Subsoil exploiration
- 1. NAME OF THE TOPIC : SUBSOIL EXPLOIRATION MADE BY : Miss.DHARA DATTANI (ME TRANSPORTATAION) LECTURER AT ATMIYA INSTITUTE OF TECHNOLOGY AND SCIENCE FOR DIPLOMA STUDIES,RAJKOT,GUJRAT,INDIA.
- 2. SUBSOILEXPLORATION??? Theprocessof collection soil datafor the Assessment(MATCHING)soil properties at asite throughseries of laboratory andfield investigation iscollectively calledSub-soil Exploration Enablesthe engineersto drawsoil profile indicating the sequenceofsoilstrataand the properties of soilinvolved. 2
- 3. OBJECTIVES • Toselectfoundation for givenstructure. • Toselectconstructiontechniques. • Todeterminesafedepth offoundation. • Todeterminebearingcapacityofsoil. • Toinvestigatesafetyof existingstructures and suggestsremedial measures. 3
- 4. PLANNING A SUB-SOIL EXPLORATION PROGRAMME – • TWO IMPORTANT ASPECTS OF PLANNING – 1. Depth of exploration – 2. Lateral extent of explanations 4
- 5. DEPTH OFEXPLORATION – The depth of exploration required a particular site depends on degree of variation of the subsurface data n the horizontal and vertical directions. – As per IS;1892-1979 “CODES OF PRACTICE FOR SUBSURFACE INVESTIGATION FOR FOUNDATION” depends on Type of Structure Intensity of Loading ShapeandDisposition of LoadedArea SoilProfile PhysicalProperties of Soil 5
- 6. – The depth of foundation should be at least equal to to the significant depth. – DEFINITION : – Significant Depth : This depth up to which the increase in stress due to structural loading causes shear failure or excessive settlement of foundation is known as significant depth. 6
- 7. Sr.No. Types offoundation Depth ofexploration 1 Isolated spread footings 1.5B, B= Width 2 Raft footings 1.5B 3 Pile foundation 1.5B, B=Width of pilegroup 4 Adjacent footings with clear spacing lessthan twice the width 1.5L L=Lengthof footing 5 Roadcuts Equal to thebottom width of thecut 6 Fill 2m below theground level or equal to height of the fill whichever ishigher. 7
- 8. Lateral extent of explorations8 UNIT NO OR SPACING OFBOREHOLES Small&less important buildings 1at centre maysuffice. Compact buildings ( coveringan area of 0.4hectares) At least 5( 1 at centre &4 at corner). Largemultistoreyed buildings At all important locations,spacing should be 10to 30m. Highways Along centre line, spacing should be 150 to 300m. Concrete dams Spacinggenerally varies between 40to 80m.
- 9. Failures 9
- 10. Leaning Towerof Pisa and Sinkholes 10
- 12. Reconnaissance 12 • Geotechnical engineer makesavisit to thesite for careful visual inspection. • Geotechnical engineer should study soil maps, aerial photographs of an existing buildings. • INFORMATION OBTAINED DURING THISSTAGE General topography of the site Typeof vegetation atsite Drainage pattern existing atsite Location of high floodmarks Evidence of landscapeslides Settlement cracks inexisting structures atsite
- 13. Preliminary Explorations 13 • Testsare connected with conepenetratometers and sounding rods for strength and compressibility of soils. • Geophysical methods are usedfor locating boundaries of different strata. E.g.electrical resistivitymethod. AIM 1. Todetermine depth, thickness, composition of each soil stratumat the site. 2. Todetermine depth of bed rock and ground water table.
- 14. Detailed explorations 14 AIM Todetermine engineering properties of soils in differentstrata. ACTIVITIES Extensiveboringprogramme,sampling,testingofsamplesin laboratory. Fieldtests(vanesheartests,plateloadtests,permeabilitytests)are conductedto determinepropertiesofsoilinnaturalstate.
