TREATMENT OF EXPANSIVE CLAYS THROUGH COMPACTION CONTROL.
ABSTRACT: This paper presents the results of an experimental study to explore the effects of initial moisture content and compaction energy on swelling characteristics of expansive clay. Remolded samples of clay procured from an expansive soil area in Pakistan were prepared by compacting them at varying moisture content and dry density. The density of the samples was varied by changing the compacting effort in the range of standard and modified proctor methods with moisture content varying between 10 (Percent) and 25 (Percent) . Swell tests were carried out using standard odeometer apparatus through which swell potential (percent swell) and swell pressure of the compacted clay samples were determined. The results of the study revealed that the swell potential of the compacted samples decreases considerably when the compaction moisture content is increased at a particular density. On the other hand, the swell potential has an increasing trend with increasing compaction density at specific moisture content.
The swell pressure decreases with increases in moisture content and increases with increase in compaction density. Based on the experimental results, it is inferred that the compaction of swelling soils slightly on wet of optimum and avoiding over compaction may help mitigating their swelling potential which otherwise can cause severe distress to civil engineering structures, especially to lightly loaded structures.
Key words: compaction, swell potential, swell pressure.
Swelling clays are famous to cause severe damage to structures resting on them. They are considered potential natural hazard, which can cause extensive damage to civil engineering structures if not adequately treated. Such soils have caused severe distresses to various civil engineering structures, especially to lightly loaded structures such as single to double storey houses, pavements, walkways, floors, canal linings etc (Chen 1975). In Pakistan, there are many areas which are covered with such soils and have caused severe damages to lightly loaded structures (Farooq, 1996). The problems associated with expansive soils include heaving and cracking of various civil engineering facilities constructed on such soils. There are several methods that can be used to minimize or eliminate the detrimental effect of expansive soils before and after the construction of structures (Basma et al, 1998).
These methods include chemical stabilization, replacement with non swelling soil, prewetting, moisture and density control during compaction process, surcharge loading, mixing with non- swelling soil and use of geosynthetics. This study is aimed to investigate the effects of compaction moisture content and compaction energy on swelling behavior of expansive soils.
Improvement in the engineering properties of soil is always desirable whenever the soil is used as constructional material such as construction of earth structures like dams, backfill material for roads, runways and railways, embankments etc. There are numbers of methods available to improve properties of soil but the most economical and the simple method is by applying mechanical energy to the soil mass. Application of mechanical energy along with addition of specified amount of water results in the denser packing of the soil particles. Compaction has generally positive effects for ordinary soils but when swelling soils are encountered beneath the foundations of the structures, their over compaction may lead to upward heave, on subsequent addition of moisture, causing uplift of the structures especially the lightly loaded structures (Seed et al., 1962; Komornik, 1969).
The initial placement moisture content and density are important parameters in affecting the swelling behavior of swelling clays (El-Shobby and Rabba, 1981; Yevnin and Zaslavsky, 1970). Sridharan (2003) conducted series of swell tests on different clay samples compacted at optimum moisture content and different degree of compaction. The results of his investigation showed that both the swell potential and swell pressure increases as the compaction energy is increased. However, the compaction moisture content was kept equal to optimum moisture content (OMC) for each energy level. This paper presents the results of a comprehensive experimental study in order to investigate the effects of varying moisture content and compacting effort on swelling properties of expansive soils.
MATERIALS AND METHODS
Test Sample: The material used in this investigation was procured from Nandipur, an expansive soil area located about 50 km on north-east of historic city of Lahore- Pakistan. Field visit was carried out to collect the soil samples and to make visual observations. The visual observations of the site indicated the presence of surface cracks which is a typical indication of swelling soils. The disturbed and block samples were collected from near the surface. The block sample was used to find out natural moisture content (NMC) and in-situ density of the soil sample. Table-1 shows the physical properties of the test sample.
Four series of tests were conducted to explore the effects of compaction moisture content and compaction density on swelling behavior of the test soil. In each series of test, the moisture content was varied from 10 (Percent) to 25 (Percent) at a particular level of compaction density. Four different level of compaction density was selected by varying the compaction energy in the practical range of field compaction which is generally specified in term of standard or modified Proctor density. Four different levels of the compaction energy which was selected to vary the test density are summarized in Table 2. In order to determine the compaction parameters corresponding to compaction energy, compaction tests were conducted on soil sample imparting the specified compacting effort as presented in Table 2.
