Earthwork Slips and Subsidences upon Public Works / Their Causes, Prevention, and Reparation

by John Newman

CHAPTER XI.

Notes upon the Failure of Dock and other Walls from a Forward Movement of the Earth Filling or Backing.—Consideration of the Causes of such Action and some Preventive and Remedial Measures.

The most serious consequences often result from slips or “boils” of sand or loose soil during the construction of docks, and from the sliding forward or subsidence of quay walls, as in addition to the great expense of restoring the damaged wall must be considered the loss of receipts caused either by delay in opening the docks or from interference with the traffic. In this chapter no attempt is made to lay down rules and to declare that if they are followed failures from pressure, subsidence, or a slip of earth, can be obviated by careful attention to them. Many of the most renowned engineers and contractors throughout the world, whose scientific attainments and practical knowledge are universally and deferentially acknowledged by those competent to form a judgment, have had anything but pleasant experiences in the construction of docks and similar works.

The two banes of dockworks may be said to be:—

1. Defective foundations.

2. “Boils,” or the rushing up of loose soil and water in the foundations.

To say that a failure of a dock-wall cannot but be the result of defective engineering is a statement which it would be difficult to substantiate. To disturb the earth below the required depth of the foundations at intervals over the area of such works in treacherous soils in order to ascertain its character would simply be to offer a general invitation to it to commence movement, vide p. [208]. It would be worthy to rank with the erudite report of a local committee formed to declare the best means of preventing floods in a district, which, after deep consideration reported: “That the committee were of opinion the floods would be permanently removed provided the engineer diverts all rivers, streams, watercourses, and ditches, to the next valley, and closes the up-stream hole in the hill, or causes the water to be discharged elsewhere, and reduces the rainfall within the limits required from time to time by the inhabitants of the district.”

Inasmuch as it is impracticable to ascertain the exact character and condition of any earth over a large area in such a location as that usually occupied by a dock, and to be certain that no unstable seams exist, and as the forces of nature are infinitely more powerful than those of man, it cannot be said that a failure is absolutely impossible. All that can be done is to render improbable any slip or subsidence by the employment of every known means to promote stability justified by the importance of the work and with a due regard to economy.

Dock-works are peculiarly liable to slips and subsidences because of their position necessarily being near to the sea, an estuary, river, or lake, and therefore in usually treacherous superficial soil of an insecure, water-charged character. The foundations are also at but comparatively little depth below the lowest water-level, and although they are not subject to the frequent vibration railway cuttings and embankments have to withstand, they are jarred by blows from vessels and falls of heavy goods, vibration from cranes and machinery, and also are liable to be suddenly and unevenly surcharged. The earth backing may also become quickly water-charged, and a force caused by the tide varying in height has to be considered in earth embankments for docks or reclamation purposes, particularly in sandy or loose soil, namely, that of the foundations being “blown” up by the upward pressure of the tidal or permanent head of water above the level of the bottom of the foundation; thus inside an enclosure embankment, or cofferdam, a depth of water of 30 feet would give an upward pressure of, say, 0·85 ton per square foot, and when the excavation has extended to a depth of 30 feet an insistent natural weight of a column of sand 1 foot square upon the foundation has been removed by the excavation equal to 1·65 ton, vide Chapter V. Thus the upward and relieved pressure 0·85 + 1·65 equals 2·50 tons per square foot, 0·85 ton of which constantly varies. It is obvious that in very permeable soils, such as sand, the hydrostatic pressure will be most felt, and should the range of tide be considerable the strain upon the earth will change and an active disturbing force be constantly present. This varying disturbing agent must be seriously considered in all dockwork or excavation near the sea or a tidal river, and also approximate to water, for there will always be danger of the water percolating, and when the foundations are not sufficiently deep of their being undermined. The weight and depth must therefore be sufficient to balance or exceed any upward pressure of the water in the soil due to its head level. The bottom of the foundation will almost invariably be below the depth at which water is usually found in the locality, and it should not be forgotten that sooner or later the earth upon which dock-walls rest is nearly certain to become wet and perhaps saturated, and that when saturated any additional water must flow away. In sandy or loose soils this may become dangerous. Care must also be taken in loose soil that it is not loaded by any temporary works so as to force the earth in any direction, or a slip will occur and any trenches may become filled, and every effort should be made to cause equal loading and pressure and to prevent unequal settlement.

The selection of the site of a dock is generally confined within narrow limits such as railway connection, roads, location of the want of dock accommodation, currents, the proper position of the entrances, &c., &c.; nevertheless the nature of the soil should be considered, for upon a tidal river a few hundred feet one way or other may cause the works to be upon various kinds of earth each more or less treacherous, and when intermixed perhaps more unstable. Even upon one side of a comparatively narrow river the earth may be in a much more stable condition than upon the other; and when an old watercourse or a dry ditch passes across the site of a dock it must not be disregarded, as failures and damage have occurred because the same section of a wall was continued throughout. It is then necessary to adopt a system of construction suitable for softer or less stable ground, and especially so if the wall has to be built upon a firm stratum overlying sand, as a diminution in the thickness of the bed may cause a settlement and the wall to be pushed forward; for there is always danger in sandy soils, unless the superimposed layer is of considerable thickness, that the latter may be pierced and the whole site be in jeopardy from a flow of the sand.