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Subsurface exploration

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About Subsurface investigation, Depth of foundation, Significant depth, Types of investigation, Steps involved, Methods of boring, Types of samples and samplers, Core recovery and RQD.

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Subsurface exploration

  1. 1. By, Ms. Shruthi Hiremath M E (Geotechnical Engg.) Assistant Professor Sanjivani College of Engineering Kopargaon
  2. 2. Investigation of the underground conditions at a site for the economical design of the substructure elements. Purpose of Exploration ❖ To determine the general suitability of the site. ❖ To find the nature of each stratum and engineering properties of the soil and rock, which may affect the design and mode of construction of proposed structure and foundation. ❖ To find out the sources of construction material. ❖ To ensure the safety of surrounding existing structures ❖ To locate the ground water level and possible corrosive effect of soil and water on foundation material. ❖ To predict the settlements ❖ Selection of suitable construction technique. ❖ Selection of type and depth of foundation.
  3. 3.  Foundations of Multi-storeyed Buildings (IS: 1892,1979)  Earth and rockfill Dams (IS: 6955, 1973)  Power House Sites (IS: 10060, 1981)  Canals and Cross Drainage Works (IS: 11385, 1985)  Ports and Harbours (IS: 4651 – Part 1, 1974)
  4. 4. Exploration program involves location and depth of borings, test pits or other methods to be used, and methods of sampling and tests to be carried out to determine the stratification and engineering properties of the soils underlying the site. The principal properties of interest will be the shear strength, deformation, and hydraulic characteristics of soil. The program should be planned so that the maximum amount of information can be obtained at minimum cost. The actual planning of a subsurface exploration program includes the following steps:  Gather all available information  Reconnaissance  Preliminary exploration  Detailed exploration
  5. 5. Assemble all information on dimensions, column spacing, type and use of structure, basement requirements, and any special architectural considerations of the proposed building. For bridges the soil engineer should have access to type and span lengths as well as pier loadings. This information will indicate any settlement limitations, and can be used to estimate foundation loads.
  6. 6. This step includes visual inspection carried out at site without drilling bore holes to reveal surface and subsurface information. This includes Collection of information about adjacent sites and structures. Type of vegetation. Ground water levels that can be determined by checking nearby wells. The general topography of the site, the possible existence of drainage ditches . Soil stratification from deep cuts .
  7. 7. In this step a few borings are made to establish in a general manner  To know the stratification, types of soil to be expected, and possibly the location of the groundwater table.  If the initial borings indicate the upper soil is loose or highly compressible, One or more borings should be taken to rock, or hard strata,  A feasibility exploration program should include collection of enough site data and sample recovery to approximately determine the properties of soil, foundation design and identify the construction procedures.  To find the thickness and composition of each soil layer.
  8. 8.  Here we make a detailed planning for soil exploration in the form trial pits or borings, their spacing and depth. Accordingly, the soil exploration is carried out.  The details of the soils encountered, the type of field tests adopted and the type of sampling done, presence of water table if met with are recorded in the form of bore log.  The soil samples are properly labeled and sent to laboratory for evaluation of their physical and engineering properties.  The report is prepared with clear description of the soils at the site, methods of exploration, soil profile, test methods and results, and the location of the groundwater. This should include information and/or explanations of any unusual soil, water bearing stratum, and soil and groundwater condition that may be troublesome during construction.
  9. 9.  Generally soil exploration should be advanced to a depth up to which the increase in pressure due to structural loading will have no damaging effect (such as settlement & shear failure) on the structure. In other words, the depth at which soil does not contribute settlement of foundation. This depth is termed as significant depth. Significant depth
  10. 10.  Type of structure  Weight of structure  Dimension of structure  Disposition of the loaded area  Soil profile and layer properties
  11. 11.  It can be that depth where net increase in vertical pressure becomes less than 10% of the initial overburden pressure.  The maximum depth reached by the pressure bulb or isobar diagram drawn with an intensity of pressure varying from 1/5th or 1/10th of the surface loading intensity (i.e. 0.2Q to 0.1Q). (Where Q = Initial loading intensity).  It may be equal to one and half to two times the width or smaller lateral dimension of the loaded area.
