46 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



Kg. 5. 



[Fkbruart, 



TraiiBTerse Section. — Brentwood Hill Cutting. 



having an outfall into the open drain at each end. On the lower side a 

 puddle wall was raised, and at its back, a dry rubble wall with straw above 

 it, to prevent the sand and loam from washing into and choking the inter- 

 stices. This plan was simple and inexpensive, and was stated to have an- 

 swered its intended purpose. With respect to the general question of slips 

 in cuttings, Mr. Bruff was convinced, that want of thorough drainage was 

 the proximate cause. In the Brentwood cutting there had been two slips, 

 which, in his opinion, arose from the surface water being checked by the 

 spoil-banks, and being allowed to soak through the surface down to the 

 slopes. In most cases where slips occurred in side-lying ground, it would 

 be observed, that the upper slope almost invariably gave way first. In em- 

 bankments on side-lying ground, slips generally occurred first on the lower 

 side, which might be ascribed to want of friction, as well as to the agency of 

 water. The eft'cct of a drain above and near to a slope, in an excavation, 

 had not hitherto been sufticiently attended to ; in inclined ground, a rubble 

 catch-water drain should, in his opinion, be formed parallel to the whole 

 length of the cutting, at a distance from the upper edge of the slopes, vary- 

 ing with the depth and nature of their material ; in a depth of 20 feet, he 

 thought that it should be set back at least the lengtli of a chain. Spoil- 

 banks placed near to the slopes of cuttings, also appeared to him to be in- 

 judicious ; in the cases of the Croydon and the Blisworth slips, which had 

 been mentioned, be had no doubt, that the spoil-banks contributed as much 

 to produce slips, by checking the drainage, and by the increased quantity of 

 surface-water they threw into the slojies, as they did by their great superposed 

 weight on the edge of the banks. It was a curious fact, which he deemed 

 worthy of notice, that serious slips seldom occurred, until 2 or 3 years after 

 the completion of the earthwork ; the large sums usually left by contractors, 

 to cover one year's risk of maintenance, might therefore, he conceived, be 

 dispensed with. 



CuUings and Embankments — Great Western Railway. 

 Mr. CoLTHURST exhibited and described, three sections of the embank- 

 ment across the valley of the Brent, at Hanwell, on the line of the Great 

 Western Railway, (Figs. 7, 8 and 9.) 'ihe embankment, which was formed 

 of gravel was 04 feet in height ; it rested on vegetable soil, beneath which 



was a thickness of 4 feet of alluvial clay ; then occurred a bed of gravel, 

 varying from 3 feet to 10 feet in thickness, resting upon the Loudon clay, 

 which was traversed in all directions by slimy beds or joints. The surface 

 of the country sloped gradually towards the Brent, which was at a level of 

 about 20 feet below the south side of the embankment. Tbe subsidence of the 

 embankment commenced during the night of the 21st of May, 1837 ; the 

 next morning the foundation was discovered to have given way, and a mass 

 of earth, .'lO feet in length by 15 feet in width, was forced from beneath the 

 north or lower side of the embankment, towards the Brent. For four months 

 this protruded mass increased in dimensions, and the subsidence of the em- 

 bankment continued, until the surface assumed an undulating outline, which, 

 on being cut through, showed that tbe suljacent beds corresponded accu- 

 rately with the curvatures produced at the surface by the disturbance. The 

 state of the seams or strata beneath the surface, was ascertained by sinking 

 trenches at right angles to the embankment, to the full depth shown in the 

 sections. Figs. 8 and 9. 



The symptoms of failure in the embankment, at this period, were confined 

 to a subsidence of about 15 feet, with a fissure extending all along the top 

 of the south slope, at the side opposite to where the foundation had yielded. 

 From the dip of that fissure, Mr. Colthurst inferred the nature and inclina- 

 tion of a rupture of the ground under the embankment, as shown in tbe sec- 

 tions. Figs. 8 and 9. 



Immediately on the commencement of the slip, Mr. Brunei directed a ter- 

 race to be formed, on the swollen surface, at the north foot of the embank- 

 ment; the weiglit of the mass thus placed, succeeded effectually in stopping 

 tbe further progress of tbe subsidence, wliich up to that period had exceeded 

 30 feet. The swollen ground extended over nearly 400 feet in length, by 

 about 80 feet in width, and was elevated nearly 10 feet, with a horizontal 

 movement of about 15 feet. The general disturbance ranged to a distance 

 of 220 feet from the foot of tbe slo|)e, towards the river Brent, the south 

 bank of which was forced forward about 5 feet. 



The section. Fig. 7, showed the position of the strata, at the time of the 

 forming of the embankment. 



Tbe section. Fig. 8, showed the state of the strata, when the sliporswoUea 

 ground was being covered by the terrace on the north side. 



