Week-14 Soil Exploration PDF

GEOTECHNICAL ENGINEERING (SOIL MECHANICS) Soil Exploration By: CE Faculty Subsurface Exploration Braja Das (2016) “The process of identifying the layers of deposits that underlie a proposed structure and their physical characteristics is general...

GEOTECHNICAL ENGINEERING (SOIL MECHANICS) Soil Exploration By: CE Faculty Subsurface Exploration Braja Das (2016) “The process of identifying the layers of deposits that underlie a proposed structure and their physical characteristics is generally referred to as subsurface exploration” Murthy, V. (2007) “The field and laboratory investigations required to obtain the essential information on the subsoil is called soil exploration or soil investigation” Araro, A. (2004) “Subsurface explorations are done for obtaining the information about subsurface conditions at the site of proposed construction. It is essential for the design of structures and for planning construction techniques” Subsurface Exploration Why do we need to conduct Subsurface Exploration??? Subsurface Exploration “The success or failure of a foundation depends essentially on the reliability of the various soil parameters obtained from the field investigation and laboratory testing, and used as an input into the design of foundations. Sophisticated theories alone will not give a safe and sound design. ” Murthy, V. (2007) Subsurface Exploration Subsurface explorations involves broadly of the ff: (a) Planning of a programme for soil exploration, (b) Collection of disturbed and undisturbed soil or rock samples, from the holes drilled in the field. The number and depths of holes depend upon the project, (c) Conducting all the necessary in-situ tests for obtaining the strength and compressibility characteristics of the soil or rock directly or indirectly, (d) Study of ground-water conditions and collection of water samples for chemical analysis, Subsurface Exploration Subsurface explorations involves broadly of the ff: (e) Geophysical exploration , if required, (f) Conducting all the necessary tests on the samples of soil/rock and water collected, (g) Preparation of drawings, charts, etc, (h) Analysis of the data collected, (i) Preparation of report. Subsurface Exploration Site investigations are generally done to obtain the information that is useful for one or more of the following purposes: (a)To select the type and depth of foundation for a given structure (b)To determine the bearing capacity of soil (c)To estimate the probable maximum and differential settlements (d)To establish the ground water level and to determine the properties of water (e)To predict the lateral earth pressure against retaining wall and abutments (f)To select suitable construction techniques (g)To predict and solve potential foundation problems (h)To ascertain the suitability of the soil as a construction materials (i) To investigate the safety of the existing structures and to suggest the remedial measures. Planning a subsurface exploration programme A sub-surface exploration programme depends: a. Type of the structure to be built b. Variability of the strata at the proposed site c. Cost of the investigation and the entire project Small house in an Small house is to be built in already built-up area an newly developed area It would, therefore, be more desirable to invest some amount on sub- surface exploration than to overdesign the building and make it costlier. Planning a subsurface exploration programme Cost Increasing variability of soil The site underlain in a strata uniform deposits Aim of investigation: To get the maximum information that is useful in the design and construction of Cost Cost the project at the minimum cost STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE - It includes a - It helps in deciding future programme of visit to site site investigations, scope of work, and to study methods of exploration to be adopted, the maps and types of samples to be taken and the other relevant laboratory testing and in-situ testing. records STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: a. general topography of b. Existence of settlement the site cracks STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: c. Evidence of landslides, d. Stratification of soils from creep of slopes and deep cuts shrinkage cracks STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: e. Location of high flood f. Depth of ground water marks (wells) STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: g. Existence of springs, h. Drainage pattern swamps STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: i. Vegetation existing at j. Existence of underground water the site mains, power conduits, etc STAGES IN SUB-SURFACE EXPLORATIONS 2. PRELIMINARY EXPLORATION The aim of a preliminary exploration is to determine the depth, thickness, extent and composition of each soil stratum at the site. - It is in the form of a few borings or test pits using cone penetrometers and sounding rods to obtain information about the strength and compressibility of soils. STAGES IN SUB-SURFACE EXPLORATIONS 3. DETAILED EXPLORATIONS To determine the engineering properties of the soils in different strata It includes an extensive boring programme, sampling and testing of the samples in a laboratory Field tests: - Vane shear tests Conducted to determine - Plate load tests the properties of soil in - Permeability tests natural state STAGES IN SUB-SURFACE EXPLORATIONS 3. DETAILED EXPLORATIONS For complex projects: For small projects - Bridges where the strata are - Dams uniform - Multi-storey buildings “detailed investigations may not