1129215843isrm sm in situ deformability - 1979

In Situ Deformability of Rock 197

Suggested Methods for Determining In Situ Deformability of Rock

PART 1. SUGGESTED METHOD FOR DEFORMABILITY

DETERMINATION USING A PLATE TEST

(SUPERFICIAL LOADING)

SCOPE

1. (a) The plate test which uses surficial loading, often referred to as the uniaxial jacking test or plate jacking test, is performed in small tunnels or test adits to measure the deformation characteristics of a rock mass.

(b) Two areas, each approximately 1 m in diameter, are loaded simultaneously using jacks positioned across the tunnel. Rock mass deformations are measured in boreholes behind each loaded area and across the tunnel between each loaded area. A typical test facility is shown in Fig. 1.

(c) Incremental and cyclic loading provide data for the calculation of elastic, deformation, and unloading moduli. The creep characteristics of the rock mass can be determined from graphs of displacement versus time.

PARTICLE BOARD

TOP PL

TUNNEL ROCK 4 RESTRAINT

BASE PLATE--~ GAGE

(d) The effects of anisotropy can be determined by orienting the thrust of the jacks in any desired direction. However, it is advisable that the thrust of the jacks remains in a plane perpendicular to the axis of the test tunnel.

EQUIPMENT

2. (a) Equipment necessary for accomplishing the test includes items for: preparing the test site, drilling and logging the instrumentation hole, measuring the rock deformation, applying and restraining test loads, recording test data, and transporting various components to the test site. 3. (a) Test site preparation equipment should include an assortment of excavation tools, such as drills and chipping hammers. Blasting should not be allowed during final preparation of the test site. 4. (a) The drill for the instrumentation holes should, if possible, have the capability of retrieving core from depths of at least 10 m. Some type of borehole viewing device is desirable for examination of the instrumentation holes to compare and verify geologic features observed in the core.

:LAT JACK, APPROX. I M DIAMETER

*CONCRETE /

/ "-'-MPBX MEASURING ANCHORS (5 OR MORE PER HOLE)

MPBX SENSOR HEAD

~UBBER SLEEVE OVER LEAD WIRES

NX, 76 MM DIAMETER, CORE DRILL HOLE APPROX. 6 FLATJACK DIAMETERS D E E P ~

/ HYDRAULIC

SCREWS FOR SET UP AND REMOVAL

LEAD WIRE

PREPARED DIAMETER 1.5 TO 2 TIMES FLATJACK DIAMETEI

DATA ACQUISITION STEM ----~ 0 @ ® 0 ®

Fig. 1. Uniaxial jacking test.

MPa HYDRAULIC PUMP

~ NOTE: TtMBER PLATFORM FOR SUPPORT DURING ERECTION NOT SHOWN

198 International Society for Rock Mechanics

(b) Instruments for measuring deformations should include a reliable multiple position borehole extensometer (MPBX) for each instrumentation hole, and a tunnel diameter gauge. All instruments should be of sufficient accuracyand sensitivity to be compatible with anticipated deformations. Experimental errors in excess of 0.01 mm can invalidate test results when the modulus of the rock mass exceeds 3.5 x 104 MPa. A discussion of the ramifications of experimental error can be found in [1]. 5. (a) The loading apparatus should be capable of applying simultaneous uniform pressures to two areas on opposite sides of the tunnel, each approximately 1 m in diameter. As shown in Fig. 1, the equipment used to apply the desired loads to the prepared and instru- mented rock may consist of calibrated flat jacks and restraint columns having the capability of sustaining the maximum desired uniform pressure with a suitable factor of safety. The hydraulic pump system with necessary fittings, valves, gages, and hoses should have sufficient pressure capability and volume to apply and maintain desired pressures to within 3?/0 of a selected value throughout the duration of the test.

PROCEDURE

6. Site preparation

(a) The area selected for testing should be carefully prepared. All loose rock material should be removed by using chipping hammers and drills. In order to reduce the restraining influence of adjoining rock, an area with a diameter 1½ to 2 times that of the test pad should be prepared. The two test areas should be concentric with and in planes oriented perpendicular to the axis of the restraint column assembly.

If blasting is required for initial test surface preparation, care should be exercised to produce surfaces which are relatively free from blast damage. Detailed site preparation procedures can be found in [2].

(b) An instrumentation hole should be core drilled into each prepared test surface. Care must be exercised to insure that the two holes are coaxial with each other and with the restraint column assembly.