  12. 12. Sl. No. Type Of Foundation Depth Of Exploration 1 Isolated spread footing or Raft 1.5 B 2 Adjacent footings with clear spacing less than 2B 1.5 L 3 Pile foundation 10 to 30m OR 1.5B 4 Base of retaining wall 1.5 B(Base width) 1.5 H (Exposed height of wall face) [whichever is Greater] 5 Floating basement Depth of construction 6 Dams 1. 1.5 times of bottom width of earth dams 2. 2 times of height from stream bed to crest for concrete dams, for dams less than 30 m high 3. Upto bed rock, in all soft, unstable and permeable strata.
  13. 13. Sl. No. Type Of Foundation Depth Of Exploration 7 Roads Cuts 1. 1.0 m where little cut or fill is required 2. In cut sections, 1.0 m below formation level 3. In deep cuts, equal to the bottom width or depth of the cut 8 Road Fill 2.0 m below ground level or equal to the height of the fill whichever is greater  B = Width of the foundation  L = Length of the foundation
  14. 14.  Open Excavations (Trial pits or Test pits)  Boring Methods  Geophysical Methods
  15. 15. Trial pits are applicable to all types of soils, which provides visual inspection of soil in their natural condition in either disturbed or undisturbed state. Here depth of investigation is limited to 3 to 3.5m. There are 2 ways 1. Pits and trenches 2. Drifts and Shafts
  16. 16. 1. Pits and Trenches Pits: They are excavated at site for inspection of strata so as t0 provide necessary working space. According to IS 4453 1967, a clear working space at the bottom of the pit should be 1.2 m x 1.2 m. Shallow pits (upto 3m) do not require lateral support. For depth greater than 3m and GWT arises then lateral support in the form of sheeting and bracing is required.
  17. 17. Trenches: They can be defined as long shallow pits. It is continuous over a considerable length and provides exposure along a line. On slopes trenches are more suitable than pits.
  18. 18. Drifts: They are the horizontal tunnels made in the hill sides to determine the nature and structure of the geological strata. According to IS 4453-1980 a drift should be 1.5m wide and 2m height in hard rock. In soft rock arched roof can be provided. Shafts: Large sized vertical holes made in the geological formation are called as shafts For Circular Diameter = 2.4m (min) For Rectangular Width = 2.4m Usually done for depth greater than 4m.
  19. 19. Exploratory bore holes are excavated in relatively soft soil close to ground. The location, spacing and depth depends on type , size and weight of the structure. Bore holes are generally located at  The building corners  The centre of the site  The place at which heavily loaded columns are proposed  At least one boring should be taken to a deeper stratum When the depth of excavation is large, vertical boring methods are adopted. Samples are extracted from bore holes and tested in laboratory. GWT is located and Insitu tests are carried using bore holes. Depending on type of soil and purpose of boring the methods are classified as:
  20. 20. Sl. No. Type of project Spacing (m) 1 Multi-storey building 10-30 2 Industrial Plant 20-60 3 Highway 250-500 4 Residential Subdivision 250-500 5 Dams and Dikes 40-80
  21. 21. Auger Boring Wash Boring Percussion Boring Core Boring OR Rotary Drilling
  22. 22. 1. Auger Boring (Helical Augers)
  23. 23. Hand operated Helical Auger Mechanical Operated Helical Auger Hand operated post hole auger
  24. 24. 2. Wash Boring
  25. 25. 3. Percussion Boring
  26. 26. Rotary drilling method of boring is useful in case of highly resistant strata. It is related to finding out the rock strata and also to access the quality of rocks from cracks, fissures and joints. It can conveniently be used in sands and silts also. 4. Core Boring or rotary drilling
  27. 27.  Geo-physical methods are used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. he major method of geo-physical investigations are: gravitational methods, magnetic methods, seismic refraction method, and electrical resistivity method. Out of these, seismic refraction method and electrical resistivity methods are the most commonly used for Civil Engineering purposes. It is a non-intrusive method of “seeing” into the ground. Geophysical methods includes surface and down-hole measurement techniques which provide details about subsurface hydrogeologic and geologic conditions. These methods have also been applied to detecting contaminant plumes and locating buried waste materials. some methods are quite site specific in their performance.
