Pressure membrane apparatus for the investigation of high suctions in soils

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Pressure membrane apparatus for the investigation of high suctions in

soils

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C A N A D A

PRESSURE MEMBRANE APPARATUS

FOR

_THE_{INVESTIGATION}

O F HIGH SUCTIONS IN

SOILS

P.

J.

Williams

July

1964

I

-

I I

D I V I S I O N O F B U I L D I N G RESEARCH

.

N A T I O N A L RESEARCH C O U N C I L

.

O T T A W A • C A N A D A

(The numbering of t h i s s e r i e s i s a continuation of the DBR Building Notes, which have been discontinued. )

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PRESSURE MEMBRANE APPARATUS FOR THE INVESTIGATION

I I

by

P.

_J. _Williams

I

P r e s s u r e m e m b r a n e a p p a r a t u s (1) i s widely used f o r t h e i n v e s t i - gation of s o i l m o i s t u r e suction. The r e s u l t s obtained a r e usuaully e x p r e s s e d in t h e f o r m of m o i s t u r e content v e r s u s suction ( p F ) c u r v e s ( 2 ) . T h e y have wide application in engineering, a g r i c u l t u r e and other fields, w h e r e m o i s t u r e movement, p o r e water p r e s s u r e s and r e l a t e d phenomena a r e being c o n s i d - e r e d . The p r e s s u r e m e m b r a n e a p p a r a t u s c a n be u s e d t o much higher s o i l suction values than i s p o s s i b l e with the s i m p l e r suction p l a t e a p p a r a t u s , w h e r e t h e suction i s l i m i t e d by t h e instability of a w a t e r column t o a p r e s s u r e of about - 1 3 p s i (0.

9 2

kg/cm2).

Conventional p r e s s u r e m e m b r a n e a p p a r a t u s i s o p e r a t e d a t a i r p r e s s u r e s up t o 200 p s i (14. 1 kg/cm2) o r m o r e r a r e l y 500 p s i (35.2 kg/cm2), Such a p p a r a t u s (and a l s o one o p e r a t i n g t o 2000 p s i ( 3 ) ) a r e

available

_{c o m -}

m e r c i a l l y , F o r r e s e a r c h p u r p o s e s extension of the suction-moistur e content c u r v e s t o higher s u c t i o r , ~ is of i n t e r e s t . T h i s can b e achieved b y vapour p r e s s u r e methods, but a t m o d e r a t e suctions ( p F 4. 5 t o 5) t h e s e methods r e q u i r e c a r e f u l t e m p e r a t u r e control. F u r t h e r m o r e , in p r e c i s e w o r k the r e s u l t s m a y not be s t r i c t l y equivalent t o t h o s e obtained by p r e s s u r e m e m - b r a n e a p p a r a t u s ,

T h e a p p a r a t u s d e s c r i b e d below i s intended for u s e a t a i r

p r e s s u r e s up t o 20, 000 p s i (1406 kg/cmZ). The cylinder could withstand the u s e of higher p r e s s u r e s , but p o s s i b l e f a i l u r e of t h e p o r o u s s t e e l p l a t e , or m e m b r a n e , should the= be c o n s i d e r e d , T h i s note w a s p r e p a r e d a s a guide t o t h e operation of t h e a p p a r a t u s used a t the Division of Building R e s e a r c h but it m a y a l s o be of i n t e r e s t t o o t h e r s contemplating the u s e of a s i m i l a r a p p a r a t u . ~ .

GENERAL, DESIGN AND OPERATlON

An a p p a r a t u s of t h i s type w a s f i r s t c o n s t r u c t e d a t t h e Road R e s e a r c h Station i n t h e United Kingdom (4). The design of the DBR

a p p a r a t u s i s quite s i m i l a r a s it w a s b a s e d on the B r i t i s h one.