(c) Examination of the core and the instrumentation hole itself will assist in locating anchor points for the MPBX's. The anchors should be located so that they are not placed on joints, and so they bracket zones of structural or lithologic change. The deepest anchor should be located approximately 6 flat jack diameters below the rock surface in order to provide a fixed point to which the movements of all other anchors can be referenced. In general, the remaining anchors should be concentrated in the zone of maximum stress between the rock surface and a point approximately 3 jack diameters back from the surface. Figure 2 illustrates some recommended locations. It is desirable for the sensor head and all anchors to be attached to the side walls of the instrumentation hole. This precludes the neces- sity of monitoring the movement of the test setup components, since all measurements will be referenced in the rock.

7. Equipment installation

(a) The complete installation of a proposed type of restraining and load applying setup together with deformation measuring instrumentation is shown sche- matically in Fig. 1. A properly located wooden platform (not shown in Fig. I) allows for alignment of all test components. The space between the flat jack assembly and rock should be filled with small aggregate concrete.

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~ SENSOR H E ~ ~'~

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FEATURES

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Fig. 2. Typical anchor locations.


The concrete should be allowed to cure sufficiently to obtain adequate strength prior to commencement of the test. The space between the fiat jack and the base and top plates should have a special partical board filler (wood chips and resin) or other suitable material fabricated to accommodate the flat jack configuration on one side and the base plate on the other side.

8. Testing

(a) After all components of the instrumentation are installed in the drill holes, they should be checked (elec- tronically or mechanically). After the loading and restraining components are installed, another check should be made of the instrumentation. A final check of all mechanical, hydraulic, and electronic components should be made after the concrete pads are placed and again before the first load increment is applied.

(b) Tests should be conducted continually on a 24-hr a day basis utilizing load ranges and increments compatible with the particular design considerations under investigation.

(c) While the test is in progress, rock deformations monitored by the instrumentation should be recorded continuously or at sufficient intervals to obtain desired data. If a noncontinuous recording system is utilized, a minimum of four readings during the first hour of each load increment or decrement is recommended.

(d) The maximum test pressure, number of cycles to the maximum pressure, and number of pressure increments in each cycle will be determined by test conditions and desired information. A maximum pressure of 1.2-1.5 times that imposed by the structure is usually considered adequate. At least five pressure increments, each followed by a period of zero pressure, should be used for each cycle. A typical one-cycle loading sequence is shown in Fig. 3.

(e) The duration of each pressure increment will be determined by the creep characteristics of the rock mass. Until the behavior of the rock mass is well under- stood, at least 48 hr should be allowed for each pressure increment followed by 24 hr at zero pressure. Obser- vations during the first pressure increments can be used

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Fig. 3. Rock Surface deformation as a funct ion of bearing pressure.

200 Internat ional Society for Rock Mechanics

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to modify time requirements for successive increments. Jack pressure should be maintained within 3% of the target value for the duration of each increment. The time frame for a typical test is shown in Fig. 4.

CALCULATIONS

9. (a) Data gathered during the test may be plotted to provide a display of Deformation vs Time, Pressure, or Depth. These plots aid in the analysis of the creep, rebound, and permanent set characteristics of the rock mass. Example plots are shown in Figs 3, 4, and 5.

(b) Deformation measurements for the various load

cycles are utilized to compute deformation moduli according to appropriate formulae. Because of their simplicity, expressions based on the theory of elasticity I-3] are normally used to approximate actual field conditions.

(c) For a uniformly distributed pressure on a circular area, the displacement at any point beneath the center of the area may be expressed:

2q(1 -/~2) 1-( a2 + z2) 1/2 _ z] w=- E

qz(1 + #)[z(a 2 + z2)_1/2 _ 1] (1) E

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ROCK DISPLACEMENT ( M M )

0.102 0.203 O. 305 0.406 O. 508 0.610

A N C H O R DEPTHS

SENSOR HEAD 0 .0 M

ANCHOR ONE 0 .5 M ANCHOR TWO I. I M ANCHOR THREE 1.8 M ANCHOR FOUR 2.4 M ANCHOR FIVE 3 . 2 M ANCHOR SIX 4 . 3 M ANCHOR SEVEN 6. I M O-DEPTHS WHERE

