Soil Properties and Foundation

A brief description of soil properties and types of foundation based on it.

1. Soil Properties and Foundation Reasat E Noor (REN)

2. Introduction Soil  is a mixture of minerals, organic matter, gases, liquids, and countless organisms that together support life on Earth. Types : i ) Boulders ii) Cobbles iii) Gravel iv) Sand v) Silt vi) Clay

3. Soil Type

4. Sand Sand  is a naturally occurring granular material composed of finely divided rock and mineral  particles. Mineral Origin is Quartz (SiO 4 ) Particle size range from 0.075- 2mm .

5. Silt Silt  is granular material of a size between sand and clay, whose mineral origin is  Quartz ( SiO 4 )  and  Feldspar ( KAlSi 3 O 8  – NaAlSi 3 O 8  –  CaAl 2 Si 2 O 8 ) . S ilt particles range between 0.002-0.0625mm .

6. Clay Clay  is a fine-grained natural rock or soil material that combines one or more clay minerals such as Kaolinite ( Al 2 Si 2 O 5 (OH) 4 )  with traces of metal oxides and organic matter.  Clay particles are lesser than 0.002 mm

7. Soil Properties Properties Sand Silt Clay Water holding Capacity Low Medium to High High Drainage Rate High Low to Medium Very Low Cohesion Between Particles Very Low Medium to High Very High Organic material Content Low Medium to High Medium to High Shrink & Swell Potential Low Medium to High High

8. Soil Classification Unified Soil Classification System The first letter tells the general type of soil G: Gravel S: Sand M: Silt C: Clay O: Organic Material The second letter is supplementary description W: Well Graded P: Poorly Graded M: Silty C: Clayey L: Low Plasticity H : High Plasticity

9. Soil Classification

10. Soil Classification

11. Soil Classification For Coarse Grained Soil Well Graded Coefficient of Uniformity C u >4 for gravels and C u >6 for sands. C z must be between 1-3. Poorly Graded One range of particle is present excessively in soil sample. Gap Graded Large and small range of particles are present excessively.

12. Soil Classification

13. Soil Classification Hydrometer Analysis

14. Soil Classification For Fine Grained Soil

15. Soil Classification The  liquid limit (LL)  is defined as the moisture content at which soil begins to behave as a  liquid   material and begins to flow.  The  plastic limit   ( PL ) is   defined  as the moisture content at which soil begins to behave as a  plastic   material . The plasticity index ( PI ) is a measure of the plasticity of a soil . PI = LL – PL Plastic Index Properties 0-3 Non-Plastic 3-15 Slightly Plastic 15-30 Medium Plastic >30 Highly Plastic

16. Soil Classification For Fine Grained Soil

17. Soil Classification

18. Soil Phase Diagram

19. Soil Phase Diagram Moisture Content The water content is defined as the ratio of mass of water to the mass of soils.

20. Soil Phase Diagram Void Ratio e = Vv /Vs Porosity n = Vv /V Dry Density p = W s /V Relative density I D = e max - e/ e max -e min The degree of compaction can be measured by relative density of soil.

21. Moisture Content & Dry Density

22. Soil Phase Diagram Specific Gravity D efined as the ratio of the weight of a given volume of soil solids to the weight of an equal volume of distilled water Soil Type Specific Gravity Gravel 2.65-2.68 Sand 2.65-2.68 Silty Sand 2.66-2.70 Silts 2.66-2.70 Inorganic Clays 2.7-2.80

23. Internal Angle of Friction A ngle  of  internal friction  ( friction angle ) A measure of the ability of a unit of rock or soil to withstand a shear stress. It is the  angle  (φ), measured between the normal force (N) and resultant force (R), that is attained when failure just occurs in response to a shearing stress (S).

24. Internal Angle of Friction

25. Internal Angle of Friction Soil Type USCS Min (Degree) Max (Degree) Well Graded Gravel GW 33 40 Poorly Graded Gravel GP 32 44 Well Graded Compacted Sand SW 30 38 Inorganic Silt (Low Plasticity ) ML 27 41 Inorganic Silt (High Plasticity) MH 23 33 Inorganic Clay (Low Plasticity) CL 27 35 Inorganic Clay (High Plasticity) CH 17 31

26. Internal Angle of Friction Active earth Pressure Coefficient K a =1-sin ø/1+sin ø Passive Earth Pressure Coefficient K p = 1+sinø/1-sinø Lateral Earth Pressure P a = ½* y* K a *h 2

27. Standard Penetration Test The number of blows required for drill rod to penetrate 300mm below the ground surface. First 150 mm penetration is neglected. Most commonly used in-situ test. Specially for cohesion less soils, which cannot be easily sampled

28. Standard Penetration Test Usefulness Relative density of cohesion less soils . Angle of shearing resistance of cohesion less soils . Unconfined compressive strength of cohesive soils

29. Standard Penetration Test N Value Correction From Terzaghi and Peck recommended the following correction for dilatancy. Where N R is recorded value, for N R =15, N c =N R From Peck, Hansen and Thornburn correction for overburden pressure Where is overburden pressure>24 KN/m 2

30. SPT Values & Soil Parameters N values Condition Relative Density Angle of Internal Friction Unit Weight ( pcf ) 0-4 Very Loose 0-0.2 25°-30° 70-100 4-10 Loose 0.2-0.4 30°-35° 90-115 10-30 Medium 0.4-0.6 35°-40° 110-130 30-50 Dense 0.6-0.85 40°-45° 110-140 Over 50 Very Dense 1 45° 130-150

