SHALLOW FOUNDATION NAME: INDRAJIT MITRA PAPER NAME AND CODE: SEMINAR-I AND CE 792 institute: university institute of technology, THE BURDWAN UNIVERSITY
INTRODUCTION Shallow foundations are those that transmit structural loads to the near surface soils. According to the Terzaghi, a foundation is shallow foundation if its depth is equal to or less than its width i.e d ≤ w. For most of the residential buildings or buildings with moderate height or multistoreyed building on soil with sufficient strength, shallow foundation is used from economical consideration.
Major Requirements : Near surface soil should be strong enough Foundation structures should be able to sustain the applied loads without exceeding the safe bearing capacity of the soil. The settlement of the structure should be should be within the tolerable limits.
When shallow foundation avoided : When the upper soil layer is highly compressible and too weak In the case of Expansive soils In case of Bridge abutments and piers because of soil erosion at the ground surface Soils such as loess are collapsible in nature
Types of Shallow Foundation: Spread footing: A spread footing is one which supports either one wall or one column. Spread footing may be of the following types – Strip footing Pad footing Fig: Pad Footing
Types of Shallow Foundation(cont.): Combined footing: When a spread footing supports the load of more than one column or wall. Fig: Combined Footings
Types of Shallow Foundation(CONT.): Strap footing: : A strap footing comprises of two or more footings of individual columns, connected by a beam, called a strap. Raft foundation: A raft foundation is a combined footing that covers the entire area beneath a structure and supports all the walls and columns. Fig: Strap Footings
Types of Shallow Foundation(CONT.): Fig- Raft foundations Requirements for the raft foundations: The allowable soil pressure is low, or the building loads are heavy Use of spread footings would cover more than one-half of the area Soil is sufficiently erratic so that the differential settlement difficult to control
FACTORS FOR DEPTH OF FOUNDATION: Bearing capacity of soil Ground water table Depth of frost action Depth of volume change due to presence of expansive soils Local erosion of soil due to flowing water Underground defects such as root holes, cavities, mine shafts, etc. excavation, ditch, pond, water course, filled up ground
PRESSURE DISTRIBUTION BELOW FOOTINGS The distribution of soil pressure under a footing is a function of the type of soil, the relative rigidity of the soil and the footing, and the depth of foundation at level of contact between footing and soil.
GROUND WATER TABLE AND FOOTINGS A RISING WATER TABLE HAVE FOLLOWING ADVERSE EFFECTS : Appreciable reduction in the bearing capacity Development of uplift pressure Possible ground heave due to the reduction of the effective stresses caused by the increasing pore water pressures. Expansion of the heavily compacted fills under the foundation Appreciable settlements of the poorly compacted fills
SOIL STIFFNESS PARAMETER AND FOOTING Soil stiffness is generally measured in the terms of Modulus of sub- grade reaction (K-value). Where, p = load intensity corresponding to settlement of plate (30cm x 30cm) of 0.125 cm. TABLE: K-VALUE CHANGES WITH SOIL CHARACTERISTICS
SOIL STIFFNESS PARAMETER AND FOOTING (cont). Foundation Size Effect on Modulus of Sub grade Reaction in Clayey Soil : Foundation Size Effect on Modulus of Subgrade Reaction In Sandy Soils:
BEARING CAPACITY and footing Factors influencing Bearing Capacity: Type of soil III. Unit weight of soil Surcharge load IV. Depth of foundation V. Mode of failure VI. Size of footing VII. Shape of footing VIII. Depth of water table IX. Eccentricity in footing load Inclination of footing load Inclination of ground Inclination of base of foundation
MODES OF SHEAR FAILURE General shear failure: Seen in dense and stiff soil. Fig: Fig: General shear failure Local shear failure: Seen in relatively loose and soft soil. Fig: Fig: Local shear failure
MODES OF SHEAR FAILURE (CONT.): Punching shear failure: Seen in loose , soft soil and at deeper elevations. Fig- punching shear failure TERZAGHI’S BEARING CAPACITY THEORY: According to Terzaghi the equation for ultimate bearing capacity for a strip footing is obtained as follows, ultimate bearing capacity qf = cNC + γDNq +0.5 γBNγ
BEARING CAPACITY OF FOOTINGS (CONT.) Circular footing : qf = 1.3 cNc + γDNq +0.3 γBNγ Square footing: qf = 1.3 cNc + γDNq +0.4 γBNγ Rectangular footing: qf = (1+0.3 B/L)cNc + γDNq + (1-0.2 B/L)0.5γBNγ Ultimate bearing capacity with the effect of water table is given by, qf= cNC + γDNq RW1+0.5 γBNγ RW2 Effect of Water Table fluctuation : qf = cNC + γDNq RW1+0.5 γBNγ RW2
Effect of Water Table fluctuation :(cont.) CASE 1: Where, ZW1 is the depth of water table from ground level. CASE 2: Where, ZW2 is the depth of water table from foundation level.