ROCK DEFORMATION WAS MEASURED WHEN LOADS WERE APPLI ED AT SURFACE ( 0 - DEPTH )

5.2

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561 I I / D E E P E S T EXTENSOMETER ANCHOR

6 j d r I ~ I

I

Fig. 5. Uniaxia l displacement vs depth referenced to deepest anchor at 6.9 M P a bearing pressure.


where: wz = displacement in the direction of the applied

pressure z = distance from the loaded surface to the

point where displacement is calculated q = pressure a = radius of loaded area /2 = Poisson's ratio E = modulus of elasticity

At the surface z = 0 and the expression reduces to:

2(1 - / 2 2) wz = o - - qa . (2)

E

(d) When loads are applied with a circular flat jack with a hole in the center, the effect of the unloaded area in the center must be subtracted. Using the notat ion:

a 2 : outer radius of flat jack al = inner radius of flat jack or radius of hole

_ 2 q ( 1 - 2) Wz /2 [(a 2 + z2) l /E(a2 + z2) 1/2]

E

z2q( 1 + /2) Z2)- 1/2 + [(a 2 + _ (a 2 + z2) - 1/2]. (3) E

After substituting appropriate values for al, a2,/2, and Z, equation (3) reduces to:

= E ( K z ) . (4) Wz

If displacements Wzl and Wz2 are measured at points z I and z 2, the indicated deformation modt~lus of the material between zl and Zz may be calculated from:

Ea = q W~, W~2 ]" (5)

R E P O R T I N G OF RESULTS

10. The report should include the following:

(a) A complete geologic description of the test site including core logs, photos of core, photos of prepared test areas, and a description of local blast damage.

(b) A description of the testing apparatus including photos of installed equipment, a schematic diagram of the equipment, specifications for accuracy and sensitivity of all pressure and deformation instruments, and calibration data for all instruments.

(c) Tabulations of unreduced data. (d) Plots of deformation versus pressure such as in

Fig. 3. Information from this plot can be used to determine the shape of the stress strain curve, to obtain values for calculation of various moduli, and to determine rebound and elasticity characteristics.

(e) Plots of deformation versus time as in Fig. 4. This plot is useful for studying the creep characteristics of the rock. It should be kept during testing to establish time requirements for each load increment.

(f) Plots of deformation versus depth referenced to the deepest anchor as in Fig. 5. This deformation profile is used to identify anomalous areas with lower or higher moduli than the average. Once such zones are identified, they can be correlated with core from the instrument holes. If MPBX anchors are located properly, the moduli of these zones can be calculated using equation (5).

(g) Calculated moduli pertinent to design problems. Care should be taken to identify the depth interval in the rock mass and stress range for each modulus.

R E F E R E N C E S

1. Benson R. P., Murphy D. K. & McCreath D. R. Modulus testing of rock at the churchill falls underground powerhouse, Labrador, from determination of the in situ modulus of deformation of rock, American Society for Testing and Materials STP477, (1969).

2. Misterek D. L., Slebir E. J. & Montgomery J. S. Bureau of recla- mation procedures for conducting uniaxial jacking tests, paper presented at American Society for Testing and Materials Annual Meeting, June 24-29, 1973, Philadelphia, Pennsylvania.

3. Timoshenko S. & Goodier J. N. Theory of Elasticity. McGraw- Hill, New York (1951).

PART 2. SUGGESTED METHOD FOR FIELD

DEFORMABILITY DETERMINATION USING

A PLATE TEST D O W N A BOREHOLE

SCOPE

1. (a) This test is used to determine the in s i t u deformability characteristics of a rock mass. Successively higher bearing pressures, in loading and unloading cycles, are applied to the flattened end of a borehole and the resulting rock displacements are recorded.

(b) Elastic and deformation modulae may be derived from graphs of bearing pressure versus displacement. Time dependent (creep) properties may be determined from graphs of displacement versus time.

(c) The method allows the testing of several horizons at various depths, with a minimum of expense to gain access to each test horizon. In the limit a semi-con- tinous log of deformability as a function of depth can be obtained.

(d) The direction of loading necessarily coincides with the borehole axis, usually near-vertical, so that no information can be obtained regarding rock anisotropy. The size of the loaded area is limited by the capabilities of available drilling equipment and is usually smaller than in other plate tests (see PART 1).

(e) The method is usually employed to provide information for the design of foundations, as an alternative to the method of PART 1 where access to the proposed foundation level cannot readily by obtained by an exploratory trench or addit.


