- As the water drains from the soil pores, the load increment is gradually shifted to the soil structure.

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1 Consolidation test Introduction - When a saturated soil mass is subjected to a load increment, the load is usually carried Initially by the water in the pores because the water is incompressible when compared With the soil skeleton. - As the water drains from the soil pores, the load increment is gradually shifted to the soil structure. This transfer of load is accompanied by a change in the total volume of soil equal to the volume of water drained. This process is known as consolidation. We can understand the consolidation process by looking at the spring analogy shown in Figure 1.in which spring refers to the soil structure. عندما نو ثر بقوة على الشكل الاول حیث الص مام مغل ق ف ان الم اء س یتحمل كام ل الض غط ول یس الزنب رك حیث ان الماء لا یقبل هنا الانضغاط بینما الزنبرك یمكن ان ینضغط ولكن عندما نف تح الص مام ویب دا الم اء بالهروب ینتقل الحمل من الماء الى الزنبرك كما فى الشكل (C). Consolidation, occurs in a soil is directly related to the permeability of the soil because the permeability controls the speed at which the pore water can escape. Laboratory consolidation studies are almost entirely limited to soils of low permeability. Because time required for consolidation after a load application can be considered negligible for high permeable soils. Objective The main purpose of consolidation tests is to obtain soil data which is used in predicting the rate and amount of settlement of structures founded on clay. four terms can be Determined: 1) The preconsolidation stress, p. This is the maximum stress that the soil has felt in the past. 1

2 ) The compression index, Cc, which indicates the compressibility of a normally-consolidated soil. 3) The recompression index, Cr, which indicates the compressibility of an Over-consolidated soil. 4) The coefficient of consolidation, cv, which indicates the rate of compression under a load increment. A normally-consolidated soil is defined as a soil which, at the present time, is undergoing the application of a stress that is larger than any stress it has undergone in its history. present > p Conversely, an over-consolidated soil is defined as a soil which has experienced higher stresses in the past, present < p Equipment Consolidometer and dial indicator Loading frame (pneumatically controlled) Timing device. Some notes on the apparatus: 1) Note that the specimen is contained by a relatively rigid ring; thus compression and Consolidation are one-dimensional. ) There are porous stones at the top and bottom of the specimen so that water can drain, and thereby dissipate excess hydrostatic pressure. 3) The specimen is surrounded by a water bath at all times, to prevent the specimen from desiccating (drying out). 4) The dial indicator, accurate to 0.01 mm, measures the change in height of the specimen after the load has been applied. Procedure Calculations Cv (the coefficient of consolidation) The two common (graphical methods) for calculation are as follows: 1) Logarithm -of- time method: For a given incremental loading of the laboratory test, the specimen deformation (Y-axis)against log-time (x-axis), the following constructions are need to determine Cv: 1) Extend the straight-line portions of primary and secondary consolidations to intersect of A is represented by d100 that is, the deformation at the of 100% primary consolidation. ) The initial curved portion of the plot of deformation versus log t is approximated to be a parabola on the natural scale. Select times t1 and t on the curved portion such thst t = 4t1.let the difference of specimen deformation during time (t t1) be equal to x.

3 3) Draw a horizontal line DE such that the vertical distance BD is equal to x. the deformation corresponding to line De is do (that is, deformation at 0% consolidation). 4) The ordinate of point F on the consolidation curve represent the deformation at 50% primary consolidation, and its abscissa represents the corresponding time (t 50 ) 5) For 50% average degree consolidation, Tv = from table. C v 0.197H t 50 dr H dr H i D avg D avg =(initial dial readingfinal dial reading)/ ) Square-Root-of-Time Method: In this method, a plot of deformation against the square root of time is made for the incremental loading. Other graphic constructions required are as follows: 1) Draw a line AB through the early of the curve. ) Draw a line Ac such that OC =1.15 OB The abscissa of point D, which is the intersection of AC and the consolidation curve. Gives the square root of time for 90% consolidation (t 0.5 ). 3) For 90% consolidation, T90 =.848 (from table) H t Hdr is determined as in previous method C v 90 dr 3

4 Void ratio pressure plots: 1- Calculate the height of solids: Hs = ms / (Gs w Ar) Where: ms = mass of solids (dry mass of specimen) Gs = specific gravity. w = density of water. Ar = cross-sectional area of the ring. An idealized figure, showing void-ratio/height rela tionships is shown below. - Calculate the initial height of voids: Hv 0 = H 0 - H s Where: H 0 = initial height of the specimen 3- Calculate the initial void ratio: e 0 = H v0 / H s e 1 = H 1 /H s e 1 =e o - e 1 4

5 e = H /H s e =e 1 - e e n =e 0 - ( H)/H s. Pre-consolidation determination (pc) Casagrande (1936) suggested a simple graphic construction to determine preconsolidation pressure. 1. By visual observation, establish point a, at which the e-iog ' plot has a minimum radius of curvature.. Draw a horizontal line ab. 3. Draw the line ac tangent at a. 4. Draw the line ad, which is the bisector of the angle bac. 5. Project the straight-line portion gh of the e-iog ' plot back to intersect line ad at f.the abscissa of point f is the preconsolidation pressure, 'c. Compression index determination(c c ) Normally Consolidated Clay of Low to Medium Plasticity 1. Curve is the laboratory e-iog ' plot. From this plot, determine the preconsolidation pressure ( c ') = o ' (that is, the present effective overburden pressure). Knowing where ( c ') = o ', draw vertical line ab.. Calculate the void ratio in the field. e o. Draw horizontal line cd. 3. Ca1culate 0.4eo and draw line ef (Note: f is the point of intersection of the line with curve.) 4. Join points f and g. Note that g is the point of intersection of lines ab and cd. 5

6 Data - Mass of oven dry cake (Ms) = 89.7g. - Final Water content = 18.7 % - Specific gravity (G.S) =.71 - Mass of final wet sample ( Mw)= g. - Initial height of the sample (Hi) (or ring height) = 0 mm. - Ring Diameter = Dr =6.14cm. Load (1) = 100 kpa. Load () = 00 kpa 6

7 Elapsed time (min) Load (1) 100 kpa. Deformation (mm) Elapsed time (min) Load( ) 00 kpa Deformation (mm) Required: - Plot deformation vs Log time then obtain Cv. - Plot deformation vs time then obtain Cv. Load (kpa) (mm) H*10-3 cm* e 0 0 e o= *Different of dial reading between the start and the end of each load increment. Required - Calculate void ratio at the end of each load increment. - Plot e Vs Log P, then obtain Pre-consolidation pressure pc Compression index C c 7