Dimitrios Loukidis

Bearing capacity of strip footings on purely frictional soil under eccentric and inclined loads

The finite element method is used for the determination of the collapse load of a rigid strip footing placed on a uniform layer of purely frictional soil subjected to inclined and eccentric loading. The footing is set on the free surface of the soil mass with no surcharge applied. The soil is assumed to be elastic – perfectly plastic following the Mohr–Coulomb failure criterion. Two series of analyses were performed, one using an associated flow rule and one using a nonassociated flow rule. The first series is in accordance with bearing capacity solutions currently used in shallow foundation design practice, while the second one is consistent with the dilatancy exhibited by sands in reality. Both probe-type analyses and swipe-type analyses were undertaken. Analyses for associated and nonassociated flow rules yield essentially the same trends regarding the effective width, inclination factor, and normalized vertical force – horizontal force – moment (V–H–M) failure envelope. The results show that the incli...

SEISMIC DESIGN OF DEEP FOUNDATIONS

Detailed investigations of pile foundations affected by earthquakes around the world since the 1960s indicate that pile foundations are susceptible to damage to such a degree that the serviceability and integrity of the superstructure may be affected. Although numerous cases of seismically damaged piles are reported, the detailed mechanisms causing the damage are not yet fully understood. As a consequence, an effective seismic design of pile foundations has not been yet established in practice. Many road bridge structures supported on piles exist in southern Indiana. This is a region where the risk of occurrence of a dangerous earthquake is high due to its proximity to two major seismic sources, the New Madrid Seismic Zone (NMSZ) and the Wabash Valley Fault System (WVFS). The present study is a first step towards the assessment of potential earthquake induced damage to pile foundations in southern Indiana. Credible earthquake magnitudes for each of the two potential seismic sources are assessed for a return period of 1000 years. SHAKE analyses are performed at nine selected sites in southwestern Indiana to estimate the potential of ground shaking and liquefaction susceptibility. The soil profile and soil properties at each site are obtained from the archives of the Indiana Department of Transportation. The amplitude of the rock outcrop motion is estimated using attenuation relationships appropriate to the region, and estimated values are compared with predictions from the USGS. SHAKE analyses are performed for two earthquake scenarios: a NMSZ earthquake and a WVFS earthquake. Two sets of input motions are considered for each scenario. The liquefaction potential at those nine sites is assessed based on the Seed et al. (1975) method. Data from a total of 59 real cases of earthquake-induced damage to piles have been gathered through an extensive literature survey. The collected and compiled data have been used to identify the causes and types of pile damage, and the severity of damage. Based on the survey, damage is usually located near the pile head, at the interfaces between soft and stiff layers, and between liquefiable and non-liquefiable layers. Large inertial loads from the superstructure can cause crushing of the head of concrete piles. Imposed deformations due to the response of the surrounding soil can produce small to large cracks on concrete piles depending on the soil profile. In contrast, large inertial loads, liquefaction and lateral spreading can cause wide cracks. Few cases of steel piles are found in the literature. Steel casing seems to improve the performance of concrete piles. Numerical simulations of a concrete pile at a selected road bridge site with and without steel casing are used to investigate the effect of steel casing on the performance of concrete piles. Results from this work suggest that major credible seismic events can generate accelerations high enough to produce damage to concrete piles in southern Indiana. The potential of liquefaction and lateral spreading increase the likelihood of damage.

Assessment of Axially-Loaded Pile Dynamic Design Methods and Review of INDOT Axially-Loaded Pile Design Procedure

The general aim of the present research is to identify areas of improvement and propose changes in the current methodologies followed by INDOT for design of axially loaded piles, with special focus on the dynamic analysis of pile driving. Interviews with INDOT geotechnical engineers and private geotechnical consultants frequently involved in INDOT’s deep foundation projects provided information on the methods and software currently employed. It was found that geotechnical engineers rely on static unit soil resistance equations that were developed over twenty years ago and that have a relatively large degree of empiricism. Updated and improved static design equations recently proposed in the literature have not yet been implemented in practice. Pile design relies predominantly on SPT data; cone penetration testing is performed only occasionally. Dynamic analysis of pile driving in standard practice is performed using Smith-type soil reaction models. A comprehensive review of existing soil reaction models for 1-dimensional dynamic pile analysis is presented. This review allowed an assessment of the validity of existing models and identification of their limitations. New shaft and base reaction models are developed that overcome shortcomings of existing models and that are consistent with the physics and mechanics of pile driving. The proposed shaft reaction model consists of a soil disk representing the near field soil surrounding the pile shaft, a plastic slider-viscous dashpot system representing the thin shear band forming at the soil-pile interface located at the inner boundary of the soil disk, and far field- consistent boundaries placed at the outer boundary of the soil disk. The soil in the disk is assumed to follow a hyperbolic stress-strain law. The base reaction model consists of a nonlinear spring and a radiation dashpot connected in parallel. The nonlinear spring is formulated in a way that reproduces realistically the base load-settlement response under static conditions. The initial spring stiffness and the radiation dashpot take into account the effect of the high base embedment. Both shaft and base reaction models capture effectively soil nonlinearity, hysteretic damping, viscous damping, and radiation damping. The input parameters of the models consist of standard geotechnical parameters, thus reducing the level of empiricism in calculations to a minimum. Data collected during the driving of full-scale piles in the field and model piles in the laboratory are used for validating the proposed models.

