Effect of Shape of Footing and Water Table on Bearing Capacity of Soil

Transcription

1 Effect of Shape of Footing and Water Table on Bearing Capacity of Soil M. S. Dixit Research Scholar, Department of Civil Engineering, Government College of Engineering, Aurangabad (Maharashtra State), India, address: K. A. Patil Associate Pprofessor, Department of Civil Engineering, Government College of Engineering, Aurangabad (Maharashtra State), India, address: ABSTRACT Soil is a universally available natural material derived mostly from rocks and rocky minerals. As soil is a product of nature, possess an inherently variable and complex character. The bearing capacity is the most important soil property, which governs the design of foundation. Soft clay strata are often unable to bear the load transferred from the superstructure to the foundation. Bearing capacity and the settlement are the two important parameters in the field of geotechnical engineering. Civil engineering projects such as buildings, bridges, dams and roadways require detailed subsurface information as part of the design process. Bearing capacity is affected by various factors like change in level of water table, eccentric loads, inclined loads, dimensions of the footings, etc. The effect of water table on bearing capacity of soil determined by Terzaghi s method and IS Code method for rectangular footings are considered. KEYWORDS: Bearing capacity, Depth, water table, soil. INTRODUCTION Soil is considered by the engineer as a complex material produced by weathering of the solid rock. Soil is the most important material, which is in use for construction of civil engineering structures. Amongst all parameters, the bearing capacity of soil to support the load coming over its unit area is very important. There are various methods for calculation of bearing capacity of soil put forth by scientists like Prandtl, Terzaghi, Meyerhoff, Hansen, Vesic and others. Principal factors that influence ultimate bearing capacities are type of soil, width of foundation, soil weight in shear zone and surcharge. Structural rigidity and the contact stress distribution do not greatly influence bearing capacity. Bearing capacity analysis assumes a uniform contact pressure between the foundation and underlying soil. With other factors unchanged the type of failure of soil, depth of foundation, and effect of water table also governs the bearing capacity of soil. Soil is the most important factor in the construction world, in which the property of bearing loads coming upon has to be suitable. This property is the most significant one as the stability of the structure mostly depends on it

2 Vol. 18 [2013], Bund. G 1656 Due to increase in population and industrialization, there is increase in construction activities in the cities and industrial area. Hence, it has become necessary to carry out construction activities on marshy land, low lying area, expansive soil having swelling and shrinkage characteristics, water logged areas etc. Safe bearing capacity values are assumed depending upon type of soil encountered at proposed depth of foundation. In addition to properties of soil, width of foundation, depth of foundation, water table variation near the base of the footing, eccentricity of loading governs the ultimate and safe bearing capacity of soil. With all these factors into consideration the study was undertaken i.e. Effect of different parameters on bearing capacity of soil. Thus, based on investigations carried out, it will be possible to decide optimum depth of foundation for proposed structure, from economy and practical considerations. LITERATURE REVIEW Terzaghi was the first to propose a bearing capacity equation on the consideration of general shear failure in the soil below a rough strip footing. Using the principle of superposition, he demonstrated the effects of soil cohesion, its angle of internal friction, surcharge (soil lying above the level of footing base), soil unit weight and footing width on the ultimate bearing pressure. Later on, Brinch Hansen introduced factor that accounted for footing shape and load inclination, in the bearing capacity equation. K. Manjunath and A.S. Reddy studied the effect of depth and water table on bearing capacity of rectangular footing. They found that as depth of foundation increases, bearing capacity increases. They found that there is increase in dry density and decrease in optimum moisture content. They also studied the effect of depth of footing using geotextile reinforcement. Javad Hajiani and Nader Hataf carried out experimental and numerical investigation of the bearing capacity of model circular footing on reinforced sand. They investigated the bearing capacity of circular and ring footings on reinforced sand by conducting laboratory model tests along with numerical analysis. The effects of the depth of the first layer of reinforcement, vertical spacing and number of reinforcement layers on bearing capacity of the footings were investigated BEARING CAPACITY BASED ON LABORATORY TESTS Laboratory Tests The soil used in the study is collected from Shahnoorwadi, Aurangabad City of the proposed Mega, Nano Mall and Multiplex. The soils samples are collected at a depth of 3 m, 5m, and 6m. The aim of this work is to study the effect of different parameters on bearing capacity of the soil. The details of soil samples collected is shown in table 1 Table 1: Details of Soil Samples Collected Symbol Description of soil Depth of foundation in metre Soil-I Black cotton soil 3.0 Soil-II Kunkar soil 5.0 Soil-III Soft murum 6.0 Experimental work was planned to study the properties of soils collected for determination of ultimate bearing capacity of the soil. For the soils the basic properties such as specific gravity

