rROlO_ S7 - hi 5. Examina tion of Gardners Quarry, Relbi a .

V .M. Threader

A seismic sur vey by N. L. Matthews (appendix 1) and a field study by the writer were carried out in a basalt quarry situated on the nortQ-east flank of Cocked Hat Hill, midway between Relbia and Breadalbane [EQ165038) (fig . 11). The material won from this quarry is used in the as-mined con­dition for gravel road construction by the St Leonards Municipal Council.

GEOLOGY

The quarry is situated near the eastern edge of a Tertiary basalt flow which extends about 8 km in a NW-SE direction and one kilometre in a NE- SW direction. The eastern edge of the flow could not be determined accurately due to soil cover. The basalt rests on Tertiary sediments of the Launceston Basin (fig. 11) . The basalt is deeply weathered and is extracted by ripping.

QUALITY OF MATERIAL

The accompanying graphs (fig.12) represent particle size distributions of two samples taken from stockpiles on the quarry floor. Only -25 mm mater­ial was taken as larger particles appeared to be friable and so would break down to the smaller size with handling and compaction. The grading conforms to the specification for that maximum size and has a dust ratio of 0.67 which is within prescribed limits .

Other tests

Liquid limit Plastic limit Linear shrinkage

Result

50 26 11

~AASRA Specification Sealed roads Gravel roads

<25 <6 <2

<35 <9 <3

These results indicate that the material may be subject to excessive volume changes when wet and is therefore of marginal quality.

RESERVES

The area bounded by three fences and the broken line on Figure 13 is approximately 0 . 9 ha, and within this area the volume of rippable stone is estimated to be about 100 000 m3 and was calculated as follows:

Area 1m2) Length 1m) Volume (m 3 )

900 (shaded on 70 63 000 Section AB)

600 (shaded on 60 36 000 Section CD)

. -----100 000 -----

The basalt flow extends 10··11 kIn from Relbia to Western Junction and similar locations to Gardners quarry could be found anywhere along the flow. At the quarry there are approximately 1 200 m3 of basalt per metre of length (i . e. in a north-west to south-east direction). If such a volume is con­tinuous along the basalt margin alternative sources of supply should be easily located if required.

57

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LOCQlit~ : Gotdne r'5 Quarry 1 Re l bio

Reference No. : 4.7-71

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Pcrt icle Size (mm

Figure 12. Grading curves, Gardners Quarry, Relbia .

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RECOMMENDATIONS

A minimum of two , preferably four, diamond drill holes should be lo­cated as indicated on Figure 13 to confirm the seismic evidence, particularly with respect to the thickness of rippable stone. The depth of drilling would be approximately 15 m.

APPENDIX 1

Seismic survey rv.L. Matthe~ ~

Seismic work was undertaken at this quarry to determine whether the results could be used to help to assess reserves of material similar to that being quarried at pr esent.

Seismic spreads were laid out in the positions shown on Figure 13 . Each spr ead had a geophone inter val of 7.6 m except for Spread 2 where the in­terval was 4.6 m. An extension shot was fired about 4 . 6 m from the first geophone on the north-west end of Spread 1 . Only one end of Spread 4 was fired, so the information from this spread is less reliable . The results are summarised in Table 1.

Only Spreads 1 and 2 exhibited a third layer (V2) at one end and a cal­culation of the depth to this refractor is imprecise; the values shown (table 1) give a range of possible depths. An extension shot On the opposite end of Spread 1 failed to reveal material with a seismic velocity any greater than 1 585 m/s. It is probable that lateral variations in the degree of weathering would account for this.

CONCLUSIONS

It is apparent that the material being quarried at present is that with­in the intermediate layer (VI = ~1300 m/s) indicated in Spreads 1 and 2. Less weathered material probably occurs under one end of each spread, but the depth to this layer (V2) cannot be determined accurately although a range of depths can be given. A minimum thickness of about 14 m of I 310 mls material underlies Spread 1 and a minimum thickness of about 9 m of similar material underlies Spread 2. These depths could be much greater in each case.

In the case of both Spreads 3 and 4, slightly greater velocities were obtained for the intermediate layer than in Spreads I and 2. This may be interpreted as being slightly less weathered basalt. It is possible that it might not be rippable. In each case there is a greater than average depth of overburden.

[22 July 1975J

60

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Table 1. SEI SMIC RESULTS

Spread No.

1

2

3

4

Vo Velocity (m/s)

380

365

490

535

Thickness (m)

4 . 6- 6.7

1.0- 6.4

7.6-11.6

2.7- 8.2

VI Velocity (m/s)

1 310

1 280 or

1 830

1 525

Thickness V2 Velocity Depth to (m) (m/s) VI/V2 interface (m)

14.3-21. 3 3 050 20- 22 (one end or 26- 28 only)

9 . 1-1 5 . 2 1 920 14 - 17 24 . 4 - 28.0 (one end or 31-34

only)

.... -----/" ... -

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FRENCH

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_ _ _ _ Probable depth of decomposed oosolt

50

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---

HARD BASALT

(£l

SOil & DECOMPOSED BASALT

ili/));; Access rood

Rippobll!' basolt

-10 20 30 1.,0 50 60 70 80 90 100 120 130 11.,0 150 160 170m B

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FIGURE 13

r-------------------------------------~

::01 L----'::~:~s:"~':" ~E~._k __ i~~~~~~~~~:=__:::~_=':'~ .. j Rivt . l Tertiory sand and cloy

o , 2 , 3 4 , , 5 , • , 7 , , , 9 , 10 , 11 , 12 , 13 , " , 15 16 17k , ! !

Section through South Esk River , Cocked Hat Hill & Rose Rivulet

GARDNER'S QUARRY REL BIA Geolog ist : V. M. Threader Drawn: April 1977

TASMANIA DEPARTMEN T OF MINES 3988 Tech,Rep,Dep,Mines Tasm. 20,