The discussion on the Paper "The Effect of Subsidence due to Coal-workings upon Bridges and other Structures" was continued and concluded.

SECT. II. OTHER SELECTED PAPERS

(Paper No. 3086.)

"Trial Survey for a Railway over the Outeniqua
Mountains, Cape Colony."

By CHARLES EDWARD STEWART, Assoc. M. Inst. C.E.

In this Paper is described the trial survey undertaken by the Author to carry an approximate line over the Outeniqua Mountains on the basis of a 1 in 60 gradient and a limiting curvature of 5 chains radius.

To the north of the mountains lies the Oudtshoorn, or second plateau, to the south the George, or coast plateau, and the dividing ridge of Afgunst Neck, through which it is proposed to tunnel, is about 2 miles distant from Montague Pass Neck and 170 feet above it. The summit to be overcome is about 1,400 feet above George. In 1880 a detailed survey, on the basis of a 1 in 40 gradient, was undertaken by the Government for a line of railway over the mountains, passing through the Montague Pass, where a good roadway was constructed in 1844.

The trial survey was commenced at Afgunst Neck; the summit level being fixed at 1,460 feet on the same datum as the Government survey through Montague Pass, giving a total ascent from the point of departure on the Government line at George to a summit of 1,363 feet. A trial line was carried from the summit of the proposed tunnel to ensure a junction with the Government line on the northern slope, the distance being found to be 3 miles 10 chains with a gradient of 1 in 70. The next step in the survey was to derive an accurate section of the proposed tunnel through the dividing ridge. A section of the ridge seemed to present a series of five straight lines, the southern slope being as steep as 1 to 1. These slopes were measured with a theodolite, the lengths being taken with a tape and the horizontal equivalents were deduced from them. The flying levels were then carried over the ridge and down the south side, and the tunnel was laid out on a gradient of 1 in 60, with a summit of 2 chains of level. was calculated to be 720 yards. From the south side of Afgunst Neck the gradient traverse to George was then started by way of the

The length of the tunnel

Fig. 1.

Kayman River and Zwart River Valleys. The continuous line in the Figs. shows the gradient traverse, and the radiating lines level contour points. The side slopes of the mountain and ravines are shown by dotted lines, and the centre-line of the proposed railway by a curved line. The instruments employed for the work were a 14-inch Dumpy level provided with stadia wires, an Abney level, and a prismatic compass. The level was placed over each peg on the gradient traverse, the height of the line of collimation being taken at each peg with a tape and the distance ahead to the next peg with the stadia wires. When practicable all pegswere inserted on the 1 in 60 gradient traverse; and generally between 100 feet and 600 feet distant. Longer sights than 40 feet could not sometimes be taken owing to the undulations of the side slopes of the mountain. The 14foot levelling staff would not permit of a length of more than 600 feet, as the height of the line of collimation usually exceeded 4 feet. Owing to the rocky nature of the mountain slopes, the ground level was worked to at the pegs and not the tops of the pegs, as it would have been difficult to drive the pegs to any desired level between the clefts of the rocks. The bearing of each peg was taken with a prismatic compass placed on the top of the level tripod, and at each station the level was unscrewed from the tripod for the purpose of replacing the prismatic compass. The transverse inclinations of the ground were taken at each peg with the Abney level; they varied between 1 to 1 and 7 to 1. The Author was able, with the aid of two men, to carry on these three operations at the same time,

1

1:1.

3:1

32

Scale, 1 inch = 800 feet,

2:1

accomplishing on favourable ground as much as 4,700 feet in 1 day. The flying levels were carried on at the same time, as a check on the gradient traverse, and bench-marks were established along the line of the proposed route. The levels usually checked well with

Fig. 2.

the grade line; at times only a difference of 0.30 foot occurred in a distance of about 2 miles. At the last point on the gradient traverse near the junction of the deviation with district No. 2 of Government survey at George there was an accumulated difference between the gradient traverse calculated on the total distance and the flying levels of 7.16 feet. The flying levels closed with the Government datum at George to 1.60 foot. The closing point of the gradient traverse was actually the crossing of Zwart River, as seen by Fig. 1. The gradient traverse was continued to the point A, the line AB being taken on the level at a distance of 854 feet. All lines north of AB were taken on the level from the right bank of the stream, a staff-man remaining on each side. The point chosen for the crossing of the Zwart River is immediately

Scale, 1 inch = 800 feet.

Fig. 3.

CH

above a waterfall 60 feet high. The gradient traverse was again started at C with a fall of 34 feet towards George. The length of the proposed line round the diagram being estimated, the gradient traverse was continued down the Zwart Valley and along the mountain slope north of George. When a junction was made with the Government line at George it was found that the total distance from Afgunst Neck was sufficient to work in a 1 in 60 gradient with 50 chains to spare in length. When the Author started the 1 in 60 grade line from the summit traverse he did not expect to be able to work it in without having recourse to a zigzag on the mountain slopes north of George. The unexpected distance was obtained by the windings along both sides of the ridge of mountains which divides the Kayman River Valley from the Zwart River Valley.

Scale, 1 inch = 800 feet.

RAD S

Fig. 4.

2:1

TUNNEL 100 YO

2:1

2:1

RAD

Scale, 1 inch = 800 feet.

Fig. 2 shows a ravine in Zwart River Valley where, in order to round it, a tunnel 170 yards long had to be introduced. One side of the ravine, as seen by the radiating lines, fits a 5-chain curve well. Fig. 3 shows a ravine in Zwart River Valley; the line AB is on the gradient and across the ravine. The line BC is a "side shot" and on level, the side slopes of the ravine being 1 to 1 as shown by dotted lines. The actual slope of the bed of the ravine was found generally to be about half that of the side slopes. It was therefore easy to compute the height of the proposed formation level above the bed of the stream in the ravine. The height at this point was 50 feet, the limit for rock-filling. The amount of rock-cutting at each side of the ravine was adjusted to balance the filling. Fig. 4 shows a spur in Kayman River Valley where it was necessary to introduce a tunnel 100 yards in length. Fig. 5 illustrates a ravine in Kayman River Valley at a place locally known as "Charlie's Neck," which is a dividing ridge between the Kayman River Valley and the Zwart River Valley. A topographical plan was prepared some years ago by Mr. Ballott, Government Surveyor, George, proposing to tunnel through this ridge into the Zwart River Valley and from that point follow a gradient of 1 in 41 to George. This route was examined by the Government and pronounced impracticable.

Fig. 5.

TUNNEL

214:1

B

Scale, 1 inch = 800 feet.

The total curvature was 699 chains; the total length of the proposed route being 19 miles 25 chains. The straight portion of the line will then be 846 chains, or more than half the total length. The limiting straight between contrary curves was taken to be 150 feet. The 5-chain curvature was found to cover 21 chains for each mile of line. The length through Montague Pass, according to the Government survey, is 12 miles 64 chains'