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LBNIR PUBLICATIONS CONIPACTL_

(LENDZG SECTION)

DEPARTMENT OFMINERALS AND ENERGY

BUREAU OF MINERAL RESOURCES,GEOLOGY AND GEOPHYSICS

055313

Record 1976/71

DEFENCE FORCE ACADEMY SITE,: •UNTROON A.Cf. INVESTIGATION OF THE SUBSURFACE, 1975

by

D.G. Bennett and -G. Jacobson

The information contained in this report has been obtained by the Department of Minerals and Energyas part of the policy of the Australian Government to assist in the exploration and development ofmineral resources. It may not be published in any form or used in a company prospectus or statementwithout the permission in writing of the Director, Bureau of Mineral Resources, Geology and Geophysics.

BMRRecord1976/71

c.3

Record 1976/71

DE7ENcE. FORCE: .AQADEM•^DUNTROON

INVESTIGATION. OF THE SUBSURFACE : : 1975

by

D.G. Bennett and G. Jacobson

CONTENTS

Page

SUMMARYINTRODUCTION^

1

GEOLOGY^

1

TEE SEISMIC SURVEY: METHODS AND EQUIPMENT^

3

SEISMIC VELOCITIES AND EXCAVATION CONDITIONS^

3

RESULTS OF THE SEISMIC SURVEY^

4

CONCLUSIONS^

5

REFERENCES^

7

TABLES

:1. ' DEPTHS OF MACHINE REFUSAL,2. ' SEISMIC VELOCITIES AND EXCAVATION CONDITIONS

• PLATES

. ' LOCATIONS OF SEISMIC TRAVERSES

2.^SEISMIC CROSS-SECTIONS

SUMMARY

The Bureau of Mineral Resources, Geology and Geophysicsinvestigated the subsurface of part of the proposed site of theAustralian Defence Force Academy; Duntroon, A0.C.7. The investi-gated area is underlain mainly by irregularly weathered dacite,with Some colluvium at its eastern end. Depths of marginallyrippable material range up to 12 m, but seismic surveys indicatedtwo areas of hard rock close to the surface which are a constrainton planning the layout of buildings.

INTRODUCTION

At the request of the Department of Housing and Con-struction (DHC, now the Department of Construction), the Bureauof Mineral Resources, Geology and Geophysics (BMR) investigatedthe subsurface of part .of the proposed site of the AustralianDefence Force Academy, Duntroon, %A.C.-1".. 'The site is bounded byNortheott Drive, Fairbairn Avenue, and General Bridges Drive(Plate 1).

A soils investigation of the entire site by DHC, andgeological logging of trenches, indicated an area of hard rockclose to the surface near General Bridges Drive in the southwestpart of the site. 'A geophysical survey was undertaken in thisarea to investigate the extent of the rock, which may constrainthe layout of buildings. Other objectives of the survey were todetermine the approximate depths of rippable rock, and toestablish depths to suitable foundation conditions for buildings.The survey was carried out in November 1975, and a total of 11seismic spreads were shot over 726-m. The field party consistedof^Bennett, p.Rt. Francis, andgM\% 'Preston-Stanley.

Taylor and D.G. Bennett interpreted the seismic data.

GEOLOGY:

The proposed Academy site is underlain by volcanicrocks, the Ainslie Volcanics, of Early Devonian age (Strusz &Henderson, 1971). 'Table 1 shows the rock conditions encounteredin pits and trenches, and the depths of backhoe/bulldozer refusal.

In the steep southwest part of the site, hard dacite isclose to the surface in places above 590-m. 'Dacite crops outadjacent to General Bridges Drive (Plate 1) and, the bulldozertrenches G8, C6/D6 1 and D8 indicated depths of-1.0..-1.6m to hard,slightly weathered, medium-grained dacite, which will have to beexcavated by blasting. The seismic sections for this part of thesite (Plate 2) indicate irregular weathering of the dacite, and anortheast-trending fault associated with zones of deep weathering.

