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

Effect of Water Table on Bearing Capacity of Soil
 

where R_q^* and R_y^* are water table correction factor.


when 0 ≤ z_q ≤ D_f when 0 ≤ z_γ ≤ B.
If Z_γ > B they R_γ^* = 1
If Z_γ ≤ 0 they
If water table rise to G.L
R_q^* = 1 / 2 and R_γ^* = 1 / 2

Plate Load Test

1. Significant only for cohesionless.
2. Short duration test hence only results in immediate settlement.
   (i) q_uf / q_up = B_f / B_p
   (ii) q_uf = q_up
   ..for ∅ = soil … for C-soil
   If plate load test carried at foundation level then
   
   (iii) 
   (iv) S_f / S_p = B_f / B_p
   … for dense sand.  … for clays
   (v) S_f / S_p = (B_f / B_p)^{n + 1} 
   … for silts.
   where,
   q_uf =Ultimate bearing capacity of foundation
   q_up = Ultimate bearing capacity of plate
   S_f = Settlement of foundations
   S_p = Settlement of plate
   B_f = Width of foundation in m
   B_p = Width of plate in m
   Housels Approach
   Q_P = mA_P + nP_P
   Q_f = mA_f + nP_f
   where, Q_p = Allowable load on plate m and n are constant
   P = Perimeter A_p = Area of plate
   A_f = 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.
   N_q + N_γ 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
q_{a net} = 0.44NS = C_wkN/m²

D_w = depth of water table below G.L
D_f = Depth of foundation
B = Width of foundation
N = Avg. corrected S.P.T. no.
S = Permissible settlement of foundation
C_w = Water table correction factor
q_{a net} = Net allowable bearing pressure.

Teng's Equations
 
C_w =Water table correction factor
D_w = Depth of water table below foundation level
B = Width of foundation
C_d =Depth correction factor
S = Permissible settlement in 'mm'.

I.S Code Method
 
q_ns =Net safe bearing pressure in kN/m²
B = Width in meter.
S = Settlement in 'mm'.

I.S. Code Formula for Raft

q_ns = 0.88NSC_w

C_w: Same as of peck Henson.

Meyer-Hoffs Equation

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

B ≥ 1.2 m (where q_ns 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. q_ns = 3.6q_sR_w B > 1.2 m.where, q_ns = Net safe bearing pressure in kN/m².
4. q_ns = 2.7q_c.R_w B < 1.2 m.where, R_w = Water table correction factor.