15. It has been noticed in some mountainous districts that the clouds break against the highest main ranges, discharge themselves on the smaller ranges, and generally do not reach the inner ranges which rise on the high table-lands; therefore, consider whether by locating a railway or road near the latter, there is much less probability of a slip in the earthworks and less provision required for surface and flood waters.
16. The configuration of a district through which a railway or road must be aligned, may be such that its location becomes one more based upon placing it on soil which is less bad or treacherous, than upon firm or stable ground. This is especially the case in hilly countries contiguous to the sea or large rivers. It may be optional to construct the works upon low or valley ground upon the side of a mountain or hill, or close to the sea shore or a river bank, which may require continuous defence works to protect it from waves and erosion; or on table-land which, however, if impervious and retentive of moisture may act as a catchment reservoir between hills, and cause the ground to be always in a damp state.
The character of the soil, the magnitude, and especially the average height of the embankments, or the depth of the cuttings, the easy drainage and discharge of the rainfall, and an economically constructed, maintained, and worked line are the chief conditions to inseparably bear in mind in determining the location.
CHAPTER II.
The Probability of a Slip—Time of the most frequent Occurrence—Some Conditions under which Slips and Subsidences in Cuttings and Embankments may be expected in different Earths, such as Rock, Chalk, Sand, Gravel, Clay, &c., &c.,—Notes on the Slopes of Repose.
It is of importance to know when serious slips are most likely to happen and under what conditions they are probable, for the process of disintegration may commence immediately the earth is excavated, and be very gradual, although the soil may remain stable for many months, or even a year or two, because the earth has not had time to be affected to the point of instability.
The history of recorded slips appears to indicate that the most serious movements of earth and those most difficult to remedy occur in the following soils.
Drift Earth upon rock in sidelong ground.
Chalk Soils, as witness the slips in the early part of 1877 in the cuttings near Folkestone, and the more recent on the Calais-Boulogne Railway, and that in an embankment of chalk at Binham’s Wood, near Balcombe, in October 1853, when in a length of about 200 yards some 70,000 cubic yards of earth slipped towards a valley. Probably this is one of the most extensive recorded slips of a railway embankment of chalk; however, in this case the traffic was not stopped, but only delayed.
Clay Soils, especially the yellow clay; illustrated by the notable slip at New Cross, near London, when some 90,000 cubic yards of yellow clay moved upon the smooth surface of a shaly clay bed and covered the formation: also the brown, and boulder clay, and the lias clays, as witness the well-known recorded slips in the Midland counties of England, in which either aluminous or calcareous material may preponderate.