Subsurface Investigation
Download as PPT, PDF25 likes9,570 views
The document details the sub-surface investigation procedures necessary for building construction, focusing on the assessment of soil properties and geological features. It covers the purpose of site exploration, depth of investigation, various methods including trial pits, boring techniques, and geo-physical methods, along with the difference between disturbed and undisturbed soil samples. Finally, it addresses the importance of bearing capacity and methods for improving it, including grading soil and foundation depth adjustment.
Subsurface Investigation
- 1. Prepared By Prof. Karan S. Chauhan Assistant Professor Department Of Civil Engineering @ Shri Labhubhai Trivedi Institute Of Engineering And Technology,Rajkot Shri Labhubhai Trivedi Institute of Engineering & Technology, Rajkot Sub Surface InvestigationSub Surface Investigation Subject :- Building Construction Subject Code : 2130607
- 2. SUB-SURFACE INVESTIGATION The surface and subsurface investigation or site exploration is carried out to collect the information about physical properties and characteristics of the subsoil material as well as the details of other geological features of the site area.
- 3. Contents Purpose of site exploration Depth of exploration Methods of site exploration Soil samples and samplers
- 4. Purpose of Site Exploration The site exploration or investigation is carried out to collect the complete details of the site for the following purposes : FOR NEW STRUCTURE:- The selection of type and depth of foundation. The determine of bearing capacity of the selected foundation. The prediction of settlement of the selected foundation. The determine of the ground water level. The evaluation of the earth pressure against walls, basement, abutments etc. The provision against constructional difficulties. The suitability of soil and degree of compaction of soil.
- 5. For Existing Structure :- The investigation of the safety of the structure. The prediction of settlement. The determination of remedial measure if the structure is unsafe or will suffer detrimental settlement.
- 6. Depth Of Exploration Exploration, in general, should be carried out to a depth up to which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of foundation a depth, known as significant depth, depends upon the type of structure, its weight, size, shape and disposition of the loaded areas, and the soil profile and its properties. The significant depth may be assumed to be equal to one- and-half to two time the width of the loaded area. National Buildind Code Of India (Sp 7-1983) Suggests... As a thumb rule , the depth should be one and half times the probable width of the footing or 1.5 m whichever is more. In case of weak soils , however , the pits or the bore holes should be taken to a depth at which the loads can be carried by the soil without undesirable settlement. the number and spacing of the trail pits or bore holes to be adopted for a site will depend upon the area of plot as well as type of structure to be built.
- 7. Methods of Site Exploration 1) Trial pits OR Open Excavation 2) Subsurface soundings 3) Boring Auger boring Auger and shell boring Wash boring Percussion boring Rotary boring 4 ) Geo physical methods Seismic refraction method Electrical resistivity method Magnetic method Gravitational method
- 8. Trail Pits The excavation of trial pits is a simple and reliable method. The depth is limited to 4-5m only. The biggest advantage of method is that soil strata can be inspected in their natural condition and samples can be conveniently taken. It is easy to obtain disturbed and undisturbed samples Block samples can be cut by hand tools and tube samples can be taken from the bottom of the pit.
- 10. Subsurface Soundings This method consists of measuring the variation in the resistance offered by the soil with the depth by means of a tool known as penetrometer. The penetrometer may consist of a mild steel cone fitted loosely to a steel rod or it may be a tool known as standard split spoon sampler. The resistance offered by the soil to penetration is co-related with the engineering properties of soil , such as – density , consistency , permeability and bearing capacity.
- 12. Boring The various boring methods adopted for soil exploration are as follows : Auger boring Auger and shell boring Wash boring Percussion boring Rotary boring
- 13. Auger Boring This type of augers are commonly used for soft clayey or sandy soils or soft soils . It can be operated manually or mechanically . The hand operated augers are used up to 6 m depth and the mechanically operated augers are used for greater depth up to 15 m . The auger is vertically driven into the ground by rotating its handle and is pressed down during the process of rotation . At every 30 cm depth of penetration , the auger is drawn out and the samples of soils are collected separately for testing. Two types augers are used 1) Spiral/Helical , 2) Post hole Samples collected from auger boring is badly disturbed sample.
- 15. Auger and Shell Boring In this method ,different types of tools are used for boring. In case of soft clay , cylindrical auger having 75 to 200 mm diameter hollow tube with a cutting edge at its bottom is used . In case of hard clay , the shells with cutting edge or teeth at its lower end are used ; while in case of sandy soil or sand , the pumps are used for boring . The hand operated rigs are used 25 m depth up to 200 mm diameter and the mechanical rigs are used for 50m depth. The samples of soil are collected at regular interval for testing .
