Consolidation Experiment

8/2/2019 Consolidation Experiment

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16Consolidation Test

INTRODUCTIONConsolidation is the process of time-dependent settlement of saturated clayey soilwhen subjected to an increased loading. In this chapter, the prodedure for one-dimensional laboratory consolidation test will be described, and methods of calcula-tion of obtaining the void ratio-pressure curve (e vs. log p), preconsolidation pressure(p ), and the coefficient of consolidation (cv) will be outlined.EQUIPMENT

1. Consolidation unit2. Specimen trimming device3. Wire saw4. Balance, sensitive to 0.01 g5. Stop watch6. Moisture can7. OvenThe consolidation unit consists of a consolidometer and a loading unit. The con-

solidometer can either be (i)a floating ring consolidometer (Fig. 16-1a) or (ii) a fixedring consolidometer (Fig. 16-1b). The floating ring :ondolidometer usually consists ofa brass ring in which the soil specimen is placed. One porous stone is placed at the topof the specimen and another porous stone at the bottom. The soil specimen in the ringwith the two porous stones are placed on a base plate. A plastic ring surrounding thespecimen fits into a groove on the base plate. Load is applied through a loading headwhich is placed on the top porous stone. In the floating ring consolidometer, compres-sion of the soil specimen occurs from the top and bottom towards the center.

The fixed ring consolidometer consists essentially of the same components, i.e.,a hollow brass plate, two porous stones, a brass ring to hold the soil specimen and ametal ring which can be fixed tightly to the top of the base plate. The ring surroundsthe soil specimen. A stand pipe is attached to the side of the base plate. This can beused for permeability determination of soil. In the fixed ring consolidometer, thecompression of the specimen occurs from the top towards the bottom.

The specifications for the loading devices of the consolidation unit vary depen-ding on the manufacturer. Figure 16-2 shows one type of loading device.

89


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90

d

c

a Specimen

:_b. ':

(a )

Soil Mechanics

LEGEND

a--Brass ringb - Porous stonec- Base plated- Plastic ringe- Loading headf - Metal ringg- Stand pipeh- Dial gauge

(b)

Figure 161. Schematic diagram of (a) floating ring consolidometer.(b) fixed ring consolidorneter.

During the consolidation test, when load is applied to a soil specimen, thenature of variation of side friction between the surrounding brass ring and thespecimen are different for the fixed ring and the floating ring consolidometer, andthis is shown in Fig. 16-3. In most cases, a side friction of 10%of the applied load is areasonable estimate.PROCEDURE

1. Prepare a soil specimen for the test. The specimen is prepared by trimmingan undisturbed natural sample obtained in Shelby tubes. The Shelby tubesample should be about 1/4 in. to 1/2 in. (6.35mm to 12.7 mm) larger indiameter than the specimen diameter to be prepared for the test.

Note: For classroom instructional purposes, a specimen can be mold-ed in the laboratory.

2. Collect some excess soil that has been trimmed in a moisture can formoisture content determination.

3. Collect some of the excess soil trimmed in Step 1 for determination of thespecific gravity of soil solids, Gs'


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Consolidation Test 914. Determine the weight of the consolidation ring (WI)'5. Place the soil specimen in the consolidation ring. Use the wire saw to trim

the specimen flush with the top and bottom of the consolidation ring.Record the size of the specimen.

6. Determine the weight of the consolidation ring and the specimen (W2).7. Saturate the lower porous stone on the base of the consolidometer,8. Place the soil specimen in the ring over the lower porous stone.9. Place the upper porous stone on the specimen in the ring.10. Attach the top ring to the base of the consolidometer.11. Add water to the consolidometer to submerge the soil and keep it

saturated. In the case of the fixed ring consolidometer, the outside ring (at-tached to the top of the base) and the stand pipe connection attached to thebase should be kept full with water. This needs to be done for the entireperiod of the test.

Figure 162. Consolidation loading assembly. In thisassembly, two specimens can besimultaneously tested. Lever arm ratio forloading is 1:10.


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92 Soil MechanicsSpecimentop

Specimentop

.' . "" :::.:: ',.: . . . .,:'.~:':.

....

.-:',

Specimenbottom

Specimenbottom

(a) (b)

f = friction/unit contact area

Figure 16-30 Nature of variation of soil-ring friction perunit contact areas in (a) fixed ring consolidometer.(b) floating ring consolidometer.

12. Place the consolidometer in the loading device.13. Attach the vertical deflection dial guage to measure the compression of

soil. It should be fixed in such a way that the dial is at the beginning of itsrelease run. The dial gauge used should be calibrated to read as 1small div= 0.0001 in. (0.00254mm).

14. Apply load to the specimen such that the magnitude of pressure, p, on thespecimen is 112 ton/It! (47.88 kN/m2). Take the vertical deflection dialguage reading at the following times, t, counted from the time of the loadapplication. 0 min, 0.25 min, 1min, 2.25 min, 4 min, 6.25 min, 9 min, 12.25min, 20.25min, 25min, 36min, 60 min, 120min, 240 min, 480min and 1440min (24 hr).

