Description and installation of piezometers for measuring pore-water pressures in clay soils

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Description and installation of piezometers for measuring pore-water pressures in clay soils

CANADA Ser TKL B92 no- 37 c . 2 BLDG'

I3ESCRIF!PEO'PS ABIl

INSTALLATION

OF

PIEZOB'EETERS

FOR MEA.SURINIG PORE-IT-4TER PRESSURES

IN

CLAY SOILS

by

M.

Boqozuk ANALYZED

The engineering p r o p e r t i e s of saturated c l a y

s o i l s may be g r e a t l y affected by w a t e r , In certain situations

a n

increase

in

pore water pressure may suffTciently reduce the strength of

the

subsoil t o cause a serious foundation failure. A h o w l e d g e of these pore pressures therefore is of t h e utmost importance in designing certain engineering

structures on saturates c l a y s o i l s , This Note discusses t h r e e methods f a r measuring pare water pressures in t h e f i e l d u s e d

by t h e Soil Mechanics S e c t i o n , Division of B u i l d i n g Research, N a t i o n a l Research Council,

I.,

POROUS

STOKE PIEZOMETER

WLTfE CASING

The

construction

and

i n s t a l l a t i o n of

this piezometer was f i r s t d e s a r i b e d by C a s a p n d a

(1949).

It

consists of a porous stone t i p (Pig,

1)

i n s t a l l e d to %he required depth end connected to the surface w i t h a s m a l l

diarne-ber saran tube. Pore w a t e r pressure readings a r e made by measuring the head

of

w a t e r in the saran tube.

Materials

1. Norton Porous Stone Tube,

No,

E l 2 0 O.B. = 1.5 ,In,,

I.D.

= 1.0

in.,

length of tube 12 in. t o 24 in.

2. Rubber s t o p p e r ,

Bo.

5 .

3. Neoprene

or

rubber

bushing,

318

in.

I.D. by 5/16

in.

w a l l , 4

in,

long,

5 , S a t u r a t e d sand, between No. 20 and No,

35

mesh, 6 , Piezometer s e a l , either b e n t o n i t e or AM-9 chemlcal

gel

7 , Rounded p e b b l e s , 318

i n .

(approximate d i a m e t e r ) . 8. Tamping hammer to f f t caslng.

9 , 2-in, 1.D. casing, IISae f i r s t 1 0 - f t section of c a s i n g s h o u l d n o t be provided w i t h

a

c o u p l i n g or drive s h o e at

i t s l o w e r end.

10, Pipe cap, brass collar f o r s a r a n tube and wooden d i s c ,

11, Driving r a m , t o drive c a s i n g .

12. Auger or drill equipment t o c l e a n and wash casing,

13,

Pump and w a t e r s u p p l y .

Assembly of Piezometer Point

One end of t h e saran t u b e is bevelled on t h e ou-kside w i t h

a h i f e and l u b r i c a t e d w i t h water. It is then i n s e r t e d a b o u t 1 fn, into t h e r u b b e r bushing, which is t h e n i n s e r t e d about 3 in. i n t o one end of t h e porous t u b e , !then with a t w i s t i n g motion t h e saran

t u b i n g is forced a f u r t h e r

3

in, i n t o t h e w b b e r bushing.

!Ibis may r e q u i r e considerable effort; a s t r a p wrench f a r

gripping the saran t u b e (Casagrande

1949,

p.199) m a y be h e l p f u l , F i e l d I n s t a l l a t i o n

!The cased h o l e is advanced to a depth o f 2 ft belaw t h e

b o t t o m o f the porous t i p (Fig, 21.

To

facilitate d r i v i n g , t h e casing may b e cleaned w i t h a hand auger or d r i l l bit a s it _is being d r i v e n , Ro cleaning or washing s h o u l d be done, however d u r i n g t h e last 10 ft, iChis will assure a tight contact between the bottom 10 ft of casing and the s u r r o u n d i n g soil, When in p o s i t i o n , the i n s i d e o f t h e casing is washed c l e a n to the b o t t o m , and t h e wash water entirely r e p l a c e d by c l e a r water. !This is done

water in t h e t o p

u n t i l

a l l of t h e cloudy water is r e p l a c e d .

