VARIATION OF STRENGTH OF COHESIVE SOIL WITH MOISTURE CONTENT AND TIME

Transcription

1 5th 5th INDIAN GEOTECHNICAL CONFERENCE 17th 19th DECEMBER 215, Pune, Maharashtra, India VARIATION OF STRENGTH OF COHESIVE SOIL WITH MOISTURE CONTENT AND TIME J.Maity1, B.C.Chattopadhyay2 ABSTRACT Strength of compacted soil greatly depends on the moisture content at which soil has been compacted. Subgrade is constructed at a designed placement moisture content and density, before the placement of the road structure and it is understood that the subgrade will retain its engineering properties at placement condition with time during which road is open to traffic. Satisfactory performance is thus possible, only if, placement strength and deformation characteristics of the subgrade soil remain unaffected with time. Maintenance of strength with time is thus very important criteria for satisfactory performance of the road itself. However little quantitative information is available in literature regarding nature of variation of strength of compacted soils of various characteristics with time. The subgrade of roads used below both flexible and rigid pavements are made by placing suitable soil in specific thickness and compaction, laid in layers after mixing specified water so that moisture content of mixed becomes equal to Optimum Moisture Content (OMC) and compacted to reach specified maximum dry density as achieved in laboratory by standard compaction test. In India strength of such soils are generally measured by California Bearing Ratio (CBR) value. For laid subgrade in worst condition of submergence CBR value are determined in laboratory. The efficacy of adopted road design depends directly on the value of CBR achieved while placing the subgrade and stability of road structure becomes totally dependent on maintaining the CBR value when the road is open to traffic under all weather condition. Most of the damages caused in pavements may be traced due to loss of strength of the subgrade which may be caused by several reasons. One of the common reasons may be due to variation of moisture content of the subgrade from central portion of the road to edges under adverse seasonal effect due to flooding or heavy raining. The other major cause is due to loss of strength of the subgrade soil with time. Apart form strength loss there can be decrease in the deformation modulus of the soil with such variation in moisture content or with aging effect. In view of above a detail investigation is initiated to study the strength and corresponding initial tangent modulus of the locally available silty clayey soil. Proctor compaction test has been conducted to determine the maximum dry density (MDD) and optimum moisture content (OMC) of the chosen cohesive soil. From the compaction curve, six moisture contents have been selected, i.e. one at OMC (.5%), two moisture contents (22.5 and 23.5%) on dry side of OMC and three moisture contents (27, and 33%) on wet side of OMC. With these six moisture contents, several numbers of unconfined compressive strength (UCS) tests have been conducted at different time interval of day, 1 day, 7days, 14 days, days, and days. From each test, stress-strain plots have been made to find UCS value and 1 2 Maity_J, Civil Engg. Dept, MSIT, Kolkata, India, joymaity1975@yahoo.co.in. Chattopadhyay_B.C., Civil Engg. Dept, MSIT, Kolkata, India, ccbikash@yahoo.com.

2 J.Maity, & B.C.Chattopadhyay initial tangent modulus value. These results have been analyzed in this paper to study the effect of time and moisture content on the strength and the initial tangent modulus of the soil. Keywords: Subgrade, California Bearing Ratio test, Unconfined compressive strength test, Aging effect, Initial tangent modulus.

