Week2_CE427-Subsurface-Exploration-Lecture.pdf

CE427 Foundation Engineering Subsurface Exploration 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. (200...

CE427 Foundation Engineering Subsurface Exploration 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” 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 already Small house is to be built in an built-up area 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 uniform strata deposits Aim of investigation: To get the maximum information that is useful in the design and construction of the Cost Cost project at the minimum cost  STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE - It includes a visit to - It helps in deciding future programme of site site and to study investigations, scope of work, methods of the maps and other exploration to be adopted, types of samples to relevant records be taken and the laboratory testing and in-situ testing. By: Vera Karla S. Caingles  STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: a. general topography of the site b. Existence of settlement cracks  STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: c. Evidence of landslides, creep d. Stratification of soils from deep of slopes and shrinkage cracks cuts  STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: e. Location of high flood marks f. Depth of ground water (wells)  STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: g. Existence of springs, swamps h. Drainage pattern  STAGES IN SUB-SURFACE EXPLORATIONS 1. SITE RECONNAISSANCE The information about the following features is obtained in reconnaissance: i. Vegetation existing at the site j. Existence of underground water 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 the - Plate load tests properties of soil in natural state - Permeability tests  STAGES IN SUB-SURFACE EXPLORATIONS 3. DETAILED EXPLORATIONS For small projects For complex projects: where the strata are - Bridges uniform - Dams - Multi-storey buildings “detailed investigations may not be required and generally the design is “detailed investigations” 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 size coarser than 0.075mm Coarse-grained soil Soil with particle size finer fine-grained soil than 0.075mm  Sampling in soil Sampling Undisturbed Disturbed Sampling Sampling  Sampling in soil Disturbed Samples Undisturbed Samples Are representative samples which Are those that represent the in-situ contain all the constituents in their condition of the soil in all respects, proper proportions, but the structure such as structural arrangement of of soil is not the same as in the in- particles, water content, density and situ conditions stress conditions. The various laboratory tests that can The various laboratory tests that can be conducted on such soil samples be conducted on such soil samples are: are: 1. Mechanical properties 1. Shear strength 2. Atterberg limits 2. consolidation 3. Specific gravity 3. in-situ density and water content 4. Chemical analysis 4. permeability  Sampling in soil The amount of sampling depends on: a. Time constraint b. Topography c. Cost factors d. Reason for sampling By: Vera Karla S. Caingles  Sampling in soil How to Collect Disturbed Soil Samples 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 How to Collect Undisturbed Soil Samples 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 How to Collect Undisturbed Soil Samples  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 mixed with gravel even if the strata lies above Hand Auger the water table.  Exploratory Borings in the Field HAND OPERATED AUGERS  Exploratory Borings in the Field ✓ Fligths act as a crew conveyor to bring the soil to POWER DRIVEN AUGERS 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 removed while taking samples or conducting standard penetration tests. The drilling rig can be mounted on a truck or a tractor. Holes (a) Plugged while advancing the auger (b) plug removed and sampler inserted may be drilled by this method rapidly to depths of to sample soil below auger 60m or more.  Exploratory Borings in the Field POWER DRIVEN AUGERS  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 SHELL AND AUGER METHOD  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 WASH BORING  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 ROTARY DRILLING  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 PERCUSSION DRILLING  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  Soil Exploration Report 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 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 Soil Exploration Report 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