BEARING CAPACITY ACCORDING TO INDIAN STANDARD CODES General shear failure: qf = c Nc sc dc ic + q (Nq-1) sq dq iq + 0.5γ B Nγ sγ dγ iγ W Local shear failure: qf = ⅔ c N'c sc dc ic + q (Nq-1) sq dq iq + 0.5γ B N'γ sγ dγ iγ W Shape factors for different shapes of footings:
BEARING CAPACITY ACCORDING TO INDIAN STANDARD CODES (cont.) Depth factors: Inclination factor : Values of W: Water table remain at or below a depth of (Df + B), then W= 1. Water table located at depth Df or likely to rise above the base then, W= 0.5 If Df < Dw < (Df + B), then W be obtained by linear interpolation dc = 1 + 0.2 Df/B √Nφ dq = dγ = 1 for ф < 10° dq = dγ = 1 + 0.1 Df/B √Nφ for ф > 10° ic = iq = (1- α /90)² iγ = (1- α /ф)²
SETTLEMENTS OF SHALLOW FOUNDATION The total settlement of a footing in clay may be considered to three components (Skempton and Bjerrum, 1957) Immediate Settlement: Values for influence factors, If : S = Si + Sc +Ss
Settlement s of shallow foundation(cont.) Primary Consolidation: The primary consolidation settlement Sc is given by the following formula: Sc = Values of 𝜆 for different types of soil :
Settlement s of shallow foundation(cont.) Secondary consolidation: Secondary consolidation settlement is more important in the case of organic and highly-compressible inorganic clays which is given by, Ss = Cα = Secondary Compression Index = Fig: void ratio vs. time (log scale)
CORRECTION ON TOTAL SETTLEMENT FOR DEPTH AND RIGIDITY Effect of Depth of Foundation: Corrected settlement = Scorrected = Sc x Depth factor Fig: Fox’s correction curves for settlements of flexible Rectangular footings of BxL at depth D
CORRECTION ON TOTAL SETTLEMENT FOR DEPTH AND RIGIDITY 2) Effect of the rigidity of foundation: Rigidity factor = = 0.8 TABLE: Permissible uniform and differential settlement and tilt for footings
PLATE LOAD TEST DETERMINATION OF SETTLEMENT: LOADING SYSTEMS: There are two loading set-up : Fig: set up for gravity loading platform Fig: set up for reaction loading platform DETERMINATION OF SETTLEMENT: According to Terzaghi and Peck (1948):
PLATE LOAD TEST(cont.) Table: Values of index n for different soils: According to Bond (1961): Table: Values of index n for different soils: DETERMINATION OF BEARING CAPACITY: Bearing capacity can be obtained from the load settlement curve that can be plotted from settlement data.
Fig : Load- settlement curves PLATE LOAD TEST(cont.) Fig : Load- settlement curves o obtained from test From the corrected load settlement curves (given below)the ultimate bearing capacity in case of dense cohesionless soils or cohesive soils can be obtained without difficulty (curves D and B ) as the failure is well defined.
Fig : Corrected Load–Settlement curve (in log-log scale) PLATE LOAD TEST(cont.) Fig : Corrected Load–Settlement curve (in log-log scale) The bearing capacity of sands and gravels increases with the size of footings.
The following conclusions can be drawn , they are - Shallow foundations are used when the soil has sufficient strength within a short depth below the ground level. Terzaghi’s equation is generally used for computation of bearing capacity of soil. For design purpose, it is usually necessary to investigate both the bearing capacity of soil and the settlement of a footing. Plate load test is used to determine the ultimate bearing capacity and settlement of a footing in field. There are another tests like S.P.T and C.P.T also used to determine ultimate bearing capacity.
REFERENCES IS 6403: 1981 (Reaffirmed 2002): Code of practice for determination of breaking capacity of shallow foundations IS:1888:1982 (Reaffirmed 1995) : Method of load test on soils IS 1080 - 1985 (Reaffirmed 1997): Code of practice for design and construction of shallow foundations in soils (other than raft, ring and shell). IS 2950 (Part1) -1981 (Reaffirmed 1998): Code of practice for design and construction of raft foundations - part 1 design. IS 8009 (Part 1) - 1976 (Re affirmed 1998): Code of practice for calculation of settlements of foundations part-1(swallow foundations subjected to symmetrical static vertical loads). IS 8009 (Part 2) - 1980 (Re affirmed 1995): Code of practice for calculation of settlements of foundations part-2(deep foundations subjected to symmetrical static vertical loading). IS 9214 - 1979 (Re affirmed 1997): Method of determination of modulus of subgrade reaction (k-value) of soils in field. Soil mechanics and foundation: Punmia, Jain and Jain. NPTEL – Advanced foundation engineering.
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