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IS 13414 (1992): monitoring of rock movement usingmulti-point borehole extensometers-Guidelines [CED 48: RockMechanics]

IS 13414:1992

Ivldian Stamiard

MONITORING OF ROCK MOVEMENTS USING MULTI-POINT BOREHOLE EXTENSOMETERS-GUIDELINES

UDC 624’121’54

BUREAU OF INDIAN STANDARDS MANAK RHAVAN. 9 11AHADlJII SHAH ZAFAR MARG

NEW DELHI 110002

July 1992 Price Group 3

Rock Mechanics Sectional Committee, CED 48

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft f?naliTed by the Rock Mechanics Sectional Committee had been approved by the Civil Engineering Division Council.

Rock movement is monitored to ensure safe and economic construction of some civil and mining works. It helps in receiving a forewarning of impending failure and taking corrective measur-es. One of the methods to measure rock movement is by using borehole extensometer. Rorehole exlensorneters are used to measure rock movements that may take place as the result of surface and underground excavation, foundation loading, movement of natural slopes or in-situ testing.

In reporting the results of a test or analysis made in accordance with this standard, if the final value, observed or calculated, is to be rounded off, it shall be done in accordance with IS 2 : 1960 ‘Rules for rounding off numerical values ( revised )'.

IS 13414:1992

MONITORING OF ROCK MOVEMENTS USING MULTI-POINT BOREHOLE EXTENSOMETERS-GUIDELINES

I SCOPE

This standard describes the method of rock movement measurement using multi-point bore- hole extensometer (MPBX). This covers the measurement of static rock movements that may take place as a result of surface and underground excavations, foundation loading, movement of natural slopes or in-situ testing.

2 REFERENCES

The Indian Standard IS I 1358 : 1987 ‘Glossary of terms and symbols applicable to rock mecha- nics’ is a necessary adjunct to this standard.

3 TERMINOLOGY

For the purpose of this standard the definition given in IS 11358 : 1987 shall apply in addition to the following.

3.1 Rock Movement

It is the relative displacement of the rock mass between the anchorage point and the collar of the cxtensometer which may be caused due to one or more than one of such processes as loosening, squeezing, swelling, creeping, destressing, etc. It may or may not cause strata separation.

4 INSIRU.%lENT/EQUlPMENT

4. I General

The borehole extensometer is an instrument used for monitoring movement of rock mass at several points aro;ind underground openings, rock slopes and other excavations. This instrument consists of anchors embedded in a borchole at different depths, movement transferring elements which are fixed to the anchors and extend up to a reference head connected to bnrehole collar. Each movement transferring element is covered by a rigid PVC pipe, so that these elements are free to transfer the displacements of the rock mass from anchor positions to borehole collar position. The relative displacement bctwecn the collar and the move- ment transferring elements is measured by means of a mcchanic:il or an elcctricol read out unit. The relative displacement may also be monitored through a computerised data acquisition system.

4.2 Classification

4.2.1 Depending upon the movement transferring element between anchors and the borehole collar

a)

b)

( see Fig. IA ):

Wire type borehole extensometer - connec- ting element is wire.

Rod type borehole extensometer - connec- ting element is rod.

c) Probe type borehole extensometer -- no direct connection between the anchors and the borehole collar.

4.2.2 Depending upon the number of anchors ( see Fig. 1B ):

a) Single point borehole extensometer ( SPBX ) - anchorage at one point.

b) Double point borehole extensometer ( DPBX ) - anchorage at two points.

c) Multipoint borehole extensomer ( MPBX ) - anchorage at more than two points.

4.2.3 Depending upon the reading facility:

a) Mechanical MPBX or SPBX - with direct reading facility.

b) Electra-mechanical MPBX or SPBX - with

4.2.4

a)

b)

remote and direct reading facility.

Electrical MPBX or SPBX - with remote reading facility only.

Depending on anchor types:

Mechanical type with expansion shell type and prong type anchors ( see Fig. 2 ).

Grouted type in which the anchors are grouted into the borehole while keeping the movement transferring elements free within rigid PVC pipes.

4.3 Multi-point Borchole Extensometer ( sc’c Fig. 3 )

A multi-point borehole extcnsometcr comprises following four main components:

4 Anchors,

b) Rods or wires,

4 Collar head, and

4 Displacement sensors.

1

+-TO RECEIVER ’ TO WEIGHT

- ELCT CABLE WITH DEPTH MARKING

BORE HO1

_ PRGBE

AOU TYPE WIRE TYPE PROBE TYPF.

