1. Foundation Engineering Site Investigations - Virncheepuram Naveen

2. Standard Penetration Test Introduction: Most commonly used In-situ test Especially for cohesion less soils, which cant be easily sampled

3. Useful in finding out: Relative density of cohesion less soils. Angle of shearing resistance of cohesion less soils. Unconfined compressive strength of cohesive soils

4. INSTRUMENTS 1. Drilling equipment for boreholes 2. Split-spoon sampler 3. Drive-weight assembly 4. Cathead 5. Hammer a) Safety Hammer b) Donut Hammer c) Automatic Hammer

5. 1. Drilling equipment for boreholes: Any drilling equipment is acceptable that provides a reasonably clean hole, which is at least 5 mm larger than the sampler or sampling rods, and less than 170 mm diameter.

6. 2. Split spoon sampler It is a sampler for obtaining a disturbed sample of soil and consists of-o Driving shoe : Made of tool-steel, about 75 mm long o Steel Tube : 450 mm long, split longitudinally in two halves o Coupling : 150 mm long, provided at the top o Check Valve o 4 Venting Ports : 10 mm diameter

7. 3. Drive weight assembly Hammer of 63.5 kg A driving Head (Anvil) A guide permitting a free fall of 0.76 m and over lift capability of at least 100 mm.

8. 4. Cathead Operating at approximately 100 rpm Equipped with suitable rope and overhead sheave for lifting drive-weight

9. 5. Hammer a) Safety Hammer Closed system Delivers approximately 60% of the maximum free fall energy Highly variable energy transfer

10. b) Donut Hammer Open system Delivers approximately 45% of the maximum free fall energy Highly variable energy transfer

11. c) Automatic Hammer Safest system Delivers approximately 95 - 100% of the maximum free fall energy Consistent and effective energy transfer Increased production

12. Procedure 1. Drilling of borehole 2. Driving the Casing 3. Assembling equipment 4. Penetration testing 5. Handling sample

13. 1.Drilling of borehole Drill the borehole to the desired sampling depth and clean out all disturbed material. The equipment used shall provide a clean borehole, 100 to 150 mm in diameter, for insertion of the sampler to ensure that the penetration test is performed on undisturbed soil. Casing shall be used when drilling in sand, soft clay or other soils in which the sides of borehole are likely to cave in.

14. 2.Driving the Casing Where casing is used, it shall not be driven below the level at which the test is made or soil sample is taken. In the case of cohesion less soils which cannot stand without casing, the advancement of the casing pipe should be such that it does not disturb the soil to be tested or sampled; the casing shall preferably be advanced by slowly turning the casing rather than by driving, as the vibration caused by driving may alter the density of such deposits immediately below the bottom of the borehole.

15. 3.Assembling equipment Attach the split-spoon sampler to the drill rod and lower into the hole until it is sitting on the undisturbed material. Attach the drive weight assembly. Lift the 63.5 kg hammer approximately 0.76 m and allow it to fall on the anvil delivering one seating blow. Mark the drill rod in 3 successive .15 m increments to observe penetration.

16. 4. Penetration testing Raise and drop the hammer 0.76 m successively by means of the rope and cathead, using no more than two and one forth wraps around the cathead. The hammer should be operated between 40 and 60 blows per minute and should drop freely. Record the number of blows for each .15 m of the penetration. The first 0.15 m increment is the "seating" drive.

17. The sum of the blows for second and third increment of 0.15 m penetration is termed "penetration resistance or "N-value". If the split spoon sampler is driven less than 45 cm (total), then the penetration resistance shall be for the last 30 cm of penetration (if less than 30 cm is penetrated, the logs should state the number of blows and the depth penetrated). If the no. of blows for 15cm drive exceeds 50, it is taken as a refusal and the test is discontinued. Tests shall be made at every change in stratum or at intervals of not more than l-5 m whichever is less. Tests may be made at lesser intervals if specified or considered necessary.

18. The intervals be increased to 3 m if in between vane shear test is performed.( as per IS:2131-1963) . The entire sampler may sometimes sink under its own weight when very soft sub-soil stratum is encountered. Under such conditions, it may not be necessary to give any blow to the split spoon sampler and SPT value should be indicated as zero.

19. 5. Handling sample Bring the sampler to the surface and open it. Remove any obvious contamination from the ends or sides and drain excess water. Carefully scrape or slice along one side to expose fresh material and any stratification. Record the length, composition, colour, stratification and condition of sample. Remove sample and wrap it or seal in a plastic bag to retain moisture. If the sample can be removed relatively intact, wrap it in several layers of plastic and seal ends with tape.

20. Corrections: Dilatancy correction Overburden pressure correction Gibbs and holtz correction (1957) Peck, hansen and thornburns correction Peck and bazaraas correction

21. Dilatancy correction Silty fine sands and fine sand below the water table develop pore pressure which is not easily dissipated. Pore pressure increases the resistance of the soil thus, Penetration Number (N) also increases This correction is applied when observed value of N exceeds 15

22. Dilatancy correction Terzaghi and Peck (1967) recommended the following correction-

23. 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.

24. Gibbs and holtz correction (1957)

25. Peck, Hansen and Thornburns correction

26. Peck and bazaraas correction One of the most commonly used corrections According to them,

27. FACTORS COMMENTS Attitude of operators Blow counts for the same soil using the same rig can vary, depending on who is operating the rig, and perhaps the mood of operator and time of drilling. Overdrive sampler Higher blow counts usually result from an overdriven sampler. Sampler plugged by gravel Higher blow counts result when gravel plugs the sampler, resistance of loose sand could be highly overestimated. Plugged casing High N-values may be recorded for loose sand when sampling below groundwater table. Hydrostatic pressure can cause sand to rise within the casing.

