CHAPTER I.
Introduction—General Considerations—Enumeration of the Primary Causes of Slips and Subsidences in Cuttings and embankments, and Earthworks Constructed to Contain or Exclude Water—Some Dominant Principles to be Remembered in Determining the Location of Earthworks.
Earthslips and subsidences may be caused by the terrible power of an earthquake or other dreaded subterranean destroying force, upheaving, cracking, and shattering the earth’s crust and dealing death and havoc in its awe-inspiring course. They may also originate from the untiring efforts of the meanest rodents or the most minute crustaceous animals burrowing passages for aqueous action, the chief agent of the instability of the surface soils of the earth.
The ceaseless mutability of the created elements has been thus magnificently described:—
“For know, whatever was created needs
To be sustain’d and fed; of elements
The grosser feeds the purer, earth the sea,
Earth and the sea feed air, the air those fires
Ethereal, and as lowest first the Moon;
Whence in her visage round those spots, unpurg’d
Vapours not yet into her substance turn’d.
Nor doth the Moon no nourishment exhale
From her moist continent to higher orbs.
The Sun, that light imparts to all, receives
From all his alimental recompense
In humid exhalations, and at even
Sups with the ocean.”
It is the constant absence of peace and rest in the earth that produces instability, however great, however small, for the disastrous landslips that have occurred in Switzerland and other parts of Europe, in India, and recently at Quebec, and in all regions of the world were caused by the same disintegrating operations as those which generate an earth-slip of comparative insignificance. In proceeding to a practical consideration of earthslips and subsidences, it may be well to call attention to the complexity of the subject, the character and conditions of earth and the impairing elements being so very variable and numerous that it is impossible to determine any rules even for a particular soil; and, moreover, it is necessary to separately consider slips in cuttings and those in embankments, as movement is somewhat differently created, and it does not necessarily follow because earth stands well in a cutting that it will do so in an embankment, or vice versâ.
It may be said every kind of earth will slip or weather under certain conditions, even the hardest rock if superimposed upon an unstable stratum; therefore, some of the main questions to be considered are:—
I. The Probability of the Occurrence of a Slip.
II. The Effect of a Slip.
III. Should every Precaution be taken to prevent a Slip when a Cutting is being Excavated or an Embankment being Deposited; or is it better to Repair a Slip as it happens?
It is obvious in railway cuttings and embankments a mere crumbling of the surface may be disregarded, but in dock, canal, or any works containing or expelling water, the smallest movement, crack, or aperture must immediately receive due attention.
In order to effectually remedy a disease it is necessary to ascertain its character. Many of the primary causes of slips in cuttings and embankments are, therefore, here enumerated; but, of course, they are not named in their order of importance, which cannot be established.