A deep bench mark for precise levelling

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A deep bench mark for precise levelling Peckover, F. L.

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CANAL Ser THL B92 no. 11 c. 2

mt

A

DEEP

BENCH MARK FOR PRECISE LEVELLING

by

F.

.L,

Peakover

The c o n s t r u c t i o n of large

or

spec1 a1 s t r u c t u r e s , and t h e Investigation

of problems connected with t h e v e r t l c a l movement of s t r u c t u r e s r e q u i r e t h a t a bench mark be u n a f f e c t e d by s u r f a c e and

s o i l movements and y e t be economical in i n s t a l l a t i o n and o p e r a t ion, This N o t e d e s c r i b e s a method of i n s t a l l i n g a deep

bench mark w i t h o r d i n a r y m a t e r i a l s and

t h e equipment of a m a l l machine shop.

February, 1952

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A

DEEP

BENCH 1.IARK FOR PRECISE

LEVELLING

A bench mark i s a survey point f i x e d to a r e l a -

t i v e l y permanent o b j e c t t o prsv5de a r e f e r e n c e l e v e l f o r

a s long as is required.

The provision sf bench marks

i n

engineerfng c o n s t r u c t i o n 5 s usually a simple and T a f r l y r o u t i n e proce- d u r e .

In

the construction of hlghways o r s m a l l b u l l d h g s which are r a p i d l y e r e c t e d , a bench rnar~k I s e a s i l y o b t a i n e d

by using a nearby rock, tree or b u l l d i n g .

In

t h e

constructSon of l a r g e or s p e c i a l s t r u c t u r e s and f cr inve s-

t f g a t i o n d p u r p o s e s , however, it o f t e n b e c o r n s necessary to

keep a r e c o r d of e l e v a t i o n s

i n

a more precLse manner, For this purpose, more c a m f u l l y chosen.heneh marks are r e q u l r s d and the u s u a l p r a c t i c e is 50 locate them on b e d r o c k or

l a r g e s t r u c t u r e s ,

B e d r o c k or s t r u c t u ~ e s resting upon it provide

the most r e l i a b l e r e f e r e n c e l e v e l , When these are not a v a i l a b l e , choice of an a c c u r a t e d a t u m c . a l l s f o r

considerable c a r e , S m a l l buildings o r other i n s t a l -

l a t i o n s r e s t i n g at s h a l l o w d e p t h s are often f o m d to move up and down, slowly but a p p r e c i a b l y , m d aTe t h e r e f ore

quite u n s u i t a b l e f o r u s e as a datum over a p e r i o d of more than a f e w days, Such movements of t h e g ~ o u n d are

b r o u g h t a b o u t b y m o f sture and volume changes in c e r t a i n

sails, c a u s e d mainly by changing rainrall a d --ather .

P n v a r i o u s seasons of t h e y e s r , These raoverrent;~ may be

g r e a t enough to a f f e c t structures f currdsd a2 depths of

as m u c h as 12 o r 15 f e e t , Consequently, bench marks must

be f o u n d e d , as a g e n e r a l y u l e , at depths g m a t e r than this if r e a s o n a b l e accuracy is t o be o b t a l n e d over an

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When t h e r e i s no s u i t a b l e structure a v a i l a b l e t o c a r r y the bench mark to t h e necessary d e p t h a s p e c i a l i n s t a l l a t i o n m a J hdve to be made, Such an i n s t a l l a t f o n is c a l l e d a "deep bench m a ~ k ,

his'

note deserilbes a d e s l g n of a deep bench nark which can be p r e p a r s d

w i t h o r d f n u y tools and t h e assS_stance of a Small machflae shop, Components of a Deep Bench Nark

