Jorge G. Zornberg

INFLUENCE OF MATRIC SUCTION ON THE RESULTS OF PLATE LOAD TESTS PERFORMED ON A LATERITIC SOIL DEPOSIT

The performance of foundation systems on unsaturated soil deposits is considerably influenced by variations of the negative pore-water pressure (i.e., matric suction) distribution within the soil mass due to local microclimate conditions. Although significant understanding has been gained in the last few decades on the behavior of unsaturated soils, the use of unsaturated soil mechanics concepts in the interpretation of field tests has not been incorporated into the state-of-practice. As a consequence, conservative foundation engineering principles are used in practice in regions where unsaturated soil conditions prevail. For example, direct use of field test results (e.g., from plate load tests) obtained during a dry season in unsaturated soil deposits may lead to the selection of unconservatively high design parameters at the site. On the other hand, ignoring altogether the beneficial impact of matric suction on the bearing capacity of soils may lead to unnecessarily expensive foundation systems in tropical, arid climates. This paper investigates the influence of soil suction on the results of plate load tests conducted at a depth of 1.5 m on a structured, naturally occurring lateritic soil. Ten tests were carried out under different soil suction conditions. Matric suction was monitored during plate load testing using tensiometers installed at the bottom of the testing pit up to a depth equal to one plate diameter (0.80 m), which is generally recognized as the influence zone in which significant stress variation occurs (Terzaghi and Peck 1948). The results provide insight into the influence of soil matric suction on ultimate bearing capacity and settlement rate of plate load tests performed on lateritic soils. Background Although the notion that the presence of negative pore-water pressures (matric suction) in the soil influences the behavior of foundations is not new, there is only limited information reported in technical literature dealing with the quantification of this problem. A brief description of previous research on this topic regarding shallow footings is presented below. Bearing capacity loss associated with the soil saturation has been commonly accounted for through the use of different values of unit weight of the soil (� ) that arise due to total or partial submersion. Other soil parameters such as the effective cohesion ( c� ) and the effective internal friction angle ( �� ) are usually assumed to play a minor role. Considering the submerged unit weight of the soil is about 50 % of the moist unit weight, Terzaghi and Peck (1948) stated that the bearing capacity of shallow footings could be approximately reduced by 50 % if the water level rises from a depth equal to the footing width below the footing to the surface. Meyerhof (1955) proposed an analytical model for estimating the bearing capacity of shallow foundations based on the variations of the soil unit weight and taking into consideration the possibility of partial submersion. The groundwater table is assumed to lie between the foundation base and the lower portion of the soil failure surface. The value of unit weight of soil to be used is defined as:

Behavior of Geogrid-Sand Interface in Direct Shear Mode

The contribution of transverse ribs to the soil-geogrids interaction under pullout mode has been well documented. However, the contribution of transverse ribs to the soil-geogrid interaction under direct shear mode is, at best, unclear. Consequently, this paper presents the results of a comprehensive direct shear testing program aimed at evaluating the contribution of transverse ribs to the interface shear. The direct shear tests involved Ottawa sand and several polyester geogrids with a variety of material tensile strength, percent open area, and aperture pattern. The test results show that the shear strength of sand-geogrid interfaces under direct shear mode is significantly higher than that of sand-geotextile interfaces. Analysis of shear displacement-strength response of the interfaces indicates that, in addition to interface shear components due to sand-rib friction and sand-sand shear at the location of the openings, the transverse ribs provide additional contribution to the overall sand-geogrid interface resistance. Specifically, analysis of the results reveals that the transverse ribs of the geogrid used in this study provide approximately 10% of interface shear resistance. This contribution is positively correlated with the tensile strength and the stiffness of geogrid ribs, but is negatively correlated with the percent open area of the geogrid. A simple model is proposed to quantify the contribution of transverse ribs to the interface shear strength under direct shear mode.