  28. 28. In this method, shock waves are created into the soil at their ground level or a certain depth below it by exploding small charge in the soil or by striking a plate on the soil with a hammer. The radiating shock waves are picked up by the vibration detector (also called geophone or seismometer) where the time of travel of the shock waves gets recorded. A number of geophones are arranged on surface , The shock waves travels directly from the shock point along the ground surface and are picked first by the geophone. The other waves which travel through the soil get refracted at the interface of two soil strata. The refracted rays are also picked up by the geophone. If the underlying layer is denser, the refracted waves travel much faster. As the distance between the shock point and the geophone increases, the refracted waves are able to reach the geophone earlier than the direct waves. By knowing the time of travel primary and refracted waves at various geophones, the depth of various strata can be evaluated, by preparing distance-time graphs and using analytical methods. Seismic refraction method is fast and reliable in establishing profiles of different strata provided the deeper layer have increasingly greater density and thus higher velocities and also increasingly greater thickness. Different kinds of materlais such as gravel, clay hardpan, or rock have characteristic seismic velocities and hence they may be identified by the distance-time graphs. The exact type of material cannot, however, be recognised and the exploration should be supplemented by boring or soundings and sampling.It is not suitable for the ground with hard strata overlying soft strata.
  29. 29.  The electrical resistivity is resistance of the material to the passage of electrical current. Each soil has its own resistivity depending upon its water content, compaction and composition; for example, it is low for saturated silt and high for loose dry gravel or solid rock. The test is conducted by driving four metal spikes to serve as electrodes into the ground along a straight line at equal distance. A direct voltage is imposed between the two outer electrodes, and the potential drop is measured between the inner electrodes. The mean resistivity Q (ohm-m) is computed from the expression: D = Distance beween the electrodes (m) E = Current Flowing between outer electrodes (amps) I = Potential drop between inner electrodes (Volts) Ώ = 2П D E Ohm-m I
  30. 30. Non-Representative samples soil samples in which neither the in-situ soil structure, moisture content nor the soil particles are preserved. They are not representative. They cannot be used for any tests as the soil particles either gets mixed up or some particles may be lost. These Samples are obtained through wash boring or percussion drilling. Disturbed soil samples Disturbed soil samples are those in which the in-situ soil structure and moisture content are lost, but the soil particles are intact. They are representative and can be used for grain size analysis, liquid and plastic limit, specific gravity, compaction tests, moisture content, organic content determination and soil classification test performed in the lab. These samples are obtained through cuttings while auguring, grab, split spoon (SPT), etc.
  31. 31. Undisturbed soil samples Undisturbed soil samples are those in which the in-situ soil structure and moisture content are preserved. They are representative and also intact. These are used for consolidation, permeability or shear strengths test (Engineering properties). In sand is very difficult to obtain undisturbed sample. These samples are obtained by using Shelby tube (thin wall), piston sampler, surface (box), vacuum, freezing, etc.
  32. 32. Where, D1 – Inner diameter of cutting edge D2 – Outer diameter of cutting edge D3 – Inner diameter of Sampling tube D4 – Outer diameter of sampling tube Causes of Soil Disturbances  Friction between the soil and the sampling tube  The wall thickness of the sampling tube  The sharpness of the cutting edge  Care and handling during transportation of the sample tube To minimize friction  The sampling tube should be pushed instead of driven into the ground  Sampling tube that are in common use have been designed to minimize sampling  disturbances.
  33. 33. 1. For obtaining good quality undisturbed samples, the area ratio should be less than or equal to 10%. 2. It may be high as 110% for thick wall sampler like split spoon sampler and may be as low as 6 to 9% for thin wall samples like Shelby tube
  34. 34.  The inside clearance allows elastic expansion of the sample when it enters the sampling tube.  It helps in reducing the frictional drag on the sample, and also helps to retain the core.  For an undisturbed sample, the inside clearance should be between 0.5 and 3%.
  35. 35.  Outside clearance facilitates the withdrawal of the sample from the ground.  For reducing the driving force, the outside clearance should be as small as possible.  Normally, it lies between zero and 2%.  Co Should not be more than Ci.
