1843.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



47 



Fig. 1. 





Ittfajit-BivjEg amwi* ' 



"T"' 



lirent Embankment ut Hnnwell, Great Western Railway. 



-m 



The acclion, Fig. 9, gave the form of the terrace, and of the ground beneath 

 it during the further suhsidence; hut all under the cmhankment in Figs. 8 

 and 9, must he considered as inferential from the appearances ahove. The 

 rupture of the ground beneath the embankment, was indicated by the crack 

 near the upper part of the south slope. 



In a letter received recently from Mr. Bertram, one of the engineers on 

 the Great M'estern l!oil«ay, it was stated, that the Brent embankment had 

 Kuhfided very little for several years ; indeed from the nature of the material 

 there was naturally less sinking than in loosely formed clay embankments; 

 a coating of ballast from G inches to 9 inches in thickness, applied once a 

 vear, was found sufiicient for all purposes. The slips which occurred in em- 

 iiankments formed of clay, occasioned trouble at first, by their immediate 

 cflfect on the road ahove, and the difficulty of adding material to them. Mr. 

 Bertram had found in many such instances, in the London clay district, that 

 a temporary measure, of forming the softened mass which had slipped down, 

 into large raised beds or ridges from 8 feet to 12 feet wide, by dressing with 

 the spade, surface punning, &c., had the effect of keeping rain-water out, 

 allowing I be raised parts to dry, and retaining the mass in its place, until 

 better weather and matured arrangements, permitted the more permanent 

 proceeding, of forming an extended footing and working up the mass with 

 additional material, so as to fill up the space with an increased slope. 



When the Acton cutting slipped about three years since, Mr. Bertram was 

 induced (from the difficulty of bringing gravel to the spot, and the quantity 

 of surplus stuff in the cutting), to try burnt clay for the drains, for forming 

 an open backing to collect water, and also for mixing with tlie soft clay in 

 punning up agaui ; from what he then saw, he gave a decided preference to 

 that material, over any kind of gravel, for mixing with clay, to retain it in its 

 place. When gravel was used, there was generally a slight subsidence and 

 opening at the top, hut witli burnt clay neither occurred. The usual system 

 pursued, was to form with that mixed material, continuous abutments and 

 revetments, upon the original face, and in all cases to make sure of thorough 

 drainage from the back. 



He had always been able to trace an immediate connexion between the 

 courses of septaria and the slips at Acton. Those courses were not suffi- 

 ciently open to act as natural drains : he had made many surface and deep 

 drains leading from them, but the quantity of water drawn ell', was not equal 

 to that which was obtained by the means before described. 



Pg.lO. 



A, .Snoil bank. 



B, GraTtl. 



C, Clay. 



I), Brii k.ranh, nith lonmy sind. 

 E, CUy rarleimted ivllh uatcr-markl. 

 K, Dark coloured clay. 

 G, Upraised bank or dam of clay. 



At Ruscombc, he Iiad removed the gravel stratum from the top, laying 

 bare and well draining the surface of the clay, using the gravel as a footing 

 or buttress below, at such portions of the cutting as had been forced up by 

 previous slips; when there was under drainage from longitudinal culverts, 

 that plan answered very well. 



At that portion of the Sonning cutting, which slipped so suddenly 2 years 

 ago, the stratum of gravel was fouiul to be broken into, by an upraised bank 

 or dam of clay, at G (I'ig. 10), which after much wet weather, kept a rescr- 

 voir of water penned back, until it broke out the mass of clay, down to the 

 next stratum at D, and so out at E ; the dam G had been cut across at dif- 

 ferent points in the slope for the purpose of drainage, and when that was 

 done, all that portion of the cutting became particularly dry. A drain was 

 led from the back of the dam G in ilressing off the slip. Tliat continued to 

 bring away a great deal of water, which previously had some other outlet, 

 over the lowest point of the bank. 



Mr. Sibley thought, the causes of the subsidence of the llanwell em- 

 bankment were very obvious. In laying out the foundations of the Lunatic 

 Asylum, in that immediate vicinity, and in the formation of a deep sewer, 

 with a soil pit 20 feet in diameter, and 20 feet in depth, at the side of the 

 Brent, he bad ample opportunity for examining the strata, and it ajipeared 

 to him, that had a trench been made in the direction of, and at the foot of 

 the embankment, the marshy piece of land where it was situated, would have 

 been sufficiently drained, to enable it to carry the weight of the mass laid 

 upon it. 



The trustees of the Uxbridge road had their great store of gravel m fields 

 to the west of this embankment, and excavations had been going on therefor 

 about half a century. The springs in that neighbourhood, accumulated in a 



^ ,fm\j^mW^!^H'^, 



_^dF 



y' '.<^M 



TransYcm 'pction of Sonnin 5 Cuiiius, Great Weatetn Railway .—The dotted lines show the form of the tllp.) 



