Consolidation Test
OBJECTIVES
To determine The coefficient of volume compressibility, mv To determine The coefficient of consolidation, Cv, for that load increment
Introduction
When loaded with external stresses, pore water pressure in saturated cohesive soils will go up. As the water in the pore spaces of the soil under higher pressure than the surrounding area, water will drain out and the pwp will dissipate with time resulting reduction in volume of the soil. Time taken for dissipation of pwp depends on the coefficient of consolidation of the soil and ,if it is low (as in cohesive soils),the dissipation of excess pwp takes place very slowly. Consolidation is defined as the time dependent reduction of the volume of a soil due to the expulsion of pre water. Practical example of this situation is loading of a footing on a clay layer. Pwp in the clay layer will be increased when the footing is loaded and with time water will drainout reducing the pwp in the clay layer and causing volume reduction of the clay layer. Reduction in the volume of the clay layer can cause settlement of the footing with time.
Theory
Consolidation of a saturated soil occurs due to expulsion of water under static, sustained load. The consolidation characteristics of soils are required to predict the magnitude and the rate of settlement. The following characteristics are obtained from the consolidation test.
Apparatus
Strain controlled triaxial load frame Dial gauge Soil trimming tools like fine wire saw, knife, spatula, etc Water content cans Weighing balance, accuracy 0.01 g. Water reservoir to saturate the sample Oven
Procedure
The metal ring was cleaned and dried. its diameter and height were measured. the mass of the empty ring was taken. The ring was pressed into the soil sample contained in a large container at the desired density and water content. The ring was to be pressed with hands. The soil around the ring was removed. The soil specimen should project about 10mm on either side of the ring. Any voids in the specimen due to the removal of large size particles should be filled back by pressing the soil lightly. Trimed the specimen flush with the top and bottom of the ring. Any soil particles sticking to the outside of the ring were removed. Weigh the ring with the specimen. Take a small quantity of the soil removed during trimming for the water content determination. The porous stones were satuareted by boiling them in distilled water for about 15min. The Consolidometer was assembled. Placed the bottom porous stone, bottom filter paper, specimen, top filter paper and the top porous stone, one by one. Position the loading block centrally on the top porous stone. Mount the assembly on the loading frame. Centre it such that the load applied is axial. In the case of the lever loading system, counterbalance the system Set the dial gauge in position. Allow sufficient margin for the swelling of the soil. Connect the mould assembly to the water reservoir having the water level at about the same as the soil specimen. Allow the water to flow into the specimen till it is fully saturated. The initial reading was taken of the dial gauge. An initial setting load was applied to the assembly so that there is no swelling. Allow the setting load to stand till there is no change in the dial gauge reading or for 24 hours. The final gauge reading was taken under the initial setting load.
The first load increment was applied to apply a pressure of 13.816 , and start the stop watch. Record the dial gauge readings at 0, 0.25, 1, 2.00, 4.0, 8.00,15.00, 20.00, 60.00, 120.00 and 1440 minutes After the last load increment had been applied and the readings taken, decrease the load to 1/4 of the last load and allow it to stand for 24 hours. Take the dial gauge reading after 24 hours. Further reduce the load to 1/4 of the previous load and repeat the above procedure. Likewise, further reduce the load to 1/4 previous and repeat the procedure. Finally reduce the load to the initial setting load and keep it for 24 hours and take the final dial gauge reading. Dismantle the assembly. Take out the ring with the specimen. Wipe out the excess surface water using a blotting paper. The mass of the ring with the specimen was taken The specimen was dried in the oven for 24 hours and determine the dry mass of specimen.
Laboratory Consolidation Test and Analysis Data obtained from one increment in a conventiional multi increment Consolidation Test
Sample thickness is 20 mm at the start when the load is 0 kN/m2 . At the beginning of the load increment 60 – 120 kN/m2 , sample has already settled 3.744 mm and the sample thickness is 16.256 mm.
Discussion All soils are compressible so deformation will occur whenever stress is applied to soils. Soil minerals and water are both incompressible. Therefore, when saturated soils are loaded, the load first acts on the pore water causing pore water pressures that are in excess of the hydrostatic pressures. The excess pore water pressures are largest near the application of load and decrease with distance from the loading. The variations in excess pore water pressure cause total head gradients in the soil which, according to Darcy’s Law, will induce water to flow from locations of high total head to low total head. The excess pore water pressures dissipate as water flows from the soil and, to compensate for the applied stress, the stress is transferred to the soil minerals resulting in higher effective soil stress. The flow of water from the soil also causes reductions in the soil volume and settlements at the ground surface. Fine-grained soils have very low permeability so they can require substantial periods of time before the excess pore water pressures fully dissipate. This process of time-dependent settlement is referred to as consolidation. Terzaghi’s theory for one-dimensional consolidation provided the means to calculate the total amount of consolidation settlement and the consolidation settlement rate. In practice, engineers obtain representative soil samples, conduct consolidation tests and use Terzaghi’s consolidation theory to predict the total settlement and time rate of settlement for embankments and foundations.
A 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 soil layer in the ground. A conventional consolidation test is conducted over a number of load increments. The number of load 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. Increments in a conventional consolidation test are generally of 24 hr duration and the load is doubled in the successive increment. In this practical class one load increment of a multi increment consolidation test is conducted and the data will be analysed to obtain the compressibility characteristics of the soil.
The compressibility characteristics of the soil are;
Parameters needed to estimate the amount of consolidation settlement Parameters needed to estimate the rate of consolidation settlement in the field.
Using the data from a single load increment of the test, only the coefficient of volume compressibility mv can be estimated. Data from all the load increments should be combined to draw the e vs log σ graph and to obtain the compression index Cc - the other parameter used to estimate the consolidation settlement. The rate of consolidation settlement is estimated using the Coefficient of consolidation Cv. This parameter is determined for each load increment in the test. In this laboratory assignment, the coefficient of consolidation should be estimated using two methods - the root time method (Taylor's method) and the log (time) method - Casagrande's method.
Reference
http://www.iitgn.ac.in/research/stl/consolidation.php http://www.durhamgeo.com/testing/soils/consolidation-testing.html http://www.ngi.no/no/Innholdsbokser/Referansjeprosjekter-LISTER-/Beskrivelseav-labforsok/Oedometer-test/ http://aboutbuildingengineering.com/2012/09/one-dimensional-consolidationtesting.html