  36. 36.  The friction on the inside wall of the sampling tube causes disturbances of the sample.  Therefore the inside surface of the sampler should be as smooth as possible.  It is usually smeared with oil before use to reduce friction.  The non – return value provided on the sampler should be of proper design.  It should have an orifice of large area to allow air, water or slurry to escape quickly when the sampler is driven.  It should close when the sample is withdrawn
  37. 37.  The degree of disturbance depends upon the method of applying force during sampling and depends upon the rate of penetration of the sample.  For obtaining undisturbed samples, the sampler should be pushed and not driven L = length of the sample within the tube and H = Depth of penetration of the sampling tube  Rr = 96 – 98 % for getting a satisfactory undisturbed sample
  38. 38. ❖ Split spoon Sampler ❖ Scraper Bucket Sampler ❖ Shelby tube or Thin Walled Sampler ❖ Piston Sampler
  39. 39.  It has an inside diameter of 35mm and an outside diameter of 50mm.  Has a split tube which is held together using a screw- on driving shoe at the bottom end and a cap at the upper end.  4 vent posts are provided to improve recovery of sample.  The thicker wall of the standard sampler permits higher driving stresses than the Shelby tube but with higher levels of soil disturbances. It is used in SPT test.  Split spoon samples are highly disturbed.  They are used for visual examination and for classification tests.
  40. 40. Split Spoon Sampler
  41. 41.  Scraper bucket can be used in case of Sandy soil containing pebbles (gravels) also below water table, it is difficult to use split spoon sampler.  Driving point is attached at the end.  It has vertical slit in the upper portion of the sampler.  As the sampler rotates the cutting of disturbed sample is collected in slit.
  42. 42.  It is a thin-walled seamless steel tube of inside diameter 50 to 76.2mm, outer diameter upto 125mm and length of 600-900mm.  The bottom end of the tube is sharpened. The tubes can be attached to drilling rods.  The drilling rod with the sampler attached is lowered to the bottom of the borehole and the sampler is pushed into the soil, when the required depth is reached it is twisted to 360’ twice.  For sandy soil length of tube = 5xdia to 10xdia  For Clayey soil length of tube = 10xdia to 15xdia  The sheared soil sample inside the tube at the bottom is then pulled out and the two ends of the sampler are sealed and sent to the lab.  The samples can be used for consolidation and shear tests as it is undisturbed.
  43. 43. Shelby Tube Sampler
  44. 44.  When sampling very soft and sensitive clays to get high quality undisturbed samples, they tend to fall out of the sampler. Then piston samplers are used.  They consist of a thin wall tube with a piston. Initially, the piston closes the end of the thin wall tube. The sampler is lowered to the bottom of the borehole and then the thin wall tube is pushed into the soil hydraulically past the piston.  Later the pressure is released through a hole in the piston rod. To a large extent, the presence of the piston prevents distortion in the sample by not letting the soil squeeze into the sampling tube very fast and by not admitting excess soil.  Consequently, samples obtained in this manner are less disturbed than those obtained by Shelby tubes.
  45. 45. Piston Sampler
  46. 46.  It may be necessary to core rock if bedrock is encountered at a certain depth during drilling. It is always desirable that coring be done for at least 3 m. If the bedrock is weathered or irregular, the coring may need to be extended to a greater depth.  For coring, a coring bit is attached to the core barrel and core barrel is attached to the drilling rod. The cutting element in the bit may be diamond, tungsten, or carbide.  The coring is done by rotary drilling. Water is circulated and cuttings are washed out
  47. 47.  Rock cores obtained by such barrels can be fractured because of torsion. To avoid this problem, one can use double-tube core barrels. On the basis of the length of the rock core obtained the following quantities can be obtained for evaluation of the quality of rock
  48. 48. RQD was developed in 1964 by D. U. Deere*. It is determined by measuring the core recovery percentage of core chunks that are greater than 100 mm in length. Core that is not hard or sound should not be counted even if they are 100 mm in length. RQD was introduced for use with core diameters of 54.7 mm. It is a leading indicator for low-quality rock zones. Today RQD is used as a standard parameter in drill core logging and forms a basic element value of the major mass classification systems.
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About Subsurface investigation, Depth of foundation, Significant depth, Types of investigation, Steps involved, Methods of boring, Types of samples and samplers, Core recovery and RQD.


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