3 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India VARIATION OF STRENGTH OF COHESIVE SOIL WITH MOISTURE CONTENT AND TIME J.Maity, Assistant Professor, Civil Engg. Dept, MSIT, Kolkata -15, joymaity1975@yahoo.co.in. B.C.Chattopadhyay, Professor, Civil Engg. Dept, MSIT, Kolkata ccbikash@yahoo.com. ABSTRACT The loss of strength of the subgrade may be due to variation of time and moisture content of the subgrade under adverse seasonal effect. Besides this there is also decrease in deformation modulus of the soil with moisture content and time. A detail investigation is initiated to study the strength and corresponding initial tangent modulus of clayey soil. Several numbers of unconfined compressive strength (UCS) tests have been conducted at different time interval with different moisture contents. From the results of each test, the effect of time and moisture content on the strength and the initial tangent modulus of the soil have been studied. INTRODUCTION Quick and cost effective infrastructure development and maintenance-free long life of such infrastructure are essential for economic growth of a country like India. Through different government schemes like Pradhan Mantri Gram Sadrak Yojona, Golden Quadrilateral etc in India over last two decades huge amounts of construction of roads are already made and being made for economic growth of the country. Though majority of roads constructed in India are flexible road and small percentage of road are made of rigid pavement because of high initial cost involvement in construction. However both the types of road require proper subgrade. Subgrades are generally constructed with locally available cohesive soil by placing such soil at placement dry density near maximum dry density at optimum moisture content. The value of maximum dry density and optimum moisture content generally are found in laboratory as per standard codal provision[1]. Design and construction of pavement structure over the subgrade is generally based on the California Bearing Ratio (CBR) of the subgrade, in soaked condition [2]. However performance of pavements depends not only on the quality of the subgrade but also on the uniformity of quality of subgrade over the whole section during service period. As road structure is open to nature, they can be subjected to extreme variation in atmospheric condition like temperature variation, rainfall of wide range, pollution etc. In many cases subgrade is subjected to moisture migration due to variation of moisture content after placement particularly during rainy or dry temperature seasons or time effect. The sub-grade moisture near the edges becomes different from that in the central area. During winter moisture content near the edges is lesser than the central portion. As a result the compacted soil making the sub-grade is subjected to variation in moisture content and its engineering property will vary. As a result modulus of deformation may also change considerably in the sub-grade both in space and time. Similarly maintenance of the strength of compacted soil, developed during placement, with time is also very important for satisfactory performance of the road sub-grade under loading.

4 J.Maity, & B.C.Chattopadhyay Thus, performance of flexible and rigid pavement is directly dependent on maintenance of the strength and modulus of deformation with time after placement. However little information on variation of deformability under moving loads on road surface on the sub-grade soil with time or moisture content variation are available. It is felt that detailed experimental investigation for study the variation of initial tangent modulus of the compacted cohesive soil of different placement moisture of content and also to investigate the effect of time i.e., ageing on the change of above modulus may be useful in actual practice. OBJECTIVES In the present investigation, the variation of the strength of compacted soil and corresponding initial tangent modulus with moisture content and time elapsed has been studied. Soil has been compacted under standard Proctor test at various chosen moisture contents, and then at each moisture contents several identical soil samples have been subjected to unconfined compressive strength test at different time intervals after preparation of sample for the study of the effect of ageing on strength and initial tangent modulus value. MATERIALS For this experimental study, a blackish gray clayey soil is chosen and has been collected from the area near Mechheda, in East Midnapur, West Bengal. The soil is classified as CH, as per IS soil classification [3]. The physical properties of clay as determined according to IS codal provision are given in Table 1. Table 1: Summary of Physical Properties of Clay Colour Brown Classification (IS) Effective diameter, D1 (mm) Coefficient of uniformity, Cu Liquid Limit (%) Plasticity Index (%) CH Maximum dry density (gm/cc) 1.5 Optimum moisture content (%).5 UCS (N/cm2) at OMC 11 METHODOLOGY In this study, six numbers of moisture contents have been chosen 22.5%, 23.5%,.5%, 27%, %, and 33% respectively. At these chosen moisture content, soil samples have been compacted in Proctor s mould and seven number of identical compacted soil samples were extracted and marked with dates. For obtaining cylindrical soil samples three tubes were inserted inside the Proctor mould and machine extractor was used for extracting the samples from the tubes. Then all samples were put into plastic bags with labeling and were kept in several numbers of desiccators for preserving them for UCS test. Proctor compaction test has been conducted to determine the maximum dry density (MDD) and optimum moisture content (OMC) of the chosen cohesive soil. From the determined compaction curve, six moisture contents have been selected, i.e. one at OMC (.5%), two moisture contents (22.5 and 23.5%) on dry side of OMC and three moisture contents (27, and 33%) on wet side of OMC. With these six moisture contents, several numbers of unconfined compressive strength (UCS) tests have been conducted at different time interval of day, 1 day, 7days, 14 days, days, and days. From each test, stress-strain plots have been made to find UCS value and initial tangent modulus value. These results have been analyzed to study the effect of time and moisture content on the strength and the initial tangent modulus of the soil. The UCS tests [4] have been performed on clayey soil for various moisture contents and time intervals is given in Table 2. Table 2: Details of various moisture contents and time intervals. Soil type Cohesive soil Moisture content (%) 22.5, 23.5,.5, 27, and 33. Time interval (day), 1, 7, 14,, and. RESULTS AND DISCUSSIONS From results of Unconfined Compressive Strength tests conducted on clayey soil for different moisture contents and time intervals, stress vs strain curves have been drawn. The values of UCS and also the initial tangent modulus have been obtained directly from these curves for placement

