Effect of Water Cement Ratio on Cohesion and Friction Angle of Expansive Black Clay of Gombe State, Nigeria

Transcription

1 Effect of Water Cement Ratio on Cohesion and Friction Angle of Expansive Black Clay of Gombe State, Nigeria Ochepo, J., Stephen O. D. and Masbeye, O. Department of Civil Engineering, A. B. U. Zaria, Nigeria ABSTRACT This paper examines the effect of water cement ratio on the cohesion and friction angle of expansive black clay of Gombe state in Nigeria. The black clay was obtained from Gombe state, and remolded with predetermined amount of water. The remolded sample was mixed with cement slurry which was prepared in a manner as to obtain cement contents of 4, 8, and 12% of the dry weight of the soil. The water-cement ratio (W/C) values used was in the ranged shear strength test were carried out on the specimen using shear box method, in order to determine the cohesion and friction angle of the cement admixed clay. Each specimen for the shear strength parameter test were prepared by placing the paste in a square metal mold 60mm x 60mm x 25 mm and carefully subjected to vibration so as to remove air bubble. The mold and the specimen were sealed up to prevent moisture loss and cured for 7, 14 and 28 days respectively. Results show that cohesion and friction angle decreased with increased watercement ratio at all cements content and curing age. Similarly, cohesion and friction angle increased with increasing cement content as well as curing age for all water cement ratio. Overall improvement on the cohesion and friction angle of the base soil at 0.4 water cement ratio, 12 % cement content and 28 days curing is about 81.8% and 70.2 % respectively. A W/C ratio of 0.4 and 12 % cement content is recommended for application in stabilization techniques such as the deep mixing method (DMM). KEYWORDS: Water-cement ratio, Black Cotton Soil, Angle of internal friction, Cohesion INTRODUCTION Expansive soils are soils which are characterized with cyclic movement (i.e. swelling and shrinkage due to seasonal variation). These cyclic behaviors are as a result of the dominant presence of expansive clay mineral, montmorillonite in the soils. The swelling and shrinkage behavior of the soil is usually in an uneven pattern and of such a magnitude as to cause extensive damage to the structure and pavements resting on them (Nelson and Miller, 1992). In Nigeria, one

2 Vol. 17 [2012], Bund. S 2600 problematic soil which exhibit expansive behaviors is the black clay (Black cotton soil) covering about 104,000 km 2 in the North-Eastern part of the country. Ola (1983) reported 70% montmorillonite in the Nigerian black cotton soils. The soil therefore swells excessively when it absorbs water during the wet season, exerting many kilo-newton per square area of swelling pressure and shrinks extremely, developing cracks, because of evaporation of water during the dry season. Adeniji, (1991) reported that cracks, often measuring 70 mm wide and 1.0 m deep may develop when the soil shrinks and may extend up to 3.0 m in case of high deposit. Similarly, when wet, the soil has high index properties, its bearing value and strength is low. This dual characteristic of the expansive soil causes distress in the structure founded on the soils. An estimate of the damages caused by expansive soils on civil engineering structures such as highway embankment and building in the United State is about one billion dollars annually. Worldwide, the cost is many billions of dollars (Gourley et al., 1993). In the Northeastern part of Nigeria where these soil are found, construction of roads generally poses a great problem because of high volume change, low bearing value and severe cracking during dry and wet season. The lack of good drainages and construction materials couple with large volume of traffic most of which are overloaded further complicates the problem. To control the severity of distress caused on structures founded on this soils, many different modification and stabilization techniques with additives such as lime and fly ash have been applied. (Osinubi, 1995; 1998; Basma et. al, 1991; Chummar, 1987; Desai et. al,1977); The effectiveness of lime and cement in treatment of cohesive soils with expansive properties have been reported, (Chen, 1988; Hausmann, 1990; Osinubi, 1995). These chemical additives have been found effective in improving expansive properties such as the control of volume change, increase strength, decrease plasticity index and swell and shrinkage strain potential of expansive soils and fine-grained cohesive soils. (Hausmann, 1990; Osinubi, 1998a,b; 1999a,b; Osinubi and Katte, 1997; 1999). In this study, the effect of water-cement ratio on shear strength parameter (cohesion and angle of internal friction) of expansive black clay from the Gombe state of Nigeria was evaluated. The shear strength tests were conducted using shear box method as described in BS (1990). MATERIALS, METHODS AND TEST RESULTS Materials Soil: The clay used in study was the expansive black cotton soil obtained from Deba in Gombe State in the North Eastern part of Nigeria. The soil was collected by method of disturbed sampling from a depth not less than 0.5 in order to avoid the top soil. The soil samples were placed in plastic bags and sealed to avoid loss of moisture during transportation. In the laboratory, the soil was dried and pulverized to obtain particles passing sieve through British Standard No. 4 sieve, (4.75 mm aperture). Cement: Ordinary Portland cement used for this study was obtained from the open market.

