-METHODST10

DETERMINATION OF THE ONE-DIMENSIONAL CONSOLIDATION PROPERTIES

OF SOILS

1 SCOPE This method covers the determination of the rate and magnitude of consolida-tion of a soil specimen after being saturated. The specimen is in the form of a disc. restrained laterally, and loaded and freely drained axially.

2 APPARATUS 2.1 A metal consolidation ring, preferably of high-grade stainless steel, non-corro-

sive in relation to the soil tested. The inner surface must be highly polished or be coated with a low-friction material. The ring must be provided with a cutting edge to facilitate preparation of the specimen. The thickness of the ring must be such that it rigidly confines the sample laterally under the greatest load applied during testing, with a change in diameter not exceeding 0,03 per cent. The in-side diameter of the ring should be at least 6 mm smaller than the diameter of the undisturbed sample to be tested to allow for at least 3 mm of material to be trimmed away (see 3.1) (Figure ST10il).

2.2 Porous plates for placing on tt1e top and bottom surfaces of the test specimen, e.g, si'liered fused aluminium oxide, silicon carbide or sintered metal which is not attacked by the soil or soil moisture. They must be of suitable porosity so that the soil will Mot extrude into the pores. The diameter of the top porous plate must be 0,02 to 0,05 mm less than that of the ring, A chamfer on the upper edge is an advantage to minimize the danger of binding, should tilting occur. The diameter of the bottom plate must be large enough to support the ring and specimen adequately. The plates must be sufficiently thick to prevent breaking, usually 6 to 13 mm thick,

2,3 A consolidation cell made of suitable non-corrosive material, within which tt1e test specimen assembly is placed. The cell must be capable of being filled with water to a level higher than tt1e top of the upper porous plate (Figure ST10/1).

2.4 Extensiometer: a dial gauge or other measuring device to measure changes in thickness of the specimen throughout the test, accurate to 0,002 mm.

2.5 A loading device for applying vertical loads to the specimen with a rigid bed for supporting the consolidation cell. The device must be capable of maintaining specified loads for long periods of time with an accuracy of 1 per cent of the ap-plied load and must permit application of a given load increment Within a period of two seconds without impact. The apparatus must be capable of accommo-dating a. compreSSion of at least 75 per cent of the specimen thickness. The force applied to the test specimen must be applied centrally to a loading cap covering the top porous plate through some form of spherical seating.

2.6 A moisture room or plastic bags tor storing samples. Special methOds Ora TMH6, Pretoria, South Africa, 1984 51

2.7 A spatUla 150 mm long with a sharp, straight edge for trimming the sample. 2.8 A balance to weigh up to 1 000 g, accurate to 0, 19. 2.9 A drying oven capable of maintaining a temperature of 105 to 110C. 2.10 A timing device or stop watch readable to one second. 2.11 Means of measuring the height of the test specimen or depth of the consolida

lion ring to an accuracy of 0,1 mm. 2.12 A trimming bench (for use with block samples). 2.13 Filter paper, e.g. Whatman No 54 (or similar). One paper must be cut 10 the

size of the inner diameter of the sample ring and another to the outer diameter. 2.14 A watch glass. 100 mm in dj~meter or other container for moisture content de~

terminations.

3 METHOD 3.1 Preparation of tut specimen

This test may be carried out on remoulded or undisturbed soil samples. Place a tared consolidation ring with the cuttlng edge downwards on the sample and start cutting away the soil at the sides of the ring with the spatula to such an ex-tent that the ring can be slowly pushed over the core, the last fraction of soil be-ing pared away by th~ cutting edge of the ring, No unnatural voids should be formed against the inner surface of the ring. Continue with this operation until the soil protrudes a few milfimetres above the top of the ring. Cut the ring free a few millimetres below the bottom of the ring. Trim the top and bottom surfaces carefully, using the spatula as a straight-edge, until they are flush with the edges of the ring. Should any occasional small inclusion interfere with the trimming operations, remove it and fill the cav-ity completely with matenal from the parings. At all stages prevent loss or gain of moisture by the specimen as far as pos-sible. Organic soiis and soils easily damaged may be cut with a sampling tube {undis-turbed) and carefully pushed into the consolidation ring in which case the ring and tube must have the same diameter (see 5.1). Weigh the two dry filter paper rounds (prepared as in 2.13) and the ring with the specimen filter paper.

