1.1.1                            ESTIMATION OF SETTLEMENT

• General

Prediction of settlement is an important part of foundation design to ensure the future stability and serviceability of the structure supported by the foundation. The prediction of settlement comprising immediate settlement, primary consolidation settlement and secondary consolidation settlement should be:

• based on the results of a proper site investigation and appropriate laboratory or field tests identifying the conditions of the groundwater and the ground that contribute to the settlement of the foundation;
  • based on the principles of mechanics or established empirical methods proven with adequate correlation; and
  • applicable to Hong Kong soils and in conformity with case histories.

(2)                    Foundations on Granular Soils

Methods for computing immediate settlements of foundations on granular soils are based on theory of elasticity, empirical correlations or full-scale loading

tests. Empirical correlations between foundation settlement and results  of insitu tests such as standard penetration tests generally provide an acceptable solution for estimating the settlement of a shallow foundation on granular soils. Based on the theory of elasticity, the settlement of a shallow foundation can be calculated using an equation of the following form:

S = qnet Bf ‘ Fo e            E

s

Where

S_e = immediate settlement

q _net = mean net foundation bearing pressure (the net foundation bearing pressure is the total foundation bearing pressure less effective overburden pressure at the base of the foundation)

B_f’    =  effective width of the foundation E_s = Young’s modulus of soil

F_o    =  a coefficient whose value depends on the shape and dimensions of     the foundation, the variation of soil stiffness with depth, the thickness of compressible strata, Poisson’s ratio, the distribution of ground bearing pressure and the point at which the settlement is calculated. Reference should be made to GEO Publication No. 1/2006 for determination.

(3)                   Foundations on Fine-Grained Soils

For fine-grained soils, immediate settlement may be estimated using the same equation for granular soils. In addition to the immediate settlement, consolidation settlement should also be considered. An estimate of the consolidation settlement can be made using the settlement-time curve obtained from oedometer tests or other sources of reference that suit the conditions of the site. Consolidation settlement may be considered to consist of primary consolidation and secondary consolidation stage.

The primary consolidation settlement of a soil layer due to an applied loading depends on the relative magnitudes of the initial vertical effective stress acting on the soil and the effective preconsolidation pressure, and can be estimated as follows:

For s

‘ = s

‘< s

‘ + Ds

S = H (CR log sn0 ‘ + Dsn )

n0            p            n0                n                      p            s

sn0 ‘

For s

‘ < s

‘ < s

‘ + Ds

S = H (CR log sn0 ‘ + Dsn + RR log

sp ‘ )

n0            p            n0                n                      p            s

sp ‘                      sn0 ‘

For s

‘< s   ‘ + Ds  < s ‘     S = H (RR log sn0 ‘ + Dsn )

n0            n0                n            p                      p             s

sn0 ‘

Where	σv0′	=	initial vertical effective stress in the soil layer
	σp’	=	effective preconsolidation pressure, which is the maximum vertical effective stress that has acted on the soil layer in the past and can be determined from laboratory oedometer tests
	Δσv	=	change in vertical effective stress due to the fill and future imposed load on the soil layer to be considered
	Sp	=	ultimate                primary concerned	consolidation	settlement	of	the	layer
	Hs	=	thickness of the soil layer to be considered
	CR	=	compression ratio, equal to the slope of the virgin compression portion of the ε-logσ ‘ plot as shown in Figure 2.5
		=	Cc 1 + e0
	RR	=	recompression ratio, equal to the average slope of the recompression portion of the ε-logσ ‘ plot as shown in Figure 2.5
		=	Cr 1 + e0
	Cc	=	compression index which can be estimated from laboratory
			oedometer tests
	Cr	=	recompression index which can be estimated from laboratory
			oedometer tests
	e0	=	initial void ratio of the layer