3 Vol. 17 [2012], Bund. S 2601 Index properties of base soil Methods The laboratory tests to determine the index properties of the natural soil (base soil) were conducted in accordance with British Standards BS 1377 (1990). The results are shown in Table 1. Preparation of cement-admixed clay The black cotton soil sample used for the direct shear test were remolded with predetermined amount of water. The remolded samples were then mixed with cement slurry which was design so as to obtain cement contents of 4, 8 and 12% of the dry weight of the soil. The water-cement ratio (W/C) values were in the range The remoulding water content (W), which is the water content of the remolded clay prior to the addition of cement slurry and the water content of the clay slurry was design so as to obtain a clay-water-cement mixture with water content equal to the optimum mixing clay water content ( C w,opt ) (Lorenzo et al., 2006). The optimum mixing clay water content ( C w,opt ) is defined as the total clay water content of the clay-water-cement mixture that would yield the highest possible improvement in strength of cured cement-admixed clay. Based on the results of unconfined compression and Oedometer compression tests, Lorenzo et al., (2006) reported that the C w,opt falls near the liquid limit of the base clay. Mixing was done using a hand mixer until there was a homogeneous clay-water-cement paste. Preparation of specimen and test Because of the high workability of the clay-water-cement paste, each specimen for the shear strength parameter test were prepared by placing the paste in a square metal mold 60mm x 60mm x 25 mm (which had its inside coated with a thin film of petroleum jelly to prevent adhering to the mold) and carefully subjected to vibration so as to remove air bubble. The mold and the specimen were sealed up to prevent moisture loss and cured for 7, 14 and 28 days respectively in the humidity room. After each curing period, the specimen were removed from the mold and placed in the shearbox. The shearbox was assemble with the two halves securely clamped together, fit into the base plate and placed securely in position in the carriage of the machine. The shearing of the specimen was carried out as described in BS (1990). Preliminary results and soil classification DISCUSSION OF RESULTS The index properties and particle size distribution of the natural soil are summarized and shown in Table 1 and Fig. 1 respectively. From the combine results of the Atterberg limits and the sieve analysis, the soil was classified as A-7-6 and CH in accordance with AASHTO and the Unified Soil Classification System (USCS) respectively. The values of the cohesion and angle of internal friction of the soil are 20 kn/m 2 and 11 0 respectively. Linear shrinkage and plasticity index values were 20 % and 50 %. Using the criterion for swelling potential and degree of expansion based on plasticity index and linear shrinkage after Chen (1988) and Altmeyer (1955) as given in Table 2, and this results shows that the degree of expansion of the soil is critical and its swelling potential is very high.

4 Vol. 17 [2012], Bund. S 2602 Table 1: Index Properties of Natural Soil Quantity Property Natural moisture content (%) Percent passing Sieve No. 200 Liquid Limit (%) Plastic Limit (%) Plasticity Index (%) Linear Shrinkage (%) Specific Gravity AASHTO Classification USCS Classification Cohesion C (kn/m 2 ) Angle of internal friction θ ( 0 ) Color Dominant clay mineral A-7-6 CH Grayish Dark Montmorillonite Table 2: Potential Expansiveness S/No Plasticity Index Swelling Linear Shrinkage Degree of Expansion (%) Potential (%) Low 0-5 Non-Critical Medium 5-8 Marginal High >8 Critical 4 35 and above Very high - - Percentage Passing (%) Particle Size (mm) Figure 1: Particle size distribution of natural black cotton soil Effect of water-cement ratio on cohesion of cement admixed soil The effects of water-cement ratio on cohesion of the cement admixed soil are shown in Fig.

5 Vol. 17 [2012], Bund. S a-c respectively. Generally it is observed that cohesionn decreased with increased water-cement ratio at all cement content and curing age. The decreasee in cohesion of the soil at higher water- and cement ration may be attributed to the hydration of cement which caused the flocculation coagulation of the soil particles into lager sized aggregates or grains with consequent reduction in the clay size particle of the soil. This reaction was fasterr at low water-cement ratio and decrease with increasing water-cement ratio. At high water-cement ratio, the high quantity of water results in weak bond between the soil and the hydration productss which bind the soil, thereby resulting in soft and less stiff material. Cohesion (kn/m 2 ) 7 Days Curing 14 Days Curing 28 Days Curing Water Cement Ratio Figure 2a: Variation of cohesion with water-cement ratio (at 4 % cement content) Cohesion (kn/m 2 ) 7 Days Curing 14 Days Curing 28 Days Curing Water Cement Ratio Figure 2b: Variation of cohesion with water-cement ratio (at 8 % cement content)

