Victoria University Antarctic Research Expedition Science and Logistics Reports 1980-81: VUWAE 25
ASSISTANCE TO SCOTT BASE
ASSISTANCE TO SCOTT BASE
Explosives for Ice Blasting (Alex Pyne).
Explosives have been used previously by NZARP events working on the sea ice. The main uses have been for seismic studies or to make access holes in sea ice to sample the sea water and sea floor. It was for both these purposes that members of Event 14 participated in an explosives course, arranged through the Department of Labour (Explosives Division), prior to departing for Antarctica.
In previous seasons sea ice thickness has been quite variable. In open parts of the Sound it is between 2 and 3m, but in a few places, notably Explorers Cove and the bay at Ferrar Glacier Snout in New Harbour, the ice has been up to 5m thick because it is several years old. The VUWAE portable motorised 12 inch diameter auger was not expected to penetrate overthickened ice so explosives were to have been used to complete the access holes. This season, however, nearly all the sea ice in the Sound was first-year ice a little over 2m in thickness, which was well within the auger's capability. Consequently, no explosives were used to make sea ice holes but were used for the seismic refraction programme and to blast the Scott Base ice quarry.
The explosives were obtained from the US Navy Support Force stores at McMurdo Station and will be partly replaced by stock donated to the expedition by ICI NZ Ltd. The ICI stock will be used by next year's expedition continuing the work on the sea ice. Electric detonators were not available from the McMurdo stores so plain No.6 detonators initiated with safety fuse were used. Plain detonators were considered to be the much safer option in antarctic conditions where static electricity is frequently developed in the dry air and from blowing snow. A reinforced detonating chord (10g/m P.E.T.N.) was used to initiate the main charges of either AN GD 95 or a 60% seismic grade gelignite.
Ice has been excavated from the ice cliff near Scott Base for several years for the base fresh water supply, since the recent installation of the "reverse osmosis" plant, ice has only been used for the kitchen and as a standby water Supply. In previous years the US Naval Support Force has blasted the ice cliff to reduce the ice to manageable size for the Scott Base ice melters.
This season in mid-October it became necessary to blast the ice quarry again when the reverse osmosis unit was inoperative. Event 14 volunteered to set the charges, to experiment blasting hard antarctic ice and to supply ice quickly for the base. The Navy was not expected to be able to do the job for several days. The quarry was blasted on 15 October and again on 19 November.
The ice cliff is composed of hard blue and white ice layers with less dense ice and granule snow layers composing the upper 4 metres. The layers dip 30 towards the road at the base of the cliff, on 15 October previous quarrying had formed a vertical ice face approximately 8m high and 12m wide. page 29 A snow slope continued away from the top of the face at an angle of 30°.
To blast the face two sets of charges were laid. On the snow slope above, a track (0.5m wide) was dug parallel to the face and 5m up slope from the face edge. Ten vertical holes (4 inches diameter, 2.6m deep) were drilled from the track across the face in the layers of granule snow and thin ice (less than 0.3m). Two cartridges (55m dia.) of AN 95 were lowered to the base of each hole and the holes stemmed with snow and water which quickly began freezing. Thirty-two kg of AN 95 was used for the top ten charges.
Four horizontal holes were drilled lm above the quarry floor, and about 3m apart across the face. These holes were drilled 2.6m deep into hard ice. Three cartridges of AN 95 were loaded at the end of each hole and also stemmed with snow and water. Setting the charges for this blast took six people most of the day, the major proportion of time being used to drill the holes and set the top 10 charges on the slope above the face. In total 51 kg (two cases) of AN 95 high explosive was used.
The resulting blast removed a slab of snow-ice from across the top of the face which was about 12m long, 5m wide and 3m thick. Large blocks of hard ice were also dislodged from the face by the bottom charges. The blast on October 15 was not as successful as expected but nevertheless kept the base supplied with water for just over a month.
The ice cliff was again blasted on 19 November when a different and quicker method of setting the charges was employed. The floor of the ice quarry was again cleared leaving a vertical ice face about 8m high and 12m wide. The 920 loader was used as a platform to drill 12 nearly horizontal holes, 4m long and 4 inches diameter, into the face. Three rows, each of four holes were drilled about 1m, 3m and 5m above the quarry floor, evenly spaced across the entire face. Thanks to the help from several Scott Base staff, drilling and loading took less than half a day.
Four cartridges of AN 95 were loaded at the base of each hole giving a total loading of 77 kg (3 cases) for the face. The holes were stemmed as previously with a snow and water slush.
This blast on 19 November was very successful. The entire face to 4m deep was dislodged and broken into "loader size" blocks. All of the ice was useful hard ice because contaminant soft snow had been removed in the previous blast. There was very little "fly ice" and it is concluded that the 3 cases of AN 95 was just sufficient for the estimated 400 cubic metres (4 × 105kg) of ice dislodged.
This season's experience has shown that for future blasts an excellent result would be obtained if two sets of charges were used. The smaller set of charges should be loaded into vertical holes (3-4m deep) across the top of the face. These would remove snow and porous ice and perhaps develop a vertical shear plane parallel to the face. The larger set should be loaded into horizontal holes drilled 4 metres deep into the face. This set would dislodge and break up the potentially useful ice. The operation could be carried out relatively quickly if the loader was used.page 30