TASK SUMMARY

EFFECT OF CREEP ON THE SETTLEMENT-TIME RELATION

DURING PRIMARY CONSOLIDATION OF CLAY

ADVANCED CIVIL AND ENVIRONMENT ENGINEERING

Taufiq (10-8705-601-88)

Master Degree Course Faculty of Civil and Environment Engineering

YAMAGUCHI UNIVERSITY2010

0

EFFECT OF CREEP ON THE SETTLEMENT-TIME RELATION DURING

PRIMARY CONSOLIDATION OF CLAY

This paper discusses several approaches to calculation of settlements of

foundation soils with creep – two approaches based on Hypothesis B and

one based on Hypothesis A. Hypothesis A ignores compression in the

primary consolidation period, therefore underestimates the total

settlement. Hypothesis B considers creep compressions in both primary

and the secondary consolidation periods and is logically correct, but

faces difficulties in implementation. The paper is emphasized on a

simplified approach to calculation of settlements of soils with creep, to

clarify the effect of creep on the settlement-time during the primary

consolidation where the results for saturated clay were analyzed by

Yin`s EVP Model.

Introduction

Soft soils have been encountered in Japan and many places of the world.

Soft marine soils are often encountered in the civil constructions such

as: reclamation, seawalls near-shore, and foundations on reclaimed land.

The soft marine soils are problematic foundation soils for civil

engineering purposes due to low shear strength and high time-

dependent compressibility. The skeleton of soft soils exhibits time-

dependent stress-strain behavior such as creep, relaxation, strain-rate

effects.

Creep is the continuous compression of the soil skeleton under a

constant effective stress. Soils creep is due to mainly: a). viscous

squeezing out of adsorbed water in double layers on clay particles and

b). viscous re-arrangement/deformation of clay particles. Adsorbed

water isn`t free water and can`t flow freely under gravity or hydraulic

gradient. Under certain effective stress, however, the adsorbed water

will move out slowly. At the same time, the clay particles (plate

structure) move closer and rearrange to a new equilibrium position,

resulting further deformation or compression of the soil. This

compression occurs slowly with the time. According to the mechanism,

the creep will occur whenever effective stresses exist in the soils or are

acting on the clay particles with adsorbed water, independent of the free

1

pore water in voids or the consolidation process.

A simple one-dimensional consolidation model consists of rectilinear

element of soil subject to vertical changes in loading and through which

vertical (only) seepage flow is taking place. There are three variables:

1. the excess pore pressure ( )

2. the depth of the element in the layer (z)

3. the time elapsed since application of the loading (t)

For 1-D straining condition, there are two methods for the calculation of

consolidation settlement with creep which are Hypothesis A and

Hypothesis B. The first Hypothesis (Hypothesis A) assumes that creep

occurs after “primary” consolidation. Therefore, the total settlement is

equal to primary consolidation only before time at the end of primary

consolidation (tEOP) and equal to the “primary” consolidation settlement

plus “secondary” consolidation (creep) settlement after tEOP. It is

commonly appreciated in the international geotechnical community that

Hypothesis A is logically wrong but simple in calculation and still used by

most consulting firms.

Hypothesis B assumes that creep occurs during whole

consolidation/compression process, during and after “primary”

consolidation. According to these, the total settlement is equal to instant

consolidation plus creep settlement from the starting time. The latter

method is considered to be logically correct, but complicated in

calculation. Furthermore, the paper is compared to a fully coupled

consolidation analysis approach based on Hypothesis B and using a 1-D

elastic visco-plastic model (Yin and Graham 1989, 1992) for soil

skeleton.

2

One of consolidation methods of consolidation evaluation is Terzaghi`s

one dimension consolidation theory. According to this theory any

increase in total stress primarily causes excess pore water pressure. By

continuous drainage, stress transfers to soil aggregates. After the excess

pore water pressure has been dissipated the effective stress will be

constant.