A deep bench mark r e q u i r e s essentflally a f i r m connectSon w i t h a n m d e ~ l y b g deep s t r a t u m of soil or rock which will move n o

more than a n e g l i g i b l e amount. Thfs connection

must

be u n a f f e c t e d

by surface and seasonal soil movements, which are usually verticalo

One method of achieving this is -t;o e x ~ e n d a r o d i n t o the ground f o r

a s u f f i c i e n t d e p t h and t o p r o t e c t it f r o m movements of t h e surroun- ding s o 5 1 by "1 o u t e r casing. If t h e l o w e r end of' the rod is firmly

seated, then a c c u r a t e l e v e l s may ,be o b t a t n a d f r o m t h e t o p end,

A deep bench mark of t h f s t y p e 4 s shorn diagrammatically in F f g , 1 m d d e t a E l e d

in

F i g , 2 , The c e n t r a l rod 03" this I n s t a l - l a t i o n is a t t a c h e d at the l o w e r end to a steel shoe, d e t a i l e d I n

F l g ,

3

{a) which s e r v e s I n i t i a l l y as- a d r f v e point and l a t e r as a

su9t;able connection w i t h the ground at d e p t h . A l l b u t the l o w e r extremity of the c e n t r a l r o d i s e n c l o s e d

in

a p l p e casing whficla

r i s e s and f a l l s w l t h the surrounding ground, l e a v i n g the rod

mar-

r e c t e d . Corrosion and f r f c t L o n between t h e rad and c a s f n g are

m i n l m f z e d b y f i l l 9 n g t h e intervenir,g space w l t h heavy a f l., T h e

upper end o f the rod I s p r o t e c t e d a g a l n s t damage by a durable cover,

the lid of which 1s removed when l e v e l readfngs are t a k e n ,

A b i l l of necessary m a t e r i a l s f o r t h e i n s t a l l a t i o n i s given i n Table 1,

Preparing f o r I n s t a l l a t i o n

The l o c a t i o n of a deep bench mark s h o u l d be c h o s e n c a r e f u l -

ly

wf th two ob j e c t s in mind;

maximum

use and

m i n i m u m

d i s t w b a n e e ,

For maximum use it should be p l a c e d s o t h a t i t may be s i g h t e d e a s i l y from a number of a n g l e s , The p a s s i b X l i t y of df s k u r b m c e , however, r e q u i r e s t h a t it s h o u l d be inconspicuous and p r o t e c t e d from o r d i n a r y

p e d e s t r i m or v e h i c l e t r a f f i c *

It

shoul5 be l o c a t e d on l e v e l ground

and at l e a s t a f e w feet away f r o m the n e a r e s t tree,

When the site is chosen, a number of' p r e p a r a t o r y s t e p s

s h o u l d be taken. First, a l i m i t i n g d e p t h I s chosen and this s h o u l d

be the smaller of t w o f i g u r e s : the d e p t h at which k t 9s t h o u g h t t h a t bed rock lies, o r the d s p t h to which it fs e s t i m a t e d that the bench mark c a n be d r i v e n ,

If

the l a t t e r f i g w e governs, it s h o u l d

be at l e a s t 25 feet and p r e f e r a b l y more. The l e n g t h

of

c e n t r a l r o d and casing are chosen accordfngly, w i t h due a l l o w a n c e f o ~ unforeseen c o n d i t i o n s ,

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Heans of d r i v i n g t h e installatfon to the r e q u i r e d d e p t h are a l s o chosen i n advance, In s o f t s o i l reasonable p e n e t r a t f o n can be o b t a i n e d w i t h a heavy sledge hammer. Tn t h i s case the casfng pipe

should be p r o t e c t e d by hammering o n a hardwood pad resting on

r e g u l a r p i p s cap. F o r o r d i n a r y d r i v i n g b e t t e r r e s u l t s may be obtained by usfng a d r i v e connection s i m f l a r to t h a t shown in Fig.

3

( b ) ,

This can be made of p a r t s which are e a s i l y o b t a i n a b l e ,

A 1 P s t of s u g g e s t e d tools for f i e l d use is given in Table 2 . Installat Ion P r o c e d u r e

The following details of procedure w i l l vary depending on l o c a l condftlans and personal choice.