'TABLE 1

. DEPTHS OF MACHINE REFUSAL

BACKHOE PIT BULLDOZER TRENCH

No. Depth of No. Depth of ROCK TYPE ANDRefusal Refusal WEATHERING

/ (M) (m)

-1.1 J8 -1.5' Moderately-slightlyweathered ashstone

2.3 Moderately weatheredashs tone

3 J30.7 D8 Highly-moderatelyweathered, closelyjointed dacite

4 2.6 Moderately weathered,quartz-veinedrhyolite

5 Moderately weathered,quartz-veined dacite

6 Highly weathereddacite

7 72.0 Extremely weathereddacite

8 Moderately weathereddacite

9 :105 Highly weatheredrhyolite

G8^:1.0. ^weathered .dacite

C6/D6^-1.2'^Moderately-slightlyweathered dacite

- 3-

In the southeast part of the proposed Academy site,pits exposed deep, mainly clayey, colluvial soils overlyingweathered dacite. Backhoe refusal was at depths of -2.0 m or morein the four pits (5, p, 7, s) excavated here, and the rock isprobably rippable to about one metre below backhoe refusal.

In the northern part of the site, pits showed thatclayey and sandy colluvial soils overlie weathered fine-grainedvolcanic rocks - Thyolite and ashstone. Depths of rippablematerial range from -1.5'm (bulldozer trench J8) to more than 2.6m (Pit 4) and are irregular. 'Groundwater seepages were observedin the sandier soils, and might cause drainage problems inexcavations.

• THE SEISATI:0 ;SURVEY!:i METHODS AND EQUIPMEXT

The seismic refraction method (Dobrin, 1952) was usedto investigate the subsurface in the southwest part of the site.The locations of the four traverses are shown in Plate :1.

Each spread consisted of 23 geophones placed in astraight line, with the 24th geophone used as a reciprocal. Forthe 'centre-line traverse a .3-m geophone spacing was used; foreach of the cross-traverses 3-m and 4-m spacings were used togive a complete coverage. Five shots were fired for each spread, .

11

^

^one in the centre of the spread, one at half a geophone spacingfrom each end, and one at a distance of 35 to 40 m from each end.

The equipment used was a 24-channel SIE PSU-19 refrac-tion seismograph with 8 Hz GSC-20D geophones. 'Interpretation wasbased on the reciprocal method (Hawkins, 1961).

11SEISAI.I•C IVELOGIMUIS.' :AND^.:CONDITIONS:

Seismic velocities are roughly related to excavationconditions (Caterpillar Tractor Co0,:1966). Table 2 shows anapproximate relationship between seismic velocity and rippabilityin the Canberra district. '

-4-

' TABLE 2

SEISMIC VELOOITIES -:AND EXCAVATION CONDITIONS.......^.....^.

VELOCITY. : EXCAVATION CONDITIONS:

(m/ s)

Less than

1100-1500

Greatei

1000

than 1600

Rippable

Marginal

Blasting .

- Tip or blast

'GEOLOGY:

Soil; completelyto highly weatheredrock.

Highly-moderatelyweathered rock;possibly corestonesof fresher rock

Moderately-slightlyweathered rock;fresh rock

RESULTS OF THE SEISMIC SURVEY

The seismic sections are shown on Plate 2.

Traverse '1 (centreline): This traverse shows a thinsurface layer, generally less than 1 m, of soil with a seismicvelocity of 300 m/s. At the intersection point with traverse 4this layer thickens in a pocket to a depth of 2.5:m.

The underlying layer varies considerably in depth andseismic velocity: under the western end of the traverse itsseismic velocity is between 1200 and 1650 .m/s, which mayrepresent rippable material; between chainages 105 and 140; hardrock with a seismic velocity between 1600 and 2300 m/s will haveto be blasted for excavations; east of this the second layerthins gradually, but its seismic velocity indicates that itshould be rippable.

The bedrock velocity also varies. 'Up to chainage 230the bedrock has two main velocities: one about 5000 m/s, theother between 3400 and 3700 m/s. At chainage 230, the seismicvelocity changes abruptly from 3700 m/s on the west to 1800 m/son the east. The velocity of 1800 m/s represents moderatelyweathered rock, possibly in a fault zone.

