Vijay K. Puri

The bearing-capacity of a strip foundation on geogrid-reinforced sand

Abstract Laboratory-model test results for the bearing capacity of a strip foundation supported by a sand layer reinforced with layers of geogrid are presented. Based on the present model test results, the bearing-capacity ratio with respect to the ultimate bearing capacity, and at levels of limited settlement of the foundation, has been determined. For practical design purposes, it appears that the bearing-capacity ratio at limited levels of settlement is about 67–70% of the bearing-capacity ratio calculated on the basis of the ultimate bearing capacity.

FOUNDATION ON STRONG SAND UNDERLAIN BY WEAK CLAY WITH GEOGRID AT THE INTERFACE

Abstract A number of laboratory model test results for the ultimate bearing capacity of a surface strip foundation supported by a strong sand layer of limited thickness underlain by a weak clay with a layer of geogrid at the sand-clay interface has been presented. The tests were conducted at one relative density of compaction of sand and one undrained shear strength of clay. Two types of geogrid were used. Based on the model test results presented, it appears that the optimum height of the strong sand layer should be about two-thirds that of the foundation width for obtaining the maximum benefit from the geogrid reinforcement.

Static and dynamic active earth pressure

SummaryThe dynamic active earth pressure on retaining structures due to seismic loading is commonly obtained by using the modified Coulombs approach which is known as the Mononobe-Okabe method. This method has generally been used for cohesionless soils only. A general solution for the determination of total (i.e. static and dynamic) active earth force for a c-ϕ soil as backfill was developed by Prakash and Saran in 1966 based on the simplifying assumption that adhesion between the wall-soil interface is equal to the cohesion of the soil, that the surface of the backfill is horizontal, and that the effect of the vertical acceleration can be neglected. This note presents an improved method for calculating the static and dynamic active force behind a rigid retaining wall based on its geometry, inclination of the backfill, surcharge, strength parameters of the backfill, and the adhesion between the wall face and the soil. The effects of adhesion, inclination of backfill, and vertical components of seismic loading for a typical retaining wall are discussed.

Geotechnical aspects of oil-contaminated sands.

Contamination due to chemicals and oil spills can influence the engineering behavior of soils. The results of an investigation conducted to study the effects of oil contamination on compaction characteristics, shear strength, one-dimensional compression, and hydraulic conductivity of a sand are presented in this article. The test results indicate that the compaction characteristics are influenced by oil contamination. The angle of internal friction of sand based on total stress condition was found to decrease with the presence of oil in the pore spaces. One-dimensional compression characteristics of sand are significantly influenced by oil contamination, resulting in a decrease in the value of the constrained modulus with increase in the degree of oil saturation. Hydraulic conductivity was observed to be a function of the initial viscosity and the degree of oil saturation.

A laboratory investigation into the settlement of a foundation on geogrid-reinforced sand due to cyclic load

SummaryLaboratory model test results for permanent settlement of a shallow square foundation supported by geogrid-reinforced sand and subjected to cyclic loading are presented. During the application of the cyclic load, the foundation was subjected to a sustained static load. Tests were conducted with only one type of geogrid and at one relative density of compaction of sand. Based on the model test results, the nature of variation of the permanent settlement of the foundation with the intensity of the static loading and the amplitude of the cyclic load intensity are presented in a non-dimensional form.

Cyclic load-induced settlement of a square foundation on geogrid-reinforced sand

Abstract The permanent settlement of a square model foundation supported by geogrid-reinforced sand and subjected to cyclic loading has been evaluated. The foundation was initially subjected to an allowable static load and the cyclic load was superimposed on it. For a given static allowable load, the ultimate permanent settlement increases with the increase of the amplitude of the cyclic load. The results indicate that the ultimate permanent settlement is markedly reduced due to geogrid reinforcement.

Interference effect of two closely-spaced shallow strip foundations on geogrid-reinforced sand

SummaryGeotextiles and geogrids are now being used extensively in many civil engineering construction works. This study presents some laboratory model test results for the ultimate bearing capacity of an isolated, and two closely-spaced, strip foundations resting on unreinforced sand, and sand reinforced with layers of geogrid. Based on the model test results, the variation of the group efficiency with the centre-to-centre spacing of the foundation has been determined.