The casing is then jacked up 2 ft, S a t u r a t e d sand is

slowly poured

in

t o fill t h e bottom 2 ft of t h e open hale, The sand s h o u l d n o t b e poured in t o o q u i c k l y , as it may arch and b l o c k t h e casing, making it impossible t o g e t t h e porous p o i n t

to t h e b o t t o m of t h e h a l e . The volume of sand needed s h o u l d b e computed and closely controlled, T h e tamping hammer may be

used t o check t h e height of sand in %he hole.

! h e standpipe of t h e piezometer is connected t o a

small tank and t h e porous p o i n t imel-sed a f e w feet b e l o n t h e

t o p af the c a s i n g , It is filled by applying a

vacuum

t o t h e tank. R small reservoir of water is similarly o b t a i n e d in t h e

t a n k by means of which an excess head may b e maintained in t h e porous point as it is l o w e r e d i n t o place.

With t h e point resting on t h e sand in the bat-tom of

the hole, t h e casing is p u l l e d to an elevation equal to that

of

the

top o f t h e potnt. S a t u r a t e d sand is then poured in t o fill %he space a r o u n d iL, The amount of sand u s e d s h o u l d a l s o

be computed and controlled,

The casing is $lim p u l l e d up 1 more f o o t to its f i n a l p o s i t i o n , Enough s a t u r a t e d sand is poured in to fill t h e open hole and t h e b o t t o m 2 ft o f casing, It is then tamped by means

of t e n b l o w s of the hammer dropped 6 in, !Ibis provides a sand

plug minimizing the effects of t h e overlying seal. Sealing t h e Piezometer

E i t h e r of t w o methods may be used f o r s e a l i n g t h e piezome t e r :

(a) Bentonite seal

P r e p a r e b e n t m i t e balls or sausages by w e t t i n g t h e

b e n t o n i t e clay t o j u s t above the plastic l i m i t and rolling by hand, (As t h i s is a time-consuming job, t h e y s h o u l d be pre- pared and s t o r e d in jars b e f o r e %he a c t u a l i n s t a l l a t i o n is

started,) B e b a l l s are dropped through the w a t e r to the

bottom o f t h e casing, Five

3-in.

l a y e r s , each one w e l l tamped, provides an effective s e a l .

To determine t h e number of b e n t o n i t e balls r e q u i r e d

to make one 3-in. l a y e r , t h e water level in the casing is Lomered

3

in. from the t o p , then enough balls are dropped t o raise t h e

water l e v e l again t o t h e t o p , This is f o l l o w e d by a

3/4-in.

l a y e r of the rounded p e b b l e s to p r e v e n t t h e hammer from s t i c k i n g ,

The hammer is next l o w e r e d o n t o t h e p e b b l e s and twenty b l o w s

a r e a p p l i e d by dropping t h e hammer 6 in. This process is r e p e a t e d until five l a y e r s o f b e n t o n i t e a r e in place,

A 2-ft plug of g r a d e d sand is added next and tamped i n t o p l a c e , f o l l o w e d by a n o t h e r bentonite s e a l of five 3-in. l a y e r s ,

This second seal s h o u l d b e capped with a b o u t 3 ft of s a n d and t h e

r e m a i n d e r of the hole l e f t open if desired.

Ibl AM-(4 miezometer s e a l

A simpler and l e s s time-consuming method of s e a l i n g

the p o r o u s s t o n e piezometer is w i t h a chemical marketed as

AT7!-9 (Lambe

1959),

A m i x t u r e of AN-9, w a t e r , and s u i t a b l e catalysts will polymerize to form an impermeable g e l , A I - i n , l a y e r o f bentonite seal is f i r s t placed as j u s t d e s c r i b e d t o

prevent t h e clay-chemical mix f r o m invading t h e sand around t h e p o r o u s p o l n t , Then a p r e d e t e m i n a t e d amount of s l u r r y of clay

a n d c a t a l y z e d chemical is poured into %he casing forming t h e s e a l ,

! b e f o l l o w i n g materials a r e required to place a 4 - f t s e a l of ARt-9 in a 2-in, and 4-inB casing (Lambe 1958).

