Test: Bearing Capacity - Civil Engineering (CE) MCQ

Concepts:

The maximum pressure a soil can withstand without undergoing settlement in excess of the permissible value for the structure is called allowable bearing capacity or net safe bearing pressure or safe bearing pressure.

The maximum pressure a soil can withstand without the occurrence of shear failure is called ultimate bearing capacity of soil (q_u).

The net pressure at the base of foundation in excess of initial overburden pressure which a soil can withstand without shear failure called net ultimate bearing capacity.

Or

Q_nu = q_u­  - Overburden pressure(q)

When factor of safety is applied on the net ultimate bearing capacity, then it is called net safe bearing capacity (q_ns).

Safe bearing capacity: q_s = q_ns + Effective overburden pressure

Given,

bulk unit weight (γ_b) = 20 kN/m3

Undrained shear strength (S = C) = 25 kPa

Bearing Capacity N_c = 5.7, General Shear Failure

We know q_u = CN_c

q_u = γ D_f = 25 × (5.7) 

20 × D_f = 142.5

D_f = 7.124 m

Ultimate bearing capacity for a strip footing is

For pure clay, N_c = 5.14, N_q = 1 and Nγ = 0 (∵ assuming smooth footing)

Footing is on the ground surface i.e. D = 0

q_u = c_uN_c

qu = 5.14 c_u

qu = (π + 2)c_u

As per Terzaghi's method, ultimate bearing capacity of strip footing is given by the equation

q_u = CN_c + γD_f.N_q + 0.5.B.γN_γ 

Where, B = Width of footing (Least lateral dimension)

Df = Depth of footing below G.L.

γD_f = Surcharge at foundation level.

N_c, N_q, N_γ are bearing capacity factors which depend upon frictional angle of soil.

For purely cohesive soils, ϕ = 0° 

N_c = 5.7, N_q = 1 and N_γ = 0

IS code specification for permissible settlement:

(i) Total Permissible settlement:

• For isolated footing on clay = 65 mm
• For isolated footing on sand = 40 mm
• For raft footing on clay = 65-100 mm
• For raft footing on sand = 40-65 mm

(ii) Permissible Differential settlement:

• For isolated footing on clay = 40 mm
• For isolated footing on sand = 25 mm

(iii) Permissible angular settlement:

• For high framed structure < 1/500
• To prevent all type of minor damage < 1/1000

Note: For multi-storeyed buildings having isolated foundations on sand, the maximum permissible settlement is 60 mm [ For multistorey buildings having isolated foundations take the higher load as compare to single storey buildings having isolated foundations. So that deflection caused by multistorey building having isolated foundation higher than 40 mm (from the safer side)]

Plate Load Test:

• It is a field test, which is performed to determine the ultimate bearing capacity of the soil and settlement under a given load.
• This test is done for the selection and design of the shallow foundation.
• For performing this test, the plate is placed at the desired depth, then the load is applied gradually and the settlement for each increment of the load is recorded.

The limitations of the plate load test are:

1. It has a limited depth of influence. It could only give the bearing capacity of soils with depths up to two times the width of the bearing plate.
2. It may not provide information on the potential for long-term consolidation of foundation soils.
3.  There is a scale effect as the size of the test plate is smaller than the actual foundation. For example, the bearing capacity of sands and gravels increases with the size of the footing.
4. To gain access to the test position, excavation is carried out which causes significant ground disturbance. The change in ground stress leads to the change of soil properties which the test is planned to investigate.
5. In the case of cohesive soils, this test does not give the ultimate settlement.

SPT test can be conducted to determine:

a) Relative Density of sands

b) Angle of internal friction

c) Unconfined compressive strength of clays

d) Ultimate bearing capacity on the basis of shear criteria

e) Allowable bearing pressure on the basis of settlement criteria

In this test, the split spoon sampler is driven by dynamic mechanism of hammer. This test is conducted either at every 2 to 5 meter interval or at the change of stratum.

Note:

The weight of the hammer is 63.5 kg.

The height of free fall is 750 mm or 75 cm.

The inner and outer diameter of the sampler is 35 mm and 50.5 mm respectively.

SPT test can be conducted to determine:

(a) Relative Density of sands

(b) Angle of internal friction

(c) Unconfined compressive strength of clays

(d) Ultimate bearing capacity on the basis of shear criteria

(e) Allowable bearing pressure on the basis of settlement criteria

In this test, the split spoon sampler is driven by dynamic mechanism of hammer. This test is conducted either at every 2 to 5 meter interval or at the change of stratum.

Note:

The weight of the hammer is 63.5 kg.

The height of free fall is 750 mm or 75 cm.

The inner and outer diameter of the sampler is 35 mm and 50.5 mm respectively.

Concept;

Dilatancy correction:

It is to be applied when N_o obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when N_o > 15) using the equation

N₀ - SPT value after overburden correction

Calculation:

Given: N₀ = 21

N = 15 + 1/2(21−15)

⇒ N = 18

Concept:

Plate load test: 

• The plate load test is a field test, which is performed to determine the ultimate bearing capacity of the soil and the probable settlement under a given load.
• This test is very popular for the selection and design of shallow foundation.

Procedure:

• Excavate test pit up to the desired depth. The pit size should be at least 5 times the size of the test plate (B_p).
• At the center of the pit, a small hole or depression is created. The size of the hole is the same as the size of the steel plate.
• A mild steel plate is used as a load-bearing plate whose thickness should be at least 25 mm thickness and size may vary from 300 mm to 750 mm. The plate can be square or circular. Generally, a square plate is used for square footing and a circular plate is used for circular footing.
• A column is placed at the center of the plate. The load is transferred to the plate through the centrally placed column.
• The load can be transferred to the column either by the gravity loading method or by the truss method.
• At least two dial gauges should be placed at diagonal corners of the plate to record the settlement. The gauges are placed on a platform so that it does not settle with the plate.
• Apply a seating load of 0.7 g/cm² or T/m2 and release before the actual loading starts.
• The initial readings are noted.
• The load is then applied through the hydraulic jack and increased gradually. The increment is generally one-fifth of the expected safe bearing capacity or one-tenth of the ultimate bearing capacity or any other smaller value. The applied load is noted from the pressure gauge.
• The settlement is observed for each increment and from the dial gauge. After increasing the load-settlement should be observed after 1, 4, 10, 20, 40, and 60 minutes and then at hourly intervals until the rate of settlement is less than .02 mm per hour. The readings are noted in tabular form.
• After completing the collection of data for a particular loading, the next load increment is applied and readings are noted under new load. This increment and data collection is repeated until the maximum load is applied. The maximum load is generally 1.5 times the expected ultimate load or 3 times of the expected allowable bearing pressure.