Foundations for Seismic Loads

Design of foundations in earthquake prone areas needs special considerations. Shallow foundations may experience a reduction in bearing capacity and increase in settlement and tilt due to seismic loading. The reduction in bearing capacity depends on the nature and type of soil and ground acceleration parameters. In the case of piles, the soil-pile behavior under earthquake loading is generally non-linear. The nonlinearity must be accounted for by defining soilpile stiffness in terms of strain dependent soil modulus. Several behavioral and design aspects of shallow foundations and pile groups subjected to earthquakes have been critically reviewed. Introduction Structures subjected to earthquakes may be supported on shallow foundations or on piles. The foundation must be safe both for the usual static loads as well for the dynamic loads imposed by the earthquakes and therefore the design of either type of foundation needs special considerations compared to the static case. Shallow foundations in seismic areas are commonly designed by the equivalent static approach. The observations during 1985 Michoacan-Guerero earthquake (Pecker, 1996) and the Kocaeli 1999 earthquake have shown that initial static pressure and load eccentricity have a pronounced effect on seismic behavior of foundations. The soil strength may undergo degradation under seismic loading depending on the type of soil. Pore pressure buildup and drainage conditions may result in decrease in strength and an increase in settlement. Most foundation failures due to earthquake occur due to increased settlement. However, failure due to reduction in bearing capacity have also been observed during Niigata earthquake (1964) in Japan and Izmit earthquake (1999) in Turkey (Day, 2002).Prasad et al (2004) made an experimental investigation of the seismic bearing capacity of sand. A practical method to account for reduction in bearing capacity due to earthquake loading was presented by Richard et al,(1993). The settlement and tilt of the foundation must also be considered. For analysis and design of pile groups under seismic loading a simple approach to account for nonlinear behavior of soil-pile system under dynamic loads and group efficiency factors are presented in the paper. In this approach strain dependent soil modulus is used to define soil-pile stiffness and radiation damping. GSP 160 Dynamic Response and Soil Properties Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 2 Shallow Foundations The response of a footing to dynamic loads is affected by the (1) nature and magnitude of dynamic loads, (2) number of pulses and (3) the strain rate response of soil. Shallow foundations for seismic loads are usually designed by the equivalent static approach. The foundations are considered as eccentrically loaded and the ultimate bearing capacity is accordingly estimated. To account for the effect of dynamic nature of the load, the bearing capacity factors are determined by using dynamic angle of internal friction which is taken as 2-degrees less than its static value (Das, 1992). The settlement and tilt of the foundation may then be obtained using the method proposed by Prakash (1981). Building code such as International Building Code (2006) generally permit an increase of 33 % in allowable bearing capacity when earthquake loads in addition to static loads are used in design of the foundation. This recommendation is based on the assumption that the allowable bearing pressure has adequate factor of safety for the static loads and a lower factor of safety may be accepted for earthquake loads. This recommendation may be reasonable for dense granular soils, stiff to very stiff clays or hard bedrocks but is not applicable for friable rock, loose soils susceptible to liquefaction or pore water pressure increase, sensitive clays or clays likely to undergo plastic flow (Day, 2006). Figure 1. Failure surface in soil for seismic bearing capacity (After Richards et al, 1993) Richards et al (1993) observed seismic settlements of foundations on partially saturated dense or compacted soils. These settlements were not associated with liquefaction or densification and could be easily explained in terms of seismic bearing capacity reduction. They have proposed a simplified approach to estimate the dynamic bearing capacity que and seismic settlement SEq of a strip footing. Figure 1 shows the assumed failure surfaces. The seismic bearing capacity (quE) is given by Eq. 1: quE = cNcE + qNqE + 1⁄2 γ BNγE (1) where, γ = Unit weight of soil q= γDf and Df = Depth of the foundation NcE, NqE, and NγE = Seismic bearing capacity factors which are functions of φ and tanψ = kh / (1-kv) kh and kv are the horizontal and vertical coefficients of acceleration due to earthquake. For static case, kh = kv, = 0 and Eq. (1) becomes qu = cNc + qNq + 1⁄2 γBNγ (2) GSP 160 Dynamic Response and Soil Properties Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 3 in which Nc, Nq and Nγ are the static bearing capacity factors. Figure 2 shows plots of NγE/Nγ, NqE/Nq and NcE/Nc with tan ψ and φ. tan ψ = kh/ (1-kv), tan ψ = kh/ (1-kv), tan ψ = kh/ (1-kv), Figure 2. Variation of N qE /Nq , NγE /Nγ and NcE/Nc with φ and tan ψ(After Richards et al 1993) Figure 3. Critical acceleration * h k (After Richards et al, 1993) GSP 160 Dynamic Response and Soil Properties Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 4 Seismic Settlement of Foundations Bearing capacity-related seismic settlement takes place when the ratio kh/(1 kv) reaches a critical value (kh/1 – kv)*. If kv = 0, then (kh/1 