APPARATUS (e.g. Figs 1 and 2)

2. Equipment for drilling, cleaning and preparing the test hole including:

(a) A drill or boring machine to produce a test hole of diameter at least 500 mm 1. to the maximum depth of investigation.

(b) Casing as necessary to stabilize the walls of the hole.

(c) Groundwater lowering or other equipment to allow preparation of the bearing surface and installation of the bearing plate in dry conditions.

(d) A bottom auger, reaming bit or hand tools to prepare the bearing surface flat (+ 5 mm) and perpendicular to the hole axis (+3°).

(e) Equipment to remove debris from the hole. (f) Equipment for taking core samples to a depth

of at least 3 m below the bearing surface, the diameter of the exploratory hole to be less than 109/o that of the bearing plate. 3. Equipment for installing and bedding-in the bearing plate including:

(a) equipment for lowering the plate into the test hole (b) materials and ancilliary equipment for preparing

a bedding layer beneath the plate, for example of cement mortar and plaster of paris. 4. A circular bearing plate of diameter at least 500 mm and sufficiently rigid to distort by not more than 1 mm under the test conditions. 2 5. A loading column to transmit the applied force from the reaction system to the test plate, such that:

(a) it resists buckling and carries the applied load without distortion sufficient to affect test results

(b) it is hollow to take the measuring column. (c) the resultant load acts centrally to the bearing

plate (+3 mm) throughout the test. 6. A loading and reaction system including for example a. hydraulic jack, reaction piles or anchors and ancillary equipment, such tha t

(a) load is applied axial to the loading column. (b) loads can be varied throughout the required range

and can be held constant to within 2~o of a selected value for a period of at least 24 hr.

(c) the travel of the loading jack should be greater than the sum of anticipated displacements of the test plate and reaction beam.

(d) the reaction system should be of appropriate materials, design and construction to satisfy these requirements and to ensure safe operation of the test equipment.

(e) reaction anchors should if used be located further than l0 test hole diameters from the bearing plate. 7. Load measuring equipment, for example a load cell or proving ring, to measure the applied load with an accuracy better than __+ 2 ~ of the maximum reached in the test. 8. Equipment to measure displacement of the centre of the bearing plate 3 in a direction axial to the test

* N u m b e r s refer to N O T E S a t t he e n d o f the text.

hole, such that: (a) The system should have a range greater than the

maximum plate displacement in the test, and an overall accuracy better than + 0.05 mm.

(b) The system reference beams, columns and clamps should when assembled be sufficiently rigid to meet this requirement.

(c) The reference anchors for displacement measurements should be rigidly installed at a distance greater than 10 test hole diameters from the loading plate and reaction anchors. 9. A timing device to measure test durations of up to 48 hr, reading to l see.

P R O C E D U R E

10. Test site selection

(a) The test site is selected to allow testing at the actual foundation level with loading in the direction of foundation loading, alternatively testing of rock considered typical of anticipated conditions.

(b) Attention should be given not only to the test hole location, but also to suitable locations for reaction and reference anchors, to groundwater and other conditions that may influence the conduct of the test.

(c) Selection of horizons for loading should be checked before the test starts, by examining in detail a core taken from beneath the proposed bearing surface.

11. Drilling and preparation

(a) Test hole and anchor locations are accurately marked out and the holes drilled to the required elev- ations. The test hole is cased as necessary to ensure stability throughout the test. Exploratory core is taken to a depth of at least 3 m below the proposed test horizon, and the choice of horizon confirmed or modified. Detailed geotechnical logs of all boreholes should be prepared by examining core and/or the walls of the hole.

(b) When groundwater is encountered in the test hole, steps should be taken to lower the water table (for example by pumping from well points surrounding the test area) for long enough to allow installation of the bearing plate.

(c) The bearing surface is trimmed flat (___ 5 ram), and its elevation recorded. All debris should be removed.* One or more layers of mortar or plaster scree, total thickness less than 30 mm, are placed to cover the bearing surface and the bearing plate installed before the last layer of scree has set. The delay between excavation of the bearing surface and installation of the equipment should not exceed 12 hr. 5

(d) Reaction and reference anchors are installed and the equipment assembled and checked. A small seating load (approximately 5~ of the maximum test value) is applied and held until the start of testing.

(e) The water table should be allowing to return to its normal elevation before the start of testing.