3 Vol. 18 [2013], Bund. G 1657 were determined, sieve analysis was carried out and consistency limit i.e. liquid limit and plastic limits were determined for classification of the soil. Standard Proctor test and direct shear tests were conducted to determine maximum dry density, optimum moisture content, cohesion and angle of internal friction ( ). The test results of soils tested for these properties are as shown in table 2. Table 2: Geotechnical Properties of Soil Sr. No. Properties of Soil Samples Depth in meter Grain size distribution Gravel size (4.75 mm to mm) in % Sand size (0.075 mm to 4.75 mm) in % Silt and clay size (below mm) in % 2. Consistency limits Liquid limit % Plastic limit % Plasticity index % Non plastic 3. Specific gravity Compaction characteristics Maximum dry density in gm/cm 3 Optimum moisture content % 5. Shear parameters Cohesion in kn/m 2 Angle of internal friction ( )

4 Vol. 18 [2013], Bund. G 1658 Photograph 1: Sample of black cotton soil collected from Shahanoorwadi, Aurangabad at 3m depth Photograph 2: Sample of Kunkar soil collected from Shahanoorwadi, Aurangabad at 5m depth

5 Vol. 18 [2013], Bund. G 1659 Photograph 3: Sample of murum collected from Shahanoorwadi, Aurangabad at 6m depth Based on the laboratory test results and as per bureau of Indian standards the soil is classified as CH i.e. clayey soil with high compressibility. Soil III is classified as gravelly soil i.e. coarsegrained soil, containing majority of soil particles greater than gravel size. EXPERIMENTAL ANALYSIS Based on the laboratory experimentation carried out the values of, c and are obtained. These parameters are important to determine ultimate and safe bearing capacity of soil. Based on this data, effect of shape of footing, effect of depth of footing, effect of water table on bearing capacity of soil is studied and is discussed in following paragraphs. Effect of Shape of Footing on Bearing Capacity The shape of footing is an important parameter, which governs the ultimate bearing capacity of the soil. In general strip, square, rectangular and circular shaped footings are used. For soil-i by keeping other parameters constant, the effect of shape of footing on ultimate bearing capacity of soil is studied. The values of ultimate bearing capacity for soil-i is determined by methods given by Terzaghi and Bureau of Indian Standard. These values are tabulated in Table 3 Table 3: Effect of shape of footing on ultimate bearing capacity of Soil-I for local shear failure Method of analysis Shape of footing Strip Square Rectangular Circular Terzaghi I S code

6 Vol. 18 [2013], Bund. G 1660 In Table 3 it is found that, in case of local shear failure and by following Terzaghi s analysis the ultimate bearing capacity determined is highest for square shaped footing and least for strip footing. The square and circular shaped footings have fairly same values of ultimate bearing capacity. In case of IS code method, the ultimate bearing capacity determined is highest for square shaped footing and least one for strip footing. This effect of shape of footing on ultimate bearing capacity determined by methods given by Terzaghi and Bureau of Indian Standard is different for different footings, due to combined effect of values of shape factors and different values of bearing capacity factors. If comparison is made between ultimate bearing capacity determined by Terzaghi s method in case of local shear failure for soil-i for strip footing it is found to be 5.98% higher than that obtained by IS code method. Similarly for square shaped footing, rectangular shaped footing and circular shaped footing the ultimate bearing capacity values determined by Terzaghi s method are found to be higher than that obtained by IS code method by 12.91%, 9.60% and 13.23% respectively. Soil-II is a c - soil, having cohesion equal to kn/m 2 and an angle of internal friction equal to 25 o. For soil II by keeping other parameters constant, the effect of shape of footing on ultimate bearing capacity of soil is studied. The values of ultimate bearing capacity for soil-ii are determined by methods given by Terzaghi and Bureau of Indian Standard. These values are tabulated in Table 4. Table 4: Effect of Shape of Footing on Ultimate Bearing Capacity of Soil-II for Local Shear Failure Method of Shape of footing analysis Strip Square Rectangular Circular Terzaghi I S code In Table 4 it is found that, in case of local shear failure and by following Terzaghi s analysis the ultimate bearing capacity determined is highest for square shaped footing and least for strip footing.. In case of IS code method, the ultimate bearing capacity determined is highest for rectangular shaped footing and least one for strip footing. This effect of shape of footing on ultimate bearing capacity determined by methods given by Terzaghi and Bureau of Indian Standard is different for different footings, due to combined effect of values of shape factors and different values of bearing capacity factors. If comparison is made between ultimate bearing capacity determined by Terzaghi s method in case of local shear failure for soil-ii for strip footing it is found to be 12.8% higher than that obtained by IS code method. Similarly for square shaped footing, rectangular shaped footing and circular shaped footing the ultimate bearing capacity values determined by Terzaghi s method are found to be higher than IS code method by 13.4%, 5.55% and 13.8% respectively. Soil-III is a non-cohesive soil hence the type of failure possible is general shear failure. For non-cohesive soils the value of cohesion is less and hence neglected. Most of time, this cohesion is apparent cohesion. For soil-iii by using Tezaghi and IS code method, the values of ultimate bearing capacities are determined. These values obtained are tabulated in table 5. The values tabulated in table are in kn/m 2.