The results of the compaction tests for all four cases are summarized in Table 2. Modified compaction was performed according to ASTM D 1557 and the Standard compaction was carried out according to ASTM D 698. The two other compaction tests, i.e., Reduced Standard and Reduced Modified levels of energy are selected to obtain 90-95 (Percent) of the densities obtained through standard and modified Proctor tests, respectively. It can be observed from the test results (Table 2) that maximum dry density varies from 14.5 kN/m3 to 17.5 kN/m3 with corresponding optimum moisture content (OMC) varying from 25 (Percent) to 14 (Percent) when compaction energy is varied from 202 kN/m3 to 2556 kN/m3. The material is the same in all four cases but its compaction parameters change considerably due to variation in compactive effort. The results of the compaction tests also validate the fact that with increase in compactive effort, the maximum dry density increases and the respective OMC value decreases.
Table: 1 Physical properties of test sample
Characteristics###Values and Description
Natural moisture content (Percent)###7.0
Field dry unit weight (kN/m3)###17.7
Gravel content (Percent)###0
Sand content (Percent)###2
Silt content (Percent)###58
Clay Content (Percent)###43
Liquid limit (Percent)###55
Plastic limit (Percent)###23
Unified Soil Classification Symbol###CH
Table: 2 Compaction Test Conditions and Compaction Test Parameters
Compaction level###No.of###No of###Weight of###Height of###Compaction###dmax###OMC
Swell test: As mentioned earlier, four series of swell tests were conducted. For each series, the samples were compacted at specific energy level as given in Table 2 but varying the compaction moisture content from 10 (Percent) to 25 (Percent) . Standard consolidometer was used to perform the swell test. Many researchers (Komarnik and David 1969; Mowafy and Bauer 1985) have described two parameters to evaluate the swelling properties of expansive soils, which are swell potential and swell pressure. Swell potential is defined as the percentage increase in relation to the original height of the specimen, whereas the swell pressure is designated as the pressure which is required to prevent the swelling of the sample. The swell potential of each test specimen was determined by placing the remolded specimen in oedometer ring (1.5 x 0.75 inch) with porous stones placed on the top and bottom of the specimen. The oedometer ring was assembled in consolidation frame.
The specimen was then loaded to a seating pressure of 7 kPa so as to seat the loading platen on the sample properly. The specimen was then flooded with water and allowed to swell under the seating load. Deformation readings were taken at 0, 1, 2, 4, 8, 15, 30,60, 120, 240, 480, 1440 min, and then every 4 hr on subsequent days until no further changes in readings were observed and full swell was attained. The increase in vertical height of the sample, expressed as a percentage of its original height, due to increase in moisture content was designed as the swell potential.
The swell pressure of the specimen was determined by loading the swelled specimen in various increments (Sridharan et al., 1986). In each increment of loading, complete consolidation of the specimen was ensured by giving sufficient time after the application of incremental load. The specimen was loaded until whole of the swell of the specimen is reduced to zero and the specimen attained its original height. At this stage, the swell pressure was calculated as the load required in bringing the specimen to its original height divided by the area of the specimen.
RESULTS AND DISCUSSION
Fig. 1 depicts the typical time history of swell potential of samples compacted at four moisture content levels (10 (Percent) - 25 (Percent) and modified compaction energy. Similarly, time histories of swell potential for different energy levels have also been observed. It is evident from all the figure that the rate of development of swell potential is higher in the initial period of time and then gradually diminishes and becomes almost negligible when the time elapsed is exceeded 24 hours. Fig. 1 reveals that the swell potential decreases rapidly as the compaction moisture content increases. However at specific compaction moisture content, the swell potential increases as the compaction energy increases. These observations can be better seen in Fig.2 which summarizes the results of all four series of tests.
At the lowest compaction moisture content (10 (Percent), the swell potential varies from 8.3 (Percent) to 5 (Percent) when the compaction energy level changes from modified Proctor (2556 kJ/m3) to rduced standard (202 kJ/m3).
In case of highest moisture content (25 (Percent) , the swell potential is reduced to less than 1 (Percent) for all compaction energy levels.
In Fig.2, swell potential vs compaction energy level line has also been shown at respective optimum moisture content. The OMC values for each compaction level have been indicated on the line within brackets. From the line of OMC in Fig. 2, it can be readily observed that at a specific compaction energy level, for compaction moisture content which is more than OMC, the swell potential tends to be significant and as the compaction moisture is more than OMC the swell potential decreases. This observation implies that if the field compaction is performed slightly on wet of optimum, the swell potential of the compacted soil may be controlled.
The effect of compaction energy on the swelling pressure for different compaction moisture content is presented in Fig. 3. For all moisture content except 25 (Percent) , the swell pressure increases with increase in compaction energy level. For a particular energy level, the swelling pressure decreases with increase in moisture content, e.g. swell pressure varies from 280 kPa to 20 kPa when the moisture content is varied from 10 to 25 (Percent) for modified compaction energy (2556 kJ/m3).
