William N. Houston

Laboratory Filter Paper Suction Measurements

Results of filter paper measurements for both matric and total suction values are reported for a sand, silt, and clay. Calibration curves for the matric and total suction are given for the filter paper used in this study, and comparisons with the results of several other researchers are made. For low suction values, the matric suction is more easily measured than the total suction using the filter paper method. This difference is due primarily to the insensitivity of the water content of the noncontact filter paper to changes in suction when the suction is low. A pressure membrane, saturated salt solutions, and tensiometers were used in developing the filter paper calibration curves. Potential pitfalls in making filter paper measurements and limitations of the method are discussed. A procedure for obtaining filter paper suction measurements is given.

Geotechnical engineering practice for collapsible soils

Conditions in arid and semi-arid climates favor the formation of the most problematic collapsible soils. The mechanisms that account for almost all naturally occurring collapsible soil deposits are debris flows, rapid alluvial depositions, and wind-blown deposits (loess). Collapsible soils are moisture sensitive in that increase in moisture content is the primary triggering mechanism for the volume reduction of these soils. One result of urbanization in arid regions is an increase in soil moisture content. Therefore, the impact of development-induced changes in surface and groundwater regimes on the engineering performance of moisture sensitive arid soils, including collapsible soils, becomes a critical issue for continued sustainable population expansion into arid regions.In practicing collapsible soils engineering, geotechnical engineers are faced with (1) identification and characterization of collapsible soil sites, (2) estimation of the extent and degree of wetting, (3) estimation of collapse strains and collapse settlements, and (4) selection of design/mitigation alternatives. Estimation of the extent and degree of wetting is the most difficult of these tasks, followed by selection of the best mitigation alternative.

Prediction of the soil-water characteristic curve based on grain-size-distribution and index properties

The grain-size-distribution (GSD) of a soil is intimately related to its pore size distribution and hence, the GSD holds a close relation with the soil-water characteristic curve (SWCC). In addition, the plasticity index (PI) is a measure of the water holding capacity of the soil and therefore, it plays an important role in shaping the SWCC. This paper presents two sets of statistically derived equations that describe the SWCC of non-plastic and plastic soils. Data from 154 non-plastic soils and 63 plastic soils were analyzed. Soil samples were collected as part of the National Cooperative Highway Research Program (NCHRP) 9–23 project entitled Environmental Effects in Pavement Mix and Structural Design Systems . Samples were obtained from underneath paved roads of 30 sites located throughout the United States. The soil samples were subjected to laboratory testing that included index testing and SWCC testing. SWCCs were determined using a newly developed pressure plate device capable of overburden pressure application, continuous measurements of moisture content, and volume change monitoring. In addition to the collected field data, a database of published soil index properties and SWCCs was incorporated to the analysis. Each SWCC data set was fitted with Fredlund and Xing curve, which provided an S-shaped curve with four parameters, a f , b f , c f , and h rf . Using multiple regression analysis, equations were derived for these four parameters based on predictors derived from GSD and PI. The equations presented in this paper are useful in predicting the SWCC of any given soil without carrying out actual SWCC testing and they can easily be incorporated into computer codes to solve various unsaturated soil mechanics problems such as determining moisture beneath covered areas.

An Oedometer-Type Pressure Plate SWCC Apparatus

A new oedometer device for SWCC determination has been evaluated by testing a wide range of soil types, and recommendations for best practices for use of this device have been made. The new device allows application and control of the net normal stress, σ − u a , in addition to the matric suction, u a − u w . Thus it can be used to simulate a range of overburden stresses and various other stress states of interest. It also allows the measurement of specimen volume change and the use of a single specimen for determination of the SWCC. The evaluation includes issues of temperature control, air diffusion through high air entry ceramic disks, and study of potential sources of error in water content determination for the SWCC. It is concluded that the new oedometer device can be used to obtain an accurate SWCC for a single specimen up to 1500 kPa . General recommendations for appropriate use of the equipment and corrections to SWCC data obtained using axis translation pressure plate-type devices are made.

Matric Suction Prediction Model in New AASHTO Mechanistic-Empirical Pavement Design Guide

Equilibrium moisture beneath highway pavements is critical to pavement design because it directly affects the strength and stiffness of pavement systems. Moisture is related to soil suction by means of the soil water characteristic curve (SWCC). Previous research has indicated a correlation of suction with Thornthwaites moisture index and soil type; however, these suction correlations exhibit large variability. Under NCHRP Project 9-23, Environmental Effects in Pavement Mix and Structural Design, sponsored by FHWA, soil samples were collected from beneath 30 pavement sections throughout the United States. The SWCCs and index properties of collected samples were measured at Arizona State University. The in situ degree of saturation was obtained and the corresponding in situ soil suction was found from measured SWCCs. On the basis of the field and laboratory data, two models were developed to predict equilibrium suction under pavements: one for granular non-plastic materials and another for fine-grained plastic materials. These models were adopted in the new AASHTO Mechanistic–Empirical Pavement Design Guide because they exhibited good results, with variability within acceptable limits. Model development is presented.