7 Vol. 18 [2013], Bund. G 1661 Table 5: Effect of Shape of Footing on Ultimate Bearing Capacity of Soil-III for General Shear Failure case Method of analysis Shape of footing Strip Square Rectangular Circular Terzaghi I S code In Table 5 it is found that, in case of Terzaghi s analysis the value of ultimate bearing capacity is lowest for circular footing and maximum for strip footing. This is due to combined effect of cohesion equal to zero and effect of shape factors while calculating the ultimate bearing capacity of the soil. Whereas in case of IS code method, the ultimate bearing capacity is maximum for square footing and minimum for strip footing. If comparison is made for strip footing the ultimate bearing capacity is maximum i.e kn/m 2 by Terzaghi s method and kn/m 2 by IS code method. In case of square footing the ultimate bearing capacity is maximum i.e kN/m 2 by IS code method and kn/m 2 by Terzaghi s method. In case of rectangular footing the ultimate bearing capacity is maximum i.e kn/m 2 by Terzaghi s method and kn/m 2 by IS code method. In case of circular shaped footing the ultimate bearing capacity is found to be kn/m 2 by Terzaghi s method and kn/m 2 by IS code method. Thus, it is found that for soil-iii for strip shaped footing the value of ultimate bearing capacity obtained by Terzaghi s method is found to be 14.5% higher than that obtained by IS code method of analysis. For square shaped footing the value of ultimate bearing capacity obtained by Terzaghi s method is found to be 0.780% lower than that obtained by IS code method of analysis. For rectangular shaped footing the value of ultimate bearing capacity obtained by Terzaghi s method is found to be 2.67% higher than that obtained by IS code method of analysis. For circular shaped footing the value of ultimate bearing capacity obtained by Terzaghi s method is found to be lower than that obtained by IS code method of analysis. Effect of depth of footing on bearing capacity The depth of footing is important parameters, which governs the ultimate bearing capacity of the soil. Ultimate bearing capacity is determined by using equation given by IS The shape of footing taken is rectangular footing with length =1.5 m and width = 1m.The values of ultimate bearing capacities determined by IS code method are as shown in table 6. Table 6: Effect of Depth of Rectangular Footing on Ultimate Bearing Capacity Type of soil Depth of foundation Ultimate bearing Type of failure in meter capacity kn/m 2 Black cotton soil Local shear failure Kunkar soil Local shear failure (44.35) Soft murum (73.95) General shear failure In Table 6, the values in parenthesis indicate the percentage increase in ultimate bearing capacity in comparison with 3 meter depth of foundation. Thus, for soil, having high compressibility having good value of cohesion and lesser angle of internal friction the percentage increase in ultimate bearing capacity in comparison with 3m depth, for depths of 5 m and 6m are found to be 44.35%, and 73.95% respectively.

