Shallow Foundations - 2

# Shallow Foundations - 2 | Foundation Engineering - Civil Engineering (CE) PDF Download

Effect of Water Table on Bearing Capacity of Soil

where Rq* and Ry* are water table correction factor.

when 0 ≤ zq ≤ Df when 0 ≤ zγ ≤ B.
If Zγ > B they Rγ= 1
If Zγ ≤ 0 they
If water table rise to G.L
Rq* = 1 / 2 and Rγ* = 1 / 2

1. Significant only for cohesionless.
2. Short duration test hence only results in immediate settlement.
(i) quf / qup = Bf / Bp
(ii) quf = qup
..for ∅ = soil … for C-soil
If plate load test carried at foundation level then

(iii)
(iv) Sf / Sp = Bf / Bp
… for dense sand.  … for clays
(v) Sf / Sp = (Bf / Bp)n + 1
… for silts.
where,
quf =Ultimate bearing capacity of foundation
qup = Ultimate bearing capacity of plate
Sf = Settlement of foundations
Sp = Settlement of plate
Bf = Width of foundation in m
Bp = Width of plate in m
Housels Approach
QP = mAP + nPP
Qf = mA+ nPf
where, Qp = Allowable load on plate m and n are constant
P = Perimeter Ap = Area of plate
Af = Area of foundation

### Standard Penetration Test

Significant for Granular Soils

1.  and
where, N1 = Overburden pressure correction
N0 = Observed value of S.P.T. number.
= Effective overburden pressure at the level of test in kM/m2.
2. For Saturated    fine sand and silt, when N1 > 15
N2 = 1/2(N1 - 15) + 15
where, N2 = Dilatancy correction or water table correction.
Nq + Nγ related to N value using peck Henson curve or (code method)
Teng's formula relate N value with reading capacity of granular soil.

Pecks Equation
qa net = 0.44NS = CwkN/m2

Dw = depth of water table below G.L
Df = Depth of foundation
B = Width of foundation
N = Avg. corrected S.P.T. no.
S = Permissible settlement of foundation
Cw = Water table correction factor
qa net = Net allowable bearing pressure.

Teng's Equations

Cw =Water table correction factor
Dw = Depth of water table below foundation level
B = Width of foundation
Cd =Depth correction factor
S = Permissible settlement in 'mm'.

I.S Code Method

qns =Net safe bearing pressure in kN/m2
B = Width in meter.
S = Settlement in 'mm'.

I.S. Code Formula for Raft

qns = 0.88NSCw

Cw: Same as of peck Henson.

Meyer-Hoffs Equation

qns = 0.88NSCwCd
where, qns = Net safe bearing capacity in kN/m2.
B < 1.2 m

B ≥ 1.2 m (where qns is in kN/m2.

### Cone Penetrations Test

1.  where, = Static cone resistance in kg/cm2c = Compressibility coefficient  = Initial effective over burden pressure in kg/cm2.
2.  where, 'S' = Settlement.
3. qns = 3.6qsRw B > 1.2 m.where, qns = Net safe bearing pressure in kN/m2.
4. qns = 2.7qc.Rw B < 1.2 m.where, Rw = Water table correction factor.
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## FAQs on Shallow Foundations - 2 - Foundation Engineering - Civil Engineering (CE)

 1. What are the different types of shallow foundations?
Ans. Shallow foundations can be classified into five main types: 1) Strip foundations: These are continuous footings that distribute the load of a structure evenly over a wide area. 2) Pad foundations: These are individual footings that support isolated columns or point loads. 3) Raft foundations: Also known as mat foundations, these are large slabs that distribute the load of a structure over a wider area, suitable for weak or soft soil conditions. 4) Stepped foundations: These are used when the ground slopes, and the foundation needs to be built at different levels to maintain a level surface for construction. 5) Combined footings: These are footings that support multiple columns or wall loads and are designed to distribute the load to avoid excessive settlement.
 2. What factors should be considered in the design of shallow foundations?
Ans. Several factors should be considered in the design of shallow foundations, including: 1) Soil properties: The type, strength, and settlement characteristics of the soil should be evaluated to determine the appropriate foundation design. 2) Load requirements: The magnitude and distribution of the loads from the structure need to be considered to ensure the foundation can adequately support them. 3) Site conditions: Any existing structures, utilities, or site constraints should be taken into account during the foundation design process. 4) Water table: The depth and fluctuation of the water table can affect the stability and performance of shallow foundations, so it must be considered in the design. 5) Construction feasibility: The construction methods and equipment available should be considered to ensure that the chosen foundation design can be effectively implemented on-site.
 3. How is the bearing capacity of a shallow foundation determined?
Ans. The bearing capacity of a shallow foundation is determined by conducting a geotechnical investigation, which involves soil testing. Some common methods used to determine bearing capacity include: 1) Standard Penetration Test (SPT): This test involves driving a split-spoon sampler into the ground and recording the number of blows required to penetrate the soil a certain distance. The blow count is then correlated with the soil's bearing capacity. 2) Cone Penetration Test (CPT): This test involves pushing a cone-shaped penetrometer into the ground and measuring the resistance. The recorded data is used to determine the soil's bearing capacity. 3) Plate Load Test: This test involves applying a known load to a steel plate placed on the ground surface and measuring the resulting settlement. The load is increased until the desired settlement is reached, and the bearing capacity is calculated based on the applied load. 4) Pressuremeter Test: This test involves expanding a cylindrical probe in the ground and measuring the pressure required for expansion. The pressure data is used to determine the soil's bearing capacity. 5) Laboratory Testing: Various laboratory tests, such as triaxial tests or direct shear tests, can be performed on soil samples to determine their strength properties and ultimately estimate the bearing capacity.
 4. What are the advantages of using shallow foundations?
Ans. Shallow foundations offer several advantages, including: 1) Cost-effective: Shallow foundations are generally more economical compared to deep foundations, as they require less material and construction time. 2) Construction simplicity: Shallow foundations are relatively easy to construct and require less specialized equipment and expertise compared to deep foundations. 3) Limited excavation: Shallow foundations typically do not require extensive excavation, which can reduce the impact on the surrounding environment. 4) Less settlement: Shallow foundations distribute the load over a larger area, resulting in less settlement compared to concentrated loads on deep foundations. 5) Suitable for various soil types: Shallow foundations can be designed to accommodate different soil conditions, including cohesive soils, non-cohesive soils, and mixed soils.
 5. What are the limitations of using shallow foundations?
Ans. Despite their advantages, shallow foundations have some limitations, including: 1) Limited load capacity: Shallow foundations may not be suitable for structures with heavy loads or large overturning moments, as they have a relatively lower load-bearing capacity compared to deep foundations. 2) Soil sensitivity: Shallow foundations are more susceptible to soil movements and settlement caused by changes in moisture content, which can affect their stability. 3) Depth limitations: Shallow foundations are typically limited to depths of up to 3 meters, beyond which deep foundations are usually required to achieve sufficient bearing capacity. 4) Potential for differential settlement: Shallow foundations may experience differential settlement, where different parts of the foundation settle at different rates, leading to structural issues. 5) Vulnerability to expansive soils: Shallow foundations may be prone to damage in areas with expansive soils that undergo significant volume changes due to moisture variations. Proper soil investigation and design considerations are necessary to mitigate these risks.

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