Geological and Geotechnical Investigations for Structure

Transcription

1 Indexed in Scopus Compendex and Geobase Elsevier, Chemical Abstract Services-USA, Geo-Ref Information Services-USA ISSN , Volume 05, No. 02 April 2012, P.P Geological and Geotechnical Investigations for Structure M. S. DIXIT 1 and K. A. PATIL 2 1 Government College of Engineering, Aurangabad (M. S.) Civil Engineering Department, Government College of Engineering, Aurangabad (M. S.) manishsdixit@gmail.com, kapatil111@yahoo.com Abstract: Any structure constructed on the earth is supported by the soil underlying it. Foundation is an interfacing element between superstructure and the underlying soil that transmits the loads supported by the foundation and its self weight. Foundation design requires evaluation of safe bearing capacity both immediate and long term settlement. The weak and compressible soils are subjected to problems related to bearing capacity and settlement. Soil is considered by the engineer as a complex material produced by the weathering of the solid rock. Soil being a product of nature, its properties may vary from place to place and it posses an inherently variable and complex character. Soil is the most important factor in the construction of civil engineering structures. Soil provides the base for the structure. The performance of soil as a construction material depends mostly on local environmental conditions and structural conditions. During construction of any structure the properties of soil play very important role. The most important soil property in concern with civil engineering is its bearing capacity. In construction of any structure most important components are the supports on which the structure stand and the soil which provides base for it. The soil sample and the core sample is collected from Latur and laboratory investigations were being carried out on the soil and rock samples. The paper deals with the study of effect of different parameters on bearing capacity of soil and allowable rock pressure. Keywords: soil, bearing capacity, allowable rock pressure, core sample. 1. Introduction: 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 loads from a superstructure are required to be transferred properly to the deeper strata of soil through foundation. During the load transferring process, due to the stress developed in the soil particle and the shear strength of the sub-soil, the strata starts to mobilize. Hard soil strata can bear sufficient load from foundation due to high shear strength and settlement occurs very slowly due to low compressibility whereas soft soil strata has low shear strength and settlement is fast due to high compressibility when load is transferred from the foundation. The problem of constructing highly loaded structures on soft soil can be overcome by adopting ground improvement techniques. 1.1 Literature Review: Bruce L kutter, et.al (1) studied the strength parameters for bearing capacity of sand. They used the result of centrifuge tests and triaxial tests to test the validity of the methods for determining the friction angle (φ ) for bearing capacity calculations. Huang and hong (2) proposed a method for predicating bearing capacity ratio of reinforced sand with high tensile stiffness reinforcement at ultimate footing load. They reported the linear relationship between settlement reduction factor and bearing capacity ratio. Junhwan lee and Rodrigo salgodo (3) estimated the bearing capacity of circular footings on sand based on cone penetration test. They presented the values of normalized limit unit bearing capacity both in the form of equation and chart. They also provided the values for settlement of 25 mm for various types of soil. More D.M. et.al (4) carried out the studies of bearing capacity by using plate load test. They pointed out that the bearing capacity of the foundation depends upon various factors such as depth, water table condition and presence of weak layers. They also narrated that the # Copyright 2012 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.

2 215 M. S. DIXIT and K. A. PATIL bearing capacity determined by the actual plate load test and that by the empirical formulae are not comparable. Perkins and madson (5) presented a design approach to bearing capacity determination based on the relative density approach. The approach was calibrated using bearing capacity results from studies carried out for a period of twenty years. Shiva Datta et.al (6) determined the bearing capacity of cobble-gravel mixed soil strata. The load settlement behaviour of cobble gravel dominated strata largely depends upon the amount and type of filter material i.e. sand, silt and clay present in the strata. 1.2 Bearing Capacity based on Laboratory Tests: Laboratory Tests: The soil used in the study is collected from MIDC Area Latur, Maharashtra State. The soils samples are collected at a depth of 1 m, 1.25 m, 1.5 m, 1.75 m, 2 m, 2.25 m and 2.50 m and core samples are collected at a depth of 3 m, 4m, 5m and 6 m. The aim of this work is to study the effect of different parameters on bearing capacity of the soil and allowable rock pressure. 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 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 (φ ). The test results of soils tested for these properties are as shown in table 1. 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. 1.3 Experimental Analysis: Based on the laboratory experimentation carried out the values ofγ, c and φ are obtained. These parameters are important to determine ultimate and safe bearing capacity of soil. Based on this data, effect of depth of footing, is studied and is discussed in following paragraphs 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 2. Figure 1 shows effect of depth of footing on ultimate bearing capacity for soil for local shear failure and general shear failure case. In table 2, the values in parenthesis indicate the percentage increase in ultimate bearing capacity in comparison with 1 metre depth of foundation. The percentage increase in ultimate capacity in comparison with 1 metre depth, for depths of 1.25m, 1.5m, 1.75m, 2m are found to be 6.02%, 25.61%, 41.92%, 66.69% respectively up to 2m depth it is a local shear failure and more then 2m depth it is a general shear failure. Figure 2 shows effect of depth of footing on ultimate bearing capacity for soil for local shear failure and general shear failure case. In table 3, the values in parenthesis indicate the percentage increase in ultimate bearing capacity in comparison with 1 metre depth of foundation. The percentage increase in ultimate capacity in comparison with 1 metre depth, for depths of 1.25m, 1.5m, 1.75m, 2m are found to be 3.74%, 29.78%, 47.50%, 73.13% respectively up to 2m depth it is a local shear failure and more then 2m depth it is a general shear failure. 1.4 Determination of Allowable Rock Pressure for Foundation: Geology of the Site: The site of the proposed industry is M.I.D.C. area Latur, Dist-Latur( Maharashtra)lies on upper cretaceous to lower Eocene age and which are commonly known as deccan trap basalts. The main rock types of deccan trap basalt are i)compact basalt and ii)amygdaloidal basalt. These two types of rocks are occasionally associated by the rocks like tachylite and volcanic breccia. From civil engineering point of view amygdaloidal basalt is most suitable rock type. This is because it is almost free from cooling joints, the gas cavities which are present in amygdaloidal basalt are invariably filled-completely or partially by secondary minerals like zeolite, quartz, calcite etc. it is having fairly good strength also. The gas cavities are not interconnected. Thus the rock is almost impervious with sufficiently high strength, it therefore serves good foundation grade rock. Compact basalt is having higher strength than amygdaloidal basalt. Tachylite is basaltic glass, it is redish, greenish or blackish in colour.it may be present in the form of pockets,layers,of about 0.5 to 1.0m thickness or in the form of veins. It is closely jointed and strength when fresh varies from about 100kg/cm 2 to 200kg/cm 2, on exposure to atmosphere and in presence of moisture it