15. The next day, add more load to the specimen such that the total magnitudeof pressure on the specimen becomes 1 ton/ft." (95.76 kN/m2). Take the ver-tical dial gauge readings at similar time intervals stated in Step 14. Note,here we have Ilp/p = 1 (where IIp = increase of pressure and p = the ex-isting pressure).

16. Repeat Step 15 for soil pressure magnitudes of 2 ton/ft.' (191.52 kN/m2), 4ton/ft.' (383.04 kN1m2) and 8 ton/It" (766.08 kN1m2), etc. Note: In all cases(Ilp/p) = = 1.

17. At the end of the test, remove the soil specimen and determine its moisturecontent.

CALCULATION AND GRAPHThe calculation procedure for the test can be explained with reference to Tables 16-1and 16-2and Figs. 16-4, 16-5and 16-6which are the laboratory test results for a lightbrown clay.


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Consolidation Test CONSOLIDATION TEST(Time VSjI vertical dial reading)

93

Description of soil ~ ~ ~Pressure on specimen ~ ~ ,1ft 2.Clock time of load application '6_: 3_5_a_m .. _

Time after load Jt Verticalapplication, t dial reading

(min) (mino.S) (in)0 0 O.003~

D. '25" 0.5 0.0(;5tf\.0 1.0 D . 0 6 C J J2.25' ,.S 0.0731

t t - z, 0.795C o . : 2 . . 5 2-.5 o.o~339.0 3.0 o.o'Bh$?

12.2E:; 3.5 ().081'Dllo L f - 0.0922-

20.2S" I f , S - c.Oq4 I2..5" 5 0.OQ5/f310 ~ O.Oq 7~r o o 7.7S- O. loolf

120 10.95 o , to f C f" 2 - L f o /5. t .f '1 0, (D2...'7L f ?;D 21. C J / O. ( O L f < 8 '{4'10 37.1S' 0.105,

Table 16-1


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94

l i iw. . . .2o-~C

-o.,pc tS-:1C Jc oC Jco.~"0.--rncoC J

-~e nzo(J

>?

~- = - ~~: r : '0 ~;:].. ! e : " "'-I

OJ..c:

Eu o

q

Soil Mechanics

1\0IV)


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Consolidation Test

0.07

Ctil 0.08I:"tl< aQ)~ 0.09< aCl

0.10

0.11

0.06A'4~~ ,,

~

~ ~~Ir.~

~~ t - . . . . . .\' . . . .t--- J..\1\"1--- --- --- I\~r-S 0

0.07

0.08

tilI:"tl 0.09< a~cois 0.10

0.11c

0.12 o 2 4 6 8 10 12

Figure 16-4_ Plot of dial reading vs. '" time for the testresults given in Table 16-1. Determination ofto o by square-root-of-time method.

0.12

_ ..... ~oo + - _

0.1 0.2 0.5 1000 20000 100Time (min)--Iog scale

Figure 16-5. Logarithm-of-time curve fitting method forthe laboratory results given in Table 16-1.

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96 Soil Mechanics1. Collect all the time vs. vertical dial readings data. Table 16-1 shows the

results of a pressure increase from p = 2 ton/fP to p + flp = 4 ton/ft.'.2. Determine the time for 90% primary consolidation, ~o, from each set of

time vs. vertical dial readings. An example of this is shown in Fig. 16-4which is a plot of the results of vertical dial reading vs. \l'time given in Table16-1. Draw a tangent AB to the initial consolidation curve. Measure thelength BC. Plot the point D such that the length of CD = 1.15 times thelength BC. Join AD. The adscissa of the point of intersection of the line ADwith consolidation curve will given~. In Fig. 16-4,~ = 4.75 min ", sot90= (4.75)2 = 22.56 min. This technique is referred to as the square-root-of-time-fitting method (Taylor, 1942).

3. Determine the time for 50% primary consolidation, t50, from each set oftime vs. vertical dial readings. The procedure for this is shown in Fig. 16-5,which is a semilogarithmic plot (vertical dial reading in natural scale andtime in log scale) for the set of readings shown in Table 16-1. Project thestraight line portion of the primary consolidation downwards and thestraight line portion of the secondary consolidation backwards. The pointof intersection of these two lines is A. The vertical dial reading correspon-ding to point A is d100 (dial reading at 100% primary consolidation). Selecttimes t , and t2 = 4t}. (Note t] and t2 should be within the top curved portionof the consolidation plot.) Determine the difference in dial readings, X, bet-ween times t] and ~, Plot line BC which is vertically X distance above thepoint on the consolidation curve corresponding to time t; The vertical dialgauge reading corresponding to line BC is do, i.e., the reading for 0% con-solidation. Determine the dial gauge reading corresponding to 50% primaryconsolidation as

d o + dIQOdo = = --2--

(16.1)

The time corresponding to d s o on the consolidation is L s o This is thelogarithm-of-time curve fitting method (Casagrande and Fadum, 1940). InFig. 16-5, tso = 4.9 min.

