An Apparatus for Investigation of the Distribution of Movement with Depth in Shallow Soil Layers

Publisher’s version / Version de l'éditeur: Building Research Note, 39, p. 10, 1962-10-01

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An Apparatus for Investigation of the Distribution of Movement with

Depth in Shallow Soil Layers

Williams, P. J.

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AN APPARATUS FOR INVESTIGATION

OF T m

OF

IN

by

P

.

.

T h i s note describes the construction and operation

of

instrument for measuring soil movements

in

a

ground surface to

a

Recent publications discuss observations on the occurrence and nature of these movements

(1,2,3),

to experhental investigations of this type (4). PREPARATION

OF

Portable battery-operated resistance bridge, (e,g, Tinsley, modified type

4907x1.

Polyethylene tubing

-

d.,

d.

One spring steel strip, 15 cm. x

1.2

in

Approx.

Two single strand, straight wire, resistance strain gauges ( e . g . Baldwin S , R ,

-

R e sistance

-

61

2 to 3 rn. four-conductor [gauge 22) cable, f l e s b l e . 4 spade lugs,

4 Dimensions are given in inches when theae are commonly used in c o m e r cial de scription.

G l a e s tube - 15 or 16 mm. i. d.

Polyethylene tubing

-

*,

*

Epoxy Compound:

Components Obtainable from

F i e l d Preparations

H y s o l (Canada) Ltd.

,

P.

0.

53,

R,

Toronto 1 7 , Ontario. Thiokol Liquid Polymer 3 Nagaltuck Chemical Co..

Elrnira, O n t a r i o .

Activator DMP 30 ( R o b and Haas)

A length o f the polyethylene tubing (518 in. i. d, ) i s inserted in the ground, The lower end i e heat sealed before ineertion and the upper end closed with a rubber stopper. During insertion the tube is held straight by inserting a wooden rod.

Construction

of

The probe is used to measure curvatures of the tube. W h e n

i n s e r t e d into the tube the prabe takes up the s a m e curvature, Strains

e e t up in the metal strip are measured by the strain gauges, and provide a measure

of

Electrical resistance strain gauges operate on the p r i n c i p l e

that the change in resistance of a w i r e on undergoing strain is approximately

proportional to the strain. A s in the prabe, two gauges are commonly u s e d in most applications, t h e i r resistance usually being measured in a form of wheatstone b r i d g e circuit ( F i g . lb). B y using two gauges e r r o r s due to

temperature change a r e avoided. W i t h the two gauges as arranged in the probe,

the reading8 obtained represent mice the strain in the strip. In practice

i t is not usually necessary to c o n s i d e r the curvature/strain/resistance

relationship, since the probe is calibrated d i r e c t l y f o r curvature in t e r m s of bridge reading. The Tinsley bridge mentioned above is moderately priced

and in combination with the S R 4 gauges specified has proved satisfactory.

M o s t commercially produced battery-operated bridges for s t r a i n gauge

*

reading would probably be satisfactory. It is necessary that the bridge range be appropriate f o r tke resistance changes developed in the gauges used, for a l l likely. curvatures of the probe.

A pair of gauges must be selected, which, when connected acrose the bridge terminals, permit a zero reading. (In the case of the Tinsley B r i d g e , modified 4?07X, there w i l l be a slight difference in resi5- tance between the t w o gaziges.) Tke gauges are carefully mounted one on

each s i d e of the cleaned s t r i p with the contact cement, The leads are s o l d e r e d ta the canduetor cable, Sleeving covers the junction and the bared conductor.

It is important that t b e s l e e v i l g is loose and does not g r i p the l e a d wire. _The

two ends of the sleeving are closed w i t h a small dab of Vaseline. Tape is

w o m d around the narrower part af the strip and the conductor cables.

The polyethylene tubing ( 114 in, i. d. ) is passed over the con- ductor cable,

and

stricted if necessary by a w i r e binding.

The glaas t u b i ~ g is placed vertically in a clamp, The strip

is placed in the tube with its ehd caught in a small groove previously c o t

diametrically in a cork, closing the lower end of the glaes tube. The atrip is 3eld vertical by a split cork which holds the polyethylene tubing aboz~t 1 c m beyond its junction with the metal strip. Before finally placing

the

cork in position the epoxy-polymer-accelerator mix is poured in to f i l l the

g l a s s tube. After it h a s set, the glass is shattered and completely removed. T p e probe may be sanded smooth and to exact size; i t must not be excessively

strained, and ca3 be carried in a protective tube. Marks a r e made at inter- vals of 3 , 7 5 c m on the polyethylene tubing ( 114 in. i . d. ) and n u m b e r e d con-

secutively. "1" is coincident with the top

of

(!'guide line") i a marked down the length of the tubing.

CALIBRATTOV

O F

(1) A 1- to 2-m length of polyethylene tubing is fixed f i r m l y ,

without k i n k i ~ g but with two or more curves. At least 10 crn at one end

s.clo::ld be straight. A Sine continu.ous w i t h this part of the tube, serves

as the X axis, f o r the determination of y ordinates f o r points down the t u b e . It is helpful i f the tube is so arranged that i t does not cross the

X

prevents movement or d i s tortian.