The Mechanics of Friction Fatigue in Jacked Piles Installed in Sand

The limit shaft resistance of displacement piles decreases with the number of axial loading cycles applied during pile installation (e.g., jacking, driving) or due to the nature of superstructure loading. This study examines the mechanisms governing the phenomenon of friction fatigue along piles jacked in sandy soils. Finite element analysis (FEA) of a thin disk of soil surrounding the pile shaft is performed. The pile installation process is modeled as a sequential combination of cylindrical cavity expansion and vertical shearing along the pile-soil interface. The soil disk is subjected to several vertical shearing cycles to simulate the successive application and removal of jacking loads. An advanced soil constitutive model based on two-surface plasticity and critical state theory is used in the FE simulations. FEAs examine the development and evolution of the normal stress acting on the pile shaft, which multiplied by an appropriate friction coefficient yields the value of mobilized shaft resistance. Results show that, in the absence of loading cycles, the normal stress attains values considerably larger than the in situ vertical stress. However, with subsequent application of loading cycles, the normal stress decreases at rates that increase with decreasing relative density and in situ vertical stress.

On the Pseudo-Coupled Winkler Spring Approach for Soil-Mat Foundation Interaction Analysis

Analysis of the interaction between mat foundations and the supporting soil is often performed with the soil being represented by vertical translational springs. This study sheds light on what must be the distribution of spring stiffness coefficients under a mat in order to get the same mat deflections and bending moment diagrams as in the case of a mat resting on a three-dimensional linear elastic continuum. For this purpose, the spring stiffness coefficients were directly back-calculated at each mat node from the results of finite element analyses that treat the soil as a 3D continuum. Parametric runs are performed to investigate the influence of the soil elastic properties, slab geometrical characteristics and column load configurations on the back-calculated spring stiffness spatial distributions. The computational results of the present study indicate that spring stiffness distributions often assumed currently in practice may lead to a significant underestimation of the mat bending moments.

Moisture Migration Under Mat Foundations in Nicosia Marl

The paper investigates the moisture changes occurring due to rainfall and evaporation under a mat foundation in an expansive soil of Cyprus called Nicosia marl. For this purpose, laboratory experiments were performed on samples of Nicosia marl for the determination of the hydraulic and water retention characteristics. A field water infiltration experiment was performed at a site where pairs of volumetric water content and matric suction sensors were installed at two different depths, in order to back-calculate the in situ hydraulic characteristics. Based on the experimental data and back-calculation results, finite element analysis of moisture migration underneath a mat foundation over the course of a hydrological year were performed using rainfall and evaporation input boundary conditions at the free ground surface consistent with Nicosia climatic data. Series of parametric analyses are performed varying the values of foundation embedment depth, the van Genuchten model parameters, and the vertical and horizontal hydraulic conductivities. The parametric study sheds light also on the effectiveness of two ground improvement approaches that are frequently used as remedies in the case of expansive clays, namely prewetting and soil replacement by gravel fill.

Bearing Capacity in Sand Under Eccentric and Inclined Loading Using a Bounding Surface Plasticity Model

The bearing capacity of strip footings on sand under inclined or eccentric loading is investigated using finite element analysis. The sand behavior is simulated using a bounding surface plasticity constitutive model based on critical state theory that accounts for strain softening, pre-failure non-linearity and material anisotropy. The parametric study focuses on the rate of decrease of the bearing capacity with increasing load inclination or load eccentricity for various values of sand relative density. The numerical predictions are compared with experimental data from centrifuge tests, as well as predictions from existing design equations.

Numerical investigation of gravity wall - granular soil interaction

The active earth pressure developing in granular backfills is directly related to the internal friction angle. It is well known that the internal friction angle is a function of the level of mean effective stress and the level of shear strain in the retained soil. Both these variable change as the wall moves away from the backfill due to the compliance of the foundation soil. In this paper, simulations of the response of the wall-foundation-backfill system are performed using the finite element method in order to study the interaction between wall movement and active earth pressure. The granular soil is modeled using a bounding surface plasticity constitutive model for sands based on critical state soil mechanics. The finite element analyses show that the wall system reaches a minimum active stress state associated with a peak soil friction angle at wall crest displacement less than 0.5% of the wall height, relevant to a serviceability limit state rather than an ultimate limit state. By the time the ULS is reached the friction angle mobilized in parts of the retained soil mass has approached the critical state friction angle value due to the intense shear straining.