Discussion 2x34z

DISCUSSION 6. Discuss the importance and relevance of the test, the parameters obtained, shortcoming and possible ways of improvement.

This experiment was carried out to determine the coefficient of permeability (k) for a given soil sample using falling head method. The falling head permeability test is a common laboratory testing method used to determine the permeability of fine grained soils with intermediate and low permeability such as silts and clays. The falling head test was found to be more suitable than the constant head test because of its simplicity and the inability of the latter to allow water flow at measurable pressure heads. The falling head permeability test involves flow of water through a relatively short soil sample connected to a standpipe which provides the water head and also allows measuring the volume of water ing through the sample. Before starting the flow measurements, the soil sample is saturated and the standpipes are filled with de-aired water to a given level. The test then starts by allowing water to flow through the sample until the water in the standpipe reaches a given lower limit. The height of water in the glass standpipe is recorded in every 5 minutes time interval for about 50 minutes. Using the recorded readings the graph

ln

h0 h1

( )

against

(t 1−t 0 ) was plotted. Then the coefficient of permeability, k was

found by the gradient of the graph.

The Importance and Relevance of the Test The knowledge of permeability is important for the following engineering problems:

         

To find out the rate of consolidation and settlement of a saturated soil under load To compare the permeability of two soils or in comparing the permeability of a soil in horizontal and vertical directions in deg of projects where vertical sand drains are used. Estimating the quantity of underground seepage through earthen dams and canals. Investigating problems involving pumping seepage of water for underground constructions. Analyzing the stability of earth dams, earth retaining walls, upstream and downstream slopes and retaining structures. To estimate ground water flow. To plan the method of lowering the ground water table. To calculate the uplift pressure and piping. To design the grouting. For soil freezing tests.

The factors which affect the permeability are:

      

Properties of pore fluid Size and shape of particles Void ratio of soil Structural arrangement of soil particles Degree of saturation Adsorbed water Stratification

Error Analysis Shortcomings

Parallax error is occurred where the observer’s eyes are not perpendicular to the scale while reading the height To overcome this error, the observer’s eye must be 90o perpendicular to the water level in order to get the accurate measurement. Possible Ways of Improvement.

1 2

The saturation of the specimen is important in obtaining consistent results. During test,  there should be no volume change in the soil  there should be no compressible air present in the voids of soil i.e. soil should be completely saturated.  The flow should be laminar and in a steady state condition.

Determination of the coefficient of permeability There are several ways for determining the coefficient of permeability k. They grouped as follows: .i)Laboratory methods A)Constant head permeability test  The coefficient of permeability of a coarse-grained soil can be determined in the laboratory using a constant-head permeability test.  The test includes a cylindrical soil specimen that is subjected to a constant head as shown in Figure

•

The length of the soil specimen is L and its cross-sectional area is A.

•

The total head loss (hL) along the soil specimen is equal to the constant head, which is the difference in elevation between the water levels in the upper and lower reservoirs as shown in the figure.

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A constant head implies that we have reached a steady-state condition in which the flow rate is constant (i.e., does not vary with time).

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Using a graduated flask, we can collect a volume of water (V) in a period of time (t ).

•

From this we can calculate the flow rate

•

( q= V/t).

•

Coefficient of permeability K = ql/Ah

B)Falling head permeability test This is further discussed under theory ii) Field methods A field permeability test consists of pumping out water from a main well and observing the resulting drawdown surface of the original horizontal water table from at least two observation wells. When a steady state of flow is reached, the flow quantity and the levels in the observation wells are noted. Two important field tests for determining permeability are: A)Unconfined flow pumping test B) confined flow pumping test

Advantages. The advantages of laboratory testing for measuring the permeability of soils are: a. The water content and density of the soil to be tested can be carefully controlled. b. Samples can be saturated prior to testing more readily in laboratory tests. c. High hydraulic gradients may be employed, which permits measurement of lower permeabilities in a shorter time. d. Laboratory methods are usually much more economical than field methods, especially for slowly permeable soils where long testing times are required to measure low volumes of flow.

2. Disadvantages. The disadvantages of laboratory testing are:

a. Important macro-features of natural soil deposits, such as drying cracks and alluvial soil stratification, are difficult to model with small laboratory scale specimens. Laboratory specimens are limited in size. b. To test undisturbed or in situ soils, samples must be obtained, shipped, and handled by the laboratory. Undisturbed samples are expensive to obtain. Sample disturbance may affect test results. c. Horizontal permeability is difficult to measure in laboratory specimens. Usually, vertical permeability is measured.

CONCLUSION Permeability can be defined as the ability of a porous medium to allow the flow of a fluid through it, typically expressed as the coefficient of permeability (k). This property is necessary for the calculations such as seepage through earth dams or under sheet pile walls, the seepage rate from waste storage facilities (landfills, ponds, etc.) and the rate of settlement of clayey soil deposits. −6

−1

In this experiment, the coefficient of permeability (k) is found to be 2 .169× 10 ms h1 ln gradient of the graph h2

( )

from the

t −t against ( 2 1 ) .

Finally we can conclude that this experiment was carried out successfully with the use of Falling Head Permeability Apparatus and the objective to determine the coefficient of permeability (k) for a given soil sample was achieved.