cross hole seismic investigation for characterization

7/26/2019 Cross Hole Seismic Investigation for Characterization

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26th US Symposiumon Rock MechanicsRapid City, SD / 26-28 June 1985

Crosshole seismicnvestigationor characterization

of the rock oundation t the siteof a large ockfill dam

T.L. BY

Norwegian Geotechnical nstitute, Oslo, Norway

1 INTRODUCTION

The Norwegian Geotechnical Institute (NGI) is engaged in research in

the field of geodynamic investigation of rock and soil quality.

The results from this work can be used to determine the in situ

modulus of deformation and other rock mass properties, and to locate

major joints and fractured zones.

This paper describes how the cross hole seismic method can be used

to investigate weak rock. The material under consideration forms the

foundation rock for a large rock fill dam called the Oddatjrn dam.

More spescfically, the quality of the foundation for the central core

was evaluated by means of acoustic measurments through 40 meter long

boreholes.

The Oddatjrn dam is one of several dams constructed to establish

Norways largest hydro power reservoir called Bisj. A number of

smaller lakes have been regulated into one big reservoir. These lakes

are connected by a system of tunnels.

The B1Asj reservoir is part of the Ulla-Frre Hydro Power Scheme in

south-westNorway. Th project utilizes the water power roma number

of rivers in a 2000 km mountaineous region 80 km north-east of the

town of Stayanger. Atmospheric low pressures, steadily coming in

from the Atlantic, give high precipitation, up to 2000 m pt. year, in

this region.

The State Power System, one of the four directorates of the Norwegian

Water Resources and Electricity Board, is responsible for the planning

and construction work of the project. Construction was started in

1974 and will continue towards 1990.

The contribution of the project to the national firm power production

capacity will amount to 5663 GWh pt. year. The total cost of the plan-

ning and construction work will be approximately US $ 2000 in 1984

prices.

2 THE ODDATJRN DAM

The Oddatjrndam s a rock fill damof 5.7 mill. m volumewith a tra-

ditional central core of moraine material, figure 1. The construction

work was started in the beginning of the Ulla-Frre project and the dam

will be completed in 1987-88.

The rock in the dam foundation consists partly of a hard, massive

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Figure 1. OddatjrnDams a rock fill damof 5.7 mill m volume nd

has a traditional central core of moraine material.

gneis. The left abutment is of variable quality and includes some

highly fractured pegmatites which are not very suitable for foundation

purposes.

The quality of the rock which forms the foundation of the central

core, is further diminished due to blasting operations immediately above

the left abutment in a quarry used for dam material production. Core

drillings have located permeable zones down to more than 80 meters be-

low the surface. Some major weakness zones cross the foundation of

the central core.

The smallest distance from the quarry to the dam abutment is down to

50 meters and the maximum charge weight pt. interwall was 4000 kg of

explosives.

The maximum peak ground vibration level is unknown. However, as the

results from the seismic investigations will illustrate, the rock

quality was definitely affected by the close proximity of the blasting.

The footing for the central core was to be fully grouted. The

quality of this anti-seepage grouting is traditionally investigated by

means of core rillings and water leekage tests. To complement these

tests, the State Power Board ordered cross hole seismic measurements

of the rock conditions.

The cross hole seismic monitoring through the grout curtain was to

take place in several steps:

phase 1: Before grouting (August 1984)

phase 2: After grouting (June 1985)

phase 3: After magazine filling

The,winter at Oddatjrn lasts from October until June. There are no

activities at the project during this season.

This aper describes the results from phase 1. The results were

used to irove and to control the grouting effectiveness. Zones of

extremely low quality, i.e. highly fractured zones were located to

assist the grouting crew. As we shall see, the accordance between the

seismic survey results and the grout consumption were good.

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Poer supply

B & K 2807

Disc storage

Nicotef &O9&

o o digitart

o o o o oscittoscope

Firingbattery

B & K 8101 B & K 8101

Etectricat detonator

Figure 2. Cross hole seismic investigation, principle sketch.

3 CROSS HOLE SEISMICS FOR CONTROL OF THE LEFT ABUTMENT OF THE ODDS-

Seismic or sonic methods for investigation of rock quality, make it

possible to predict the in situ mechanical properties of rock masses.

One special approach is the elastic characterization of rocks for

foundation engineering. Other purposes are prediction of fracture

frequency, localization of weak or poor zones, grouting control and

assessment of blast damage adjacent to rock faces.

The technique involving the propagation of seismic waves between

boreholes, the cross hole method, was chosen because of its reliabi-

lity and simplicity. Such seismic investigations as described are now

established as a regular service at the NGI and the cross hole seismic

device has proved to be dependable.

To investigate the grout-curtain zone in the left abutment from

1030-1060 m.a.s.l., three 5" diameter boreholes were permanently estab-

lished. The boreholes were 40 meter long with a 60 inclination.

The NGI device for borehole seismics normaly require only 48-76 m

diameter holes. The device exploits 2 Brel & Kjr hydrophones as

signal receivers. This make it necessary to_keep water in the holes.

As the rock in the actual area was highly fractured and unable to

retain water, and because of the wish to keep the boreholes permanent

during the remaining dam construction work, it was decided to line

the boreholes with 3" diameter PVC plastic tubes.

