SETTLEMENT Srilanka have more residual soils.so if we


 

SETTLEMENT CHARACTERISTICS OF RESIDUAL SOILS

 

V.Yathurshanan

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University of Moratuwa,Moratuwa,[email protected]

Prof.Kulathilaka, S.A.S

University of Moratuwa,Moratuwa

 

Abstract: Srilanka have more residual
soils.so if we want construct on residual soils we must know about
characteristics about residual soils. Most of the countries have sedimentary
soils that’s why soil concepts develops for sedimentary soils. We do not do
experimental for residual soils widely. But in construction concept of
sedimentary soils use for residual soils.

When we
applied tezaghi’s concept for residual soils we take 20mm thick sample for
laboratory testing but in field residual soil characteristics vary within small
areas.so this will affect our values, we draw the settlement versus square root
time graph will be continually curve. And settlement taking place much faster
than predicted. Because of residual soils are partially saturated so
consolidation settlement happed by air dissip/ation. When we draw settlement
versus stress (log scale) there is virgin consolidation part is linear as
sedimentary soils.so there is no reason for use conventional method using on
residual soils. (B.L.Thennekoon, 1998)

From
previous research using by oedometer when we draw compression curve plotted
using log scale and linear scale they found for some soil same got linear curve
have very close to linear except to high stress. There is thus no increse in
compressibitity with stress.the compressibility either remains constant or
decrease as stress level increase.there is no longer trace, on the linear
plots,of the prconsolidation pressure determined frome the log plots.there is
simply no significant change of slope at the pressures identified as
preconsolidation pressures according to the standard casagrande construction. (D.Wesley, 2010)

Keywords:  residual soils, sedimentary soils, terzaghi’s
concept, oedometer

 

1.   
Introduction

Properties of soils are most important to
construction. If we understand main things on soil properties we can avoid
construction failure. For this reason we have to study about soil properties.

The
differences that have been identified between residual soils and sedimentary
soils are due to various reasons. Although these differences are under
discussion, almost all the geotechnical concepts are developed by studying
sedimentary soils. (D.Wesley, Behaviour and geotechnical properties of
residual soils and allophane clays, 2009) This could be
correct if all soils that we need to deal with are sedimentary soils. Since
that is not the case it is needed to identify the differences between these two
soil groups. When considering the settlement characteristics of soils the
phenomena of consolidation plays a major role. So it is significant to analyse
the consolidation test results accurately. The conventional method of analysing
consolidation test results has become irrelevant to residual soils due to these
differences. The facts causing those differences can be discussed as the
variations in the soil formation process, structure of soil particles, soil
mineralogy and topographic influence

2.     
Formation Process

Residual soils are formed by chemical
and physical weathering from patent rock then lie on parent rock. Sedimentary
rocks formed by chemical and physical weathering from parent rock then
transported by wind and river then deposited. But sedimentary soils are
undergoing various additional processes such as erosion, hardening,
cementation, primary consolidation, secondary consolidation, etc. (D.Wesley, Behaviour and geotechnical properties of
residual soils and allophane clays, 2009)

 

Figure 1: Soil formation process (D.Wesley, Behaviour and
geotechnical properties of residual soils and allophane clays, 2009)

Figure 2: Diagrammatic
representation of soil formation processes. (D.Wesley, Behaviour and
geotechnical properties of residual soils and allophane clays, 2009)

 

3. Major difference between
sedimentary soil and residual soil

3.1 Homogeneity

After formation of this soils. If we
consider homogeneous sedimentary soil is more homogeneous than residual soils.
Because when forming of sedimentary
soils are undergoing various additional processes such as erosion, hardening,
cementation, primary consolidation, secondary consolidation, etc. From this
sedimentary became complex than residual soils. When sedimentary rocks are
undergoing to various process, sorting process occurring on sedimentary rock so
sedimentary rocks became homogeneous than residual soils. (D.Wesley,
Behaviour and geotechnical properties of residual soils and allophane clays,
2009)

3.2 Stress history

When formation of sedimentary soils
there occurring loading and unloading process this process may create stress
history on sedimentary soils.so we can identify over consolidated soil and normally
consolidated soil. The above
discussed concepts, stress history, virgin consolidation line, over
consolidation and normal consolidation cannot be applied to residual soils. (D.Wesley, Behaviour and geotechnical properties of
residual soils and allophane clays, 2009)

3.     
Structure and Mineralogy

Structure and mineralogy
are used for identify soil is sedimentary or residual (D.Wesley,
Geotechnical Engineering in Residual soils, 2010).when we consider the
structure there are 2 types micro structure and macrostructure  Which can be seen by naked eyes called as macrostructure.
Which cannot see naked eyes called microstructure.