T h e s a m p l e l i e s a t t h e bottom of a s t a i n l e s s s t e e l cylinder ( F i g u r e 1). The a i r is c o m p r e s s e d b y a piston which t r a v e l s down t h e cylinder. T h e a i r p r e s s u r e r i s e s a p p r o x i m a t e l y i n v e r s e l y p r o p o r t i o n a l t o t h e height of t h e enclosed air column. During t h e test., w a t e r p a s s e s f r o m t h e s a m p l e t o t h e e x t e r i o r t h r o a g h the m e m b r a n e and p o r o u s p l a t e a t the b a s e of t h e cylinder. When equilibrium is r e a c h e d , no f u r t h e r

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t h e m o i s t u r e content of the sample. As in the conventional p r e s s u r e m e m b r a n e a p p a r a t u s , t h e m e m b r a n e p o r e s a r e s a t u r a t e d and t h u s prevent

e e s c a p e of t h e c o m p r e s s e d a i r . T o achieve sufficiently high p r e s s u r e s , h e cylinder i s f i r s t filled with a i r c o m p r e s s e d t o about 1000 p s i

(70. 3 kg/cm2) ( t h i s m a y be supplied f r o m a c o m m e r c i a l cylinder) through t h e inlet p o r t shown in F i g u r e 1. The piston i s held above t h e p o r t a t t h i s stage. The piston head includes a B r i d g e m a n s e a l , t o p r e v e n t a i r leakage around the piston. Details of the piston and s e a l a r e shown in F i g u r e 2. A s t h e air p r e s s u r e i s r a i s e d , t h e piston head i s f o r c e d back onto the teflon w a s h e r causing t h i s t o expand and p r e s s i n c r e a s i n g l y tightly against t h e cylinder wall. In t h e a r r a n g e m e n t a t DBR, loading of t h e piston i s c a r r i e d out with a hydraulic jack s y s t e m which automatically m a i n t a i n s the load constant within 2 t o 3 lb during t h e c o u r s e of t h e t e s t .

T h e following differences between the DBR a p p a r a t u s and t h e B r i t i s h one m a y be noted:

1. The B r i d g e m a n s e a l includes a shaped teflon w a s h e r a s shown in F i g u r e 2 ,

2 . Since t h i s s e a l does not become effective until a i r p r e s s u r e s of 500 t o 1000 p s i ( 3 5 . 2 t o 70. 3 kg/cm2) a r e r e a c h e d , a r u b b e r ' 0 ' r i n g i s placed i n t h e piston head t o a c t a s a s e a l a s t h e p r e s s u r e i s r a i s e d initially.

3 . T h e piston shaft (which i s somewhat s m a l l e r than t h e cylinder b o r e ) is held l o o s e l y by t h e b r a s s bushing at the top of the cylinder. T h i s e n s u r e s t h a t .slightly non-axial loading of t h e piston will not r e s u l t in 'bindingt o r gouging of t h e piston and cylinder wall.

Axial loading is achieved by having t h e b a s e of t h e cylinder r e s t i n g on a s t e e l ball. The jack s y s t e m a l s o includes a pivot s o t h a t the loaded cylinder l i e s between two b a l l joints. Working drawings of t h e DBR a p p a r a t u s a r e available f r o m t h e Division of Building R e s e a r c h , Ottawa.

OPERATING PROCEDURE

1. T h e a p p a r a t u s i s d i s a s s e m b l e d and cleaned thoroughly.

2. A c i r c u l a r p i e c e of m e m b r a n e , slightly l a r g e r than t h e '0' r i n g in t h e cylinder b a s e , i s cut and soaked in w a t e r until soft (about

30 m i n ) .

3 . T h e m e m b r a n e i s placed on the cylinder b a s e and e x c e s s w a t e r

removed. The s a m p l e , which m u s t not exceed about 1 /5 in.

(5. 1 m m ) i n t h i c k n e s s , and should have a d i a m e t e r slightly l e s s than t h e b o r e of the c y l i n d e r , i s placed on the m e m b r a n e . It

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should be cut f r o m t h e m a t e r i a l t o be t e s t e d with a s l i t t l e disturbance of the s o i l s t r u c t u r e a s possible. R e a s s e m b l y of the cylinder m u s t be c a r r i e d out quite quickly t o avoid drying of t h e m e m b r a n e . Soil

parti.clea m u s t not come into contact with the cylinder wall. A s m a l l quantity of silicone g r e a s e i s placed on t h e piston head.

4. T h e n u t s a r e t i g h t e n e d t o 1 5 f t l b ( 2 . 0 7 m k g ) t o r q u e . C a r e m u s t b e taken that the cylinder does not t u r n in r e l a t i o n t o t h e bolts.

5. T h e cylinder m u s t be c a r e f u l l y placed under the jack with t h e piston c o r r e c t l y aligned with t h e a x i s of t h e jack. The cylinder position m a y be established with t h e aid of a plumb line, and then m a r k e d on the floor. The m a x i m u m t r a v e l of t h e piston when moved b y t h e jack should be such that t h e piston. head does c o t quite r e a c h the s a m p l e .

