Consolidation

OBJECTIVES:  

Determining the coefficient of volume compressibility for the load increment Determining the coefficient of consolidation by using  root time (Taylor’s) method and  log time (Casagrande’s) method For a given soil sample

APPARATUS:  consolidometer (oedometer)

THEORY

Consolidation is defined as the reduction of the volume of a soil due to the expulsion of water. There will be a dissipation of pore water pressures in the process. The laboratory consolidation test is performed on an undisturbed sample of a cohesive soil to determine its compressibility characteristics. The soil sample is assumed to be representing a layer in the ground. aboratory consolidation test is conducted over a number of increments. The number of increments should cover the stress range from the initial stress state of the soil to the final stress state the soil layer is expected to experience due to the proposed construction. PROCEDURE

!. First a specimen of "#mm in diameter and $#mm in thic%ness was ta%en. !!. Then it was placed in a metal cylinder with having two porous stones either side. (Top and bottom) !!!. The specimen was then placed in the apparatus. !&. ' dial gauge was placed to measure the settlement. &. Dial gauge reading was set to ero. &!. ' load of $"%*mm$ was applied on the specimen. &!!. The specimen was %ept under water during the test. &!!!. Dial gauge readings were ta%en starting from #."minutes.

CALCULATIONS:

Coefficient of compressibility mv

∇h  =   ∇σ   × H  

mv

#.+,$mm   =  $  $" KN  * m × $#mm 

Coefficient consolidation a) -y suare root time (Taylor’s) method t ,#

T 

=

=

#.0" min

/* $

t ,#

=

C v  × t  d 

$

T ,#

× d 

$

C v

=

C v

 $# mm   # .+1+ ×    $     =

t ,# $

# ./$$"

2 2  b) -y the og(time) 3 casergrander’s method δ #

δ "# t "# T "#

C v

= 44444 =

4444 44444

= 44444 =

=

2 2

4 4 44 4

T "#

× d 

t "#

$

#./$$" min

DISCUSSION: 'lmost all the civil engineering applications in construction have a great connection with the soil. -ecause every structure transmit its load finally to the ground. There fore it is very important to have an idea about the behavior of soil that we are going to use for construction pro5ect. 6hen considering construction7 settlement is one of the most important thing which has to be ta%en into account. 8ettlements of the structures are depending on soil conditions. 8pecially consolidation. Consolidation is one of the main processes which occur after applying a load on soil. Consolidation can be defined as the reduction of the volume of a soil due to the expulsion of water. 6hen a load is applied on a soil sample the pore water ta%es the load first. There fore pore water pressure increases. 6ith the time the water in the soil sample under the load is dissipated and gradually pore water pressure decreases. Then the effective pressure increases and ta%es the extra load. !f we want to carry out a consolidation test in a site first we have to ta%e samples through several bore holes. Then samples should be labeled and transferred to a laboratory for testing. Then the sample should be chec%ed from ero to the maximum load that can be exerted on the soil due to particular construction and settlements should be recorded.!f the settlements are permissible then it is not needed to change the designs7 otherwise we might have to change the design or go for a costly alternative li%e a pile foundation. The accuracy of this test is very important. -ecause overestimating the soil may lead to disasters and under estimating of soil may lead to unnecessary expenditures. !n this test we obtain Coefficient of Consolidation9C v7 Coefficient of &olume Compressibility9 m v 7 which are reuired to obtain the rate of consolidation . -y using the rate of consolidation we can determine the settlement that can be occur in a particular time. !n some situations the construction is not used until its full settlement occurs. There fore we if the settlement for the usable time is permissible then no design changes are needed.

6hen the test is undergo7 following reasons cause to errors. !. !mproper sampling of soil specimens. !!. Ta%ing disturbed soil samples. !!!. Disturbing the sample when placed in the ring. !&. !mproper ways of transport. &. Tilting of two porous stone layers. &!. :alfunctioning of consolidometer due to friction and other mechanical defects. &!!. :isreading of dial gauge. &!!!. :isreading of time. To avoid these things happen following steps can be ta%en !. ;sing correct techniues in ta%ing samples. !!. ;sing proper pac%ing and transport methods. !!!. ;sing accurate Consolidometer. !&. :inimiing human errors.

Error analysis


EXPERIMENT NO. !

CONSOLIDATION TEST AND ANALYSIS