3.2 Determination of moisture content, volume and relative density Use the material trimmed from the specimen to determine the moisture content and the apparent relative density (TMH1, Method A 12T). Measure the thickness of the consolidation specimen with callipers or by other means. If this cannot be done due to the nature of the soil, the height of the ring must be accepted as the height of the specimen.

3.3 ~bIy of .pparatu.

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Wet the porous plates by placing them in a saucer of water so that not more than haH of the depth of the plates is submerged. Center the bottom plate in the consolidation ceU and place the ring. complete with specimen, centrally on top of the porous plate with a fitter paper against the top and bottom faces of the specimen. Then place the top porous plate and loading cap centrally on top of the specimen.

Special methods Draft TMH6. Pretoria. South Africa, 1984

Place the consolidation cell in position on the bed of the loading apparatus and carefully bring the counterbalanced loading beam to a level position with the appropriate load-:transmit1ing member in contact with the loading cap. Clamp the dial gauge into position for recording the movement between the base of the consolidation cell and the loading cap. The pointer of the gauge should be arranged at such a position in its range of travel as to allow for a small amount of swelling of the specimen. the remainder of the range of travel being taken to allow for compression. Note the dial gauge reading and the time

3.4 L ding procedure Apply an initial pressure to the specimen, depending upon the ty~ } of soil and equivalent to the estimated effective overburden pressure. The initial pressure should be large enough to prevent swelling of the specimen but should not compress the soil. For stiff soils a pressure of 50 kPa should be enough, whereas a pressure as low as 10 kPa may be necessary for soft, sensitive soils. Fill the cell with distilled water at room temperature as soon as the initial pres-sure has been applied and take the dial gauge reading or set the gauge to zero (see 5.3). Start the stop watch and take dial gauge readings after 0; 0. 1; 0,25; 0,5; 1; 2; 4; 8; 15 and 30 minutes; 1; 2 ; 4; 8; 24, etc. hours. Continue with the readings until the slope of the characteristic secondary portion of the compres-sion versus log-of-time plot is apparent (or the compression versus square root of time) (see 5.4). Allow the specimen to consolidate at the specific loading to at least 90 per cent of consolidation. i.e. until too is reached (see 4.4.1). A!ternatively apply 6, 12. 25, 50. 100 and 200 per cent of the maximum field load and take the dia! gauge readings for each load as described above. load-ing should be increased until a linear plot of the curve of compression versus log of pressure (or versus log of time) becomes a straight line and at least until twice the anticipated stress due to overburden and structllre combined is reached.

3.5 Unloading procedure On completion of the compression gauge readings under the maximum applied pressure, remove the load from the test specimen and remove the consolida-tion cell from the apparatus. Remove the ring with specimen and filter papers and weigh these. Dry them in an oven at 105 to 110 C to constant mass and reweigh to determine the moisture content

4 CALCULATIONS 4.1 Calculate the initial moisture content of the specimen and its moisture content

after consolidation to the nearest 0.1 per cent. 4.2 Calculate the apparent relative density of the materia! (see TMH1, Method

A12T). 4.3 Compressibility characteria11ca

The compressibility characteristicS may be illustrated by plotting the compres-sion of the specimen as the ordinate on a linear scale and the corresponding applied pressure, p, in kPa, as the abscissa on a logarithmic scakl.

Special methods Draft TMH6. Pretoria. South Africa. 1984 53

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The compression may be indicated direCtly by plotting the actual 'thickness of the specimen, or as a strain by plotting the percentage reduction in thickness, or as a voids ratio, Calculate the thickness, H, of the specimen in mjllimetres for each load increment by subtracting the compression of the specimen (reduction in thickness). as recorded by the compression gauge, trom the lnitial thickness measurement. Should a voids plot be preferred, the voids ratio is calculated as follows:

H-H., e=--

Ha

where H = thickness of specimen in mm as calculated abOve He :::: equivalent height of the solid particles in mm and calculated as foHows .:

Max 1000 Ho=----

Gs.d .. x A

where

Ms = dry m~ss of specimen in grammes G, = relative density of the soil particles A