- 15. Methods of soilExploration 15 Exploration methods Direct Methods SemiDirect In Direct Methods Testpits, Trial pits, Trenches Borings •Auger •Auger and shell •Wash Boring •Percussiondrilling •Rotary Drilling Sounding or penetration Testsand Geophysical methods
- 16. Testpits Depth upto3m Uneconomicalat greaterdepths. Supportsarerequired at greater depths. Especiallyin caseof weakstrata Problemswith GWTandthe sameshould belowered Opentype Exploration Soilsareinvestigated in naturalcondition Soilsamplesarecollected for determiningstrength andEngineeringproperties 16
- 17. 18 1 2 3 4 Walls ofthe test pit indicate four layers (1) Clayeysilt (2) Sandy silt (3) Clean sand (4) Sandygravel 17
- 19. DRIFTSANDSHAFTS • DEFINITION: • DRIFTS: • Horizontaltunnel madein the hill sideto determinenatureandstructureof geological formation. – Drifts arealsoknown asaudits. • SHAFTS: • Largeverticalholesmadeinthe geologicalformation. – Thesemayberectangularor circular insection. 19
- 20. SHAFTS DRIFTS 20
- 21. Boring – Making or Drillingaboreholeinto the ground with a view to obtaining soil or rock samples fromspecified or known depth known as BORING 1. Augerboring 2. Augerandshellboring 3. Washboring 4. Percussiondrilling 5. Rotarydrilling 21
- 22. Auger boring SOILAUGER is a device that is useful for advancing a bore hole in to a ground. Augers may be hand operated or power driven ; the former are used for relatively small depths (<than 3 to 5m), while the latter are used for greater depths (up to 60 to 70m in case of continuous-flight auger) Auger boring is convenient in case of partially saturated sands, silts and medium to stiff cohesive soils. 22
- 23. 23 PROCESS As soon as the auger gets filled with soil, it is taken out and the soil sample is collected The soil samples obtained from this type of boring are highly disturbed
- 24. Shell andAuger – Used widely in India – The shell, ( called a sand bailer) is a heavy duty pipe with a cutting edge. – The shell is raised and let fall in a hole. The soil is cut, enters the tube which is emptied when full. – Shell is used when auger boring becomes difficult 24
- 25. Soil augers 25
- 26. WashBoring It can be used in all types of soils Boulders and rocks can’t be penetrated The method consists in first driving a casing through which a hollow drill rod with a SHARP CHISEL OR CHOPPING BIT at the lower end is inserted water is forced under pressure through the drill rod which is alternatively raised and dropped and also rotated 26
- 27. Typical set up for Washboring 27
- 28. The resulting chopping and jetting action of the bit and water disintegrates the soil The cutting are forced up to the ground surface in the form of soil – water slurry through the annular space between the drill rod and the casing The change of soil stratification could be guessed from the rate of progress and the colour of wash water He samples recovered from the wash water are almost VALUELESS for interpreting the correct geotechnical properties of soil 28
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- 30. Rotary Drilling Suitable for rockformations. Adrill bit, fixed to the lower end of adrill rod, isrotated by power while being kept in firm contact with the hole. Drilling fluid or bentonite slurry isusedunderpressure which brings up the cuttings tothe surface. Evenrock coresmaybe obtained by using suitable diamond drill bits. 30
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- 32. Percussion Drilling The method cannot be used in loose sand and is slow in plastic clay The formation gets badly disturbed by impact 32
- 33. 33 A heavy drill bit is suspended from a drill rod or a cable and is driven by repeated blows Water is added to facilate the breaking of stiff soil or rock The slurry of the pulverised material is bailed out at intervals
- 34. SOIL SAMPLING AND TYPE OF SOIL SAMPLES – The process of obtaining soil samples from the desired depth at the desired location in a natural soil deposit with engineering properties of the soil called soil sampling. – Determination of GWT is also considered as a part of soil sampling. The devices used to obtain soil samples are called soil samplers. Types of soil samples Disturbed samples Undisturbed samples 34
- 35. Types of soil samples Disturb samples – Natural structure of the soil gets partly or fully disturbed – It is used to determine index properties of soil, example : grain size, plasticity, sp.gravity etc. – Samples obtained by : core cutter, standard spoon sampler, scraper bucket, spring core catcher , etc Undisturbed samples – Natural structure of the soil and water content is preserved. – It is used to determine shear strength, permeability, compressibility, shrinkage limit, etc – Samples obtained by : Shelby tube , piston sampler,chunk samplers. 35
- 36. Extent of sample disturbance – Dimensions of Cutting edge – Inside wall friction – Non-return valve 1) Inside clearance Ci=((D1-Di)/Di)x100 2) Outside clearance Co= ((D2-D4/D4)x100 3) Area ratio Ar= ((D2 2-D1 2)/D1 2)x100 36
- 37. Area ratio (Ar)=((Do 2-Di 2)/Di 2)x100 – The degree of disturbance of the sample collected by various methods can be expressed as term called area ratio Di= Inner diameter of the cutting edge D0= outer diameter of the cutting edge. – Ar should be less than 10% 37