8 Vol. 18 [2013], Bund. G 1662 Table 7: Effect of Depth of Rectangular Footing on Safe Bearing Capacity Type of soil Depth of foundation Safe bearing Type of failure in meter capacity kn/m 2 Black cotton soil Local shear failure Kunkar soil Local shear failure (44.30) Soft murum (71.54) General shear failure In Table 7, the values in parenthesis indicate the percentage increase in safe bearing capacity in comparison with 3 metre depth of foundation. Thus, for soil, having high compressibility having good value of cohesion and lesser angle of internal friction the percentage increase in safe bearing capacity in comparison with 3m depth, for depths of 5m, and 6m are found to be 44.30%, and 71.54% respectively. Effect of Water Table on Bearing Capacity The change in moisture content of the soil affects the properties of the soil. Similarly, if soil gets submerged its ability to support the load coming over its unit area is reduced when the water table is above the base of the footing, the submerged weight is used for the soil below the water table for computing the surcharge. The water table corrections are applied to determine the ultimate bearing capacity of the soil. The values of safe bearing capacities determined by using factor of safety of 3 by IS code method and Terzaghi s method for rectangular footing. Table 8: Effect of water table on safe bearing capacity of soil for rectangular footing by IS code method Type of soil Safe bearing capacity in kn/m 2 Type of failure Without water table correction Water table reach upto the base of the footing Water table reach upto the ground level Black cotton soil (13.5) (49.14) Local shear failure Kunkar soil Local shear (4.22) Soft murum (15.66) (42.26) (50.58) failure General shear failure From table 8, it is found that for soil-i having value of Ø equal to 19.5 o, the percentage decrease in safe bearing capacity of soil due to water table corrections is found to be 13.5% and 49.14%. For soil-ii having value of Ø equal to 25 o the percentage decrease in safe bearing capacity of soil due to water table corrections is found to be 4.22% and 42.26%. For soil-iii having value of Ø equal to 35 o the percentage decrease in safe bearing capacity of soil due to water table corrections is found to be 15.66% and 50.58%. Thus, it can be concluded that the effect of water table correction on safe bearing capacity is predominant for non-cohesive soil. Safe bearing capacity of non-cohesive soil is reduced to about 50 % when water table may reach up to ground level.

9 Vol. 18 [2013], Bund. G 1663 Table 9: Effect of Water Table on Safe Bearing capacity of Soils for Rectangular Footing by Terzaghi s Method Type of soil Safe bearing capacity in kn/m 2 Type of failure Without water table correction Water table reach upto the base of the footing Water table reach upto the ground level Black cotton soil (9.7) (33.7) Local shear failure Kunkar soil Local shear (1.3) Soft murum (15.30) (39.9) (50.56) failure General shear failure From table 9, it is found that for soil-i having value of Ø equal to 19.5 o, the percentage decrease in safe bearing capacity of soil due to water table corrections is found to be 9.7% and 33.7%. For soil-ii having value of Ø equal to 25 o the percentage decrease in safe bearing capacity of soil due to water table corrections is found to be 1.3% and 39.9%. For soil-iii having value of Ø equal to 35 o the percentage decrease in safe bearing capacity of soil due to water table corrections is found to be 15.3% and 50.56%. Thus, it can be concluded that the effect of water table correction on safe bearing capacity of soil is predominant for non-cohesive soil. Safe bearing capacity of non-cohesive soil is reduced to about 50% when water table may reach up to ground level. Geological Aspects The site of the proposed Mega, Nano Mall and Multiplex at Shahanoorwadi, Aurangabad lies on basic volcanic rocks of upper cretaceous to lower Eocene (about 120 m.y. before to 60 m.y. before) age these are well known as Deccan trap basalts. There are two main rock types in deccan trap basalts and they are compact basalt and amygdaloidal basalts. The site is present on the confluence and in between two westerly flowing nalas. The area is practically a plain area except negligible slope towards west. There are practically no exposures of rock due to the presence of thick overburden of weathered products of local rocks. All the weathering products are residual and no transported deposits. Excavation rectangular in shape and about 6m deep has been made at this site. The observations of the vertical cuts of this excavation revealed the existence of geological succession of this area. It is seen that the upper layer is black cotton soil. It is underlain by a layer of kunkar soil, below kunkar a layer of murum is present and at the bottom of the excavation weathered basalt is present. The thickness of black cotton soil varies from place to place. The average thickness of black cotton soil layer is about 2.5m to 3.0m. At some places it is upto 3.2 m. The thickness of kunkar soils is from 1.5 to 1.8m and of soft murum 0.5 to 1.0m. The rock present at the bottom of the excavation is weathered compact basalt. It is blackish grey in color and aphanitic in nature. It is practically free from gas cavities cooling joints are seen in the exposures of the rock and are three sets of broadly spaced joints. Black cotton soil which is the weathering product of basalts under hot and dry climate has a strength about 100 to 150 kn/m 2 when it is dry. In presence of water it changes its properties and becomes semi plastic to plastic and also losses its strength upto 90%. It does allow water to pass through it till gets saturated water. In presence of moisture it expands and in absence of moisture it contracts. As a result vertical tensional cracks are developed in black cotton soil. Due to all these reasons, it is undesired formation in the foundation excavation of civil engineering structure.