"" area of the specimen (mm2)

d .. == density of water (g/cml The coefficient of volume compressibility. M . in m2/MN, is calculated for a pres sure increment of 100 kPa in excess of the present effective overburden pres-sure at the sample depth. It is obtained from the semi-logarithmic plot men-tioned above, as follows:

dH 1 My :;- 1000

dp H where

dp =- 100 kPa dH = change in thickness corresponding to -the increment of 100 kPa H = thickness of the specimen under the present effective overburden

pressure. When the voids ratio, e, is plotted:

de 1 000 M.,;;::;;_._-dp 1 +eo

where

dp = 100 kPa de - change in voids ratio corresponding to the increment of 100 kPa eo :;;; voids ratio under the present effective overburden pressure. When the percentage reduction in thickness is plotted, Mo, may be obtained di rectly from the curve.

SpedaI methods Craft TMH6. Pretoria. South Africa, 1964

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The coefficient of volume compressibility may be calculated similarly, if re-quired, for any pressure increment.

4.4 Consolidation coefficient The consolldation coeffICient may be determined by using 1hefollowing two methods. Only one method need be used if it has been found to be satisfactory.

4.4.1 Squa ..... root-of .. tlmeflttfng method (see 5.4) Draw a straight line which best fits the early portion of the plot between com pression gauge readings and the square root of elapsed time, and extend it to intersect the ordinate of zero time. This intersection is considered to be the cor~ reeted zero point Draw a straight line through the corrected zero point and through points with abscissae 1,15 times as great as those of the besHit line previously drawn (see Figure 5T10/ll). The intersection of this line and the curve drawn through the plotted results is taken as the 90 per cent primary compreSSion point. Its abscissa, \'too. is then read off and the consolidation coefficient, C", computed from the following for~ mula;

TFF! C,. = - (m2 per year)

1

where H "" the longest drainage path T ::::: time factor,

This leads to: 0,111 H2

C. = (m2 per year) tsc

where H ::::: average thickness for the load increment in mm

~ = time for 90 per cent consolidation in minutes (see 5.5),

4.4.2 logartthm-of-time fitting method (see 5.4) When the compression gauge readings are plotted against the loganthm of time, the two straight portions of the parabolic curve are extended to intersect at the point of 100 per cent primary compression (see Figure ST10/1ll), The corrected zero point is located by marking off the difference in ordinate be~ tween any two points on the initial portion of the curve, with times in the ratio of 1 :4, and marking off an equal distance above the upper point Check the cor-rected lero point by using another pair of points on the curve. With the zero and 100 per cent points known, the 50 per cent primary compres-sion point is located and its time, tso (min). obtained. The consolidation coeffi-cient. Cv, is then computed as follows:

0,026 HZ! Cv ::::: (nf per year)

!so Special methods Ora:ft TMH6. Pretoria. Sooth Africa, 1964 55

where H ;:; average thickness for the load increment in mm t50 :::: time for 50 percent consolidation in minutes (see 5.S). (Cv will change as H changes:)

4.5 Compression ratlos Calculate the initial compression ratio, rn. and the secondary compressiQn ratio. I r~, asfoHows:

4.5.1 Square-root fitting method: do - d.

r, "" ---. d,- d,

r _ 10 (d~ - d~) Q - 9(d., - d,)

45.2 logarithm fitting method:

ro "' ---do - d, ds - d,DO

rp:::: ---d" - d,

f , :::: 1 -- (ro + To) where

d,OO =

df

correctea zero point compression gauge reading at 90 per cent primary compression by square-root fittmg method compression gauge reading at 100 per cent primary compression by log fitting method compression gaug.e reading at zero time corrected when necess-ary for deformation due to the elasticity of the apparatus final compression gauge reading .

4.6 Report the identification and descriptIon of the sample and state whether it is undisturbed. remoulded . compacted or otherwise prepared.

56

Report the initial moisture content, initial wet bulk density and the apparent rel-ative density of the solids. Report the results as the plot between compression (or strain) or voids ratio and the logarithm of the applied pressure. Report the compression ratios, and the consolidation coefficient, Cv, in m2 per year with each plotted time--compression curve.

Special methods Draft TMH6. Pretona. South Afrtca. 1984

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5 NOTES 5,1 Precautions should be taken to mi.nimize disturbance of the soil or changes in

moisture and density during sample preparation; in particular vibration. distor-!Ion, and compression should be avoided.