After s o d or o t h e r s u r f ace covering I s removed o v e r

an

area about 2 feet s q u a r e , bore w i t h a 4-inch p o s t h o l e auger to a d e p t h of

3

f e e t arid i n s t a l l

the

b-fnch p3pe and c o u p l i n g to support the sLdes of t h e hole. Boldtng Ohis p i p e v e r t i c a l l y in the hole9 b a c k f i l l . the space surrounding t h e pipe w i t h dry sand t o k e e p it firmly

in

p l a c e , The t o p of the coupling s h o u l d be

an

inch or t w o below t h e ground

surf ace,

Proceed with a 1%-inch bore hale, uslng a carpenter's auger, below t h e 4-Pnch p i p e to a d e p t h where the ground is soft enough

to

commence d r i v i n g , Then l o w e r the 1 - i n c h p i p e En t h e prepared h o l e w i t h drEve p o i n t in position ( see Flg.

3

( a ) ) , A l o e s e l g connec.ted $-inch pipe m a g be used t o keep the d r i v e p o i n t in place while cas2ng

is l o w e r e d and then removed. An a c c u r a t e a c c o u n t of the length of p i p e b e l o w ground s h o u l d b e k e p t f r o m t h i s time,

Using a d r i v e connection or hammer, d r i v e t h e p o i n t v e r t i c a l - ly i n t o %he ground, a d d i n g l e n g t h s of 1-inch p i p e as r e q u l s e d . S t o p

d r i v i n g at t h e first sign of solfd resistance so as n o t to d i s p l a c e the dpf ve point)

or

at a maximum d e p t h chosen

In

advance, R e m o m the drive connection and f i l l the 2-inch p l p e v e q slowly with heavy ofla

Estfrnate the l e n g t h of t h e c e n t r a l p o d r e q u i r e d to reach a f i n i s h e d e l e w a t l o n of

4

i n c h e s below top of &-Inch coupling, Cut the %-inch p i p e t a this l e n g t h , allowing f o r couplings and l e a v i n g

it t h r e a d e d at b o t h ends. Then lower I t w f t h l n t h e 1-fnch p i p e , a d d b g s u c c e s s i v e lengths, Add one e x t r a l e n g t h and by means- of this, s c r e w the $-Inch p i p e i n t o the r e c e s s p r o v i d e d f o r

it

In

the

d r i v e shoe,

NOH d r i v e the 1-inch p i p = about 2 feet, using d r i v e c o n n e c t i o n or an i m p r o v i s e d yoke and automobile jacks, making s u r e

t h a t t h e c e n t r a l r o d remafns atationarg. This will disconnect the casing p i p e f r o m t h e d r i v e shoe. Cut and t h r e a d the 1-inch p l p e

close t o $pound l e v e l , About

1

inch b e l o w t h e thread, prepare h o l e s

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Dr*Sve the 1-fnch c a s i n g down again to a l e v e l 8 inches

b e l a w t o p of 4-inch p i p e coupling. Then remove top Length of c e n t r a l r o d andpinch top end o f r o d t o

ram

h o i n t on which l e v e l s can be

read. T u r n in centering screws by hand t o g u i d e c e n t r a l rod and screw c e n t e r i n g f l a n g e on 1-inch pipa. Then when count~rsunk plw is screwed into position

in

4-inch

p i p e coupling, t h e bench mark

is

ready for u s e .

This design of a bench mark s h o u l d require a m i n i m u m

b u t

nevertheless i m p o r t a n t amount of maintenance. In spring and f a l l it is s u g g e s t e d t h a t the top of the i n n e r rod be coated w i t h grease and oil added to the casing to guard a g a i n s t c o r r o s i o n ,

In a d d i t i o n ,

the i n s t a l l a t i o n should be

caref

u l l g inspe cted for damage, Any subsidence of t h e surrounding g o m d should be made good so t h a t t h e bench mark w i l l remain idcanspicuoua m d not be s u b j e c t to damage.

The difference in l e v e l between t h e t o p of the central rod and t o p of' 'the p i p e casing s h o u l d be checked.