1A

ANCHORS ‘- - u j$

SINGLE POINT TWO POlNl MULTI PIIINI

18

FIG. 1 ROCK MOVEMENT MONITORING APPAIZATUS ( BASIC TYPES )

LESS THAN HOLE DIA CAPP. l/,OlA,

EXPANSION SHELL

I A Prongs Type ‘I B Expansion Shell with Screw Anchorage Plug Type Anchorage

FIG. 2 DII+ERENT T,

4.3.X Anchors

4.3.1.1 Anchors are reference points in a bore- hole and consist of expansion shells ( as used in rock bolting), spring loaded wedges, prongs or simply lengths of twisted steel bar fixed to the borehole wall using cement, chemical or resin and grout ( Fig. 2 ).

4.3.1.2 These anchors represent the displacement of the rock mass along the longitudinal ltxis of the borehole. Grouted types of anchors are preferred in areas susceptible to vibrations due to blasting where the rock mass is highly jointed.

4.3.2 .Roads or Wires

4.3.2.1 Rods or wires are the movement trans- ferring element of the anchors embedded in the borehole. Rods or wires arc laid in rigid PVC pipes for their free displacement. Spncer-s should be used to align and control the position of these PVC pipes. Rods should be threaded on both the sides with a provision of joining two

2

YPES OF ANCHOKAGE

such rods using a coupler. The rods may be in lengths of 2 m, 1 m, 0’5 m, 0’25 m and O’I m as per the requirements.

4.3.2.2 The use of either wire or rod in a borehole eutcnsometcr depends on the following factors:

a) The length oi‘ time ol’ the observations,

b) Degree or precision,

c) Maximum depth of the ;~nchor, and

d) The direction of the borehole.

4.3.2.3 Wires may bc used li,r short-term observations up to 3 months. Wires should not be used for depths in excess of IO m. Similarly, wiI-es may be used only in vertical boreholeS. Wires requir-c tile :cpplication of it lixed tension ;I! each observation. Mild steel rods may bc LISCCI for observations up to one year. Stninlcss steel rods itre essential if obsel-tatIons ;LIL‘ required for periods exceeding 011e year. Stainless steel rods arc essential for precise nlcasurernents.

II

1s 13414 : 1992

GROUTED PAD

READOUT UNIT

BEARING PLATE

3A Bore Hole Extensometer at a Installation

EXPANSION TYPE SHELL ANCHOR

- DRILL HOLE

REFERENCE El

It-- THREADED ROD 38 - MULTI-POINT WIRE TYPE BOREHOLE

EXTENSOHETER

ANCHOR ROD GROUl

DRILL HOLE

WALL 7

MOUTH 11

.OCK

ROD

dbPOLYTHENE TUBE

_.. REFERENCE -

‘- TUNNEL WALL

COLLAR

3 C - MULTI POINT ROD TYPE BOREHOLE EXTENSOMETEf3 WITH GROUTED ANCtlORS

t;lG. 3 MULTI-POINT BOKEIIOLE ~ITENSOMETEl

IS 13414 : 1992

4.3.2.4 Recommended dimensions of rods and tension in wire and recommended for borehole extensometers:

a) Rods - 5 mm dia for depths less than 30 m, and

- 11 mm dia for depths greater than 30 m

b) Wires - 0’5 mm to 1’5 mm dia.

c) Wire tension shall be as follows:

9

ii)

For in-situ measurements - suitable tension may be applied through coil or leaf spring

For other measurements - suitable con- stant tension ( that is the member should be capable to withstand the tension ) may be applied through dead weight or spring balance.

4.3.3 Collar Head

The collar head should be made of non-rusting stable material such as stainless steel. It is fixed to the mouth of the borehole. It should have boles to receive movement transferring elements. In case of the wire type MPBX, the wires should be taken out of the collar head, reference points fixed on both the collar head and the wires and readings between the reference points be taken. In the case of the rod type MPBX, the rod lengths should be so adjusted that the heads of the rods remain within the holes of the collar head. The external face of the collar head should be used as the reference surface and the distance between this reference surface and the face of the rods should be read by a depth gauge in the case of the mechanical MPBX. Electrical sensors bet- ween the collar head and the rods should be con- nected in the case of the electrical MPBX.