28. FACTORS COMMENTS Overwashing ahead of casing Low blow count may result for dense sand since overwashing loosens sand. Drilling method Drilling technique (e.g., cased holes vs. mud stabilized holes) may result in different N-values for the same soil. Free fall of the drive weight is not attained Using more than 1-1/2 turns of rope around the drum and or using wire cable will restrict the fall of the drive weight. Not using correct weight Driller frequently supplies drive hammers with weights varying from the standard by as much as 10 lbs.

29. FACTORS COMMENTS Weight does not strike the drive cap concentrically Impact energy is reduced, increasing N-values. Not using a guide rod Incorrect N-value obtained. Not using a good tip on the sampling spoon If the tip is damaged and reduces the opening or increases the end area the N-value can be increased. Use of drill rods heavier than standard With heavier rods more energy is absorbed by the rods causing an increase in the blow count.

30. Correlations between spt and soil properties - Relative Density - Effective Stress Friction Angle - Unconfined Compressive Strength *Some correlations require the raw N-values whereas others use the corrected N-values.

31. Relative Density SPT N-Value Relative Density 0-4 Very loose 25-32 4-10 Loose 27-35 10-30 Medium 30-40 30-50 Dense 35-45 Over 50 Very dense >45

32. Unconfined Compressive Strength Of Cohesive Soils Consistency Very Soft Soft Medium Stiff Very Stiff Hard SPT N-value 30 qu 400

33. Advantages Relatively quick and simple to perform. Provides a representative soil sample. Provides useful index of relative strength and compressibility of the soil. Able to penetrate dense layers, gravel, and fill. The SPT equipment is rugged, and the test can be performed in a wide range of soil conditions.

34. Disadvantages The SPT does not typically provide continuous data, therefore important data such as weak seams may be missed. Limited applicability to cohesive soils, gravels, cobbles boulders. Somewhat slower than other sample methods due to sample retrieval. The greatest disadvantage to SPTs is the lack of reproducibility of the test results

35. Precautions The drill rods should be of standard specification and should not be in bent condition. The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear. The drop hammer must be of the right weight and the fall should be free, frictionless and vertical. The height of fall must be exactly 750 mm. Any change from this will seriously affect the N value.

36. Static Cone Penetration Test

37. Static Cone Penetration Test Introduction: Cone Sleeve Driving force Procedure Interpretation of results

38. Cone (dutch cone) Base area = 10 cm2 Apex angle = 600 To measure tip resistance Related to un-drained shear strength

39. Cone specifications Diameter = 35.7 < d < 36 (mm) Cone height = 31 < hc < 31.3 (mm) Cylindrical extension = 4.5 < he < 5.5 (mm)

40. CONE (10 cm2)

41. Cone (15 cm2)

42. Cylindrical sleeve specifications Area = 150 cm2 Height =13.4 cm To measure frictional resistance

43. Procedure Cone pushed at 10 mm/sec (35 mm) Cone withdrawn & sleeve pushed on to the cone and driven together.

44. Cone penetrometer

45. HAND OPERATED (30kn)

46. ENGINE OPERATED (200kN)

47. Advantages: Speed Economy Detailed & precise data

48. Disadvantages Soil sample not obtained Depth limited Most useful in coarser, permeable soils ie: sands

49. Approximate relationship between point of resistance of cone and penetration number i. Gravels qc=800N to 1000N ii. Sand qc=500N to 600N iii. Silty sand qc=300N to 400N iv. Silts and clayey silts qc=200N Where qc is point resistance of cone in KN/m2

50. Dynamic cone test

51. Dynamic cone test Introduction This test is conducted by driving the cone by blows of a hammer. The number of blows for driving the cone through a specified distance is a measure of the dynamic cone resistance.

52. Characteristics Dynamic cone test is performed by using a 50mm cone without bentotite slurry or by using a 65mm cone with bentotite slurry. The driving energy is given by a 65kg-hammer falling through a height of 75cm The number of blows for every 10cm penetration is recorded. The number of blows for every 30cm penetration is recorded as dynamic cone resistance.

53. Cross-section of dynamic cone: Holes of 3mm dia are provided . Distributed for a height of 150mm Sleeve of dia 60mm dia

54. Dimensional Details(Small scale DCP)

55. Correlation between standard penetration test N-value : Ncbr=1.5N for upto 3m depth Ncbr=1.75N for depth between 3 to 6m Ncbr=2.0N for depth grater than 6m Where Ncbr dynamic cone resistance in KN/m2

56. Correlation between the dynamic cone resistance of 65mm diameter cone without using bentonite slurry and the SPT number(N): Ncbr=1.5N for upto 4m depth Ncbr=1.75N for depth between 4 to 9m Ncbr=2.0N for depth grater than 9m Where Ncbr dynamic cone resistance in KN/m2

57. Precaution If the skin friction is to be eliminated, the test is conducted in a cased bore hole. When 65mm cone with bentonite slurry is used , the set-up should have arrangements for circulating slurry so that the friction on the driving rod is eliminated.

58. Reference Dr.K.R.Arora Slideshare.com Howstuffworks.com Southern Earth Sciences,.Inc; Available at: authorstream.com