2-in. casing 4-in, c a s i n g

ATrT-9 powder (20,0$ o f s o l u t i o n 120 1680 gm

( 3 , 7

1 b ) by weight e x c l u d i n g kaolin ) Water

(7'9,3;$

of s o l u t i o n by weight 675

rn

6700 gm ( 1 4 , 8 l b ) Kaolia (52% of s o l u t i o n w e i g h t ) 1100 gm 4400 ~ ; 6 n ( 9 - 7 l b ) Potassium ferricyanlde (KF) 0 0 2 3 D 0 3 2 gm 10,01$ of s o l u t i o n by w e i g h t ] D M E N ( ~ ; ) f m e t h y l a m i n o p r o p i o n i t r i l e ) 8 . 4 &n 3 7 * 6 gm (0,405 o f s o l u t i o n by weight) Ammonium persulphate (AP

1

( 0 , 2 5 $ of s o l u t i o n by weight)

I

(ti) Procedure

1, In a c o n t a i n e r (preferably mon-metallic) dissolve t h e AM-9

powder

in

the water. Then add the k a o l i n and mix into a s l u r q ,

2, Dissolve the potassium ferricyanide in

50

cc water, add to

t h e s l u r r y , and m i x w e l l .

(Note: ! h e potassium ferricyaniae r e t a r d s t h e g e l time which

may be increased or decreased depending on the amount usea, see

Fig. 3 . )

t h e t h e and mix I

4, Lower t h e

p i n .

pipe into the casing and clamp

it

so t h a t

the

b o t t o m end is

a

few inches above the b e n t o n i t e c l a y seal, Put

the f u m e 1 in t h e upper end of the pipe,

5.

Dissolve the ammonium p e r s u l p h a t e in a l i t t l e w a t e r , add it

t o the slurry, and stir w e l l ,

6 . Immediately pour a l l t h e s l u r r y down t h e pipe,

7 . Gently remove t h e pipe taking care not to m i x the slurry

w i t h t h e g r o u n d w a t e r which has been d i s p l a c e d upward,

8, G e l time mill vary w i t h s l u r r y t e m p e r a t u r e , i.e,, a b o u t 23

minutes at 70°F and 2 h o u r s at _{43OF for t h e above f o r m u l a ,}

Until g e l a t i o n o c c u r s a Tight t e n s i o n s h o u l d b e maintained on t h e saxan tubing s o t h a t it does n o t bend in contact w i t h

the casing,

9 , l e s t for gelation by lowering a small weight on t h e end of a string i n t o t h e casing, G e l a t i a n may be delayed by l o w e r

temperatures and by some common impurities in the groundwater. 10, After g e l a t i o n , b a c k f i l l

with

sand or other convenient material

and then pour water i n t o t h e casing,

( R o t e : Ih some instances it may be more convenient to prepare

the slurry u p t o step 3 in t h e laboratory and t a k e it t o t h e site, The _{amanium p e r s u l p h a t e may then} be a d d e d a n d the s l u r r y p o u r e d down %he casing, The gel t i m e may also b e checked in t h e laboratory beforehand

using

t h e same groundwater and at t h e same %emperatuxe as in t h e f i e l d f o r p r e p a f i n g t h e g e l , )

(lii

1

PFezometer cap

!The piezometer may b e capped by p v l l i n g the standpipe t h r o u g h t h e wooden d i s k and tightly clamped as shown

in

Fig. 2 .

A ptpe plug w i t h t w o small holes d r i l l e d in

it

to allow a i r to escape completes t h e cap p r o v i d e s good protection to t h e

standpipe.

The non-metallic piezometer may be i n s t a l l e d at r e -

l a t i v e l y shallow depths w i t h o u t a casing. The f allowing a d d i t i o n a l equipment is required:

1. E a r t h a u g e r ,

36

in. in aiarneter.

2, Appropriate tamping hanrmer (Fig. 4 ) .

1. The p o r o u s p o i n t and saran tubing is prepared and assembled f o l l o m i n g t h e procedure o u t l i n e d above,

2. Auger a )$-in. diameter hole to a depth 1 ft below t h e re-

q u i r e d b o t t o m elevation o f the porous tip, Thoroughly clean t h e h o l e and b a c k f i l l with 1 f t of s a t u r a t e d sand,

3. Install the porous point using t h e tamping hammer t o centre

it

in t h e h o l e .

4, Backfill w i t h sa-turated sand t o an elevation of 1 f2: above t h e

t o p of t h e p o r o u s point,

5, Seal t h e piezometer w i t h e i t h e r t h e bentonite c l a y s e a l or t h e

AN-9 gel, following 'the i n s t r u c t i o n s o u k l i n e d above.