Centrifuge Permeameter for Unsaturated Soils. I: Theoretical Basis and Experimental Developments

A new centrifuge permeameter was developed with the specific objective of expediting the measurement of the hydraulic characteristics of unsaturated soils. The development, theoretical basis, and typical results associated with using the centrifuge permeameter for concurrent determination of the soil-water retention curve (SWRC) and hydraulic conductivity function ( K function) of unsaturated soils are presented in this paper. Components developed for the centrifuge permeameter are described, including the centrifuge, permeameter, water flow control system, and instrumentation used to concurrently and nondestructively measure the infiltration rate (flow pump and outflow transducer), volumetric water content (time domain reflectometry), and matric suction (tensiometers) in flight during steady-state infiltration. A companion paper focuses on definition of the SWRC and K function for a clay soil using the procedures described in this paper. While conventional geotechnical centrifuges are used to reproduce t...

Failure Mechanisms in Sand over a Deep Active Trapdoor

An experimental testing program was undertaken to investigate failure mechanisms induced by the active movement of a deep rectangular trapdoor underlying a granular soil. Reduced-scale models were tested under normal gravity as well as under an increased gravitational field using a centrifuge facility. Some models were used to evaluate the performance of both flexible and rigid pipes undergoing a localized loss of support. Failure mechanisms in the longitudinal direction of the models were characterized by a single, well-defined failure surface that developed within the limits of the trapdoor. However, failure mechanisms in the transverse direction of the models were characterized by multiple failure surfaces extending outside the limits of the trapdoor. Significant dilation of the soil located immediately above the trapdoor was identified in the failure of the models. The pattern of the failure mechanisms was found to be affected by the stress level and backfill density. Higher stress levels were found to lead to well-developed failure zones. The influence of backfill density was found to be more relevant in models involving flexible pipes. Pipes embedded within loose backfill were severely damaged after loss of support, while pipes embedded in dense backfill experienced negligible deformations. These results indicate that damage to pipelines caused by ground loss of support can be significantly minimized by controlling the compaction of the fill.

TESTING OF REINFORCED SLOPES IN A GEOTECHNICAL CENTRIFUGE

An evaluation of the use of centrifuge modeling as a tool for analyzing the behavior of reinforced soil slopes is presented in this paper. A review of the state-of-the-art indicates that previous centrifuge studies have focused mainly on the performance of reinforced soil vertical walls and that limit equilibrium approaches (used in the design of reinforced soil slopes) have not been fully validated against the failure of models in a centrifuge. As part of an evaluation of the conditions of similarity governing the behavior of reinforced soil structures at failure, scaling laws are specifically derived by assuming the validity of limit equilibrium. It is demonstrated that an Nthscale reinforced slope model should be built using planar reinforcements having 1/N the strength of the prototype reinforcements in order to satisfy similarity requirements. A description of the experimental testing procedures implemented as part of a recent centrifuge testing program is presented, and an example dataset from this investigation is used to illustrate typical results. These include the g-level at failure, visual observation of failure development, and post-failure analysis of reinforcement breakage. The pattern observed in the geotextile reinforcements retrieved after testing indicates that the boundary effects were negligible.

PERFORMANCE OF GEOSYNTHETIC-REINFORCED WALLS SUPPORTING BRIDGE AND APPROACHING ROADWAY STRUCTURES

This paper describes a unique field application in which a geosynthetic-reinforced soil system was designed and constructed to support both the foundation of a two-span bridge and the approaching roadway structure. The reinforced soil system not only provides bridge support, but it was also designed to alleviate the common bridge bump problem. This structure was considered experimental and comprehensive material testing and instrumentation programs were conducted. These programs would allow assessment of the overall structure performance and evaluation of Colorado Department of Transportation and AASHTO design assumptions and procedures for reinforced soil structures supporting both bridge foundations and approaching roadway structures. Large-size direct shear and triaxial tests were conducted to determine representative shear strength properties and constitutive relations of the gravelly backfill used for construction. Three sections were instrumented to provide information on external movements, internal soil stresses, geogrid strains, and moisture content during various construction stages and after the structure opening to traffic. Results from a pilot (Phase I) instrumentation program and some preliminary results from a more comprehensive (Phase II) instrumentation program are presented in the paper. The results suggest that current design procedures lead to a conservative estimation of both the backfill material strength and horizontal earth pressures, and that the overall performance of this structure, before its opening to traffic, has been satisfactory.

