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Colliery Engineering

May, 1958

Borehole surveying

An accurate indication of borehole deviation is required in order to ensure that a complete ice wall is formed. If a hole wanders too far from the vertical, it may be necessary to bore another hole to complete the freezing circle, or circulation of brine may have to be continued for a much longer period before sinking can commence. If a borehole is found to have penetrated into the actual excavation zone, it has to be cut out of the brine circuit as the excavation approaches, otherwise shotfiring damage may result in the serious loss of freezing solution.

The plotted survey of the boreholes is shown in Fig. 3 and it will be noticed from this plan that the boreholes did not deviate to any great extent.

The Gebhardt instrument which was used for the survey is shown in Fig. 4. It consists basically of a pendulum suspended by a universal joint. The deflection of this pendulum from the longitudinal axis of the instrument is recorded at regular intervals of depth and time, thus giving a measure of the borehole deviation from the vertical when multiplied by an amplifying factor; the magnitude of the amplifying factor depends on the interval of depth for which the recordings are taken.

A clockwork mechanism operates a trip gear at intervals of 5 min. and imparts a sharp: downward thrust to the pendulum. This movement causes the steel tip of the pendulum to pierce a strip of paper mounted at the bottom of the instrument. The paper strip is mounted on spools which rotate slightly after each thrust of the pendulum thus exposing an unmarked portion of paper.

Rods for lowering the instrument are screwed together by a special socket and they are fitted with a rib and slot arrangement to ensure correct alignment.

At the end of the downward traverse of the borehole, the rods are turned 180 deg., and then readings are commenced on the upward traverse. Therefore, for each depth, two readings are obtained and the average values of the co-ordinates are taken.

A special vernier scale is used for measuring the co-ordinates.

No. 20 borehole calculation is reproduced in Figs. 5 and 6, and from this it can be seen that the line joining the centre of the borehole to the shaft centre is used as a base line, with the shaft centre being regarded as North. The base line is recorded on the paper strip by two fixed pointers which give the North-South line.

Readings were taken of each borehole at five minute intervals every 12 metres of depth. This interval is considered to be ample time in which to add two 6-metre rods and to allow the pendulum to steady itself before taking the next reading.

Freezing

On completion of the boring, the 5-in, diameter brine tubes were inserted in the holes. As they were lowered, each joint was tested to a pressure of 700 lb. per sq. in. and when the complete columns were in position, they were subjected to a further static test of 300 lb. per sq. in. for a period of 24 hours.

The testing of all tubes and joints is extremely important as a loss of brine into the surrounding strata would mean that freezing by normal methods might have to be discontinued. This is due to the fact that brine has a lower freezing point than water and if it was allowed to flow into the strata, then the strata would become much more difficult to freeze.

During testing, and while the freezing holes were surveyed, the freezing plant had been erected in readiness for coupling up.

After the survey was completed and the position of the bores found to be satisfactory, the internal pipes were suspended inside the freezing tubes. Then, after testing the pipe joints, connections were made to the circulating mains in readiness to commence freezing.

Fig. 7 gives a view of the brine mains in the foreshaft showing the top of the freezing tubes with the inlet and return brine pipes. A view of the freezing plant is shown in Fig. 8.

Circulation was commenced by the middle of April and the ice wall was deemed strong enough to start sinking by July 30, 1956.

A guide as to how the freezing is progressing is obtained by carefully noting the temperature of the brine before and after it has passed through the freezing tubes. At the start of freezing there will be a considerable difference between the two temperature readings but as the freezing progresses, the difference in temperature should gradually decrease.

Another means of estimating the condition of the ground is to watch the make of water from the pilot hole. This hole is fitted with a perforated lining and as freezing progresses, water in the hole is forced upwards due to the expansion of the water-bearing strata on freezing. This flow should gradually decrease until the ground is completely frozen. Irregular variations in the rate of flow would suggest that the ice wall was not complete and that the water was escaping through it.

The initial sinking stages

As stated previously, the shaft top is situated on an existing dirt tip which would be unsuitable for the foundations of the concrete winding tower, or for the construction of a fan drift. Below the existing dirt tip is a bed of boulder clay on which the main foundations will rest. In view of this, it was decided that the first 36 ft. of lining should be of a temporary nature which would later be removed when the sinking was completed. This part of the shaft was lined with internal-flanged cast-iron tubbing of 25-ft. diameter which could easily be removed.

A section of this temporary lining is shown in Fig. 2. As shown, the space between the back of the tubbing and the foreshaft wall was filled with concrete and reinforced with 6 in. by 5 in. R.S.J.s and ½ in diameter rods; the reinforcing rods were hooked over flat plates on the back of the tubbing. This additional support was used to avoid any possible slipping of the tubbing.

Figs. 10 and 11 illustrate the method used when putting in the first stages of the lining. The first 8 ft. was excavated and ring No. 6 was carefully positioned. Rings 5, 4, 3, 2 and 1 were then built up to the top, the reinforcing bars put in place and the backing filled with concrete. The reinforcing girders can also be seen in Fig. 10.

Excavation was then continued and each ring positioned in turn. The space between the excavated ground and the tubbing was grouted with cement after the addition of every two rings. In all, 24 rings were required for this initial excavation.

(To be continued)

Drawings and Photographs accompanying the article