6 Vol. 17 [2012], Bund. S 2604 Cohesion (kn/m 2 ) 7 Days Curing 14 Days Curing 28 Days Curing Water Cement Ratio Figure 2c: Variation of cohesion with water-cement ratio (at 12 % Cement content) EFFECT OF CEMENT CONTENT AND CURING AGE ON COHESION OF CEMENT ADMIXED SOIL From the Fig.2a-c and Fig.3a-c below it is observed that there is a general increase in cohesion values with increasing curing age and cement contentt for all water cement ratio. This is in agreement with Bergado et al. (1996). It was reported from the results of undrained triaxial test that cohesion and friction angle increase with increasing curing time and cement content. Peak values of 40, 80 and 110 kn/m 2 were obtained for 28 days curing at 4 %, 8 % and 12 % cement conten respectively for 0.4 water cement ratio. These aree significant improvement from values of 35, 43 and 56 kn/m 2 respectively obtained for 7 days curing. The improvement on the 7 days cohesion is about 46 % and 50 % for 8 % and 12 % cement contents. Overall improvement on the cohesion of the base soil at 12 % cement content and 28 days curing for 0.4 water cement ratio is about 81.8%.

7 Vol. 17 [2012], Bund. S 2605 Figure 3a: Variation of cohesion with cement content (at 0..4 water cement ratio) Figure 3b: Variation of cohesion with cement content (at 0. 6 Water cement ratio)

8 Vol. 17 [2012], Bund. S 2606 Figure 3c: Variation of cohesion with cement content (at 0. 8 water cement ratio) Effect of water-cement ratio on friction angle of cement admixed soil The variation of water cement ratio with friction angle of the soil is shown in Fig.4a-c at different curing age and cement content. It is observed that for a given cement content and curing age, friction angle decreased as water cement ratio increased. Peak values of 18, 27 and 37 0 were obtained at 0.4 water cement ratio at 28 days curing period for 4, 8 and 12 % cement content respectively. Thesee values reduced to 16, 24 and 32 0 respectively at 0.8 water cement ratio. The decreases in friction angle are observed to be marginal. A possible explanation for the decreasee in friction values with increase in water cement ratio may be due to larger amount of water content making the material softer and less stiff, thus developing weak bonding between the binder and the soil particles.

9 Vol. 17 [2012], Bund. S 2607 Figure 4a: Variation of angle of internal friction with water-cementt ratio (at 4 % cement content) Figure 4b: Variation of angle of internal friction with water-cementt ratio (at 8 % cement content)

10 Vol. 17 [2012], Bund. S 2608 Figure 4c: Variation of angle of internal friction with water-cement ratio (at 12 % cement content) Effect of cement content and curing age on friction angle of cement admixed soil Figs.4a-c and Figs.5a-c show the effect of cement content and curing age on the friction angle of the soil. Generally, the friction angle increases with cement content and curing age. The increase in friction angle with increasing cement content may be due to reduction in clay size fraction as a result of ion exchange reaction between soils and cement which results in flocculation and coagulation of the soil particles into lager sized aggregates (Osinubi, 2001). On the other hand, the increase in friction angle with curing age may be duee to the secondary reaction (pozzolanic) between cement and the soil (Kedzi, 1979) ). This reaction involve the dissolution of silica and alumina which are inherently acidic, by the strong bases of cement compound from the clay minerals and amorphous materials on the surface of thee clay particles. The hydrated silica and alumina then gradually react with the calcium ion libratedd from the hydrolysis of cement to form a new insoluble compound whichh hardens when cured. The secondary reactions further deplete the amount of clay sized fraction in the soil resulting in higher amountt of aggregatee sized fraction whichh is bond in a strong matrix with the hydration products. this results are however not in agreement with Clough et al. (1981) and Balmer (1958)) who showed that internal friction angle remains relatively constant for cement treated soils regardless of cement content and curing time.

11 Vol. 17 [2012], Bund. S 2609 Figure 5a: Variation of angle of internal friction with cement content (at 0.4 % water cement ratio) Angle of Internal friction ( 0 ) 7 Days Curing 14 Days Curing 28 Days Curing Cement Content (%) Figure 5b: Variation of angle of internal friction with cement content (at 0.6 % water cement ratio)