6 .

_{T h e a i r supply t o the p o r t is then attached. N o leakage should occur}

when t h i s supply i s t u r n e d on. A s the load i s applied t o the jack, t h e sealing r i n g will p a s s t h e inlet p o r t and a i r f r o m t h e c o m p r e s s e d a i r cylinder will e s c a p e . T h e a i r supply i s t,hen t u r n e d off.

7. The jack load i s applied in the c o u r s e of 1 to 2 m i n and should not be allowed t o exceed the r e q u i r e d amount. Any slight expulsion of a i r through t h e drainage tube should c e a s e within a few minutes. T h i s i s . . a s c e r t a i n e d by placing the end of the tube in a beaker of w a t e r .

8 . F i v e days i s u s u a l l y sufficient for a s a m p l e of the s i z e named t o r e a c h

equilibrium

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T h e end of the drainage tube should r e m a i n under w a t e r during t h a t p e r i o d t o avoid p o s s i b l e drying out of the p o r o u s p l a t e and m e m b r a n e ,

9.

Determination of the m o i s t u r e content i s c a r r i e d out by the u s u a l p r o c e d u r e of oven-drying. B e c a u s e of t h e s m a l l s i z e of t h e s a m p l e an.d often l o w m o i s t u r e content, weighings should be t o n e a r e s t

0. 1 m i l l i g r a m , It i s a l s o important that t h e r e i s no delay between r e m o v a l of the s a m p l e f r o m t h e cylinder and weighing b e c a u s e of the tendency of the s a m p l e t o a d s o r b a t m o s p h e r i c m o i s t u r e .

SAFETY PRECAU'GIONS

Becairse of t h e high g a s p r e s s u r e s involved c e r t a i n p r e c a u t i o n s a r e n e c e s s a r y , Couplings between t h e e x t e r n a l a i r supply and t h e

cylinder sh.ould be s e l f -sealing i f the coupling b e c o m e s loose.

If t h e loading of the piston i s not sufficiently c l o s e t o axial, o r i f soil. p a r t i c l e s contaminate t h e cylinder wall, t h e piston m a y bind in the cylinder. T h i s c a n r e s u l t in s e r i o u s damage t o t h e wall which may silbsequently r e q u i r e repolishing. When the load i s r e m o v e d in

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n e c e s s a r y in loosening t h e piston, a s it m a y be suddenly expelled with g r e a t f o r c e . Occasionally a i r leakage m a y expel t h e b r a s s bushing which then t r a v e l s up t h e piston.

A NOTE ON THE FUNCTIONING O F THE MEMBRANE

T h e m e m b r a n e s in the p r e s s u r e m e m b r a n e a p p a r a t u s function on t h e p r i n c i p l e that t h e p o r e s a r e sufficiently s m a l l s o that t h e w a t e r that fill's t h e m i s held s o f i r m l y by s u r f a c e f o r c e s t h a t i t cannot be dislodged by the a i r p r e s s u r e . M e m b r a n e s supplied for conventional p r e s s u r e m e m b r a n e apparatus::: w e r e s a t i s f a c t o r y for the highest p r e s s u r e s r e a c h e d i n t h e

p r e s e n t a p p a r a t u s .

In the conventional p r e s s u r e m e m b r a n e a p p a r a t u s , a f a i r l y s t e a d y s t r e a m of a i r bubbles p a s s e s through t h e w a t e r i n t h e drainage tube. T h i s i s v a r i o u s l y a t t r i b u t e d t o diffusion through t h e w a t e r i n t h e m e m b r a n e p o r e s , or t o the p r e s e n c e of a n occasional exceptionally l a r g e p o r e p e r

-

mitting d i r e c t p a s s a g e of t h e a i r through t h e m e m b r a n e . Leakage of t h i s type would be s e r i o u s in t h e p r e s e n t a p p a r a t u s , s i n c e the a i r within the

cylinder i s not r e p l e n i s h e d duririg t h e t e s t . Such leakage did not occur in t h e a p p a r a t u s constructed. ( G r o s s leakage s o m e t i m e s o c c u r r e d , but t h i s w a s due t o faulty placing o r drying out of t h e m e m b r a n e during t h e

a s s e m b l y of t h e c e l l . ) T h e r e a s o n for t h e negligible a i r leakage is not c l e a r , but i s p r e s u m a b l y r e l a t e d t o t h e s m a l l a r e a of t h e m e m b r a n e c o m - p a r e d t o the volume of a i r in t h e cylinder. In t h i s r e s p e c t , t h e c y l i n d r i c a l f o r m of t h e p r e s e n t a p p a r a t u s i s advantageous c o m p a r e d t o t h e f l a t p l a t e - like design of conventional a p p a r a t u s .