Traverse. 2 (cross): The surfae layer of soil (300-450m/s) is generally a metre or less over most of the traverse, butthickens to ;205'm at chainage 50, near where a fault has beenindicated (Plate 2). On the southern side of this fault a secondlayer with velocity of 600-900 m/s (highly weathered rock) extendsto a depth of 5-6.:m. 'A layer with similar velocities was notdetected on the northern side. Underlying this layer is moder-ately weathered rock with a velocity between 1650 and 1800 m/s.The bedrock on the northern side of the fault has a velocity of4800 m/s, and was detected at a depth of 1000 On the southernside, the _bottom refractor has a velocity of 3200 m/s, and itsaverage depth is 22m.

' Traverse. a :(cTos's)::: This traverse is similar to cross-traverse :2. 'A thin surface layer of soil thickens near the fault.The second layer of highly weathered rock appears to be contin-uous. It has a velocity of 950 m/s near the fault and increasesto 1100 m/s farther away. This material should be rippable; itsthickness varies from about 7 m at the northern end to about 4 mnear the southern end. 'On the southern side of the fault a thirdzone of higher velocity, 1700-2000 m/s, overlies the bedrock. Atthe northern end of the traverse, bedrock velocity was 4500 m/sat an average depth of 7-8:m. Near the fault this velocitydecreased to 3400 m/s, and at the southern end the velocity was2700 m/s, indicating a zone of deep weathering.

' Traverse 4 -'(,Cross")::: This cross-traverse follows asimilar pattern to the other two, with a fault indicated near theintersection of the centreline traverse. 'A thin layer of soilthickens slightly at the intersection point with traverse-1.Below it, highly weathered rock or indurated colluvium up to 5 mdeep has a seismic velocity of 800 m/s, and extends roughly fromchainage 30 to 110; it should be easily ripped. 'Underlyingthese layers, the weathered rock has a velocity of 1500-1600 m/ssouth of chainage 112; north of chainage 112, a seismic velocityof 2250 m/s indicates an area of hard rock. At the southern end,hard fresh bedrock was not reached and the refracting layer has avelocity of 2400.m/s at a depth of 17 to 21.m below the surface.At the northern end, a bedrock velocity of 4800 m/s was detectedat a depth Of about 10-12 m; bedrock appears to shallow to 5 mat the far northern end.

CONCLUSIONS

1 . In the southwest part of the Academy site, seismicsections and pits indicate irregularly weathered dacite withpossibly some colluvium up to 4 m deep downslope. Depths ofrippable material vary, and may be up to 12 m in places.

-6-

The locations of two areas of hard rock close tothe surface have been indicated by the seismic resu1t8: betweenchainages 105 and 140 in the centreline traverse 1, and at thenorthern end of traverse 4. In these areas, rock at a depth ofabout one metre will require blasting.:

3. 'A fault zone about 10'm wide trends northeast-southwest, nearly parallel to the centreline traverse. The rockimmediately southeast of the fault is deeply weathered withseismic velocities of 1500-2000 .m/s to depths of up to 22-m.

'Pits have indicated generally deeper soils in thenorthern and eastern parts of the Academy site; these areas havenot been investigated by seismic survey, but depths of weatheringare expected to -be greater than those in the southwest part ofthe site.

RE1ERENCES

CATERPILLAR TRACTOR COMPANY,. 1966 - HANDBOOK OF RIPPING: A GUIDETO GREATER PROFITS. 'Peoria, Illinois, Caterpillar Tractor Co.

DOBRIN,^-- INTRODUCTION TO GEOPHYSICAL PROSPECTING.New York, McGraw-Hill: Inc . ..

HAWKINS, LOP•, 1961 - •The reciprocal method of routine shallowseismic refraction investigation. 'Geophysics,, 26(6), ,806-19.