Mobilization of Reinforcement Forces in Fiber-Reinforced Soil

AbstractFiber reinforcement represents a promising alternative in projects involving localized repair of slopes and reinforcement of thin soil veneers, where planar reinforcement (e.g., with geotextiles and geogrids) is difficult to implement. Current design methodologies allow quantification of the shear strength of fiber-soil composites in terms of the parameters that independently characterize the soil matrix and fibers. The shear strength of fiber-reinforced soil is considered to have two components, including the shear strength of the soil matrix and the tension mobilized within the fibers. Triaxial compression tests and fiber pullout tests were conducted to evaluate how the fiber tension is mobilized for varying shear strain levels. The results of this evaluation provide insights into whether the shear strength of fiber-reinforced soil is governed by the peak or residual shear strength of unreinforced soil. A revision to existing design methodology is proposed in which the individual contribution of...

Water Balance and Evapotranspiration Monitoring in Geotechnical and Geoenvironmental Engineering

Among the various components of the water balance, measurement of evapotranspiration has probably been the most difficult component to quantify and measure experimentally. Some attempts for direct measurement of evapotranspiration have included the use of weighing lysimeters. However, quantification of evapotranspiration has been typically conducted using energy balance approaches or indirect water balance methods that rely on quantification of other water balance components. This paper initially presents the fundamental aspects of evapotranspiration as well as of its evaporation and transpiration components. Typical methods used for prediction of evapotranspiration based on meteorological information are also discussed. The current trend of using evapotranspirative cover systems for closure of waste containment facilities located in arid climates has brought renewed needs for quantification of evapotranspiration. Finally, case histories where direct or indirect measurements of evapotranspiration have been conducted are described and analyzed.

Centrifuge Permeameter for Unsaturated Soils. II: Measurement of the Hydraulic Characteristics of an Unsaturated Clay

This paper presents the hydraulic characteristics of an unsaturated, compacted clay, including its soil-water retention curve (SWRC) and hydraulic conductivity function ( K function), determined using a new centrifuge permeameter developed at the University of Texas at Austin. A companion paper describes the apparatus, its instrumentation layout, and data reduction procedures. Three approaches are evaluated in this study to define the SWRC and K function of the compacted clay under both drying and wetting paths, by varying the inflow rate, the g level, or both. For imposed inflow rates ranging from 20 to 0.1 mL/h and g levels ranging from 10 to 100 g, the measured matric suction ranged from 5 to 70 kPa, the average volumetric water content ranged from 23 to 33%, and the hydraulic conductivity ranged from 2 × 10 − 7 to 8 × 10 − 11 m / s . The SWRCs and K functions obtained using the three different testing approaches were very consistent, and yielded suitable information for direct determination of the h...

Effects of infiltration and evaporation on geosynthetic capillary barrier performance

This study includes an experimental investigation of the transient movement of water in unsaturated soil layers underlain by a geocomposite drainage layer (GDL) during cycles of infiltration and evaporation. The distribution in volumetric water content with depth in a soil column having a height of 1350 mm underlain by a GDL was measured during transient infiltration. The capillary break effect was observed to affect the soil up to a height of 500 mm above the GDL, with an increase in volumetric water content up to 20% above that expected for the case of infiltration under a unit hydraulic gradient. Due to the long duration of this test (2000 h), a shorter 150 mm high soil column was also evaluated to investigate the soil–GDL hydraulic interaction during cycles of infiltration and evaporation. The capillary break was observed to have re-established itself after infiltration was stopped and the soil near the interface dried. The suction and volumetric water content measured in the soil at breakthrough were...