If

t h i s does n o t

change, the centering screws should be i n s p e c t e d

to

make a w e that they are

not

restricting movement of the rod.

A s9mple design of a deep bench mmk has been o u t l i n e d f o r use

in l o c a t f o n s where

an accurate reference l e v e l is r e q u i r e d but n e i t h e r bed rock nar deep roundations are w i t h i n reach. Available equipment may requlre a l t e r a t i o n s In some of the d e t a f l s d e s c r i b e d b u t t h e s e are n o t i m p o r t a n t as long as the b a s i c p r b c k p l e s of the

d e s i g n are r e t a i n e d ,

Table 1: Bill ,of Merterlals for Deep

Bench Mark

( In approximate o r d e r or inst allat f

on)

1

-

4 in, g a l v a n i z e d

iron p i p e , 3 ft. long.

1 -

4 in.

g a l v a n i z e d

iron

p l p e coupling.

1 ina galvanized i r o n p i p e in

4

ft. l e n g t h s , t h r e a d e d b o t h ends, w i t h c o u p l i n g s ( s e e p ,

2 f o r

choice of number of p i e c e s ) .

4 in.

g a l v a n i z e d i r o n p i p a in

4

ft- lengths, thrsaded b o t h ends,

w i t h c o u p l i n g s [ s e e p . 2 for choice sf number of p i e c e s ) . 1

-

s t e e l d r i v e shoe ( s e e Fig.

3 (a)

),

3 -

m a ~ h f n e screws, & h. long.

1

-

1 in. f l o o r f l a n g e ,

4

in. diameter.

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Table 2: Equipment Required

for

I n s t a l l a t i o n

(in

appmximate order

of u s e )

spade.

4 in.

p o s t hole auger, cargenter' s l e v e l .

16 in. carpenterf s a u g e r

f o r

boring, w i t h extension rad and T-handle.

pipe wrenches to handle

$ in.,

1 in. and

4 in.

standard plpe. heavy sledge hammr,

with

pipe cap and hardwood pad; or d r i v e c o n n e c t i o n ( s e e Fig.

3

(b)).

s m a l l s t e p l a d d e r .

.

. s m a l l f u n n e l .

p l p e c u t t e r s and threddlng d l e s

for

2

in. snd 1

in.

p i p e . centre punch.

s m a l l hand drill; b i t and t a p f a r machine

screws

s u p p l i e d .

p r . p l i e r s . hammer.

screw driver. f ila

.

small squares of canvas.

measuring taps (ft. and in,).

lumber crayon.

b u c k e t of d r y cement sand,

bucket of heavy motor flushfng oil.

note book, w i p e r rags.

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P A O T E C J f V E C A P GROUND S U R F A C E

-

DATUM L E V E L

2

C E N T R A L R O D

a

F, T

k

h X T

3-

0

c

k

zz

% T h R O D OUTER P R O T E C T / N G F q U M C A S I N G S O I L CENTRAL M O V E

c

R

z

2

D R I V E S H O E , GE ST/NG AT CONSTANT L E V E L .

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I )

c

SPC- PLUG A. .V,, k 4 ' ' P I P E L Q U P L / N L i

_ - - - -

JPATDM

- - - -

LEVEL I " F L 0 0 4 F L A N G E L E N T L R l N G S C q E l V S

-

& s ~ ~1170.V ~ %P/PE LOUPLEP IN SECT/DNS.

I" CIALV, !RON PJPE COUPLED IN SECTIONS- S T E E L D R I V E S H O E

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/ P I P E C A S I N G B E V E L T b G U / P E C E N T R A L RO _{HOLE TAPPEP} I N T O TAPPEP H O L E , P f N R I V E SA'dE ASSEMBLY F O R P R l V E N G

F I G .

3(a)

PETAILS

of

D R I V E

S H O E -

2 # ,F' W E cdU,Pt!NG ' / M P R O V / S E D 30- LB. DROP W E / G M T , MAPE of S T E E L . F L A N G E S , E S C ,