4. Complete the experimental data inCols. 1,2,8 and 9 ofTable 16-2. Columns1and 2 are obtained from time-dial reading tables (such as Table 16-1) andCols. 8 and 9 are obtained from Steps 1 and 2, respectively.

5. Determine the height of solids of the specimen in the mold as (Table 16-2)

(16.2)

Hs = heightof solidsWs =dry weight of soilspecimen

D = diameterof the specimenGs = specificgravitysoilsolidsIW = specificgravityof soilsolids


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Consolidation Test 976. In Table 16-2, determine the change in heights, 6.H, of the specimen due

to load increments from p to p + P (Col. 3). For example,p = Y2ton/It", final dial reading =0.0283 in.p + Ap= 1 ton/ft.', final dial reading = 0.0356 in.

SoA H = 0.0356 - 0.0283 =0.0073 in.

7. Determine the final specimen height, Ht(f), at the end of consolidation dueto a given loading (Col.4 in Table 16-2). For example, inTable 16-2, Ht(f) atp =Y 2 ton/ft.' is 0.9917. ~ H between p = Y 2 ton/It" and 1 ton/ft." is 0.0073 in.So, Ht(f) at p = 1 ton/ft.' is 0.9917 - 0.0073 = 0.9844 in.8. Determine the height of voids, H , in the specimen at the end of consolida-tion due to a given loading, p, as (Col. 5 of Table 16-2)

(16.3)9. Determine the final void ratio at the end of consolidation for each loading,

p, (Col. 6, Table 16-2) asH V

e - Hs (16.4)

10. Determine the average specimen height, Ht(av), during consolidation foreach incremental loading (Col. 7, Table 16-2). For example, in Table 16-2,the value of Ht(av) between p = Y 2 ton/ft.' to 1 ton/ft.' isHt(f) at p Y 2 ton/ft2 + Ht(f) at p = 1 ron/fr-

20.9917 + 0.9844

2 0.9811 in.

11. Calculate the coefficient of consolidation, Cv (Col. 10, Table 16-2) from t90(Col. 8) asCv tT =v H2

where Tv = time factor, T90 = 0.848H = maximum lenght of drainage path

(16.5)

Ht(av)~2-(since the specimen is drained at top and bottom)

So0.848 H2( )t av (16.6)


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98 Soil Mechanics12. Calculate the coefficient of consolidation, c., (Col. 11, Table 16-2), from t50

(Col. 9) asTv(so%) = 0.197 =

c =v0.197 H~(av)

4 tso(16.7)

For example, between p = 1 I z ton/ft 2 to 1 ton/ft-",

0.9881 in.; tso 56.0 sec;

c =v 0.197(0.9881)2 = 0859x 10-3 in2/sec4(56)' .13. Plot a semilogarithmic graph of pressure vs. final void ratio (Col. 1vs. Col.

6, Table 16-2). Pressure p is plotted on the log scale, and the final void ratioon the linear scale. As an example, the results of Table 16-2 are plotted inFig. 16-6. Note that the plot has a curved upper portion and, after that, evs. log p has a linear relationship.

14. Calculate the compression index, C; This is the slope of the linear portionof the e vs. log p plot (Step 13). In Fig. 16-6

0.696 - 0.6128log -40.279

15. On the semilogarithmic graph (Step 13),using the same horizontal scale (i.e,the scale for p), plot the values of c, (Cols. 10 and 11, Table 16-2). As an ex-ample, the values determined in Table 16-2 are plotted in Fig. 16-6. Note, Cvis plotted on the linear scale corresponding to the average value ofp

. P I + P 2(i.e., 2 )16. Determine the preconsolidation pressure, Pc. The procedure can be explain-

ed with the aid of the e-logp graph drawn in Fig. 16-6 (Casagrande, 1936).First, determine the point A which has the smallest radius of curvature inthe e-log p plot. Draw a horizontal line AB. Draw a line AD which is thebisector of the angle BAC. Project the straight line portion of the e-log pplot backwards to meet line AD at E. The pressure corresponding to pointE is the preconsolidation pressure. In Fig. 16-6, pc = 1.6 ton/ft.',

J


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0.60

1.2c:'f

0.8' ":M0 0.4.. .

> 0

0 0.5

Consolidation Tes t

0.85

0.80Q)

a. . . .~:" 2a> 0.70'"cu : :

99

From t90 From t50

2 5 10Pressure, p (ton/f t 2 )

Figure 166. Plot of void ratio and the coefficient ofconsolidation against pressure for the soilreported in Table 162.

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Consolidation Experiment

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