10 Vol. 18 [2013], Bund. G 1664 During the weathering of basalt lime (Caco 3 ) is formed from plagioclase present in basalt. This lime is sometimes brought to upper level by the subsurface water by capillary action on evaporation of the water, the lime is deposited in the upper part of the layer in the form of practicles or in the form of nodules. It is called as Kunkar. The soil containing kunkar is called as kunkar soil. The color of kunkar soil is dirty white. The soil present below the layer of black cotton soil is low grade soil without any cohesion amongst its particles. It allows to pass the water very quickly and also losses its strength in presence of moisture. However due to the presence of kunkar the cohesion between the particles increases and even vertical cuts in such layer stand stable. Due to weathering of basalt, first murum is formed and finally soil. In the intermediate stage of formation murum horizontal sheet joining is introduced and layer become highly porous and permeable, it holds ground water. Since the site is in between and on the confluence of two nalas the topographical situation is favorable for the presence of sub surface water. In addition to this for the reasons mentioned earlier, the ground water potential of this area is good. It is also revealed in this excavation and there is leakage of ample ground water is this excavation. Photograph 4: Layers of soil samples at Shahanoorwadi, Aurangabad (Maharashtra state)

11 Vol. 18 [2013], Bund. G 1665 Photograph 5: Layers of soil samples at Shahanoorwadi, Aurangabad (Maharashtra state)

12 Vol. 18 [2013], Bund. G 1666 Photograph 6: Layers of soil samples at Shahanoorwadi, Aurangabad (Maharashtra state) CONCLUSIONS Based on the studies carried out following conclusions are drawn: 1. The important parameters, which govern the bearing capacities of soils are: cohesion, unit weight of soil, depth of proposed foundation, width of foundation and angle of internal friction. 2. In case of local shear failure, the values of ultimate bearing capacity determined for circular and square shaped footings are found to be higher than strip and rectangular footings. This trend is different for non-cohesive soil. 3. As depth of foundation increases ultimate bearing capacity of soil increases. The effect of increase in depth on safe bearing capacity is predominant due to increase in surcharge weight. 4. In case of soil III which is gravelly soil i.e. coarser soil having negligible cohesion and angle of internal friction the percentage increased in ultimate bearing capacity in

13 Vol. 18 [2013], Bund. G 1667 comparison with 3m depth for depths of 5m, 6m are found to be % and 71.54% respectively. 5. For cohesive and frictional soil leading to local shear failure, the effect of water table correction on safe bearing capacity is less in comparison with non-cohesive soil. 6. The effect of water table correction on safe bearing capacity is predominant for noncohesive soil. Safe bearing capacity of non-cohesive soil is reduced to about 50 % when water table may reach up to ground level. REFERENCES 1. Hansen Brinch. (1961) A General Formula For Bearing Capacity, Danish Geotechnical Institute, Bulletin No. 11, Copenhagen, Denmark, Ingra, S.T. and Baecher G. B. (1983) Uncertainty in bearing capacity of sand, Journal Geotech Engg, ASCE, 109(7), Javad Hajiani, and Nader Hatef (2003) Experimental and Numerical Investigation of the Bearing Capacity of Model Circular and Ring Footings on Reinforced Sand, Geotextile and Geomembranes, Manjunath, K. A. S. Reddy (1997) Influence of Depth and Water Table on Bearing Capacity of Rectangular Footing, Soil and Foundations Vol.35, No.1, Japanese Geotechnical Society, More, D. M., Pathade, N.K., and Kulkarni, A. A. (2006) Bearing Capacity by plate load test, National Conference on Corrective Engineering Practices in Trouble some Soils, Kakinada, Nayak, N. V. (2001) Foundation Design Manual, Dhanpat Rai Publications Private Limited, New Delhi. 7. Purushothama Raj, P. (2008) Soil Mechanics & Foundation Engineering, Pearson Education, New Delhi. 8. Sridharan, A. and Srinivasmurthy, B.R. (1988) Shape and Size of Effect of Foundation on Bearing Capacity of Reinforced Soil Beds, Proceedings of Indian Geotechnical Conference, Allahabad, Terzaghi, K. (1943) Theoretical Soil Mechanics, Wiley, New York, USA 10. Temel Yetimoglu, Jonathan, T. H. Ahmet Saglamer, (1994) Bearing Capacity of Rectangular Footings on Geogrid-Reinforced Sand, Journal of Geotechnical Engineering. Vol.120. No , EJGE