If

t h e

Mi-9 g e l is s e l e c t e d , t h e quantities given for _{t h e 4 - i n , casing}

may b e used,

6 , B a c k f i l l the h o l e w i t h any s u i t a b l e materials, using the hammer to compact it, Special care

must

b e taken at t h e s u r f a c e o f

the ground

t o

ensure t h a t s u r f a c e w a t e r w i l l n o t p e r c o l a t e domi t h e hole,

7. The _{Saran s t a n d p i p e may be} supported above p o m d by a b o a r d

3 , METALLIC -''GEoIVOR'~ PIEZOME9IER

!The metallic pkesometer (Pig.

5 ) was

developed by t h e

nomegian G e o t e c h i c a l I n s t i t u t e .

It

consists of a porous

metallic t i p coupled t o a steel "E" size drill

rod

and pushed

i n t o

t h e clay s o i l t o the required elevation.

Water

pressures at

the p o i n t are r e a d by m e a s u r k g t h e head o f water i n a

palyethylene t u b e connected through t h e W l l r o d s to t h e p o r o u s tip.

1, Piezorneter w i t h bronze filters ( P i g , 51, t i g h t e n i n g

nut,

and

r t 11

r u b b e r 0 ring s e a l ,

2. Polye-thylene t u b i n g 318-in. O W L , 114-in. 1.D.

3 . Threading pin w i t h

1/8-in.

d i e for threading the t u b i n g

( T i g , 6 ) -

4.

" E ' W i l l rods ana couplings,

5.

1

b u c k e t , t a l l enough t o submerge porous tips. 6 , De-aired, d i s t i l l e d water,

7 * Tubing plugs.

8 .

4-ft

_{crowbar, l&-in hand auger, screw jack.}

Procedure

1.

B r e a d one end a f polyethylene tubing by first inserting t h e ? b e a d i n g p i n to support t h e t u b i n g , then c u t threads m i t h

t h e d i e (Pig. 63, The threading pin may be clzmped w i t h p l i e r s o r bench v i c e to facilitate t h e operation,

2 , Remove t h e tightening n u t and rubber "0" ring from t h e

p i e z o m e t e r and thread it

on

the plastic t u b e , Then screw the

t u b i n g i n t o the piezometer, r e p l a c e the rubber seal and t i g h t e n the nut,

3.

Submerge the porous tip in de-aired distilled water in t h e

container, Fill t h e piezometer and t h e plastic t u b e by

applying a vacuum t o t h e system, '?hen full, plug the t u b i n g

so that water cannot escape d u r i n g handling and a l s o to

prevent remoulded clay from entering t h e p-lezometer while

it

4+ At t h e site a hole is made through the f i s s u r e d c r u s t

of

the s o i l w i t h the c r o w t a r o x hand auger.

In

s't;ifT c l a y s the

h o l e may be augered to w i t h i n 3 ft of t h e d e s i r e d loca-hion of the porous point,

5 .

*cad one length of d r i l l r o d over the p l a s t i c t u b i n g and

screw

on

t h e piezorneter t i p . men q u i c k l y lift it o u t of t h e b u c k e t and push it down t h e prepared bole. "Eu r o d s a r e added

a s r e q u i r e d to push t h e piezometer t i p into t h e soil t o t h e desired e l e v a t i o n ,

In

q u i c k sensitive clays t h e piezorneter

may be pushed in by hand. Bowever, the screw jack may b e re-

q u i r e d .

6 , Remove the p l u g from the t u b e and begin the measurements, In some cases it may be necessary to f l u s h the system. This is

done by inserting a thin p l a s t i c tube to the b o t t o m o f t h e p i e z a m e t e r and pumping water t h r o u g h the: system.

7 . To protect t h e p l a s t i c -tube, a cover may be screwed o n t o the p i p e s ,

If

a Bourdon gauge is used

to

measure pore p r e s s u r e s ,

a special box clamped t o t h e pipe is r e q u i r e d to protect t h e i n s t r u m e n t s ,

! h e water level in t h e ptezometer s t a n d p i p e is measured

w i t h an electric p i e z o m e t e ~ probe developed for t h i s purpose,

It

c o n s i s t s o f a two-conductor cable encased in a 1/8-inc5 f l e x i b l e nylon pressure tubing leading f r o m an ohmmeter to a c o n t a c t t i p , The c o n t a c t p a i n t (Fig-

7 )

is made by soldering one c o n d u c t o r t o

a central copper p o s t which protrudes f r o m t h e h b i n g and the other t o a b r a s s collar a b o u t

1/4

in, above t h e copper t i p , A

g l y p t a l seal is used as wa3erproofing and t o i n s u l a t e t h e brass collar from t h e copper t i p , In o p e r a t i o n the _{probe is pushed down}

%he standptpe to t h e water, submerging t h e copper tTp, When t h e

brass r i n g comes

in

_{c o n t a c t with t h e water an e l e c t r ~ c c o n t a c t is} made mhlch is i n d t c a t e d by the ohmmeter, B e nylon t u b i n g is marked off in intervals of feet and i n c h e s .