12 Vol. 17 [2012], Bund. S 2610 Figure 5c: Variation of angle of internal friction with cement content (at 0.8 % water cement ratio) CONCLUSION From the results of the investigation carried out in this study, the following conclusions can be drawn: 1. Cohesion and angle of internal friction decreased with increased water-cement ratio at all cements content and curing age. 2. Similarly, cohesion and angle of internal friction increase with increased in cement content as well as curing age for all water cement ratio. 3. Overall improvement on the cohesion of the base soil at 0.4 water cement ratio, 12 % cement content and 28 days curing is about 81.8% and 70.2 % for angle of internal friction. 4. For application in stabilization techniques such as the deep mixing method (DMM), a water cement ration of 0.4 and 12 % cement content is recommended. REFERENCES 1. Adeniji, F. A. (1991), Recharge function of Vertisolic Vadose in sub-sahelian Chad Basin. Proc. of 1st Inter. Conf. on Aarid Zone Hydrogeology, Hydrology and Water Resouces, Maiduguri, pp Altmeyer, W. T. Discussion of Engineering Properties of Expansive Clays. Proc. of Am. Soc. Of Civil Eng. Vol. 81 (separate No. 658), 1955, pp

13 Vol. 17 [2012], Bund. S Balmer, G. G. (1958). Shear strength and elastic properties of soil-cement mixture under triaxial loading, Portland Cement Association Research and Development Laboratories. 4. Effect of Lime on Volume Change and Compressibility of Expansive Clays, TRR- 1295, pp Bergado, D. T., Anderson, L. R., Miura, N., and Balasubramaniam, A. S. (1996). Soft ground improvement, ASCE Press. 6. BS 1377, Method of Testing Soil for Civil Engineering Purposes. British Standard Institute, London, (1990). 7. Chen, F. H. (1988). Foundation on expansive soils. 2 nd ed., Elsevier, New York. 8. Chummar, A.V. (1987). Treatment of Expansive Soil Below Existing Structures with Sand Lime Piles, Proc. Fo sixth Int. Conf. on expansive soils, New Delhi, pp Clough, G. W., Sitar, N., Bachus, R. C., and Shafii-Rad, N. (1981). Cemented sands under static loading. Journal of The Geotechnical Engineering Division, ASCE, 107(GT6) Desai, I.D. and Oza, B.N. (1977), Influence of Anhydrous Calcium Chloride on the Shear Strength of Expansive soils,, Proc. of the First National Symposium on Expansion soils, HBTI-Kanpur, India, pp 4-1 to Gourley, C. S., Newill, D., and Schreiner, H. D. (1993). Expansive soils: TRL s Research Strategy. Proc., 1 st Int. Symp. Engineering Characteristics of Arid Soils. 12. Hausmann, M. N. (1990). Engineering principles of ground modification. McGraw- Hill, New York. 13. Kezdi, A. (1979). Stabilized Earth Roads. Elsevier Scientific Publishing Co., Amsterdam, the Netherlands. 14. Lorenzo, G. A., and Bergado, D. T. (2006). Fundamental characteristics of cementadmixed clay in deep mixing. ASCE Journal of Materials in Civil Engineering Vol. 18, No. 2, pp Nelson, D. J. and Miller, J. D (1992). Expansive soils: Problems and practice in foundation and pavement engineering, Wiley, New York. 16. Ola, S. A. (1983). Tropical soils of Nigeria in Engineering Practice Balkema Publishers, Rotterdam, Netherlands. pp Osinubi, K. J. (1995). Lime modification of black cotton soil. Spectrum Journal Vol. 2 Nos. 1 and 2, pp Osinubi, K. J. and Katte, V.Y. (1997). Effect of elapse time after mixing on grain size and plasticity characteristic. I: Soil-lime mixes. The Nigerian Society of Engineers Technical Transactions, Vol. 32, No. 4, pp Osinubi, K. J. (1998b). Influence of compaction delay on the properties of cementstabilised lateritic soil. Journal of Engineering Research, Vol. JER-6, No. 1, pp Osinubi, K. J. (1999a). Evaluation of admixture stabilization of Nigerian black cotton soil. The Nigerian Society of Engineers Technical Transactions, Vol. 34, No. 3, pp

14 Vol. 17 [2012], Bund. S Osinubi, K. J. (1999b). Influence of compaction delay on the properties of limestabilised lateritic soil. Journal of Engineering Research, Vol. JER-7, Nos. 1 &2, pp Osinubi, K.J. and Katte, V.Y. (1999). Effect of elapsed time after mixing on grain size and plasticity characteristic. II: Soil-cement mixes. The Nigerian Society of Engineers Technical Transactions, Vol. 34, No. 3, pp Osinubi, K. J. (2001). Influence of compaction energy levels and delay on cement treated soils. The Nigerian Society of Engineers Technical Transactions. Vol 35, No. 4, pp Rathmayer, H. Deep Mixing Methods for Soft Subsoil Improvement in the Nordic Countries. Proceedings of IS-Tokyo 96, The 2nd International Conference on Ground Improvement Geosystems, May 1996, Tokyo, 1996, pp Balkema ejge