be “detailed investigations” required and generally the design is based on the data from reconnaissance and preliminary exploration” Sampling in Soil Soils met in nature are heterogeneous in character with a mixture of sand, silt and clay in different proportions. Classify: Soil with particle Coarse- size coarser than grained soil 0.075mm Soil with particle fine-grained size finer than soil 0.075mm Sampling in Soil Sampling Undisturbed Disturbed Sampling Sampling Sampling in Soil Disturbed Samples Undisturbed Samples Are representative samples Are those that represent the in-situ which contain all the condition of the soil in all respects, constituents in their proper such as structural arrangement of proportions, but the structure particles, water content, density and of soil is not the same as in the stress conditions. in-situ conditions The various laboratory tests The various laboratory tests that can be that can be conducted on such conducted on such soil samples are: soil samples are: 1. Shear strength 1. Mechanical properties 2. consolidation 2. Atterberg limits 3. in-situ density and water content 3. Specific gravity 4. permeability 4. Chemical analysis Sampling in Soil The amount of sampling depends on: a. Time constraint b. Topography c. Cost factors d. Reason for sampling Sampling in Soil Basic geotechnical investigations include using a backhoe to create a test pit where you collect soil from the bucket or using hand augers to collect a sample from a vertical boring. You can use drill rigs to collect disturbed samples from great depths. Collection tools such as split- spoon samplers, Shelby tubes and macrocore push samplers are used in conjunction with the drill rig or a direct-push rig to collect the sample after the rig reaches the desired depth. Sampling in Soil Engineers use drill rigs to collect undisturbed soil samples at depth. Common sampling tools include a. long split-spoon samplers, b. piston samplers and c. a pitcher barrel sampler Piston samplers are thin-walled tube samplers that collect undisturbed samples in soft soil. The piston samples do not work well in gravel, sand or lithified sediments. Like the piston sampler, a pitcher barrel sampler is pushed into the soil to collect the undisturbed sample. These methods produce the best undisturbed samples possible; however, engineers must remember to inspect the sample for signs that the soil was disturbed during collection, especially near the top and bottom of the sample. Engineers will disregard the disturbed portions of the sample during testing. Sampling in Soil Exploratory Borings in the Field HAND OPERATED AUGERS ✓ depth of about 10 m ✓Suitable for all types of soil above the water table but suitable only in clayey soil below the water table ✓A string of drill rods is used for advancing the boring ✓Diameter of the holes normally vary form 10 to 20cm ✓Not suitable in very stiff to hard clay nor in granular below the water table ✓Not practicable in denses and nor in sand Hand Auger mixed with gravel even if the strata lies above the water table. Exploratory Borings in the Field POWER DRIVEN AUGERS ✓ Fligths act as a crew conveyor to bring the soil to the surface ✓ Used in all types of soil including sandy soils below the water table but is not suitable is the soil is mixed with gravel, cobbles, etc. ✓ The central stem may be hollow or solid. A hollow stem is sometimes preferred since standard penetration tests or sampling may be done through the stem without lifting the auger from its position in the hole. Besides, the flight of augers serves the purpose of casing the hole. The hollow stem can be plugged while advancing the bore and the plug can be (a) Plugged while removed while taking samples or conducting standard advancing the auger penetration tests. The drilling rig can be mounted on a truck (b) plug removed and or a tractor. Holes may be drilled by this method rapidly to sampler inserted to depths of 60m or more. sample soil below auger Exploratory Borings in the Field SHELL AND AUGER METHOD ✓Shell (also called as sand bailer) is a heavy duty pipe with a hard cutting edge and a flat valve which opens only inside. The length of the shell ranges from 1 to 3m or more depending on the weight required for cutting the soil. The weight ranges from 30 to 60kg or more. ✓Very useful even in dense sandy deposits or stiff to hard clay soils or even sandy soil mixed with gravel. Exploratory Borings in the Field WASH BORING ✓Its purpose is tor drill holes only and not to make use of the disturbed washed materials for analysis. Whenever, an undisturbed sample is required, the boring is stop, and the chopping bit is replaced by a sampler. The sampler is pushed into the soil at the bottom of the hole and the sample is withdrawn. ✓It is very convenient method provided the soil is either sand, silt or clay. Not suitable for the soil mixed with gravel or boulders. Exploratory Borings in the Field ROTARY DRILLING ✓ Rotary drilling is a procedure by which rapidly rotating drilling bits attached to the bottom of drilling rods cut and grind the soil and advance the borehole. ✓ Rotary drilling can be used in sand, clay, and rocks (unless they are badly fissured). ✓ Water or drilling mud is forced down the drilling rods to the bits, and the return flow forces the cuttings to the surface. ✓ Boreholes with diameters of 50 to 203 mm (2 to 8 in.) can easily be made by this technique. ✓ The drilling mud is a slurry of water and bentonite. Generally, it is used when the soil that is encountered is likely to cave in. Exploratory Borings in the Field PERCUSSION DRILLING ✓ Used for