CHAPTER 9

GROUND SETTLEMENT

 

9.1 GROUND SETTLEMENT ANALYSIS

For the GRS-RW siting on the foundation susceptible to settlement, the stress develops in the geosynthetic layers due to differential settlement between the backfill and facing has to be evaluated.

 

The magnitude of settlement of a soft ground foundation develops with time. Thus, the function of the structure has to be reflected in selecting the analytical procedure. Since many uncertainties are involved in the procedure, the calculated results should only be taken as a guide. Observational procedure should be used during construction such that the settlement prediction will be modified and improved accordingly. The procedure as presented below is limited to the settlement prior to construction.

The details of observational procedure are given in the Construction Manuals of the Japan Highway Public Corporation.

 

1. Settlement Analysis

a) Estimation of final consolidation settlement

i) e-logp curve

  (9.1)

where

Sc

: final consolidation settlement (m)

eo

: initial void ratio of considered soil layer

e1

: void ratio at the end of consolidation, corresponding to po+D p in e-logp curve

H

: thickness of considered soil layer (m)

po

: overburden stress before construction (kPa)

D p

: stress increment due to GRS-RW (kPa)

 

 

ii) For normally consolidated soil, Eq. 9.1 is written using Cc and mv:

  (9.2)

where

Cc

: compression index

 

  (9.3)

where

mv

: coefficient of compressibility (1/kPa)

 

These two methods require parameters from consolidation tests. If the consolidation test results are not available, an estimation of Cc may be made using the following equations:

  (9.4)

or

  (9.5)

where

wL

: liquid limit (%)

en

: natural void ratio (= 0.0265wn)

wn

: natural water content (%)

 

iii) Estimation based on m

The compressibility ratio, m is calculated based on natural water content wn and consolidation stress, p (Figure 9.1). The settlement is then estimated as

 

  (9.6)

 

iii) Stress Increment due to GRS-RW Loading

The Osterberg's method (Figure 9.2) is used to estimate the stress increment at the center of each soil layer. For embankment widening project, the stress increment is calculated by subtracting the stress increment of the existing embankment from that of the whole structure (existing embankment and new fill).

 

 

 

 

 

b) Residual Consolidation Settlement

i) Time factor

  (9.7)

where

Tv

: time factor

Cv

: coefficient of consolidation (cm2/min)

H

: maximum drainage path (for drainage at both ends, divide H by 2) (cm)

t

: time taken from the end of consolidation to facing installation (min)

 

 

ii) Degree of consolidation

The degree of consolidation U is determined from Tv (see Figure 9.3).

 

iii) Consolidation settlement

For inorganic clay, the following equation may be used. For organic clay, secondary consolidation has to be considered.

  (9.8)

where

D S

: consolidation settlement (m)

S

: total settlement (m)

U

: degree of consolidation right before facing installation

 

2. Stress Developed between Geosynthetic Layers and Facing

The settlement may be controlled by adjusting the time for which the concrete facing will be installed. If the consolidation settlement will exceed 10 cm, the stress developed in between the facing and geosynthetic layers has to be considered. The differential settlement and stresses may be obtained through a finite element analysis.

If the ground improvement is needed, the Manual on Soft Ground Improvement, such as that published by the Japan Highway Public Corporation, should be consulted.

 

 

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