Structure Damage Due to Expansive Soils: a Case Study Beni Lew Agricultural Research Organization P.O. Box 6, Bet Dagan 50250, Israel e-mail:
[email protected]
ABSTRACT Structure damages are becoming very common at Cuiaba/ Brazil, especially in case of low structures, probably due to expansive soil behavior. This work analyzes a typical problem where an expansive soil was found to be the cause of the problem. Soil samples collected at different depths were tested according to the conventional geotechnical investigation tests, swelling potential, x-ray diffraction analysis, scanning electron microscopy and energy dispersive x-ray technique. Moreover, the wall cracks were dated and studied together with a foundation structural analysis. All the analysis tests led to the conclusion that the soil had an average to high swelling potential due to the presence of expansive-clay minerals, like smectite and vermiculite.
KEYWORDS:
Clays; Expansive soils; Soil/structure interaction
INTRODUCTION Expansive soils are found in many parts of the world, particularly in semiarid regions with alternating wet and dry seasons. The soils in these regions experience periodic swelling and shrinkage during the alternating wet and dry seasons. Such cyclic swell-shrink movements of the ground cause considerable damage to the structures founded on them. The influence of cyclic wetting and drying on the swelling behavior of natural expansive soils is well documented (Popesco, 1980; Chen and Ma, 1987; Subba and Satyadas, 1987; Dif and Bluemel, 1991; and Day, 1994). Expansive soils pose a problem where rapid urbanization and development are occurring. As development extends into these areas, identification and quantification of the soil properties that define shrink-swell potential are essential to properly evaluate the stability of a soil as a foundation material. Soil scientists recognize that shrink-swell behavior can best be predicted by examining a combination of physical, chemical, and mineralogical soil properties (Davidson and Page, 1956; Holtz and Gibbs, 1956; Gill and Reaves, 1957; Holtz, 1959; Franzemeier and Ross, 1968; Carstea et al., 1970; Anderson et al., 1973; McCormack and Wilding, 1975; Shcafer and Singer, 1976; Ross, 1978; Karathanasis and Hajek, 1985; Thomas et al., 2000; and Sridharan and Prakash, 2000). However, no one property accurately predicts shrink-swell potential. Often most expansive soils are clayey with high content of smectite minerals.
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Studied soil properties and their proposed relation to degree of expansion are summarized in Table 1.
Table 1: Swelling potential prediction in soils. Parameter
Reference
LL (%)
Chen, 1975 Chen, 1975 Holtz and Gibbs, 1956 Holtz and Gibbs, 1956 Holtz, 1959 Thomas et al., 2000 Thomas et al., 2000
PI (%) Clay Content (%) Clay Content (%) Swell Percent (%) Swell Pressure (kPa) Activity
Skempton, 1953
Low 55 >45
27
9.0 >225
1.25
This paper deals with a 400 m2 residence built 1.00 meters above the natural soil located in Cuiaba/ Brazil- a crawl space (void) was left below the residence floor. Cuiaba is a city in a valley 15 degrees south of the equator and almost in the center (east to west) of the continent, with a climate characterized by a rainy season with an average 28°C and 170 mm/month rain (from November to April) and a dry season with an average 23°C and 30 mm/month rain (from May to October). The residence was composed of shallow foundations (around 2.5 meters below the natural ground level) with bearing capacity around 20 kPa; and it presented wall damage (longitudinal cracks of around 1.0 cm) at the end of the first wet season after the construction. In the following year new foundations were added to reinforce the structure, however, again the residence presented new wall damage at the end of the next wet season, mainly in the central area and with some repercussion to the back area. The residence central area floor level came up relative to the road during the wet season. Our study was undertaken with the hypothesis that expansive soil was responsible for the problem. It is known that no one soil property or expansive soil test can precisely predict shrink-swell potential. However, a set of properties and tests can determine shrink-swell behavior. With these premises in mind, the main objective of this study was to establish the causes of a residence crack walls before and after the addition of reinforcing structures in the Cuiaba region in Brazil.
MATERIALS AND METHODS Representative disturbed and undisturbed samples were collected every 0.5 m from three boreholes drilled up to 2.5 m depth under the residence at the end of the dry season. During sample collection the standard penetration number (SPT) was determined, according to the American Society for Testing and Materials, Designation D-1586-67 (ASTM, 1986). Laboratory analyses included particle-size distribution, liquid limit (LL), plastic limit (PL), clay content, natural moisture content, dry unit weight, degree of saturation (S), expansibility potential and clay mineralogy. Particle size distribution, LL, PL, clay content, natural moisture content, S and dry unit weight tests were also accomplished according to the standards of the American Society for Testing and Materials.
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The swelling potential (swell percent and swell pressure) of each sample was measured using the "Load-Swell Method" and the "Constant Volume Method" proposed by Al-Rawas (1993) in undisturbed samples. Mineral composition was determined by x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray method (EDX). Clay minerals were identified with a Scintag XDS 2000 x-ray diffractometer with Cu-K radiation. After conducting the tests, the diffraction pattern for each sample was matched with the standard patterns prepared by the Joint Committee of Powder Diffraction Data Service (JCPDS) for a both qualitative and quantitative evaluation. The SEM analysis was carried out using a JEOL (JSM-840), which performs morphological and micro-structural assessment and gives a full elemental description using the EDX analyzer.
RESULTS AND DISCUSSION According to geotechnical tests the soil is composed of silt with increasing N values with depth. At each borehole similar values of SPT were measured at each depth, from around N value of 5 at surface to around an N value of 45 at 5.0 m depth. Samples at different depths (from 0.5 to 2.5 meters) in the three boreholes were collected and the liquid limit (LL), plastic limit (PL), plasticity index (PI), clay content (C), natural moisture content (w), degree of saturation (S) and dry unit weight ( d) values were determined. Soil samples presented a brown-red color with silt-clay strata. Similar physical soil parameter values were observed at the three different boreholes at each depth and the average values for the three boreholes at different depths are shown in Table 2. An average value of 53.6% was obtained for the LL and mean value found for PI was of 24.5% for all the samples collected. Similar values have been reported by other researchers for expansive soils (Al-Rawas, 1999; Sridharan and Prakash, 2000; Azam et al., 1998 and Thomas et al., 2000). A comparison between Atterberg's limits (Table 1) and the studied soil samples shows that this soil can be classified as highly expandable. Table 2: Average basic geotechnical properties of the samples tested at different depths. LL (liquid limit), PL (plastic limit), PI (plastic index), C (clay content,