Similar decreasing trend of swell pressure with increase in moisture content for other energy levels can be observed from the same figure. It is noteworthy from Fig. 2 and 3 that at 25 (Percent) moisture content, the values of swelling potential and swelling pressure, respectively, are almost negligible.
Fig. 4 and Fig. 5 represent the dry density versus swell potential and swell pressure respectively. It is evident from the graph that at 10 (Percent) moisture content, when dry density is maximum, the corresponding swell potential and swell pressure values are also maximum. As the moisture content increases there is decrease in swell potential and swell pressure. Swell potential decreases from 8.37 (Percent) to about 0.63 (Percent) with change in moisture from 10 (Percent) to 25 (Percent) . Similarly swelling pressure decreases from 280 kPa to 20 kPa. Swell potential decreases about 92.5 (Percent) by varying the moisture content from 10 to 25 (Percent).
Fig. 6 plots the swell potential vs swell pressure for all the tests performed. It can be observed from the figure that linear relationships may be approximated between swell potential and swell pressure irrespective of the compaction energy level and the compaction moisture content. The linear regression analysis indicates that the swell pressure is 32.5 times the swell potential. This finding is in line with the findings of Sridharan and Gurtug (1992) that swelling pressure is 48.32 times the swell potential for swelling pressure in the range of 1000 kPa.
Implications in field compaction: The results of the study have clearly revealed that the swelling properties of expansive soils are significantly affected by compaction moisture and density. As the compaction moisture content increases, both the swell potential and swell pressure decreases. This implies that the swelling behavior of the expansive clays can be controlled by controlling their compaction moisture content and the density. This aspect can be illustrated schematically with the help of Fig. 7. The in-situ compaction density for various earth structures is generally specified as 95 (Percent) of maximum dry density (gdmax) achieved through standard Proctor or modified Proctor test depending upon the loading conditions. As indicated in Fig. 7, the specified compaction density (say 95 (Percent) of gdmax) may be achieved at moisture content ranging from w1 to w2. The w1 is dry of optimum and w2 is wet of optimum.
In compacting normal soils, achieving in-situ density higher than the specified level is generally considered beneficial as it improves the engineering properties of the soil. Similarly the compaction moisture content to be dry of optimum is considered better for loading bearing earth structures. However, in case of expansive soils, this philosophy is entirely different. Compacting expansive soils at densities higher than specified and at moisture contents which are dry of optimum favors enhancement of their swelling potential. The results of the present investigation suggest that the swelling soil should be compacted on wet of the optimum and further, over-compaction than specified level should be avoided. As shown in Fig. 7, compacting the expansive soils with compaction moisture content and dry density falling in shaded area may help mitigating their swelling potential which otherwise can cause severe distress to civil engineering structures in general and to lightly loaded structures in particular.
Conclusions: From the results of this research, following conclusions can be made.
* From the results of compaction tests at four different compaction levels, it is concluded that the maximum dry density increases from 14.5 kN/m3 to 17.5 kN/m3 and optimum moisture content decreases from 25 (Percent) to 14 (Percent) when the compaction energy is increased from 225 kJ/m3 (reduced standard) to 2556 kJ/m3 (modified Proctor), respectively.
* For all compaction energy levels, the swell potential (percent swell) significantly decreases as the remoulding moisture content is increased, e.g., for lowest energy level (225 kJ/m3), the swell potential decreases from 5 (Percent) to 0.26 (Percent) and for highest energy level (2556 kJ/m3), the swell potential decreases from 8.4 (Percent) to 0.63 (Percent) when the compaction moisture content is varied from 10 (Percent) to 25 (Percent) , respectively.
* Like swell potential, the swell pressure also decreases with the increase of compaction moisture content, e.g., for lowest energy level (225 kJ/m3), the swell pressure decreases from 115 kPa to 8 kPa and for highest energy level (2556 kJ/m3), the swell pressure decreases from 280 kPa to 20 kPa when the compaction moisture content is varied from 10 (Percent) to 25 (Percent) , respectively.
* For test samples compacted at respective OMC, the percent swell decreases from 6 (Percent) to 0.26 (Percent) and the swell pressure decreases from 240 kPa to 10 kPa when the respective compaction maximum dry density is varied from 17.5 kN/m3 to 14.5 kN/m3.
* Irrespective of remoulding moisture content and dry density, there is linear relationship between swell pressure and swell potential. The regression analysis of the swell test data indicates that the swell pressure is 32.5 times the swell potential.
* In field, compacting the expansive soils at moisture content slightly wet of OMC and avoiding over compaction may help mitigating their swelling potential which otherwise can cause severe distress to civil engineering structures in general and to lightly load structures in particular.
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Department of Civil Engineering, University of Engineering and Technology, Lahore, Pakistan Corresponding Author e-mail: email@example.com