( 2 ) b-itervals of 1 5 cm are marked on the tube and t\e y ordinales

( 3 ) A table is prepared ( F i g . 3) to give the angles

#

# 2 ;

0

e t c . _{corresponding} to the curvature at each point down the tube ( F i g . 2).

In compiling the table the following steps are taken (care should be taken t o

follow the arrangement illustrated by the example in Fig, 5 ) ;

the tube.

c) Divide by

A

=

8

sin +8 ; gin

+

+

0 8 1 92; . .

.

d) Read angle

+.

e ) The required angles

+

+l;

+ S O

4

-

baa:

0a2

-

(4) The shunt on the bridge is s e t so that the galvanometez needle is centred? with "per cent resistance change" reading 0. Readings

a r e obtained with the probe for intervals of one probe length ( 1 5 c m ) down

the tube, i. e . at each marked point.

( 5 ) F o u r probe readings at each point are made as follows:

Two series of readings progressing down the tube and two p r o g r e s ~ i n g up

the tube, taking c a s e to maintain similar orientation of the probe guide l i n e .

The 'averages f o r the readings for each point are calculated. It is e s s e n t i a l that the midpoint of the probe coincides with the points marked on the tube.

t

The average probe reading at each point is then plotted against t h e corres-

ponding angle

9

( 6 ) Individual points deviate considerably f r o m the straight-

l i n e relationship a s s u m e d to exist between angle

+

A m u c h larger n u m b e r of points can be obtained by repeating procedures

2 t e 5, for additional s e t s of paints marked (and values of y) on the same

tube.

*

obtained for the s t r h i g h t position of the probe in question w i l l be 0. Other t y p e s of b r i d g e may require a slightly d i f f e r e n t p r o c e d u r e to achieve this.

PROCEDURE FOR OBSERVATIONS

IN

*( 1) S e t s h u n t s o that galvanometer needle is centred, with "per cent resistance change" reading 0 .

( 2 ) Place probe in top of tube and record orientation of guide line. Then determine ( b y gentle bending), and

record

(3) R e c o r d readings : read probe at consecutive positions down tube, at intervals equal t o one quarter of probe length (use markings

on lead cable). Position

No. 1

oriented in the same way as the probe is moved down the tube,

For tubes apparently curved, take four s e t s

of

18O0, and take two further s e t s of readings. These should have, in general, opposite sign to that of the f i r e t t w o s e t s .

(43 Measure amount of tube exposed above ground surface.

It occasionally happens that in pas sing through a straight past

of

P R O C E D U R E F O R INTERPRETATION

OF

CURVATURE

O F

TU8E LS

(1) The s i g m of the second pair of readings f o r each probe

position (under 'after rotation through 180") are reversed 2nd the algebraic a v e r a g e of the four r e a d i n g s is calculated.

>:: This procedure prepares tile T i n s l e y b r i d g e , scch t h a t the reading

o b t a i n e d f o r the straight position of the probe in question w i l l be 0. O t h e r

(2) Separate the list of average readings into four column^

as follows: starting with the deepest reading and proceeding upwards l i s t every fourth one ( e . g . l o t h , 6th, 2nd), thus making the first column

Then begin with t h e penultimate reading and proceed as before. Make third

and four th columns similarly (Fig.

5).

( 3 ) U s i n g the form of table shown (Fig. 61, compute x and y

coordinates as indicated, f o r the f i r s t column of figures,

In

9

p s s i t i ~ n s down to and including that probe position. x and y are obtained

i n

a

+,L

@,L

(4) A second table is prepared f o r the second column of figures which is treated similarly, except that to the first angle

+,

.

the first angle

+

of

.

(5) The t h i r d and fourth columns are computed similarly, except that factors of

.

The values of x and y from all four calculations are plotted on one graph, The third point f r o m the top represents the top of

the

orientation of the curvature i n relation to the slope {i. e . convex or concave downslope) is given by the observation described on page 5 , paragraph (2). R E F E R E N C E S

1) W i l l i a m s , P.

J.

Pt.

2). 'Williams, P. 5. An investigation into processes occurring in solifluction.

Am. Jour

.

No.

33 Vfilliarns, P.

S.

ground phenomena. B u l l , P e r i g l . (In P r e s s ) .

4) IVilliams,

P.

S T R A I N SENSITIVE W l R E OF S T R A I N GAUGE

3

-

FIGURE

o

PROBE FOR

DETERMINATION

OF SHAPE

BURIED

TUBE

LEGEND

s

(ro

ZERO READ/NG)

0 = VARIABLE RESISTANCE CALIBRATED FOR

" % STRAIN CHANGE"

G

F I G U R E

b

GENERAL FORM

FIGURE

2

DIAGRAM

ILLUSTRATING

COMPUTATIONS

FOR

DETERMINING CURVATURE OF

GAL1

BRATION

TUBE

Figaxe 3

-

+

...

Probe

No.

...

-

G

Points marked on cali- bration tube, 15 c m intervals 0 1 2 3

Shunt reading

...

of

...,...

c~~

...

...

Y

Figure 4

-

*Y

_0,

+

P r o b e position After Rotation through 180 a

.

3 I

Q

+

0"

0.07

Figure 5

-

Column 2 Colwnn 3

,

Probe position Average (fram

4 s e t s )