Because of the very oor rock quality, especially down to 15 meters,

it was difficult to establish the holes. The bottom of the tubes were

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to simplify entering of the holes, equiped with steel cones. The space

around the tubes was filled with grout.

The principle of the cross hole method is illustrated in figure 2.

In very poor rock, borehole lining is preferable because of the good

acoustic coupling with the rock. Mechanical clamping is of course

preferred where the borehole quality ensures that the clamps give a

reliable and constant contact with the rock. In our Oddatjrn project,

fragments of rock fell into the boreholes from the sides of the hole.

This made a good mechanical clamping uncertain because of the variable

borehole diameter.

3.1 Device specifications

To involve the desired rock volume, electrical detonators were used as

transmitters to get a signal of enough energy. To attain a good acou-

stic coupling with the rock, the boreholes were, as mentioned, filled

with water. Brel & Kjr hydrophones with internal preamplification

were used as receivers. The transmitted waveforms were recorded on a

4 channel digital storage oscilloscope, Nicolet 4094. An example of

recorded signals is shown in firgure 3.

NGI has constructed a special device for measurements in inclined

holes. To ensure repeatability and to avoid damage on the borehole

lining, it was important to keep both transmitter and receiver in the

borehole center. Figure 4 shows this borehole seismic device.

4 INTERPRETATION OF SEISMIC SIGNALS

From research and field experience, the P-wave first arrival rise time,

the rise velocity, the amplitude, the power spectrum and the travel

time are known to be acoustic parameters all reflecting changes in the

rock mass quality.

In homogenous rock, the frequency spectrum, the rise velocity and

the rise times are the most sensitive parameters. Even small physical

contrasts can be deteced.

In poor rock, however, the rise times are long and amplitudes low.

This makes these parameters less reliable because of the very low

values and unsatisfactory signal to noise ratio.

As the signal velocity (here the P-wave velocity) is a less sen-

sitive parameter, it reflects quality variations in highly fractured

rock more reliably than rise times and amplitudes.

In the actual rock characterization project, the P-wave velocity was

used as the main acoustic parameter. Figure 5 shows the sound velo-

city as a function of borehole depth.

The cross hole seismic investigation showed that the rock down to 20

meters was heavily fractured. The sound velocity varied from 500 m/s

to 2500 m/s in this area. From 21 meters the quality increased and

the typical sound velocity were 5000 m/s.

Geological investigation suggested a major joint set crossing the

foundation rock at 20 meters. The rock above this zone seemed to be

severely affected by the production blasting in the rock quarry. Be-

low this jointed zone the rock was apparently little affected by these

ground vibrations.

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i i i i i i

Time 2,5 ms/division

I I i i i [ I

Figure 3. Examples of signals from one electrical detonator, trans-

mitted through 8 meters (top) and 16 meters of competent gneis. The

double curves illustrate the repeatability.

4.1 Control of the grout curtain quality

The main purpose of the Oddatjrn project is to record the relative

changes in the acoustic parameters before and after the grouting pro-

cess and after magazine filling.

The problem is to locate zones or areas of poor quality grouting and

to perform both a detailed and general estimation of the grouting

effect.

To evaluate the improvement of the mechanical properties caused by

grouting and thereby indirectly assessing the reduction in permeabi-

lity, the sonic investigation of the rock between the boreholes will

bear a lot of information.

The experience from the seismic investigation served as a verifica-

tion of the great quality of grout injected into the heavily fractured

area down to 20 meters.

The grout consumption in the imediate dam foundation, i.e. down to

10 meters below surface was 80.000 kg for the 40 meter long investiga-

ted area of the left abutment. In one surface injection hole the con-

sumption exceeded 13.500 kg. In some areas the consumption was so

high that it was necessary to add sand to the grout to limit the flow.

The worst zone was where the mentioned major joint set crossed the

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Figure 4. Borehole seismic device.

Top: Signal transmitter, i.e. one electrical detonator

Below: Signal receiver, i.e. special hydrophon arrangement.

core foundation. Below this zone the grout consumption decreased

and confirmed the relative high sound velocities in this area.

5 CONCLUSION

The cross hole seismic project at Oddatjrn dam showed great

variations in rock mass quality. The extremely low P-wave velocity

in the upper part of the investigated area is probably caused by the

production blasting in the stone quarry situated 50-100 meters from

the rock fill dam abutment.

The results are dependable and repeatable, figure 3. This is

ensured by means of a special borehole device for inclined water

filled holes.

The grout consumption is compared with the seismic quality interpre-

tation and a good agreement is achieved.

The next phase of the project, i.e. the acoustic assessment of the

grout curtain quality, is going to be executed during the 1985 summer

season.

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500 1000 2000 3000 /000 5000 6000

SOUND VELOCITY P-WAVE ) m/s

Figure 5. Sound velocity distribution in the

heavily fractured foundation rock at the left

abutment of the Oddatjrn Dam. The more com-

petent rock below the fracture zone starts at

a borehole depth of 20-22 meters. As the

velocity plot shows, the ground conditions

down to 15 15 meters are extremely poor.

ACKNOWLEDGEMENTS

The project at Oddatjrn dam is partly founded by the Royal Norwegian

Council for Scientific and Industrial Research (NTNF). The field work

is ordered and supported by the State Power Board.

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cross hole seismic investigation for characterization

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