Highway Subgrade Construction in Expansive Soil AreasJian-Long Zheng1Rui Zhang2and He-Ping Yang31 Professor and President, ChangSha Univ. of Science and Technology, Chiling Road 45, Changsha, Hunan 410076, China. E-mail: zjl_csust@2 Ph.D. Candidate and Lecturer, School of Highway Engineering, ChangSha Univ. of Science and Technology, Chiling Road 45, Changsha, Hunan 410076, China. E-mail: zr_csust@3Professor, School of Highway Engineering, ChangSha Univ. of Science and Technology, Chiling Road 45, Changsha, Hunan 410076, China. E-mail: cscuyang@(Accepted 22 May 2007)IntroductionExpansive soil is predominantly clay soil that undergoes appreciable volume and strength changes following a change in moisture content. These volume changes can cause extensive damage to the geotechnical infrastructure, and the damage is often repeatable and latent in the long term (Liao 19848). China is one of the countries with a wide distribution of expansive soils. They are found in more than 20 provinces and regions, nearly 600,000 km2in extent. It has been estimated that the planned highways totaling 3,300 km in length pass through expansive soils areas (Zheng and Yang 200422). Improper highway construction in such areas could well lead to great losses and damage to the environment.In 2002, the Chinese Ministry of Communications (CMOC) sponsored a research project, “A Complete Package for Highway Construction in Expansive Soil Areas,” whose primary objective was to solve expansive soil problems in highway engineering. A research group with personnel from Changsha University of Science and Technology (CUST) was set up. Comprehensive laboratory tests, field investigations, and analyses were carried out, aimed at solving highway engineering problems in several different expansive soil areas. A complete presentation of the results of this research is beyond the scope of this paper, but the research on subgradeconstruction for the Nanning to Youyi Guan (NanYou) highway will be discussed to introduce the laboratory studies performed on soil properties, classification of swelling potential, and engineering properties of expansive filler soils. Field investigations of slope failures in the Ningming area also will be described. Several new techniques for building embankments and treating expansive soil cut slopes also are presented.Site GeologyNingming Basin lies in south China. The region has in an oceanic monsoon climate with long summers and quite short winters. The annual average temperature is 22.1° centigrade and the annual rainfall is about 1,200 mm, falling mostly from April to August. There is a very obvious difference between the rainy and dry seasons. The Ningming Basin is an east–west tectonic faulted basin formed in Q2–4, with deposits of lacustrine mudshale from the Nadu formation (N y) of the Eocene, 1,500 m in thickness. Argillaceous siltstone and little siltstone are present as well. The obliquity of the stratum is about 5°–7°in the middle of the basin, but 22°at the edge. In addition, there are Jurassic and Triassic siltstone and limestone around the basin.A typical geological profile from the Ningming area is shown in Fig. 1. Ningming expansive soils, dark or light gray in color, are of residual origin from the weathering of mudshale or shale from the Nadu formation of the Eocene. Due to the parent rock’s structure, the residual expansive soils have a residual fabric (microbedding and tiny tectonic fissures) originating from the parent rock, and many included ferromanganese nodules. The weathered mudshale, once exposed, collapses rapidly into smaller prism-shaped fragments, which can continue to fragment into smaller and smaller pieces. A great many vertical, cross and horizontal tectonic joints, tectonic fissures, and weathered fissures clearly can be seen in excavated profiles, and a soft interlayer exists between the soil and mudshale layers. The active zone, about 3-m thick (CMOC 1996b10) is several meters below the surface, but nevertheless affected by seasonal climatic changes (Nelson and Miller 199213).Research on Embankment Construction and Its Engineering ApplicationIn expansive soil areas, embankments filled with expansive soils usually encounter nonuniform settlement (Hu et al. 20044). Road shoulders, which cannot be compacted, adequately, develop large longitudinal cracks, and large wave deformations appear along the cross section. The bearing capacity of the pavement decreases substantially as a result of the reduced resilient modulus of the subgrade, and slope surface failures usually occur. Embankment failures usually involve three phases: shrinkage cracks form in the slope during the dry season; water infiltrates into the soil mass through the cracks in the subsequent wet season, with soil swelling; and finally, the shear strength of the clayey soil deteriorates to the extent that the shear forces