- 38. – If its not maintain then: – The possibility of enterance of excess soil – More penetration resistance of sample – Danger of disturbance sample – All increase with increase of area ratio 38
- 39. INSIDE CLEARENCE Ci=((D1-Di)/Di)x100 – Inside clearance The soil is under great stress as it enters the sampler and has a tendency to laterally expand The inside clearence should be large enough to allow a part of lateral expansion to take place 1. For good sampling process it should be between 0.5 to 3%. 2. For sandy silts and clay – 0.5% 3. For stiff and hard clay- 1.5% 4. For stiff and expansive type of clay-3% 39
- 40. Outside clearance Co= ((Do-D2/D2)x100 Reduce friction while sampler is being driven and when it is being withdrawn after sample is collected For good sampling it should: 1. Not greater than inside clearance 2. Lies between 0 and 2% 40
- 41. Recovery Ratio Lr=1 .indicates a good recovery Lr<1 ,indicates that soil is compressed Lr>1,indicates that the soil has swelled 41
- 42. – It Represent The Disturbance Of The Soil Sample – Recovery Ratio Should Be Between 96% To 98% – Wall Friction May Reduce By Suitable Inside Clearance, Smooth Finishing, Oiling – The Non Returned Valve Should Have Large Orifice That Allows Air And Water To Escape. 42
- 43. Types of soil samplers43 Thick wall samplers Thin wall samplers Split spoon sampler Shelby tubes
- 45. – split spoon samplers is a drive shoe attached to the lower end serves as the cutting edge, a sample head may be screwed at the upper end of split spoon – The standard size of the spoon sampler is of 35mm internal and 50.8mm external diameter – The sampler is lowered to the bottom of the bore hole by attaching it to the drill rod. The sampler is then driven by forcing it into the soil by blows from a hammer – The assembly of the sampler is then extracted from the hole and the cutting edge and coupling at the top are unscrewed. The two halves of the barrel are separated and the sample is thus exposed – Samples are generally taken at the intervals of about 1.53m(5ft) 45
- 46. Thin wall sampler Used to obtain undisturbed clayey samples Outside dia between 40mm to 125mm Commonly used dia is 50.8mm or 76.2mm. The lower end is sharpened and bevlled Made up of steel, brass or aluminium 46
- 48. Stationary piston sampler – Consist a thin wall tube with a piston inside. – Piston kept at lower end of the sampler tube closed when the sampler is lowered to the bottom hole. – After the sampler has been lowered 48
- 49. Obtaining chunk sample or hand craved sample 49
- 50. Chunk sample A cylindrical container open at both the ends is used for sampling. The soil is trimmed to shape at thebottom of the test pit One end of container is closed and inverted over the soil chunk and the soil sample is removed using spatula This method is suitable for cohesive soil. 50
- 51. FIELD STRENGTH TESTS The following are the major field tests for determining the soil strength: Standard Penetration Test (SPT). Cone Penetration Test (CPT). The Plate Load Test (PLT). Vane shear test (VST). 51
- 53. Standard penetration test – Spt Is Widely Used To Determine The Parameters Of The Soil In- situ. The Test Consists Of A Driving Split Spoon Sampler Into The Soil Through Bore Hole. – The Split Spoon Sampler Is Driven Into Soil A Distance Of 450mm At Bottom Of The Boring – A Hammer Of 63.5kg Weight With Free Fall Of 760mm Is Used To Drive The Sample 53
- 54. – The Number Of Blows For Penetration Of Last 300 Mm Is Designated As The “ STANDARD PENETRATION VALUE” OR N – The Test Is Usually Performed In Three Stages. The Blow Count Is Found For Revery 150mm Penetration – The Blows For The First 150mm Are Ignored As Those Required For The Seating Drive – Test is discontinued if the number of blows exceed 50 54
- 55. USEFUL IN FINDING OUT55
- 56. CORRECTIONS DILATANCY CORRECTION • Silty fine sandsand fine sandbelow the water table develop pore pressure which isnoteasily dissipated. • Porepressure increasesthe resistance of thesoil thus, Penetration Number (N) alsoincreases • Thiscorrection isapplied when observedvalue of Nexceeds 15
- 57. CORRECTIONS DILATANCY CORRECTION – TerzaghiandPeck(1967)recommended the following correction-
- 58. CORRECTIONS OVERBURDEN PRESSURE CORRECTION . • In granular soils, overburden pressure affects the penetration resistance • If two soils, having same relative density but different confining pressures are tested, the one with a higher confining pressure gives a higher penetration number as the confining pressure in cohesion less soils increases with the depth, the penetration number for soils at shallow depths is underestimated and that at greater depths is overestimated. • For uniformity, the N- values obtained from field tests under different effective overburden pressures are corrected to a standard effective overburden pressure • Relative density or density index is the ratio of the difference between the void ratios of a cohesionlesssoil in its loosest state and existing natural state to the difference between its void ratio in the loosest and densest states.