3 Geological and Geotechnical Investigations for Structure 216 quickly disintegrates into powdered material which is called as bole say red bole. Volcanic breccia is formed at the time of volcanic activity.the fragments of the country rock are embedded in lava matrix which may be tachylitic or zeolilized.if the lava matrix is tachylitic,volcanic breccia on exposure to atmosphere and in presence of moisture quickly disintegrates into pieces, the bond of tachylite is Table 1: Geotechnical Properties of Soil removed due to its conversion into bole and other rock fragments are set free, volcanic breccia also contain lot of gas cavities of small to large sizes, many a times these cavities are partly filled by secondary minerals. It is for these reasons; from civil engineering point of view tachylite and volcanic breccia are not suitable rock type. Sr. Properties of Soil Samples Depth in metre No. 1 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 % Consistency limits Liquid limit % Plastic limit % Plasticity index % Specific gravity Compaction characteristics Maximum dry density in gm/cm 3 Optimum moisture content % Shear parameters Cohesion in kn/m 2 Angle of internal friction (φ ) Table 2: Effect of Depth of Rectangular Footing on Ultimate Bearing Capacity Type of soil Redish soil, Sand Medium to coarse grained Depth of foundation in metre Local Shear Failure General Shear Failure (6.02) (25.61) (41.92) (66.69) Type of soil Redish soil, Sand Medium to coarse grained Figure 1: Effect of Depth of Footing on Ultimate Bearing Capacity of Soil Table 3: Effect of Depth of Rectangular Footing on Safe Bearing Capacity Depth of foundation in metre Local Shear Failure General Shear Failure (3.74) (29.78) (47.50) (73.13)

4 217 M. S. DIXIT and K. A. PATIL Figure 2: Effect of Depth of Footing on Safe Bearing Capacity of Soil Factors affecting the Strength of Basalt: The factors which affect the strength of different varities of basalts are i) weathering ii) moisture contain iii) presence of openings in the rock and iv)hydrothermal alteration. Basalts on weathering under warm and humid climate i.e in kokan get converted into laterite in the area above ghat i.e. east of sahyadries, due to hot and dry climate, basalt on weathering first gets converted into murum and finally into black cotton soil. The loss in strength of basalt will naturally depend upon degree of weathering. The core recovery is the indicative of degree of weathering, high core recovery will also reflect in a higher strength and low core recovery in lower strength. Weathering also change the state of the rock from hard solid material to friable powder material in presence of moisture, the strength of material decreases. The reduction in the strength is negligible in hard solid fresh varities of basalt like compact basalt. But the varities like volcanic breccia and tachylite show considerable reduction in strength. So also in case of weathered varities of basalt there is considerable reduction in the strength. Red bole and tachylitic lava matrix in volcanic breccia may become semi plastic and plastic material in presence of moisture. Thus these rock types may loose their strength considerably in presence of moisture. Openings of any type in the rock reduce the strength of the rock. In basalts opening are in the form of gas cavities party or completely empty. Such cavities are large in number and size in volcanic breccia thus the strength of volcanic breccia is much low as compared to compact basalt for the same reason. Due to hydrothermal alteration the basalt becomes redish, greenish or yellowish in colour it becomes little more brittle also. As a result the strength of the hydrothermally altered basalt and volcanic breccia decreases considerably. In basalt flow water may percolate through the vertical and horizontal cooing joints. The weathering continues from these joint planes in all the direction.under these circumstances, there may be undecomposed hard core of the rock surrounded by weathered portion. Thus the hard core contributes to core recovery and core recovery reflects the state of weathering. As per the standard practice to call a particular layer of the rock as foundation grade rock, the core recovery must be more than 95% or at least more than 90% the thickness must be sufficient and the rock should be impervious Core Logging: The drill holes were taken by diamond drill machine of 54mm diameter up to a depth of 6m and were vertical drill holes. The summary of the results of the core logging is as follows: Table 4: Petrological Description of Core Logging Depth in m 0.0 to 2.2m 2.2 to 3.2m 3.2 to 3.7m 3.7 to 4.0m 4.0 to 6.0m Petrological Description Sand-medium to coarse grained mixed with soil. The soil quantity is slightly more. It is redish in colour Murum containing small fragments of compact basalt which are hard and durable.the effects of weathering are seen. The fragments of basalt are almost free from gas cavities. Compact basalt,blackish grey in colour, contain small to medium gas cavities which are partly or completely filled by secondary minerals like zeolite and quartz. Core recovery about 94%. Volcanic breccia,redish brown in colour and contains small to medium sized amygdals. core recovery is 84% Compact basalt, blackish grey in colour contain gas cavities which are filled by secondary minerals. Core recovery is 88% to 97%