making holes in rocks, boulders and other hard strata. ✓ The heavy chisel is alternately lifted and dropped in a vertical hole. ✓ Advantages: It can be used for all types of materials and useful for drilling holes in glacial tills containing boulders ✓ Disadvantage: the material at the bottom of the hole is disturbed by heavy blows of the chisel. ✓ It is not possible to get good quality undisturbed samples. ✓ More expensive ✓ Difficult to detect minor changes on the properties of the strata penetrated Exploratory Borings in the Field CORE DRILLING ✓Used for drilling holes and for obtaining rock cores. ✓It may be done using either a diamond studded bit or a cutting edge consisting of chilled shot. ✓The diamond is superior to the other type of drilling but is costlier. ✓The core barrel may consist of a single tube or a double tube. ✓A double-tube barrel gives a good quality sample of the rock Soil Exploration Report At the end of the soil exploration program, the soil and rock samples collected from the field are subjected to visual observation and laboratory tests. Then, a soil exploration report is prepared for use by the planning and design office. Any soil exploration report should contain the following information: 1. Scope of investigation 2. General description of the proposed structure for which the exploration has been conducted 3. Geologic conditions of the site 4. Drainage facilities at the site 5. Details of boring 6. Description of subsoil conditions as determined from the soil and rock samples collected 7. Groundwater table as observed from the boreholes Soil Exploration Report 8. Details of foundation recommendations and alternatives 9. Any anticipated construction problems 10. Limitations of the investigation The following graphic presentations also need to be attached to the soil exploration report: 1. Site location map 2. Location of borings with respect to the proposed structure 3. Boring logs 4. Laboratory test results 5. Other special presentations Soil Exploration Report In-Situ Tests In situ tests are essential for soil exploration and geotechnical investigations. They involve testing the properties of soil directly at its original location without disturbing its structure or moisture content. These tests are widely used to determine soil strength, compressibility, and other engineering properties. 1. Standard Penetration Test (SPT) Purpose: Determines soil density and strength. Procedure: A split-spoon sampler is driven into the ground by a hammer of standard weight falling from a standard height. The number of hammer blows required to drive the sampler 30 cm is recorded as the SPT N-value. Applications: Used to assess bearing capacity, estimate settlement, and determine soil type. In-Situ Tests 2. Cone Penetration Test (CPT) Purpose: Measures soil resistance to penetration. Procedure: A cone-shaped probe is pushed into the ground at a constant rate, and resistance is measured continuously. Applications: Useful for identifying stratigraphy, assessing liquefaction potential, and estimating soil properties like undrained shear strength. In-Situ Tests 3. Plate Load Test Purpose: Determines soil bearing capacity and settlement under load. Procedure: A steel plate is placed on the soil surface, and a load is applied incrementally. Settlements are recorded at each load increment. Applications: Common in shallow foundation design. In-Situ Tests 4. Vane Shear Test Purpose: Measures the shear strength of soft, cohesive soils. Procedure: A four-bladed vane is inserted into the soil and rotated. The torque required to shear the soil is measured. Applications: Ideal for measuring undrained shear strength in clay soils. In-Situ Tests 5. Pressuremeter Test (PMT) Purpose: Measures in situ stress-strain behavior of soils. Procedure: A cylindrical probe is inserted into a borehole, and pressure is applied radially. The resulting deformation is measured. Applications: Provides detailed information about soil modulus and strength properties. In-Situ Tests 6. Field Permeability Test Purpose: Assesses the permeability of soil or rock. Procedure: Water is injected into or pumped out of a borehole, and the flow rate is measured under steady conditions. Applications: Used in groundwater studies and drainage design. In-Situ Tests 7. Dynamic Cone Penetrometer (DCP) Test Purpose: Evaluates soil strength, especially in road construction. Procedure: A cone is driven into the soil using a standardized hammer, and penetration per blow is recorded. Applications: Quick assessment of shallow soil strength. In-Situ Tests 8. Electrical Resistivity Test Purpose: Determines soil moisture content, salinity, and structure. Procedure: Electrodes are placed in the ground, and resistivity is measured by passing a current through the soil. Applications: Widely used in site investigations for infrastructure projects. In-Situ Tests Benefits of In Situ Tests Reflect real soil behavior under natural conditions. Minimize soil disturbance. Provide direct data for foundation and design parameters. For engineering and construction projects, these tests ensure accurate soil characterization, optimizing safety and efficiency. References Araro, K. (2004). Soil Mechanics and Foundation Engineering. Standard Publishers Distributors, Delhi Das, B. (2016). Principles of Geotechnical Engineering (8th Edition). Cangage Learning, Boston, USA Murthy, V. (2007). Advanced Foundation Engineering. CBS Publishers and Distributors, New Delhi, Bangalore Murthy, V. (n.d). Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering QUESTIONS By: CE Faculty THANK YOU! ☺ By: CE Faculty