- 17. Wash Boring This method may be use in all types of soils except those mixed with gravel and boulders, and rocky strata. Water jet under pressure is forced into the hole through the rod and the bit, which is alternatively raised and dropped, and rotated as well The resulting chopping and jetting action of the bit and water loosens the soil at the lower end and forces the water-slurry upwards through the annular space between the drill rod and the casing. The soil water slurry is led into a settling tank where the soil particles settle where the water overflows into a sump The soil particles collected represent a very disturb sample and is not very useful for the evaluation of the engineering properties of the soil. The changes in soil strata may be indicated by the change in the rate of progress and the change in the colour of the wash water
- 19. Percussion Boring This method is used for rocks and soils having boulders ; which are broken up by repeated blows from a bit or chisel . The percussion boring method is used for making holes in rocks, boulders and hard strata. In this method a heavy drilling bit called ‘churn bit’. If the point where the drill bit strikes is above the ground water table, a water added to the hole to facilitate the breaking of stiff soil or rock. One of the disadvantages is that the material at bottom of the hole is disturbed by heavy blows of the chisel.
- 21. Rotary Boring This method is a of drilling holes in rock formations. A hollow drill bit, fixed to the lower end of a hollow drill rod, is rotated by power while being kept in firm contact with the bottom of the hole. Drilling fluid usually bentonite clay slurry is force under pressure through the drill rod and it comes up bringing the cuttings to the surface. This method is then known as core drilling or core boring. This method is a fast method of drilling holes in rock formations. In this method by using suitable diamond studded drill bits or steel bits with shots, the rock cores may be obtained. The method is then known as core drilling or core boring
- 23. Geo-Physical Method It is the method used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. It involves the detection of significant difference in the physical properties of geological formation. Major methods are….. 1. Seismic refraction method 2. Electrical resistivity method 3. Magnetic method 4. Gravitational method
- 24. Seismic Refraction method Seismic refraction is a geophysical method used for investigating subsurface ground conditions by utilizing surface-sourced seismic waves. Data acquired on site is computer processed and interpreted to produce models of the seismic velocity and layer thickness of the subsurface ground structure. The method is commonly used for measuring the thickness of overburden in areas where bedrock is at depth, and assessing ripability parameters. Applications Measures Bedrock Depth & Overburden Thickness Determines Rippability Parameters Investigates Pipeline Routes Locates Geological Structures Evaluates Sand & Gravel Deposits Defines Ancient Landfill Sites
- 27. Electric Resistivity method The electric resistivity method is based on the measurement and recording of changes in the mean resistivity of various soils. Each soil has its own resistivity depending upon its water content, compaction and composition. The test is conducted by driving four metal spikes to serve as electrodes into the ground along a straight line at equal distance. Direct voltage is imposed between two outer electrodes, the potential drop is measured between the inner electrodes.
- 29. Soil Samples 1. Disturbed samples A disturbed sample is a sample in which the natural structure of soil gets partly or fully modified and destroyed . However , by taking proper precautions , the natural water content in soil sample can be preserved . This type of soil sample should maintain the original proportion of the various particles . 2. Undisturbed samples An undisturbed sample is a sample which the natural structure and properties remain preserved . The sample disturbance depends on the design of samplers and the method of sampling . The sampling tube when forced into the ground should cause as little remolding and disturbance as possible .
- 30. Types of soil sampler Depending upon the area ratio Thick wall sampler Thin wall sampler Main types are… 1. Open drive sampler 2. Piston sampler 3. Rotary sampler
- 31. 2 1 3 4 1. Open drive sampler
- 34. Bearing Capacity of soil The max. load per unit area which the soil or rock can carry without yielding or displacement is termed as the bearing capacity of soil. Methods for determining B.C. of soil 1.Plate load test 2.Penetration Test 3.SBC from IS code 4.Analytical Method
- 35. 1. Plate Load test It is Field test to check B.C. of soil Gravity Loading Method
- 36. Truss Method
- 37. 2. Penetration Test This test is use for resistance offered by soil to penetration of standard split spoon sampler, Resistance is co-related with properties of soil such as density, bearing capacity etc.
- 39. 3. SBC based on I.S. codes I.S. 1904-1978 Table-I Used for unimportant & lightly loaded structure
- 40. Choice for select method of site exploration Nature of Soil Topography Cost of exploration
- 41. Methods for improving Bearing Capacity of Soil :- Increasing Depth of Foundation Grouting Drainage of soil Compaction of soil (Ramming, rubble compaction, flooding , vibration, vibrofloatation, preloading compaction, use sand pile) Chemical treatment