31. SPT Values & Soil Parameters

32. SPT Values & Soil Parameters N values Condition Unconfined Compressive Strength (Tsf.) 0-2 Very Soft 0-0.25 2-4 Soft 0.25-0.50 4-8 Medium Stiff 0.50-1.00 8-15 Stiff 1.00-2.00 15-30 Very Stiff 2.00-4.00 Over 30 Hard Over 4.00

33. SPT Values & Soil Parameters

34. Bearing Capacity For Strip Footing q ult = CN c +yDN q +0.5yBN y ( Tsm ) For Square Footing q ult = 1.3CN c +yDN q +0.4yBN y ( Tsm ) For Circular Footing q ult = 1.3CN c +yDN q +0.4yBN y ( Tsm ) For Raft Foundation q ult = 5.7C(1+0.3B/L) (TSM)

35. Bearing Capacity Pile Capacity P pu = A p (40N) L b /B < A p ( 400N ) Where, N = Statistical average of the SPT N 55 in a zone of about 8B above to 3B below the pile point B =Width or Diameter of pile point in meter L b = Pile Penetration Depth A p = Base are of pile P pu = Ultimate Pile Tip Capacity

36. Foundation Design Criteria Two main characteristics The foundation has to be safe against overall shear failure in the soil The foundation cannot undergo excessive settlement Location and Depth of Foundation Depth should be at least 50 cm below natural ground level Must be placed below the zone of volume change. In fine sands and silts, foundation must be placed below the zone in which trouble may caused by frost Depth of foundation in river should be below the scour level.

37. Foundation Types Shallow Foundation ( Df /B<1) Deep Foundation ( Df /B>4 )

38. Foundation Types Shallow Foundation (D/B<1) Distributes the loads on the ground laterally . Types Strip Footing/Continuous Footing Spread or Isolated Footing Combined Footing Strap or Cantilever Footing Mat or Raft Foundation

39. Foundation Types Strip Footing/Continuous Footing Provided for load bearing wall Provided for closely spaced column.

40. Foundation Types Spread or Isolated Footing Provided for supporting an individual column.

41. Foundation Types Combined Footing Supports two closely spaced column. Even distribution of loads where column is loaded eccentrically .

42. Foundation Types Strap or Cantilever Footing Economical than combined footing. The distance between the columns is large. Used where allowable soil pressure is high . The strap is designed as a rigid beam.

43. Foundation Types Mat or Raft Foundation Required where allowable soil pressure is low. Reduce differential settlement. Distance of columns is low.

44. Types of Failure of Soil

45. Types of Failure of Soil General Shear Failure Brittle type stress-strain behavior. (like dense sand) Sudden failure in soil takes place and failure surface of soil extends to the ground surface. Ultimate load can be easily located. A bulging of ground surface adjacent to foundation can be easily located

46. Types of Failure of Soil Local Shear Failure Observed in case of sand and clayey soil of medium compaction. Significant compression of the soil just beneath the foundation Ultimate load can not be easily located. Slight bulging of ground surface adjacent to foundation can be located.

47. Types of Failure of Soil Punching Shear Failure Observed in fairly loose soil. Poorly defined shear plane. Soil zones beyond the loaded area is little affected. Beyond the ultimate load failure, Load-settlement graph is linear. The failure surface in soil will not extend to the ground surface.

48. Types of Failure of Soil

49. Foundation Types Deep Foundation (D/B>4) Distributes the load vertically . Types Buoyancy rafts (hollow box foundations) Caissons Cylinders Shaft foundations Pile foundations

50. Foundation Types Pile foundations Usual length -10-20m Usual load- 30-300 ton Types End Bearing Pile Friction Pile Combination of Friction and End Bearing Pile

51. Foundation Types End Bearing Pile U ltimate bearing capacity of the pile depends entirely on the underlying material Friction Bearing Pile  In cohesion less soils, such as sands of medium to low density, friction piles are often used to increase the density and thus the shear strength.  Combination of Friction and End Bearing Pile

52. Foundation for Different Types of Soil Sand Footings : Easy to Construct and Economical Retaining Structures : Must be used because sand cannot support themselves. Deep Foundation (Piles) : Uses Friction Resistance but low in bearing capacity.

53. Foundation for Different Types of Soil (Cont.) Sand Problems Bearing capacity may cause problem but in most cases it is sufficient Excessive settlement in wet and loose deposits. Overburden Pressure is usually low.

54. Foundation for Different Types of Soil (Cont.) Silt & Clay Footings : Economic but may have problems with bearing capacity in saturated clays. Retaining Structures : Clays are self supportive up to a certain height. Must be used if it exceeds the critical height. Deep Foundation (Piles) : If bearing capacity is low piles may driven to rock

55. Foundation for Different Types of Soil (Cont.) Silt and Clay Problems Low bearing capacity. Generally low shear strength when wet. High consolidation in soft clays. Swelling is possible. Over-consolidated soil may have cracks and fissures.

56. Foundation for Different Types of Soil (Cont.) Soil Problems Solution Sand Settlement 1. Loose sand must be compacted. 2. Lowering water table may result in sand densification Bearing Capacity 1. Compaction increases cohesion and friction 2. Using Pile foundation

57. Foundation for Different Types of Soil (Cont.) Soil Problems Solution Silt and Clay Consolidation 1. Lowering water table. 2. Pre-Loading 3. Drive pile to rock Bearing Capacity 1. Compaction 2. Use of Deep foundation Expansion or Swelling 1. Treat or stabilize soil 2. Maintain Constant water table. 3. Alter soil nature 4. Include Swell pressure in design

58. Thank You all!