CALCULATION O F CYLINDER AIR PRESSURE FR.OM A P P L I E D LOAD

During loading the p r e s s u r e i n the enclosed a i r i s equal t o the load applied by t h e jack ( p e r unit c r o s s sectional a r e a of piston)

-

l e s s that due t o f r i c t i o n a t the piston s e a l . During unloading t h e p r e s s u r e equals t h a t applied b y t h e jack p l u s t h e f r i c t i o n a l f o r c e .

T h e f r i c t i o n a l f o r c e m a y be evaluated b y a c a l i b r a t i o n t e s t in which loading i s i m m e d i a t e l y followed by unloading. Two values of applied load':'* a r e thu.s obtained for any position of t h e piston. T h e d i f - f e r e n c e between t h e m r e p r e s e n t s twice t h e f r i c t i o n a l f o r c e involved.

F r o m s u h a t e s t a t a b l e of applied load during loading and c o r r e s p o n d i n g

The m e m b r a n e s w e r e obtained f r o m t h e I r r i g a t i o n E n g i n e e r i n g Corporation, Monrovia, California. S i m i l a r m e m b r a n e s a r e sold a s !Visking DiaPy s i s Tubing',

.I. 4,

.,. .I* _{The jack u s e d a t}DBR _{r e q u i r e s a s m a l l s p r i n g c o r r e c t i o n t o be}

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f r i c t i o n a l f o r c e c a n be p r e p a r e d . When t h e g a s p r e s s u r e (and hence the applied p F , i. e.

,

log of g a s p r e s s u r e in g m / c m 2 ) , i s calculated it i s u s u a l l y found that the a c c u r a c y of t h e f r i c t i o n determination i s of r e l a - tively l i t t l e significance.

ACKNOWLEDGEMENTS

T h e a p p a r a t u s used a t the Division of Building R e s e a r c h was designed by A. J. Smialowski of the Division of Applied P h y s i c s , National R e s e a r c h Council. J. D. Coleman of t h e Road R e s e a r c h Station, United Kingdom, reviewed and advised on the p r e l i m i n a r y p l a n s .

REFERENCES

1 . R i c h a r d s , L. A. P r e s s u r e m e m b r a n e a p p a r a t u s , Construction and u s e ,

Agr. Eng. 28, 1947, p. 451-460.

2 . Croney, D.

,

J. D. Coleman and P. M. B r i d g e . T h e suction of m o i s t u r e held i n s o i l and other p o r o u s m a t e r i a l s . Road R e s . Techn. P a p e r 24,

D. S. I. R.

,

R.

_{L . , H a r m o n d s w o r t h , Middx., 1952, 42 p.}

3. Croney, D. and J. D. Coleman. P o r e p r e s s u r e and suction in soil. In P o r e p r e s s u r e and suction i n s o i l s . B u t t e r w o r t h s , London, 1961,

31-37.

4. Croney, D . , J. D. Coleman and W. P. M. Black. Movement and

distribution of water in s o i l in r e l a t i o n t o highway design and p e r f o r m - ance. Highway R e s . Bd.

,

Spec. Rep. 40, 1958, p . 226-252.

5. Coleman, J. D. and A. D. M a r s h . An investigation of the p r e s s u r e

-

m e m b r a n e method for m e a s u r i n g the suction roper ties of soil. J o u r . Soil Sci. Vol. 12, No. 2, 1961, p. 343-362.

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' 0 " RtNG SEAL ---,.

pOROUS DISC OF

-

ALLOY STEEL

SCALE : FULL S I Z E

FIGURE I

PRESSURE MEMBRANE APPARATUS FOR USE AT HIGH PRESSURES (WORKING DRAWINGS AVAILABLE fR0M DIVISION OF BUILDING RESEARCH,

NATIONAL RESEARCH COUNCIL

.I

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1

-;051~

1

SCALE: TW/CE f . _5.

FIGURE 2

PISTON FOR HIGH PRESSURE APPARATUS AND BRIDGEMAN SEAL