Session 7 – 8 SETTLEMENT OF SHALLOW FOUNDATION Course : S0484/Foundation Engineering Year : 2007 Version : 1/0 Session 7 – 8 SETTLEMENT OF SHALLOW FOUNDATION

Consolidation Settlement SHALLOW FOUNDATION Topic: General Immediate Settlement Consolidation Settlement

GENERAL The settlement of shallow foundation may be divided into three broad categories: Immediate settlement, which is caused by the elastic deformation of dry soil and of moist and saturated soils without any change in the moisture content. Immediate settlement are generally based on equations derived from the elasticity theory Primary consolidation settlement, which is the result of a volume change in saturated cohesive soils because of expulsion of the water that occupies the void spaces. Secondary consolidation settlement, which is observed in saturated cohesive soils and is the result of the plastic adjustment of soil particles. This course will focus at immediate and primary consolidation settlement only.

IMMEDIATE SETTLEMENT

General Equation (Harr, 1966) IMMEDIATE SETTLEMENT General Equation (Harr, 1966) Flexibel Foundation At the corner of foundation At the center of foundation Average Rigid Foundation ; ; H =  Es = Modulus of elasticity of soil B = Foundation width L = Foundation length

IMMEDIATE SETTLEMENT

IMMEDIATE SETTLEMENT If Df = 0 and H < , the elastic settlement of foundation can be determined from the following formula: (corner of rigid foundation) (corner of flexible foundation) The variations of F1 and F2 with H/B are given in the graphs of next slide

IMMEDIATE SETTLEMENT

IMMEDIATE SETTLEMENT

EXAMPLE Problem: A foundation is 1 m x 2 m in plan and carries a net load per unit area, qo = 150 kN/m2. Given, for the soil, Es = 10,000 kN/m2, s 0.3. Assuming the foundation to be flexible, estimate the elastic settlement at the center of the foundation for the following conditions: a. Df = 0 and H =  b. Df = 0 and H = 5 m

EXAMPLE Solution: Part a. Part b. For L/B = 2/1 = 2    1.53, so For L’/B’ = 2, and H/B’ = 10  F1  0.638 and F2  0.033, so

General Equation (Bowles, 1982) IMMEDIATE SETTLEMENT General Equation (Bowles, 1982) Es = Modulus of elasticity of soil H = effective layer thickness, ex. 2 - 4B below foundation At the center of Foundation and F1 time by 4 At the corner of Foundation and F1 time by 1

IMMEDIATE SETTLEMENT For saturated clay soil

IMMEDIATE SETTLEMENT For sandy soil where: Iz = factor of strain influence C1 = correction factor to thickness of embedment foundation = 1 – 0.5x[q/(q-q)] C2 = correction factor due to soil creep = 1+0,2.log(t/0,1) t = time in years q = stress caused by external load q =  . Df

IMMEDIATE SETTLEMENT Circle Foundation or L/B =1 z = 0  Iz = 0.1 Young Modulus Circle Foundation or L/B =1 z = 0  Iz = 0.1 z = z1 = 0,5 B  Iz = 0.5 z = z2 = 2B  Iz = 0.0 Foundation with L/B ≥ 10 z = 0  Iz = 0.2 z = z1 = B  Iz = 0.5 z = z2 = 4B  Iz = 0.0

EXAMPLE A shallow foundation 3 m x 3 m (as shown in the following drawing). The subgrade is sandy soil with Young modulus varies based on N-SPT value (use the following correlation: Es = 766N) Determine the settlement occur in 5 years (use strain influence method)

EXAMPLE

EXAMPLE Depth (m) z Es (kN/m2) Iz (average) (m3/kN) 0.0 – 1.0 1.0 8000 0.233 0.291 x 10-4 1.0 – 1.5 0.5 10000 0.433 0.217 x 10-4 1.5 – 4.0 2.5 0.361 0.903 x 10-4 4.0 – 6.0 2.0 16000 0.111 0.139 x 10-4  1.55 x 10-4

CONSOLIDATION SETTLEMENT

CONSOLIDATION SETTLEMENT Normal Consolidation Over consolidation or or po + p < pc po < pc < po+p

CONSOLIDATION SETTLEMENT where: eo = initial void ratio Cc = compression index Cs = swelling index pc = preconsolidation pressure po = average effective pressure on the clay layer before the construction of the foundation =  ’.z p = average increase of pressure on the clay layer caused by the foundation construction and other external load, which can be determine using method of 2:1, Boussinesq, Westergaard or Newmark. Alternatively, the average increase of pressure (p) may be approximated by: pt = the pressure increase at the top of the clay layer pm = the pressure increase at the middle of the clay layer pb = the pressure increase at the bottom of the clay layer

CONSOLIDATION SETTLEMENT

EXAMPLE A foundation 1m x 2m in plan is shown in the following figure. Estimate the consolidation settlement of the foundation. Assume the clay is normally consolidated.

EXAMPLE po = (2.5)(16.5) + (0.50)(17.5-10) +(1.25)(16-10) = 52.5 kN/m2 2:1 method

ALLOWABLE SETTLEMENT