Mineralogy is depend on
parent rocks. Most of the residual soils are micro – structured up to various
states. The existence
of such various microstructures induces a sense of different compression
behaviours and presence of a yield pressure of soils. (D.Wesley,
Geotechnical Engineering in Residual soils, 2010)

4. Review of methodologies

There are several methods are using for
determine the settlement characteristics of residual soils. Experiments are
carried out for determine rate of consolidation to determine parameters such as
coefficient of
consolidation (Cv), compression index (Cc), recompression index (Cr),
coefficient of volume compressibility (mv), etc. For measure these parameters
settlement and time will be used.

Undisturbed
soil samples will be collected from the fields to conduct laboratory tests.
Results will be compared with tests conducted on remolded soil samples.
Consolidation tests will be carried out using both Oedometer apparatus and Rowe
Cell apparatus (with pore pressure measurements). The sample size will be
different in these two tests. Since the variation of soil characteristics of
residual soils

is larger within a
selected small area, the representativeness of Oedometer sample is
questionable. The Rowe Cell apparatus will be used as the solution which
accommodates larger, more representative samples.

For
determine coefficient of consolidation. These methods are using

·        
Taylor’s method

·        
Casagrande method

·        
velocity method

·        
Hyperbola method

 

(1)

 

 

 

 

The Uz
vs. Tv plot proposed by Terzaghi can be used to find Tv.

Figure
3: Variation of Uz with Tv and depth

 

Figure 4:    log time plot casegrande’s method

We can calculate t50 from
this graph. Then we can calculate Cv.

Figure 5: root time versus settlement graph, Taylor’s method

From this graph we can calculate t90   from this we can calculate Cv.

Coefficient of volume compressibility (mv)

 

(2)

4.1 Rowcell test

According to Rowe and Barden (1966), the
first consolidation cell that was used by Frontard in 1910, was a metal
container with a perforated base and was loaded by means of a piston. In 1919
Terzaghi devised a single drainage cell which was called an Oedometer. It does
not facilitate sufficient control over drainage and measurement of pore
pressure. Also it does not facilitate to test larger samples which is a
requirement for testing non – uniform soils.

After further researches and
modifications, the Rowe cell was introduced as a new consolidation cell.
Diaphragm loading, pore pressure measurements, horizontal drainage and
minimized wall friction are the main features that were leading to present
design of Rowe cell. Load is applied by means of hydraulic pressure acting
across a rubber diaphragm in order to achieve uniformity of loading. (K.H.Head, 1985)

 

 

Figure
6: Basic configuration of the Rowe Cell

Eight different drainage types can be performed by
using the Rowe cell as shown in Figure 7. Basically the four drainage types are
single drainage, double drainage, inward drainage and outward drainage. Rowe
& Barden (1966) also noted that the type of loading, either flexible platen
with a uniformly distributed load (free stain) or rigid platen (equal strain)
can be adapted according to the nature of sample to be tested. The important
factor in all these arrangements is that full control of drainage, initial pore
pressure and back pressure can be achieved (K.H.Head, 1985).

 

Figure
4: Different types of drainage and loading conditions in Rowe cell

Usually
properties of residual soils are varying vastly within a small area and the
representativeness of Oedometer sample has become questionable. Since Rowe cell
apparatus accommodates much larger samples to be tested, adopting Rowe cell
would be an acceptable solution for the matter of representativeness. Pressure
is directly applied via a hydraulic system which makes much accurate stressing
other than stressing in terms of loads. Also the Rowe cell can perform eight
different loading and drainage types resulting this test more flexible than
Oedometer test. (K.H.Head, 1985).

 

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