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12. Testing

(a) With the seating load applied (paragraph lid), load and displacement should be observed and recorded over a period not less than 48 hours to establish datum values and to assess variations due to ambient conditions. 6

(b) Loads and load increments to be applied during the test should be selected to cover a range 0.3-1.5 qo, where qo is the stress intensity produced by the proposed structure, v

(c) Load is increased in not less than five approximately equal increments to a maximum of approximately 1/3 the maximum for the test. At each increment the load is held constant (___3~) and displacement recorded as a function of time until it stabilizes. 7 The procedure is continued for decreasing load increments until the seating load is again reached.

(d) The procedure 12(c) is repeated for maximum cycle loads of approximately 2/3 and 3/3 the maximum for the test.

13. The equipment is removed from the test hole and further tests may be carried out on deeper horizons by re-drilling in the same hole (paragraphs 11 and 12).

CALCULATIONS

14. (a) Graphs are plotted of incremental settlement (or uplift in the case of unloading) against the logarithm of time (Fig. 3).

(b) Bearing pressure versus settlement curves are plotted for each test (Figs 4 and 5).

(c) Deformation modulae may be determined from tangents to the pressure-settlement curve. In Fig. 6 three such moduli are defined where

and

Ei is the initial tangent modulus E e is the elastic modulus obtained from a re-

loading cycle

Ey is a "yield" modulus.

206

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International Society for Rock Mechanics

• '" 451 k P a

~ 71~ kPa

-~__1180 k P a

I I I 10 100 1000

Time (minutes)

Fig. 3. Typical relationships between incremental displacement and time for various load-intensities.

(d) The modulus is calculated from the formula

dq ~ D ( 1 - v 2) I~ E =

where q is the applied pressure p is the settlement D is the plate diameter v is Poissons's ratio (between 0.1 ar/d 0.3 for

most rocks) lc is a depth correction factor given in Fig. 7.

(e) A time-dependent parameter R (known as the creep ratio) is determined for each load increment. The parameter R is defined as the incremental settlement per cycle of log time divided by the total overall settlement due to the applied pressure. The relationship between R and applied pressure may be presented graphically (Fig. 8).

REPORTING OF RESULTS

15. The report should include the following (a) Diagrams and detailed descriptions of the test

equipment and methods used for drilling, preparation and testing.

(b) Plans and sections showing the location of tests in relation to the generalized topography, geology and groundwater regime.

(c) Detailed geotechnical logs and descriptions of rock at least 3 m above and below each tested horizon.

(d) Tabulated test results, graphs o f displacement versus time for each load increment, and graphs of load versus displacement for the test as a whole (e.g. Fig. 4).

(e) Derived values of deformability parameters, together with details of methods and assumptions used

in their derivation. Variations with depth in the ground should also be shown graphically as 'deformability pro- files' superimposed on the geotechnical log of the test hole.

NOTES

1. The test hole should preferably be of sufficient diameter to allow manual inspection, and preparation of the bearing surface. Where the hole is insufficiently

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large for manual inspection it must be core-drilled to provide adequate samples for a detailed geotechnical log of ground conditions.

2. The bearing plate, if of steel unreinforced by webs, should be at least 20mm thick for a diameter of 500 mm.

3. If required, the displacement of rock at any level below the bearing plate may be monitored, using rods passing through a hole in the centre of the plate and rigidly anchored in the exploratory drillhole.

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4. When the test hole is large enough, rock trimming and installation of the bearing plate should be carried out by hand. When this is not possible, cleaning may be carried out with an auger or similar device operating at the end of a drill rod assembly, and the mortar scree placed using a tremie or bottom opening bucket.

5. Particularly when testing weaker rocks there'will be rebound, loosening and possibly swelling associated with excavation of the bearing surface and changes in groundwater conditions. This may be minimized by reducing the delay between excavation and testing to a minimum.

6. Small fluctuations in displacement are likely to result from changes in the groundwater regime, tem- perature and other environmental effects.

7. At higher applied loads the displacement may not completely stabilize in a rcasonab!c ',imc: a criterion

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that readings should continue until the rate of displacement is less than 2~o of the incremental displacement per hour may be used. This criterion may be modified to suit the purpose of the test. The final increment in any one cycle should be held for as long as practical if the displacement is still increasing.