5 Geological and Geotechnical Investigations for Structure Allowable Rock Pressure: When the foundation is supported by hard strata such as compact rock, than allowable rock pressure is determined. It is generally assumed that sound bed rock can carry any load transmitted to it. Most rocks in reality can withstand stresses even exceeding 30,000 KN/m 2 and are usually stronger than concrete. Defects like bedding plane, joints, cracks, make the rock of doubtful quality.besides weathering reduces the crushing strength in extreme cases making it a soil like soft material. If the bearing rock is of doubtful quality crushing tests are used to determine bearing values. Crushing strength is obtained by testing intact core samples. Diameter of Specimen (cm) Table 5: Allowable Rock Pressure Depth of Foundation (m) Crushing Strength KN/m 2 Allowable Rock Pressure KN/m m m m m In this case allowable rock pressure is obtained by dividing the crushing strength by a factor of safety of 10. The proposed structure is meant for chemical industry. In extreme case, by considering the seepage of the effluent below the footing a higher value of factor of safety of 10 is used. Conclusion: Based on the studies carried out following conclusions are drawn: 1. 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. 2. The percentage increase in ultimate bearing capacity in comparison with 1m depth, for depths of 1.25m, 1.5m and 1.75m and 2m are found to be 6.02%, 25.61% and 41.92% and 66.69% respectively. 3. For MIDC area Latur, for 3m, 4m,5m and 6m the allowable rock pressure values are kn/m 2, kn/m 2, kn/m 2, and kn/m 2 respectively. 4. The decrease in value i.e. at 4m depth is due to presence of rock volcanic breccia, which is having lower strength than compact basalt also it contains opening in the form of gas cavities partially or completely empty. Such cavities are large in number and size. 5. For 5m and 6m, there is increase in allowable rock pressure as compact basalt is present, which is having higher strength than amygdaloidal basalt and volcanic breccia. References: [1] Bruce L. Kutter, Abbas Abghari and James A. Cheney (1998), Strength Parameters for Bearing Capacity of Sand, Journal of Geotechnical Engineering. Vol.114 No.4. pp [2] Huang and Hong, (2000), Ultimate Bearing Capacity and Settlement of Footings on Reinforced Sandy Ground, Soils and Foundation, Vol.40 No.5, Japanese Geotechnical Society, pp [3] Junhwan Lee and Rodirigo Salgado (2005), Estimation of Bearing Capacity of Circular Footings on Sands Based on Cone Pentration Test, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 131, No.4, pp [4] 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, pp [5] Perkins S.W. and Madson, (2000), Bearing Capacity of Shallow Foundations on Sand- A Relative Density Approach, Journal of Geotechnical and Geo environmental Engineering. Vol.126 No.6, pp [6] Shiva Datta, M.S. Verma, M.S. Alam, Sunder Singh and W.A. Iraqi (2006), Determination of bearing capacity in cobble-gravel mixed soil strata, National Conference on Corrective Engineering Practices in Trouble some Soil, Kakinada, pp [7] Shashi K. Gulhati and Manoj Datta (2007), Geotechnical Engineering, Tata McGraw-Hill, New Delhi. [8] P. Purushothama Raj (2008), Soil Mechanics & Foundation Engineering, Pearson Education, New Delhi. [9] C. Venkatramaiah (2006), Geotechnical Engineering New Age Publishers, New Delhi. [10] Donald P. Coduto (2002), Geotechnical Engineering pearson education (Singapur) Pvt. Ltd. Indian Branch 482 F.I.E. Patparganj. Delhi.