5 5th 5th INDIAN GEOTECHNICAL CONFERENCE 17th 19th DECEMBER 215, Pune, Maharashtra, India water content changing from 22.5% to 33% at different time intervals of day, 1 day, 7 days, 14 days, days, and days, after preparation of the samples. The values of UCS and initial tangent modulus of compacted soils at different moisture content with different time intervals in days after compaction are given in table 3 and 4 respectively. with water content for all cases of different time interval. The rate of decrease is higher at initial stage and gradually lower with increase of time interval. day 1 day 26 7 days 14 days 22 Table: 3 Values of UCS (qu) of clayey soil at different water content (%) for different no. of days Unconfined Compressive Strength (qu) in N/cm2 22.5% 23.5%.5% 27% % 33% Table: 4 Values of Tangent Modulus (Ei) of clayey soil at different water content (%) for different no. of days No. of Days % Tangent Modulus (Ei) in N/cm2 23.5%.5% 27% % % The discussions are presented on those results in a sequential manner as given below(a) Variation of UCS value with water content: The variations in the value of unconfined compressive strength (qu) of the chosen soil samples with water content are plotted for different time intervals of of day, 1 day, 7 days, 14 days, days, and days, after preparation of the samples, is shown in figure 1. The result shows that the UCS values of the clayey soil decrease UCS (N/cm2) days Water content (%) Fig: 1. Variation of Unconfined compressive strength (UCS) with water content (b) Variation of UCS value with time period: The variations in the value of unconfined compressive strength (qu) of the chosen soil samples with time period, plotted for different moisture contents of 22.5%, 23.5%,.5%, 27%, %, and 33%, is shown in figure 2. The result shows that the values of the UCS value of clayey soil increases with time period for all cases of different water contents. Water content = 22.5% Water content = 23.5% Water content =.5% Water content = 27% Water content = % Water content = 33% UCS (N/cm2) No. of Days days No of Days 2 Fig: 2. Variation of Unconfined compressive strength (UCS) with No. of days

6 J.Maity, & B.C.Chattopadhyay 1 Water content = 23.5% Water content =.5% Water content = 27% Water content = % Water content = 33% day 1 1 day 9 7 days 8 Tangent Modulus (N/cm 2) Water content = 22.5% 1 Tangent Modulus (N/cm 2) (c) Variation of tangent modulus (Ei) with water content: The variations in the value of initial tangent modulus (Ei) of the chosen soil samples with water content are plotted for different time intervals of day, 1 day, 7 days, 14 days, days, and days after preparation of the samples, is shown in figure 3. The result shows that the values of the initial tangent modulus of clayey soil are decreases with water content for all cases of different time interval. The rate of decrease is higher at initial stage and gradually lower with increase of time interval. 14 days days 7 days Water content (%) Fig: 3. Variation of Initial Tangent Modulus (Ei) with water content (d) Variation of tangent modulus (Ei) with time period: The variations in the value of initial tangent modulus (Ei), of the chosen soil samples with time period are plotted for different moisture contents of 22.5%, 23.5%,.5%, 27%, %, and 33%, is shown in figure 4. The result shows that the values of the initial tangent modulus of clayey soil increases with time period for all cases of different water contents No of Days 2 Fig: 4. Variation of Initial Tangent Modulus (Ei) with No. of days CONCLUSIONS From the experimental study following conclusions are drawn: 1. Unconfined compression strength (UCS) of the clayey soil decreases with water content for all cases of different time interval. The rate of decrease is higher at initial stage and gradually lower with increase of time interval. 2. The UCS value of clayey soil increases with time period for all cases of water contents ranging from 22.5% to 33%. 3. The initial tangent modulus of clayey soil also decreases with increasing water content for all cases of different time interval. The rate of decrease is higher at initial stage and gradually lower with increase of time interval. 4. The value of the initial tangent modulus of clayey soil increases with time period for all cases of different water contents. REFERENCES 1. BIS 272 (Part VII) -198, Determination of Water content Dry density relation using light compaction, Bureau of Indian Standards, New Delhi, India. 2. IRC 37, 21, Guidelines for the design of flexible pavements, The Indian Roads Congress, New Delhi, India.

7 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India 3. BIS: 1498, 197, Classification and identification of soils for general engineering purposes, Bureau of Indian Standards, New Delhi, India. 4. BIS: 272 (Part X), 1973, Determination of Unconfined Compressive Strength, Bureau of Indian Standards, New Delhi, India.