Study of Expansive Soils and Residential Foundations on Expansive Soils in Arizona

Construction on expansive soils is challenging and thus prone to some problems and litigation. The engineering community makes extensive use of local experience and empirical procedures to address these problems. Although there has been extensive study of expansive soils and foundations on expansive soils, data related to performance of residential structures are limited in general and limited in the Phoenix area, in particular. In this study, an overview of the Phoenix Valley, Arizona, geotechnical practice and foundation performance related to residential structures on expansive clays, was developed through surveys and interviews with geotechnical engineers, structural engineers, and homebuilders. Using data obtained from files of Phoenix area geotechnical firms and government agencies, the existing Natural Resource Conservation Service map showing expansive soil locations throughout the Phoenix region was updated through the use of correlation developed in this study relating expansion index to common soil index properties such as Atterberg limits and percent passing the No. 200 sieve. Files of forensic investigations linked to expansive soil regions were made available for this study by several geotechnical engineering firms, and Phoenix Valley areas where forensic investigations have been identified, were mapped for comparison to regions identified in the updated map as having expansive soils. Comparison of the forensic investigation map to the updated map of expansive clay locations revealed that most of the forensic investigations were in regions identified with clays labeled as high to moderately high expansion potential, with a few forensic investigations in regions of medium expansion potential. Finally, unsaturated flow analyses were conducted for an Arizona expansive clay profile for two very different landscaped conditions of well-irrigated turf and desert landscape. The results of the numerical analyses were consistent with the reported observations and modes of failure identified through the surveys and interviews conducted with engineering and homebuilder professionals, including the finding that site drainage was found to be extremely important to good foundation performance, regardless of the type of landscape selected.

Effects of Testing Procedures on the Laboratory Determination of Swell Pressure of Expansive Soils

There is lack of agreement on laboratory procedures that should be used to measure swell pressure. In this study, effects of apparatus compressibility and applied initial net normal stress on measured swell pressure were revisited, with emphasis on undisturbed specimens. Samples covering a wide range of properties were collected from Texas and Arizona, and effects were explored by examination for consistency of trends in behavior for several “companion” sets of undisturbed specimens. Failure to account for apparatus compressibility results in unintended swell of the specimen, reducing the constant volume swell pressure by as much as half for specimens of this study. Applied initial net normal stress level also affects measured swell pressure. For specimens of this study, swell pressures measured at low initial net normal stress (token load) were found to be about 40%, on average, of swell pressures measured for specimens loaded initially equivalent to field overburden stress. This difference is believed to be primarily due to initial specimen compression. Commonly adopted sampling disturbance corrections to constant volume tests conducted at initial light confining stress (∼ 1–7 kPa) were investigated, and found to give inconsistent values of swell pressure. Comparisons of constant volume to load-back swell pressure are also presented. Findings suggest that application of an initial confining stress level corresponding to field conditions and application of apparatus compressibility corrections as the swell pressure develops are key to the consistent measurement of constant volume swell pressure, and that measured swell pressures from constant volume tests, when carefully performed, are more consistent than swell pressures estimated using methods to account for sampling disturbance.

LONG-TERM MOISTURE CONDITIONS UNDER HIGHWAY PAVEMENTS

Equilibrium moisture beneath highway pavements is critical to pavement design because moisture directly affects the strength and stiffness of pavement systems. Moisture is related to soil suction by means of the soil-water characteristic curve (SWCC). Previous research has indicated a correlation of suction with Thornthwaite Moisture Index (TMI) and soil type; however, these suction correlations exhibited large variability. Under an NCHRP project sponsored by the Federal Highway Administration, soil samples were collected from beneath thirty pavement sections throughout the United States: two from the WesTrack test facility, one from the MNRoad Project, and twenty-seven from the Long Term Performance sites. SWCCs and index properties were measured on collected samples at Arizona State University. The in-situ degree of saturation was obtained from soil index properties, dry unit weight, and moisture content, and the corresponding in-situ soil suction was obtained from SWCCs. Based on the field and laboratory data, an algorithm was developed to predict suction under the pavement using TMI, percent passing 200, and Plasticity Index.

Interpretation and Comparison of Collapse Measurement Techniques

The collapse potential of a soil can be measured directly by using simple laboratory or field tests. It has been demonstrated that stress-strain curves from laboratory response-to-wetting tests can be used to obtain reasonable estimates of strain and settlement upon wetting. When field plate load tests are used to determine soil collapsibility, the stress-strain relationship is often not evaluated. In this paper the advantages of developing a stress-strain relationship from plate load tests for estimating colaspe settlement are discussed, and a test procedure and analysis for in-situ collapse testing are addressed. Laboratory and field methods for assessing collapse potential are compared.

Resilient Modulus Predictive Model for Unbound Pavement Materials

Data from 96 resilient modulus laboratory tests on four base materials and four subgrade soils was used to develop a predictive model capable of estimating changes in resilient modulus as a function of changes in the state of stress, moisture and density. Test specimens were compacted at optimum moisture/density conditions and then dried or soaked to various moisture conditions. Both standard and modified compactive efforts were used. Plastic, subgrade-type soils were especially affected by moisture. Non-plastic, base-type materials were more sensitive to changes in the state of stress.