PART 3. SUGGESTED METHOD FOR

MEASURING ROCK MASS DEFORMABILITY USING

A RADIAL JACKING TEST

SCOPE

1. (a) This test measures the deformability of a rock mass by subjecting a test chamber of circular cross section to uniformly distributed radial loading; the conse- quent rock displacements are measured, from which Elastic or Deformation modulae may be calculated. 1.

(b) The test loads a large volume of rock so that the results may be taken to closely represent the true properties of the rock mass, taking into account the influence of joints and fissures. The anisotropic deformability of the rock can also be measured.

(c) The results are usually employed in the design of dam foundations and for the proportioning of pressure shaft and tunnel linings.

APPARATUS

2. Equipment for excavating and lining the test chamber including:

(a) Drilling and blasting materials or mechanical excavation equipment, z

(b) Concreting materials and equipment for lining the tunnel, together with strips of weak jointing material for segmenting the lining. 3 3. A reaction frame usually comprising steel rings of sufficient strength and rigidity to resist the force applied by flat jacks or pressurising fluid. 4 The frame must also act as a waterproof membrane when load is applied by water pressure. When load is applied with flat jacks the frame must be provided with smooth surfaces; hardwood planks are usually inserted between the flat jacks and the steel rings. 4. Loading equipment to apply a uniformly distributed radial pressure to the inner face of the concrete lining, including:

(a) A hydraulic pump capable of applying the required pressure and of holding this pressure constant

* Numbers refer to NOTES at the end of the text.

to within 5% over a period of at least 24 hr, together with all necessary hoses, connectors and fluid.

(b) Flat jacks, when used for load application (Fig. la), should be designed to load the maximum of the full circumference of the lining, with sufficient sep- aration to allow displacement measurements, and should have a bursting pressure and travel consistent with the anticipated loads and displacements.

(c) Water pressure, when used for load application (Fig. l b) requires water seals to contain the pressurized water between the concrete lining and the reaction frame. Special water seals are also required to allow the passage of extensometer rods through the lining and reaction frame; pressurized water should not be allowed to escape into the rock since this will greatly affect the test results. 5. Load measuring equipment comprising one or more hydraulic pressure gauges or transducers s, of suitable range and capable of measuring the applied pressure with an accuracy better than _+2~o. 6. (a) Displacement measuring equipment to monitor rock movements radial to the tunnel with a precision better than 0.01 ram. Single or multiple position extensometers conforming with the ISRM "Suggested Methods for Monitoring Rock Displacements" should be used. Directions of measurement should be chosen with regard to the rock fabi'ic and any direction of anisotropy.

(c) Measurements of movement should be related to reference anchors rigidly secured in rock, well away from the influence of the loaded zone. When using multiple position extensometers the deepest anchor may be used as a reference provided it is situated at least 2 test chamber diameters from the chamber lining. Alternatively the measurements may be related to a rigid reference beam passing along the axis of the chamber and anchored at a distance of not less than 1 chamber diameter from either end of the chamber (Fig. 1).

PROCEDURE

7, Preparation (a) The test chamber location is selected taking into

account the rock conditions, particularly the orientation of the rock fabric elements such as joints, bedding and foliation in relation to the orientation of the proposed tunnel or opening for which results are required.

(b) The test chamber is excavated to the required dimensions. 2,6

(c) The geology of the chamber is recorded and speci- mens taken for index testing as required.

(d) The chamber is lined with concrete) The reaction frame and loading equipment are assembled.

(e) The extensometer holes are accurately marked out and drilled, ensuring no interference between loading and measuring systems. The extensometers are installed and the equipment is checked.


A B

® 0)

(91 181 (tO)

' (161 (21 ", (1) ~ i (151 J

i

~----@

Fig. la. Radial jacking test; flat jack loading alternative.

1. Measuring profile. 2. Distance equal to the length of active loading. 3. Control extensometer. 4. Pressure gauge. 5. Reference beam. 6. Handpump. 7. Flat jack. 8. Hardwood lagging. 9. Shotcrete. 10. Excavation diameter. 11. Measuring diameter. 12. Extensometer drillholes. 13. Dial gauge extensometer. 14. Steel rod. 15. Expansion wedges. 16. Excavation

radius. 18. Inscribed circle. 19. Rockbolt anchor. 20. Steel ring.