4.3.4 Displucrmen~ Sensors

Displacement sensors are either of the electrical type or the mechanical type. The mechanical sensor may be a depth micrometer, a dial gauge, or vernier callipers. lzlectrical sensors may con- sist of linear variable differential transducer ( LVDT ), rotary or linear potentiometers or cantilever displacement sensing devices using vibrating wire or resistance strain gauges. The LVDT or the vibrating wire type sensors should be used for stable long-term measurements. These sensors are preferred over the mechanical type where a large number of instruments make it obligatory to use a computerised data acquisi- tion system. In all cases, where the reliablity 01 the electrical sensors has not been established, arrangements for both electrical and mechanical sensors should be preferred so that periodic cross check by a mechanical sensor 15 possible.

5 INSTAI,L,Al’ION PROCEDURE

5.1 The procedure of‘ installation and mvnltoring should be carried out strictly under the guidance of an experienced instrumentation and rock mechanics expert.

5.2 The recommended procedure of the install- ation of a Multi-point Bore Extensometer is as follows:

b)

c)

4

4

a) Decide the actual Dlace of installation. orientation and length of borehole and number of anchors in each hole by closely examining the site of installation, keeping in view the purpose of instrumentation. To avoid the effect of temperature, the collar head should be kept inside the underground opening. If the borehole ex- tensometers are to be installed ahead of an excavation face which require the collar head on the ground surface, super invar rods should be used as movement transfer- ring elements. In the case of slopes, it is suggested to use super invar rods instead of ordinary steel rods. Drill the borehole up to the required depth in the desired direction using diatnond coring drill. Study the cores and fix the depth of anchors depending upon the stability con- siderations and positions of geological dis- continuities, if any. The borehole should be cleaned before installing the instrument. Anchors should be fixed at their designated positions in the borehole by grouting ot any other suitable method of anchorage. For grouted anchorage at least 24 hours should be allowed for setting and harden- ing of the grout before installation of the electrical sensor unit. A protective covering is required for the safety of the collar head, ln case of vertically upward and upward inclined holes a breather tube of rigid PVC pipe should be inserted up to the full depth of the borehole to take out air wllilc grout is injected. LVDT, etc, and readout unit should be thoroughly checked before connecting to the movement trnnsterring rods. The readings of the instruments should bc recorded in a field note book as per the given data sheet ( see Annex A ). Every entry in the data sheet should be recorded in explicitly clear manner. &marks column should point out any specific information to be furnished to the data analysis agency.

f)

i?)

11)

6 DA’I‘A ANALYSIS

6.1 The calculation of rclalivc displaccnlcnt 01‘ anchors is the first step in data analysis. The calculation depends upon the type of dlspIacement seusor used for taking the observalictus. If ;t mechanical sensor is used for ohservutic~ns, thcll the change in reading directly gives tile chnnge itI distance between the reference head and thl: anchor. In case OK an electrical displaccincnt sensor, the reading slrnll be Gol-lccied wirh the calibration chart Lo oblain the change iii distance. The pet missible error sllollld Ilot exccetl $ 0’02 111n1.

4

IS 13414 : 1992

6.2 The calculated relative displacement should be plotted against time (see Fig. 4). Rate of relative displacement should also be calculated and the rate of change of displacement with time be plotted as shown in Fig. 5.

NO. OF DAYS ------c

FIG. 4 PLOT OF TIME VERSUS RELATIVE ROCK MOVEMENT

6.3 Following inferences can be drawn from these plots:

a) Curves 2 and 3 in Fig. 5 indicate insta- bility of the structure, whereas curve 1 indicates the stability.

b) A high rate of displacement or a sudden increase in rock movement is indicative of unstable conditions.

c) Large displacements also indicate an tmstable condition.

NO OF DAYS -

FIG. 5 PLOT OF RATE 01: RELATIVE DISPLACEMENT VERSUS TIME

ANNEX A ’ ( Clause 5.2 )

STANDARD DATA SHEET FOR ROCK MOVEMENT MONITORING USING MPBX

Name of Project:

Date of Installation:

Date of Excavation:

Type of Rock Mass:

Location:

Angle of Borehole:

Depth of Anchors:

Anchor No. 1:

Anchor No. 2:

Anchor No. 3.

Anchor No. 4:

Observations:

Date & Time of Observations

Face Chainage Observations, mm Kemarks C___----h_-..__-_-_

m An 1 An 2 An 3 An 4

NOTES

1 l’rovidc information on Joint 01 sheal- done in the remarks column. 2 Write down data and tinx of repositioning of rcferencc point on a wire or rcadjustmcnt of Iod ICII~LII if the range of mensurenlcnt has exceeded the designated panic. 3 Collect infornxttion on date and time of blast or excavation cycle, support installation, ctc, for ux in analysis.

5

Starrdsrd Mark

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Revision of Indian Standards

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Dot : No CED 48 ( 4634 )

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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