Separate –Type Consolidometer Test

The peat used in this study was taken from test site that located in

Kanagawa Prefecture (Japan) where the consolidation characteristics of

peat were compared with alluvial clay, separate-type-consolidometer

(STC) test on reconstituted Yokohama clay was also performed. Typical

particle size distribution curves and e-logp curves, also physical and

mechanical properties of samples are presented briefly in this paper.

The saturated specimen is 60 mm diameter and 100 mm thick which

divided into five subspecimens (H=20mm, D=60mm), enclosed in a

circular metal ring and sandwiched between porous stones.

Vertical static load increments are applied at regular time intervals (e.g.

12, 24, 48 hr). The load is doubled with each increment up to the

required maximum. During each load stage thickness changes are

recorded against time.

After full consolidation is reached under the final load, the loads are

removed (in one or several stages - to a low nominal value close to zero)

and the specimen allowed to swell, after which the specimen is removed

and its thickness and water content determined. With a porous stone

both above and below the soil specimen the drainage will be two-way.

After the end of primary consolidation it is known that the relationships

between the excess pore water pressure inside the specimen and

elapsed time for disturbed and undisturbed Yokohama peats, and for

Yokohama clay. At the early stage of consolidation process almost the

same excess pore water pressure as the applied consolidation pressure

is observed and dissipates with time. For the case of undisturbed, the

dissipation rate of the excess pore water pressure is higher compared

3

with the disturbed one. It is considered that the structure of soil in the

undisturbed sample is remained and so the high permeability is kept

during the consolidation process.

The effective stress in each sub specimen is different and in the

drainage side more rapidly increased. However, in the undrained side,

the effective stress at the initial consolidation is increase gradually with

the time.

Applying Yin`s Model to STC

Yin and Graham (1989, 1994) proposed a 1-D Elastic Visco-Plastic (1-D

EVP) constitutive relationship for the time-dependent stress-strain

behavior of soft soils in 1-D straining:

The one-dimensional compression and swelling characteristics of a soil

may be measured in the laboratory using the oedometer creep test data

(from the Greek: oidema = a swelling).

A cylindrical specimen of soil enclosed in a metal ring is subjected to a

series of increasing static loads, while changes in thickness are recorded

against time. From the changes in thickness at the end of each load

stage the compressibility of the soil may be observed, and parameters

measured such as Compression Index (Cc) and Coefficient of Volume

Compressibility (mv). From the changes in thickness recorded against

time during a load stage the rate of consolidation may be observed and

the coefficient of consolidation (cv) measured.

Comparison of Experimental Results with Numerical Analysis

The recorded thickness changes during one of the load stages in an

oedometer test are used to evaluate the coefficient of consolidation (cv).

The procedure involves plotting thickness changes (i.e. settlement)

against a suitable function of time [either Ötime or log(time)] and then

fitting to this the theoretical curve. An alternative to the Root-Time

method, that is particularly useful when there is significant secondary

compression (creep).

4

In the current research in order to introduce a new method to evaluate

creep settlement based on viscoelastic models, specimens obtained

from large domain of property were subjected to consolidation tests and

the results were analyzed. This method was also compared with

conventional method of creep settlement evaluation, Cα method. Hence,

when experimental data fit creep behavior of soft soil can be studied by

generalized Maxwell model. Using Terzaghi’s theory and the proposed

model, total settlement throughout the soil settlement process may be

evaluated. In this study numerical analysis based on the elastic visco-

plastic model proposed by Yin was performed by a finite difference

method.

Conclusions

The effect of creep on the settlement-time relation during the primary

consolidation can obtained several ideas:

a. The creep deformation is obviously known due to the excess pore

water pressure increases and the relaxation of effective stress occurs

under the constant void ratio. By using Yin`s EVP model considered

the constitutive relations, moreover it could stimulate the increase of

excess pore water pressure during the undrained loading stage.

b. The Yin`s model also be in conformity in the relationship between the

average strain or the excess pore water pressure and elapsed time in

Yokohama peat.

5