8. Testing

(a) The test is carried out in at least three loading and unloading cycles, a higher maximum pressure being applied at each cycle, v

(b) For each cycle the pressure is increased at an average rate of 0.05 MPa/min to the maximum for the cycle, taking not less than 3 intermediate sets of load- displacement readings in order to adequately define a set of pressure-displacement curves (e.g. Fig. 3).

(c) On reaching the maximum pressure for the cycle the pressure is held constant (___2% of maximum test presstlre) recording displacements as a function of time until approximately 80% of the estimated long term displacement has been recorded (Fig. 4). 8 Each cycle is completed by reducing the pressure to near-zero at the same average rate, taking a further three sets of pressure-displacement readings.

(d) For the final cycle the maximum pressure is held constant until no further displacements are observed. 8


:';. ~i

Approx 4m

-:~:..-,,::~ T F,: ~: ..~:,.::r..i!-: "'"a:.' .. . .-... *

• " ' "" " " "i-'i' ' " • °" " ' "'

i

i i

"':.."~ . . . ...r: """ . . ::': : ' ~ : " .~:::..~ ""~

LSj:-: ; ' f f ~ . . ~ l%" j "*'q

II , . ' . • , . . , . • . , . .

[ i )

• ~; 5i

~,o

" " 7 ,

14 "

,; j:

Fig. lb. Radial jacking test equ ipment ; i a l ternat ive loading system using water pressure.

The cycle is completed by unloading in stages taking readings of pressure and corresponding displacements.

(e) The test equipment is then dismantled, or further tests may be required having grouted the rock. 6

CALCULATIONS

9. (a) A solution is given only for the case of a single measuring circle with extensometer anchors immedi- ately behind the lining. This solution, which also assumes linear-elastic behaviour for the rock, is usually adequate in practice although it is possible to analyse more complex and realistic test configurations using for example finite element analysis.

(b) If flat jacks are used, the applied load values are first corrected to give an equivalent distributed pressure Pl on the test chamber lining:

~b Pl = 2 . rc . r 1 .P,.-

Pl = distributed pressure on the lining at radius rl

p., = manometric pressure in the fiat jacks

b = fiat jack width (see Fig. 5)

The equivalent pressure P2 at a "measuring radius" i" 2 just beneath the lining is calculated, this radius being


O

®

I

l? L - Ill l l l Illlllllll

A B I q ~

L "i I [- . . . . . . -l, ~ , F - - -

--i ®

///I- 1 A

~ i IIIIIIIIIIIIlUlIIIII ]

\ I Y A " / I /

A = AAI ÷A&2+~A3=AAI +2ABI I_ L!

I i i I

Fig. 2. Method of superposition to give displacements for equivalent uniformally distributed loading (elimination of end effects).

(c) Superposition of displacements for two "tic- ticious" loaded lengths is used to give the equivalent displacements A for an "infinitely long test chamber". 9

A -- AA1 + AA2 + AA3 = AA1 + 2. An1 (see Fig. 2 to give symbols)

(d) The result of the long duration test (Ad) under A maximum pressure (max P2) is plotted on the displace-

ment graph (Fig. 3). Test data for each cycle are pro- portionally corrected to give the complete long term pressure-displacement curve. The elastic component (A,,) and the plastic component (Ap) of the total deformation (A,) are obtained from the deformation at the final unloading:

A t = A p + A , , (see Fig. 3)

(e) The elastic modulus E and the deformation modulus V are obtained from the pressure-displacement graph (Fig. 3) using the following formulae based on the theory of elasticity"

E - P 2 " r 2 m + 1 A Ae m

v - P 2 " r z m + 1 At m

where P2 is the maximum test pressure and m is an estimated value for Poissons Ratio.

(f) Alternatively to (e) above, the moduli of undis- turbed rock may be obtained taking into account the effect of a fissured and loosened region by using the

outside the zone of irregular stresses beneath the flat jacks and the lining and loose rock.

rl Eb P2 = - - " Pl = - - " Pm"

r 2 2./t . r 2

following formulae:

=p2.rz(m__++ l l n r a~ E A e \ m r2 /

9p

6,0

3,0

L Ad I A A ' 2 A B I~ -I

. ,/

-17 q . . . . . I , - A 50 I00

m m I00 Fig. 3. Typical graph of applied pressure versus displacement.

R.M.M.S. 16/3 D

212

O/o A


I00

80

P

Prr~x

f /

I/A // L btime

Fig. 4. Typical form of graph for displacement versus time at constant applied pressure.

where i" 3 is the radius to the limit of the assumed fissured and loosened zone, and In is the Naperian (natural) logarithm.