Where the elevat-ion sf the water

in

t h e standpipe can

be o b t a i n e d by a d i r e c t r e a d i n g this s h o u l d b e aone, If the pore pressures become s o g r e a t t h a t t h e water is f o r c e d o u t %he t o p , $he excess pressure can be measured w i t h Bourdon gauge o r

a mercury manometer.

For s h a l l o w piezometers t h e s e n s i t i v i t y may b e i n c r e a s e d

a n d stiffness of measuring device a n d by de-airing provisions,

5.

DVANTAGES

AmD

_{DISADVrnTAGES OF}

_BOTH

_{PIEZO'RrniCERS}

Comparable r e s a l t s have been obtained from t h e t w o

piezometers. F o r example Pigo 8 shoms the pore water pressure

t a k e n

at

t h e same location u s i n g "Casagranaer' piezometers and

ttGeonor" ppiezometexs,

The porous s t o n e piezometer may b e u s e d in any c l a y s o i l ,

r e g a r d l e s s of corrosivity. Its greatest disadvantage is t h e t i m e required t o p r e p a r e and i n s t a l l it. The m o s t time-consuming job is t h e preparation and i n s t a l l a t i o n of t h e _{bentonite s e a l ,} but t h i s can be r e p l a c e d by the

AM-9

_{g e l . Driving the casing and} mashing the h o l e a l s o is t i m e consuming and requires c o n s i d e r a b l e

equipment. Another disadvantage i s t h a t t h e saran t u b i n g becomes

brittle in cold weather and if it breaks i n s i d e t h e casing r e p a i r s a r e very d i f f i c u l t

.

I h e "Geonor" piezometer is a simple and compact unit. It is q u i c k l y a s s e m b l e d and easily i n s t a l l e d . When the project is completed, t h e p i e m m e t e r s can b e w i t h d r a w n , d e a n e d , and re-

used, As it is metal, however, t h e r e is some q u e s t i o n as t o Whether long-term observations can b e made in corrosive c l a y

s o i l s ,

A single "Geonorrr piezometer can a l s o b e used t o m e a s u r e

p o r e w a t e r pressures at a number of d e p t h s in t h e soil, This is done by extending t h e "EM rod casing and jacking the porous point t o %he desired elevations,

In

t a k i n g t h e measurements, care must b e t a k e n t o ensure t h a t the r e a d i n g s a r e t a k e n when

t h e pore water pressures have reached equilibrium c o n d i t i o n s , The polyethylene t u b i n g is n o t affected by cold wea-Lher,

To prevent freezing of water fn t h e standpipes, an a n t i - freeze s o l u t i o n can b e used, The density of t h e s o l u t f o n m u s t b e t a k e n i n t o account when computing p o r e water p r e s s u r e s ,

Casagrande, A . (1949)* Soil mechanics in t h e design and con- s t r u c t i o n of t h e Logan A i r p o r t , Appendix. B o s t o n SocTety o f Civil a g i n e e r s , A p r i l

1949,

(Also p u b l i s h e d in B o s t o n Society o f C i v i l Engineers, Contributions to Soil Mechanics,

Zambe, T.W, ( 1 9 5 8 ) . Instructions

for

I n s t a l l i n g

AM-9

PFexometer

Seal. Nimeographed letter, Massachusetts h s t f t u t e of Technology, Cambridge, 27 October

1958.

Sambe, T.W,

(1959)

Sealing t h e C a s a p n d e pieaometer. C i v i l

Engineering, Vo1. 29, 8 0 . 4 , A p r i l

1959,

p.66.

Hote : AM-9 chemical gel may be o b t a i n e d from CpnarnFd af Canada -tea, 160 Bloor Street East, Toronto 5 , Ontario, or 5550

SARAN TUBING POLISHED HOLE PIPE THREAD BREAK EDGES A T HOLE ENTRY PASSES NOTE ;

TAMPING HAMMER TO BE USED

WITH 5'-0'' LENGTHS O F

3/i

IRON PlPE

MATERIAL : BRASS

THROUGH

FIGURE 4

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