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WHAT IS PRESUMPTIVE BEARING CAPACITY OF SOIL AND ITS VALUES? Suryakanta | November 8, 2014 | Geotechnical | 1 Comment

WHAT IS PRESUMPTIVE BEARING CAPACITY? Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footing. These are called presumptive bearing capacity values. These values are based on experience with other structures already built.

WHAT ARE THE VALUES OF PRESUMPTIVE BEARING CAPACITY? The table given below shows the presumptive bearing capacity values for different types of soils. Type of Soil / Rock

Safe / Allowable Bearing Capacity (kN/m2)

Rock

3240

Soft rock

440

Coarse sand

440

Medium sand

245

Fine sand

440

Soft shell / Stiff clay

100

Soft clay

100

Very soft clay

50

NOTES 1. For non-cohesive soils the values should be reduced by 50% if the water table is above or near base of footing. 2. These values should be used only for preliminary design. The actual bearing capacity of soil should be calculated as per the standard codes.

WHAT ARE THE LIMITATIONS OF USING PRESUMPTIVE BEARING CAPACITY VALUES?

  

Presumptive bearing capacity values are based only on visual classification of surface soil. So these values are not true representation of the actual capacity and not reliable. These values don’t consider important factors affecting the bearing capacity; such as the shape, width, depth of footing, location of water table, strength and compressibility of the soil. These values are conservative and can only be used for preliminary design or even for final design of small unimportant structures.

TABLE 1806.2 PRESUMPTIVE LOAD-BEARING VALUES

CLASS OF MATERIALS

VERTICAL FOUNDATION PRESSURE (psf)

LATERAL BEARING PRESSURE (psf/ft below natural grade)

LATERAL SLIDING RESISTANCE Coefficient of frictiona

Cohesion (psf)b

1. Crystalline bedrock

12,000

1,200

0.70

‒

2. Sedimentary and foliated rock

4,000

400

0.35

‒

3. Sandy gravel and/or gravel (GW and GP)

3,000

200

0.35

‒

4. Sand, silty sand, clayey sand, silty gravel and clayey gravel (SW, SP, SM, SC, GM and GC)

2,000

150

0.25

‒

5. Clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH and CH)

1,500

100

‒

130

For SI: 1 pound per square foot = 0.0479kPa, 1 pound per square foot per foot = 0.157 kPa/m. a. Coefficient to be multiplied by the dead load. b. Cohesion value to be multiplied by the contact area, as limited by Section 1806.3.2

DESIGN OF FOUNDATIONS - PRESUMPTIVE BEARING CAPACITY VALUES FROM CODES. For the design of foundations of lightly loaded structures and for a preliminary design of any structure the presumptive safe bearing capacity may be used. The presumptive safe bearing capacities of various types of soils are given in Table 2.2. TABLE 2.2 PRESUMPTIVE SAFE BEARING CAPACITY ( IS: 1904-1961 )

Note 1. Compactness or loosenEss of non-cohesive matenais may be determined by driving a wooden picket of dimension 5cm x 5 cm x lOcm with a sharp point. The picket shall be pushed vertically into the soil by full weight of a person and if the penetration of the picket exceeds 20 cm, the loose state shall be asaumed to exist. Note 2. No generalised values for presumptive safe bearing capacities can be given for these types of soils. In such area, adequate site investigation shall be carried out and expert advice shall be sought. Note 3. Peat may occur in a very soft spongy condition or may be quite firm and compact. While ultimate bearing capacity may be high in the compact cases very large consolidation settlements occur even under small presures and the movements continue for decades. Note 4. The strength of made-up ground depends on the nature of the material, its depth and age, and the method used for consolidating it. Note 5. The presumptive safe bearing values may be increased by an amount equal to weight of the material (virgin soil) removed from above the bearing level, that is, the base of the foundation. Note 6. For non-coheisve soils, the presumptive safe bearing values shall be reduced by 50 percent if the water table is above or near the bearing surface of the soil. If the water table is below tje bearing surface of the soil at a distance at least equal to the width of the foundation, no such reduction shall apply. For intermediate depths of the water table, proportional reduction of presumptive safe bearing value may be made.