(g) The dimensions of pressure linings can be determined directly by graph. 1 Use the load line of the greatest displacement as shown in Figs 3, 6 and 7.

/

Fig. 6. Typical graph showing total and plastic displacements as a function of direction perpendicular to the test chamber axis.

/ \

\\ J

b ~

P~

\

/

pro' ~ b = p1.2.~,7r

Pro" 7-b

q : 2 . T . ~

lq

r I -; q ~! p~: p,' -¢j-

Fig. 5. Scheme of loading showing symbols used in the calculations.

IO

T ~6 o. E5 8

o.

~o

P,

Pi A

=T-

5

~5

i '°

Pi = Pr + Ps (o = gap between steel

and concrete

r

i0

1.0

5O

I I i I00 2O0 500

o~ t

Fig. 7. Design chart for direct estimation of pressure tunnel lining thicknesses (from Lauffer & Seeber, see NOTE 1).


REPORTING OF RESULTS

10. The report should include the following: (a) A diagram giving all dimensions, photographs

and detailed description of the test equipment, full description of the methods used for test chamber preparation, lining and testing.

(b) Geological plans and section of the test chamber showing the relative orientations of bedding, jointing, faulting and any other features that may affect the test results, preferably with index test data to give further information on the mechanical characteristics of the rock tested.

(c) Tabulated test observations together with graphs of displacement versus applied pressure Ps or P2, and displacement versus time at constant pressure for each of the displacement measuring locations. Tabulated "corrected" values together with details of the correc- tions applied. See Figs 3, 4 and Table 1 (graphs are usually drawn only for the maximum and minimum displacements).

(d) Transverse section of the test chamber showing the total (At) and plastic (Ap) displacements resulting from the maximum pressure (e.g. Fig. 6). The orientations of significant geological fabrics should be shown on this figure for comparison with any anisotropy of test results.

(e) The graphs showing displacements as a function of applied pressure (e.g. Fig. 3) should be annotated to show the corresponding elastic and deformation moduli and data from which these were derived.

NOTES

1. For the design of pressure tunnel linings, the lining thicknesses in the full scale tunnel may be determined directly from the results of the test on the "model" tunnel. (Lauffer, H. and Seeber, G. "Design and control of linings in pressure tunnels and shafts." 7th Int. Conf. on Large Dams, Rome 1961, R91, Q25).

2. The recommended diameter is 2.5 m, with a loaded length equal to this diameter. Blasting is only permitted if the test results are applied directly as a "model" test to the case of a blasted full scale tunnel (see NOTE 1). Otherwise the chamber should be excavated with as little disturbance as possible.

3. When testing only the rock, the lining should be segmented so that it has negligible resistance to radial expansion; in this case the composition of the lining is relatively unimportant, and it may be of either shotcrete or concrete. Alternatively when it is required to test the lining together with the rock, the lining should not be segmented and its properties should be modelled according to those of the prototype.

TABLE 1. SUGGESTED LAYOUT FOR TEST DATA SHEET

1 2 3 4 5 4 + 5 6 7 4 + 5 + 7

Ad NR time P2 AA As A A + A a Ad corr. At

1

2 3a 3b 3c 4

5 6a 6b 6c 7 8

9a

o0

8 9

Ae Ap

E - P 2 " r 2 m + 1 A e m

v - P 2 " r 2 m + 1 A t fir/


4. Either flat jacks or a pressurizing fluid may be used to apply radial pressure to the test chamber; the two alternatives are illustrated in Fig. la and b.

5. Measurements are usually by means of mechanical guages. Particular care is required to guarantee the reliability of electric transducers and recording equipment when used.

6. To assess the effectiveness of grouting, two test chambers are usually prepared adjacent to each other. Grouting is carried out after completion of testing in the ungrouted chamber, and the equipment is then transferred to the grouted chamber.

7. Typically the maximum pressure applied in this test is from 5-10 MPa.

8. In the case of "creeping" rock it may be necessary to stop loading even though the displacements continue. Not less than 80Y/o of the anticipated long term displacement should have been reached.

9. This superposition is made necessary by the com- paratively short length of test chamber in relation to its diameter. Superposition is only strictly valid for elastic deformations but also gives a good approximation if the rock is moderately plastic in its behaviour.