles under vertical vibration were computed by two different analytical approaches — (i) Novaks frequency independent solutions with static interaction factor for parabolic soil profile and (ii) Novaks complex frequency dependent analytical solutions with dynamic interaction factor approach for layered media. From the comparison of these theories with the experimental data it was found that the Novaks frequency dependent solutions with dynamic interaction factor approach produced reasonable estimates of the experimental results. In this study the influence of excitation intensity, static load on pile and different contact condition of pile cap with soil on the dynamic behaviour of pile foundation are reported.

Reliability Coupled Sensitivity-Based Seismic Analysis of Gravity Retaining Wall Using Pseudostatic Approach

AbstractThe stability of geotechnical earth structures is often affected by associated uncertainties present in geotechnical parameters, if they are not properly accounted for. The present paper aims at quantifying these uncertainties and proposes a modification factor, namely probabilistic risk factor (Rf) for each geotechnical random variable. A gravity retaining wall is analyzed by a pseudostatic method of analysis against four modes of failure namely, sliding, overturning, eccentricity, and bearing. The effect of variation of properties of backfill and foundation soil on stability of the wall for various earthquake conditions is analyzed. Rf simultaneously identifies the effects of Pf of a gravity retaining wall subjected to earthquake loading and also the sensitivity of geotechnical random variables on different modes of failure. The geotechnical random variables are modified by Rf and applied in design. It is observed that, apart from the seismic horizontal and vertical pseudostatic acceleration coe...

Risk Factor Based Design of Cantilever Retaining Walls

Sensitivity analysis of geotechnical random variables on potential failure modes (overturning, sliding, bearing capacity and eccentricity) of a cantilever retaining wall reveals that high sensitivity of a particular variable on a particular mode of failure does not necessarily imply a remarkable contribution to the overall failure probability. The present paper aims to combine probability of failure (Pf) of each failure mode and sensitivity of the random variables to these failure modes and introduces a new factor, called Probabilistic Risk Factor (Rf) for each random variable. Pf is calculated by Monte Carlo Simulation and sensitivity analysis of each random variable is calculated based on normalized F-Statistics value. Rf is a reduction factor which takes into account the variations of random variables and hence can be directly implemented in design by the designers. The random variables (friction angle and unit weight of backfill soil; and friction angle, unit weight and cohesion of foundation soil), when divided by Rf and applied in design, yield a structure which is safe against variations of the random variables. It is observed that Rf of friction angle (φ1) of backfill increases and cohesion (c2) of foundation soil decreases with an increase in variation of φ1, while Rf for unit weights (γ1 and γ2) of both the soil and friction angle of foundation soil (φ2) remains almost constant. Finally, design guidelines for different variations of φ1 are provided based on the proposed methodology, which proves to be cost effective.

The Nonlinear Coupled Response of Single and Group Piles under Various Horizontal Excitations: Experimental and Theoretical Study

Dynamic response characteristics of reinforced concrete single piles and 2 × 2 group piles subjected to varying levels of horizontal harmonic excitation are investigated by both experimental and analytical study. Two different types of coupled vibration tests, namely, type 1—horizontal exciting force above the center of gravity (c.g.), and type 2—horizontal exciting force below the c.g. of the pile cap-loading system, are conducted in the field. The tests are conducted for different eccentricities to determine the frequency-amplitude response of piles for horizontal and rocking motion separately. The influence of excitation intensity, static load on pile, spacing of piles in group, and different contact condition of pile cap with soil on the coupled dynamic response of piles are reported. The measured responses of type 1 and type 2 are compared with the results obtained by the continuum approach of Novak with nonlinear solution. For nonlinear analysis, the boundary zone concept, which accounts for yielding of soil around the pile, is incorporated into the linear elastic-based model and the allowance is made for the separation between the pile and soil. A reasonable match between the measured and predicted response by nonlinear analysis has been observed after introducing appropriate boundary zone parameters and soil–pile separation length. The differences between the dynamic characteristics of piles for coupled vibration type 1 and type 2 in terms of frequency response curves for amplitude, stiffness, and damping constants are also discussed.

Risk Factor based Analysis of Cantilever Sheet Pile Walls

Sensitivity analysis involving different random variables and potential failure modes of cantilever sheet pile walls for different soil conditions focuses on the fact that, high sensitivity of a particular variable on a particular failure mode does not necessarily imply a remarkable contribution to overall failure probability (Pf). The present paper aims at identifying a probabilistic risk factor (Rf) for each random variable based on the combined effects of Pf of each mode of failure and sensitivity of each random variable on these failure modes. Three different soil conditions are considered: (1) cohesionless soil above and below dredge line, (2) cohesionless soil above dredge line and cohesive soil below dredge line and (3) cohesive soil above and below dredge line. It is observed that friction angle of both foundation and backfill soils are the major guiding factors for Case 1, while for Cases 2 and 3, cohesion for foundation soil dominates over the other two random variables. Thus the present paper proposes a safe and economic design by assigning different Rf for different random variables for sheet pile walls embedded in different subsurface conditions and varying positions of water table.

Prediction of boundary zone parameters and soil–pile separation under horizontal and rocking vibrations

Abstract This paper attempts to study the dynamic soil–pile interaction on the non-linear response under coupled vibration by both experimental and analytical study. Dynamic response characteristics of reinforced concrete piles under varying levels of vertical and horizontal harmonic excitation are investigated in the present study. Single and group (2×2) piles are used in the investigation. The horizontal exciting force applied on the pile cap through the oscillator causes coupled motion to the foundation. The measured response is compared by the two different analytical approaches: linear analysis – Novak’s plane strain theory with static interaction factor approach and non-linear analysis – Novak’s continuum model with dynamic interaction factor approach. For non-linear analysis, allowance is made for the separation between the pile and soil in boundary zone model. Reasonable match between the measured and predicted response by non-linear analysis is found. The test data are used to establish the empirical relationship between the extent of soil separation around the pile and the maximum vibration amplitudes under coupled vibration.

Field Test on Group Piles under Machine Induced Coupled Vibration

Dynamic response characteristics of reinforced concrete group piles with embedded pile-cap condition are investigated in the field under varying levels of coupled harmonic excitations. The piles are constructed by bored cast-in-situ method. The site is located at Indian Institute of Technology Kharagpur, West Bengal, India. The soil properties are determined by laboratory tests on both disturbed and undisturbed soil samples collected from various boreholes at the site. Two in-situ tests, namely, standard penetration tests to determine N-value and cross hole seismic tests for determining the shear-wave velocity of soil are conducted at various depths of soil layer. Forced coupled vibration tests are conducted on pile groups using Lazan type mechanical oscillator with four different eccentric moments. Both the horizontal and rocking motions of pile groups are measured simultaneously for different operating frequencies of oscillator. Two resonant peaks are observed at two different frequencies. It is also observed that as the eccentric moment increases, the resonant amplitude increases, but the natural frequency decreases for both horizontal and rocking responses. The test results on piles are then compared with the numerical approach with two different soil-pile models – (i) A linear visco-elastic medium composed of outer infinite region and an inner weaker layer with reduced shear modulus, and (ii) a boundary zone with parabolic variation of shear modulus and with a non-reflective interface. From the comparison between the test results and numerical results using the first model it is found that the prediction of nonlinear response of the pile foundation is in good agreement with the test results. For the second model, the numerical results are not found satisfactory when compared to the test results. The nonlinear parameters like separation length, shear modulus ratio, weak zone damping factor and thickness ratio are also predicted for the group piles under coupled vibration.