STRUSZ,^HENDERSON, G.A'0M'0,971 - Canberra, A0C.7. -1:50 000 geological map and explanatory notes. - Bur Miner. ReSour.. Aust, explan ,, Notesv '

• " (>2mJ OVERLYING DACITE

~ 5f(P .... .:::::-.:::::-.:::::-.:::::-

PLATE I

LOCATION MAP

LEGEND

ROAD

TRACK

~ ELEVATION CONTOUR (2m interval) ~

SEISMIC TRAVERSE

_._._-_. __ ....... _... GEOLOGICAL BOUNDARY

N

METRES 50 o 50 I I I I

100 I

AUSTRALIAN DEFENCE FORCE ACADEMY SITE, LOCATIONS OF SEISMIC TRAVERSES

155/85-299

--.... '" ~ Ib E::

'--

~ ~ ~ l.J.J l;j

--.... ~ ~ Ib

~

~ ~

~ l.J.J

~

620 WEST

615 TRAVERSE 1

Ib

~ ~ t> -.....;

it -.:

EAST

610 .6"-----___ 300 I --- .

605

600

595

590

585

580

515

615

610

605

600

595

590

585

580

1200

5000

0 20

SOUTH

-7"----___ _ __ SURF4CE 7 ----__ _

----1650

1400

2300 10 ------- \0 ---......... ,: ,.: 1000 ----__ _

i ~ _ ____:_r~---+-----5~/OOO ---:;-_ SUi9F4 : 12 5/00 ,----_ C'E

3400 3900 _

5000

,: 2600 5" - -~__ ............ l.!?E.-------_ - 4' -, 800'5 - - - - :::::_:-:_::-::::-:::-~ __ ---

3100

40 60 80 100 120 140 160 I 0 200

NORTH 615 SOUTH

220

:' -;; - 800 ---, , ,

I

I

POSSIBLEI FRACTuRE

ZONE

1800

240

--3 1900

260 280 300

NORTH 350 TRAVERSE 2 TRAVERSE 3

1 --------800 ---~-----------1800

3200

0 20 40

30() 610 SURFACE

350 350 --.... 605

~ 300 .!::: --- ---7----____ _

300

300

SURFACE Ib

600 Ii:: '-- 1-;00------~ 4

----595

~ /100

~ li...J 590

10

1700 5

/100 /-9 _______

/ -----/ 480(1 10 ____

/ W~ V

-------.6 -------_ 550

950 --1':6-

-"5--_ 1800

~ poo _---/ 4800 ii-

~------------~~---7 :::: ______ "00 -----;';0 ii '9"1

2100 PO"S I 3200 I

POSSIBLE I FAuLT ZONE

60 80 100

50 I

585 SHEAR I iFAUif 4500 ZONE? ZONE

4500

5801---------.---------r-------~--------_r--------,_--------r_------_,r_------_._ 120 140

o I

160

HORIZONTAL SCALE

o

50 I

20

AUSTRALIAN DEFENCE FORCE ACADEMY SEISMIC CROSS-SECTIONS

40 60

100 METRES I

80 100 120 /40 160

Record No. /976/7/

LEGEND

3·5 Depth to refractor (metres)

3700 SeismIc velocity in formation (metres/second)

Interpolated boundary of intermediate refractor

Bedrock boundary

615 SOUTH TRAVERSE 4

--.... '"

610

605-

600

595

~ 590 Ib Ii::

'--~ 585 ~ ~ ~ 580 "-.J ~

575

570

565

400

1500

17

350

I 400

-""'""__ ~ SURFACE ,._ -1; __ ,,- _-- ---'" 800 ----

'- 1·5 _----"j"--", 800 _-'"--------5

1·5

2250

1600

-;------- - -...,. "/ 12

1500

I 4800

/ /

------./ 21 ------ \ POSSIBLE I

FAuLT ZOI'iE 2400

4800

",/

PLATE 2

NORTH

"..-5 ./

5604---------~--------r_------~--------_r--------,_--------,_------_,r_------_, o 20 40 60 80 100 120 140 160

I55/85-305