5.2 If the sample continues to swell after water has been added under the initial pressure, the next highest pressure should be used as the initial pressure. The next load should be applied in time to prevent loss of material should the po-rous plate be lifted above the top of the cutting ring. Expansive (and dispersive) soils are likely to slake on contact with water.

5.3 If the thickness versus square root of time is plotted. use time intervals with easily obtainable square roots like 0,09; 0,25; 0,49; 1; 4; g, 16: 36; 49 min.: 1; 4; 9 hours. etc.

5.4 The two fitting methods suggested generally show reasonable agreement. In some cases the square-root plot does not produce a straight line portion and the logarithmic plot may be more satisfactory. The reverse may also be true. SometImes the squareMfoot-oHime curve IS best used for determining the cor-rected zero pOint and the logarithmic time curve for the theoretical 100 per cent consolidation point.

5.5 In the calculation of C the drainage path was taken as half the sample thick ness. The formulae may differ from those used by other authors for this reason and because ot metrication of the units . if other consolidation times, e.q. he or teo, are used, the correct time factors

HZ should be obtained from the literature. The drainage path correction, then also be omitted.

22 . may

REFERENCES 1. BS 1377:1975. BRITISH STANDARDS INSTITUTION, London. 2. ASTM Designation 02435-70.

Special methods [)fat! TMH6. PretOiia. South Africa, 1984 57

LOAO !NG PAD

CONSOLIDATIONI ~~:==:tS: RING -

SOIL SPECIMEN

(SAMPLE RING)

FIGURE ST10f1 DIAGRAMMATIC ILLUSTRATION OF A TYPICAL CONSOLlDA-TIONCELL

Special methods 58 Draft TMH6. Pretoria, Scutt'! Africa. 1984

0 N

0 0 ~ I I-

W ~ C,!)

'" ... Z w - ~ ;, r:: E u:

w w :E 'if i= ::e . I- I.J.. II.. 0 C ..:. 0

N I-- 0 0 0 II: It I UJ ILl II: It

Q ::> 0 a (f) (/)

-a -.... t/) w a: :::) CI u::

0 IX ~ W N ...... 0 W l-t) I.iJ It It 0 t)

(WWI SONlav~ 30nv!)

SpeCIal methods Draft TMH6. Pretona. South Aftlca. 1984 59

g

~ ~ ~ 4' ~ ~. 8 m :T~ > -::r :!. ~ i ~i

o ~O~EetEJ iE~6 I II! I -, O,O! ' I ! - + - ! o,IOr--- .. I l-t-I .. +1.+_- -J-.+1 ._+-+----t ------l--.-

........ ~ I i' U ' ......... ! I ; ! ' ! '

0,15 -- -++ '-to. ,--t-r-t--i-'-+ j ! I I r-.~ I :! f! i I ~ . I ~'-r ! ' !! i., J +-- .. _- -f- --

". ~ I I I I l I !! I i I e O,20f--' E I !! -1-+-+1 I ! I :I --+.--t- i-i --- --'. --t---I-. _._.. l j

ho : 2 rl~~ t .. ~J I !! -... - -=1' ___ --.. l.n 0,25 0 z ~ ! I I i , i

' 1 II ~ " .~ ~ ~ il l -,", -' -+-.--+- --- -7~,~;:~.. I I I: --I . I I --.~.

'--" --. -',---100.} PRIM~RY COMPRES],O;; -- i

~O,30 ~ w 0,35

~ ~ 0,,,,,",

o j4~1-1 - - +-

0,50t-1 --+--+-+ -+-H-t I !, t -1-+ '\ =ttl' I' 1111 I 1- ,--- i I

O,!5!5f I I I I I ! III I I I! I I --tnt ++l+-+- -- I I Ii i ,!

0,601 I lit t ! I I I ! I I! I I I I -I ' I '. i I -1-'- I t 6 18

SECONDS 304260 2 >4 6 8 10 20 4() 60 eo 100

----J MINUTES Tl ME - lOG SCALE

FIGURE ST10/1U LOG-OF-TIME FITTING METHOD

3 4 6 e 12 Z4 48 HOURS '--' ---..J

TMH 6-1TMH 6-2TMH 6-3