within the slope cannot be adequately resisted, resulting in a localized slope failure. Generally, times until failure can range from several months to several years (Zhang et al. 2005).Selection of Embankment FillerIt has been stipulated in China’s Technical Specification for the Construction of Highway Subgrades (CMOC 1996a11) that expansive soil with a high swelling potential should not be used as embankment fill material due to its poor water stability. Expansive soil with medium swelling potential can be used as filler only after it has been improved. Expansive soil with low swelling potential can be used as filler according to the climate, hydrological conditions, and the highway classification, but the side slopes and top of the embankment should be protected. In light of these specifications, nearly five million m3of expansive soils and highly weathered mudshale excavated for the construction of the NanYou highway in the Ningming area could not be used as embankment filler without being improved, because they are characterized as having moderate swelling potential.Consequently, the research group studied stabilizing expansive soils with calcium lime, Portland cement, and mixtures of lime and cement in the laboratory. The resulting clay contents, compressibility, California bearing ratio (CBR), and swelling capabilities showed that the improving effect of the lime was the greatest (Chen 20042). For light-gray expansive soil, the optimum lime content was determined to be 3%, and the optimum moisture content was 15.2%. These values were determined by compaction tests at 2,684.9 kj/m3. The resulting CBR and soakedswelling ratio satisfied the specifications for fill. However, in practice it is very hard to mix lime and expansive soil properly because the natural moisture content of the soil is so very high. The soil easily agglomerates, so the operation is complicated and requires heavy construction machinery. In addition, the high price of lime will greatly increase highway construction costs, and lime dust that inevitably escapes during spreading will do some harm to the environment. Therefore, lime stabilization is not popular in engineering practice. However, if the expansive soils must be replaced by nonexpansive material transported from several hundred kilometers away that would entail high costs and environmental degradation.According to the Chinese specification, the CBR of fill in an upper embankment (less than 1.5 m below the embankment surface) should reach 4%, and it should be 3% for the material in the lower embankment (more than 1.5 m below the surface). In light of the routine soaking that is part of the CBR test method, the CBR of expansive soils can rarely reach 3%. However, according to the change of CBR with moisture content (Fig. 3), and the change in swell percent of CBR samples with time , the research group found that if the CBR samples were not soaked, the CBR was very high at relatively low moisture content. Therefore, research was carried out on the feasibility of using expansive soils as fill, including research on evaluating their bearing capacity and the field compaction control of expansive soil fill.Changes in California bearing ratio with moisture contentEvaluating the Bearing Capacity of Expansive SoilsSubgrade “stiffness” controls the total pavement thickness, especially with flexible pavements. So, it is very important to correctly test the stiffness of subgrade materials, and the conditions (density and moisture content) at which the material is tested should be considered. The CBR testing condition stipulated in China’s Test Methods of Soils for Highway Engineering (CMOC 1996b) involves compacting the material in a mold (0.152 m in diameter and 0.120 m high) and soaking for 4 days under a surcharge weight of 50 N, corresponding to 2.7 kPa. However, for clayey materials, this procedure leads to only the upper and lower parts of the sample becoming saturated or nearly saturated, because of the low permeability of clay, especially for expansive soils, and the air entrapped in the sample. The measuredCBR then only corresponds to the saturated soil in the shallow upper part, where the soil has disintegrated under the light surcharge after soaking (Uzan 1998).However, suppose expansive soil is used as fill in the lower embankment and nonexpansive fill material is used in the upper as a moisture barrier. Then, the surface of the lower embankment would not be soaked, and the upward pressure on the surface would no longer be 50 N. Therefore, the research group studied a modified CBR test on expansive soil simulating such field conditions. The study mainly focused on weathered mudshale, whose swell potential is medium, and which formeda large percentage of excavated material in the construction of the NanYou Highway.1.Li, S. 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