Earthwork Slips in the Cuttings and Embankments of Various Railways, with their Causes and Modes of Treatment.
By John Barratt Squire, Assoc. Inst. C.E.
During the construction of the Meltham Branch Railway, near Huddersfield, between 1867 and 1869, several serious slips had to be contended with.
The first of these worthy of notice was in the embankment at Dungeon Wood (Plate 9, Fig. 1). Where the mischief occurred the embankment was 44 feet high at the centre line, but the ground sloped towards the turnpike road, and further to a small river, at an average inclination of 1 in 3. No apprehension was, however, felt by those in charge of the works that a slip would occur, as the side-long ground was apparently of a rocky nature, and the material to be tipped consisted chiefly of loose rag rock from the neighbouring cutting. The embankment was scarcely completed, however, before a considerable settlement was observed, attended by serious consequences. The turnpike road skirting the hillside at a much lower level began to move, both laterally and downwards; a row of cottages near the place soon became untenable; a toll-bar close upon the point most affected was wrecked as by an earthquake; and the ground below the road shifted, rolled up, and at one time threatened to block up the river. The prevailing opinion as to the immediate cause of the damage was, at the time, that a lodgment of water between the railway and the road had been disturbed by the railway embankment. To ascertain if this was the case, a heading was driven from the river into the hillside, under the road; but although a considerable quantity of water was met with, there was nothing like sufficient to justify this opinion. Fig. 1 shows what the Author believed to be the cause, viz., that the alluvial deposit, receiving the great weight of the railway embankment upon its wedge-shaped form, became detached, and slipped down upon the face of the rock. The railway embankment was little worse, and was made good with more material; the cottages were pulled down, the toll-bar was erected at another place, the road eased down to its altered level, and the fence walls were rebuilt.
The next slip of importance was within 4 mile of the above, in a deep cutting, and was attended with still more serious consequences (Plate 9, Fig. 2). It had, moreover, this difference from the foregoing, that the cause was apparent. The cutting was about 55 feet deep at the centre line, but in such side-long ground that, whereas the slope on the lower side only measured about 38 feet, on the higher side it measured 175 feet. The excavation on the lower side was almost entirely in shale, but on the higher side it was in shale only for about 20 feet above the formation level, all above that being in loosely bedded sandstone rock, dipping at an angle of about 30° from the horizontal, in the same direction as the slope of the hill. The consequence was, that the cutting had not long been completed before the shale began to perish by exposure to the weather, and the rock above it tumbled in. This action, it was afterwards discovered, was still further promoted by the shale extending under the rock, in a sort of dish. Thousands of tons of rock would fall in a night, portions of which were thrown into the valley for hundreds of yards, causing great consternation in the whole district. A mansion adjacent was considered unsafe to live in, its windows being smashed, the whole structure shaken, and much other damage done to the grounds, so that the owner was compelled at an early stage to remove his establishment. To effect a remedy, a retaining wall, with raking counterforts, was erected, covering the shale and supporting the rock. This wall was 12 feet thick at the bottom, and 4 feet thick at the top, about 60 feet in height, and 10 or 12 chains in length; its foundations were begun about 10 feet below the formation level of the railway; it was built of rubble masonry from rock found in the slip, and together with the counterforts contained about 17,000 cubic yards of material, and was spoken of as the largest retaining wall in England. The putting in of the foundations was attended with great difficulty, but was continued night and day, several reliable men being stationed at prominent points at all times to watch the hillside, to give alarm if they saw it breaking. The work was accomplished, however, without serious accident, and has proved a great success. Another troublesome piece of work upon this railway was the construction of the Netherton tunnel; it was 345 yards long, built for a single line, upon an inclination of 1 in 95, and was 67 feet in the soffit below the surface at the deepest point. The difficulties arose from a misconception of the material to be cut through, and consequently, to some extent, insufficient timbering; but beyond this the strata proved loose and treacherous, and were heavily weighted with some old buildings on the top, so that several subsidences occurred, which necessitated the section of the tunnel being altered. (Plate 9, Fig. 3.)