kv)* becomes equal to kh * Figure 3 shows the variation of kh * (for kv = c = 0; granular soil) with the static factor of safety (FS) applied to the ultimate bearing capacity Eq, (2), for φ = 10°, 20°, 30°, and 40° and Df/B of 0, 0.25, 0.5 and 1.0. The settlement (SEq) of a strip foundation due to an earthquake can be estimated (Richards et al, 1993) as 4 2 * ( ) 0 .1 7 4 ta n k V h S m E q A E A g A α − = (3) where V = peak velocity for the design earthquake (m/sec),A = acceleration coefficient for the design earthquake, g = acceleration due to gravity (9.81 m/sec). tan αAE depends on φ and kh* In Figure 4, variation of tan tan αAE with kh* for φ of 15° 40° is shown. Figure 4. Variation of tan αAE with kh* and φ (After Richards et al 1993) Suppose a typical strip foundation is supported on a sandy soil with B = 2 m, and Df = 0.5 m, and γ = 18 KN/m, φ = 34°, and c = 0. The value of kh = 0.3 and kv = 0 and the velocity V induced by the design earthquake is 0.4 m/sec. The static ultimate bearing capacity for this footing for vertical load will be 1,000 KN/m (Eq. 2). The reduced ultimate bearing capacity for vertical load is calculated as 290 KN/m (Eq 1). If the footing is designed using a FS = 3 on the static ultimate bearing capacity (i.e., for an allowable soil pressure of 333 kN/m), the additional settlement GSP 160 Dynamic Response and Soil Properties Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 5 due to earthquake will be 20.5 mm. This settlement reduces to 7.0 mm if FS of 4 is used. Besides ensuring that the footing soil system does not experience a bearing capacity failure or undergo excessive settlement, the foundations should be tied together using interconnecting beams (Applied Technology Council ,1978) Piles and Pile Groups under Earthquake Loadings Pile foundations are used extensively to support heavy loads. The bulk of loads are static which forms the basis for fixing the section (size), embedded length and configuration (spacing and arrangement) of the piles in the group. Dynamic loads are caused by nature, e.g. earthquakes, winds, waves and by man-made vibrations, e.g. machines, traffic and blasts. Prakash and Sharma (1990) have recommended the following procedures for design of single pile against earthquakes: 1 Estimate the dead load on the pile. The mass at the pile top which may be considered vibrating with the piles is only a fraction of this load. 2. Determine the natural frequency ωn1 and time period in first mode of vibrations. 3. For the above period, determine the spectral displacement Sd for the appropriate damping (radiation + material). 4. Using the pile displacements in (3) above, determine the bending moments and shear forces for structural design of the pile. Analysis of Pile Groups The following steps are involved in analysis of a pile group 1. Determine the stiffness and radiation damping of the single pile 2. Determine the stiffness of the pile group by applying appropriate pile-soil-pile interaction factors. 3. Determine natural frequencies of the pile group. 4. Estimate the response of the pile group for the given input motion. The pile cap displacements (translation and rotation) may be used as input parameters for superstructure analysis. Stiffness and Radiation Damping of Single Piles Stiffness of single piles in horizontal-translation and rotation may be determined as follows: Novak and El-Sharnouby (1983) and Gazetas (1991) have recommended expressions for horizontal sliding stiffness (kxo, rocking stiffness (ko) and cross-coupling stiffness kxo. Corresponding expressions for radiation damping coefficients Cx, Co and Cxo, crossdamping respectively have been developed by both the investigators. Stiffness and Radiation Damping of Pile-Group Gazetas (1991) has recommended pile-soil-pile interaction factors for both the stiffness and radiation damping. Based on experimental studies on piles, following simple expression has been developed for Group Efficiency Factors (e) in horizontal vibrations GSP 160 Dynamic Response and Soil Properties Copyright ASCE 2007 Geo-Denver 2007: New Peaks in Geotechnics 6 G r o u p s t i f f n e s s ( k ) x g = N X S i n g l e p i l e s t i f f n e s s ( k ) x ∈

Ultimate uplift capacity of model rigid metal piles in clay

SummaryLaboratory model test results for estimation of the ultimate uplift capacity of rigid metal piles embedded in a compacted near-saturated clayey soil are presented. The length-diameter ratio of the piles was varied from 10 to 15. The direction of the uplift load on the pile was varied from 0 to 50°. Based on the present test results and the results of existing model studies, an empirical relationship for estimating the pile uplift capacity has been presented.

On Seismic Design of Retaining Walls

Retaining walls have failed either by sliding away from the backfill or due to combined action of sliding and rocking displacements, during earthquakes. Performance based design of the retaining walls in seismic areas must account for these displacements, in addition to the usual factors of safety against failure in bearing, sliding and overturning. A realistic model for estimating dynamic displacement, which accounts for the combined action of sliding and rocking and takes into consideration, non-linear stiffness of soil and geometric and material damping and coupling effects is now available, Wu and Prakash (2009). This model has been used to calculate the displacement for several combinations of backfill and foundation soil conditions. Based on this study, typical design charts for preliminary design have been proposed.