- 59. CORRECTIONS GIBBS AND HOLTZ’ CORRECTION (1957)
- 60. CORRECTIONSAPPLIED 2. PECK, HANSEN AND THORNBURN’S CORRECTION – AS per IS:2131-1981 overburden given by . PECK, HANSEN AND THORNBURN’S CORRECTION – P0 = over burden pressure, (kPa) = γ x D – D = depth of testing (m) – γ = unit weight of soil at the time of testing 𝑥 = 0.77 𝑙𝑜𝑔 2000 𝑝
- 61. Static cone penetration test Method is used to determine the penetration of resistance of soil and also the skin friction values used for determining the required length of piles in given situation. Also called as Dutch cone test Used in soft or loose soils like silty sands, loose sands, layered deposits of sands, silts and clay and clayey soil. 61
- 62. Equipment for scpt – Steel cone – Friction jacket – Sounding rod – Mantle tube – Driving mechanism 62
- 63. This method is also used to skin friction values which is used to determine the length of the piles The cone is pushed only by thrust and not by driving In order to find out the cone resistance , the cone alone is pushed Later the cone and sleeve is pushed together to find out the combined frictional and point resistance of the cone. Hydraulic gauges are used for measuring pressure developed 63
- 64. Cone Penetration Test (CPT)64 Apex angle 60o Diameter 35.7 mm
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- 68. CPT Truck
- 69. Crawler Type CPT Truck
- 70. Typical CPT Data
- 71. Cone Penetration Test (CPT)71
- 72. Dynamic Cone Penetration Test IS 4968 –part I and II 1976 Cone is Driven by means of hammer blows No. of blows required for every 100 mm penetration is measured No. of blows required for 300mm penetration is taken as dynamic cone penetration resistance Height of Fall of hammer : 750 mm Weight of Hammer Cone Diameter 65 kg i)50mm (without Bentonite slurry) ii) 65mm (with Bentonite slurry)
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- 74. Geo physical methods – Stratification of soil and rocks can be determined by geo physical method of exploration which measures changes in certain physical charecteristics ex density – This methods are used for investigation of soil – Seismic refraction method – Electrical resistivity method 74
- 75. Seismic Refraction Method : – The seismic refraction method is based on the property of seismic waves to refract (or be bent) when they travel from one medium to another of different density or elasticity. – In this method, shock waves are created into the soil at their ground level or a certain depth below it. – The radiating shockwaves are picked up by the vibration detector (Geophone or seismometer) where the time of travel of shock waves get recorded. 75
- 76. – Direct waves or primary waves travel directly from shock point along the ground surface to be picked up by geophone. – Refracted waves travel through the soil and also get refracted at the interface of two soil strata. The refracted waves are also picked up by the geophone. – If the underlying level is denser the refracted waves travel much faster and at longer distances, the shock waves reach faster than the direct waves. 76
- 77. – Hence by distance-time graphs and analytical methods, the depth of various strata can be evaluated by using the time of travel of primary and refracted waves. – Seismic refraction method is fast & reliable in establishing the profile of different strata. – Different material such as gravel, clay hardsoil or rock have characteristic properties and hence can be identified by distance-time graphs. 77
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- 80. Electrical Resistivity Method The electrical 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 composition , compaction, water content etc. – In this method , four metal spikes serve as electrodes which are drive into the ground along a straight line at equal distance. – A direct voltage is imposed between the outer two electrodes, and potential drop is measured between the inner electrodes. 80
- 81. – The mean resistivity Ω (ohm-cm) is calculated by : Ω = 2 Π D V / I D = Distance between electrodes. (cm) V = Potential drop between inner electrodes. ( volts) I = Current between outer electrodes. (ampere) – The depth of exploration is roughly proportional to the electrode spacing . – So to study greater depths, the electrode spacing is increased gradually and made roughly equal to depth of exploration required. This method is know as resistivity sounding. 81
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