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

by John Newman

In addition to other recommendations, the steam navvy usually excavates the earth in small pieces of nearly uniform size, and as it will perform the work of many men it may be most useful in treacherous soil, as it may enable a cutting to be excavated in dry weather and be protected before the commencement of the wet season. The slopes should be excavated in such earth as soon as possible after the gullet has been removed so as to prevent a slip, and it may be necessary to close-sleeper or consolidate the ground in order that the weight of the machine can be supported upon soft ground.

CHAPTER X.

Notes upon the Location, Preservation, and Protection of Sea, Estuary, Reclamation, Canal, and Reservoir Embankments of Earth constructed to Contain or Expel Water.

In the first place, care should be taken in determining the site of an embankment across an estuary that there shall be no concentration or alteration of the general direction of the currents, or scouring action will be created; for the erosive and other deleterious effects of wave action upon a shore are influenced by the angle at which they are impelled against it. A prudent course to adopt is to carefully preserve the usual channels by means of bridges, particularly in ground of a loose character, such as is usually found in partly landlocked waters; for if the velocity is increased, the earth which has been deposited by the original current being reduced or impeded resulting in the suspended matter in the water falling to the bottom, will be subject to a force that will again disturb and cause it to return to its previous suspensory condition; and any disturbance of the normal currents may destroy the equilibrium of stability and alter the flow, and when they are affected it may be most difficult to restore them to their original state, for water will always endeavour to obtain the easiest channel.

It is essential to know the heaviest flood discharge of any river that may flow into an estuary, the greatest depth and velocity of the river, the normal and flood channels, their sectional area and direction, and the extent and shape of the catchment area, so as to establish the required opening to give the natural waterway; for it is important not to interfere with the tidal capacity of an estuary or the volume or flow of any upland waters into the sea, as a navigable channel may become filled with silt, especially when the shore is flat or sandbanks exist; and upon a sandy coast an embankment across an estuary with openings for a navigable channel will probably cause it to become difficult to navigate and, perhaps, impossible, without constant dredging and other works of maintenance, as an embankment may obstruct and deflect the currents and prevent them carrying away the suspended matter. An open viaduct instead of an embankment is almost always to be preferred, and may be necessary; for the power of a current to scour or move particles is greatly augmented by a small increase of the velocity, and the earth may be in such a delicately balanced condition that any increase of scouring action may destroy the seat of an embankment.

To prevent leakage and scour of the base of an embankment near a river resting upon loose soil, curtain walls are sometimes inserted upon both sides extending to a considerable depth, thereby affording security against an embankment merely resting upon a mound which may gradually erode, with the result that it must finally slip and be destroyed; and it may happen, unless the foundations are carried down into impervious soil, that water may escape underneath and undermine it; such action is obstructed by carrying the slopes a few feet below the ground so as to prevent through surface percolation.

As a rule, an embankment across an estuary with one or two openings in it is to be avoided, and it should be most carefully considered whether it will not be better to expend a larger sum and erect a pile viaduct which will not interfere with the currents or channels and only require ordinary precautions to be taken against erosion, instead of depositing an embankment with openings at the channels and the necessary protective works which may consist of pitching the slopes, covering them with fascines, mattress work, or sods, erecting short or long, low or high, groynes, as the former may be ineffectual in causing a deposit or a shoal in front of the toe of the slope and in preventing scour, for in loose silty and sandy soil of considerable depth and not sufficiently firm to resist erosion, they will probably fail by reason of the space between them being washed away; and a complete covering of the foreshore may be requisite, or a protecting apron of homogeneous impermeable soil, and continuous training walls to prevent the creation of shoals: all of which protective works will constantly need to be repaired, and the failure of any one may cause not only an embankment to give way but the remaining preservative works. Undoubtedly there are many estuaries upon which if an embankment even with many openings for channels had been deposited, it would have failed, the soil being in such a tender state that the least additional weight upon the surface would destroy the equilibrium. Also interference with the littoral currents is a risky operation in any but a hard rock bed as regards the foundations, quite apart from injury to navigable channels. When there are numerous ditches, creeks, or channels crossing the line of a proposed estuary embankment of variable stability, especially should hills be near and the earth be so porous as to be saturated every tide, without doubt the safer plan is to erect a low viaduct, as the old waterways will be an endless source of trouble should the ground be anything but firm clay, and interference with the currents will not be preventable, as either their velocity will be increased or decreased, the result being scour or fresh deposits which will affect the channels; and such alteration may change the direction of the motion of the waters, which must be prevented in front of an estuary embankment; or waves will be created by the water travelling and rushing over shoals, for the even configuration of the bottom is a wave lessener, and should there be a deep channel near an estuary embankment severe wave action may be created. The piers or piles of such a viaduct should be cylindrical, thus offering no flat surface for the waves to break against, and yet temporarily to divide them without serious shock to a structure. When of very considerable length, a projecting embankment deposited upon a flat shore of a tidal estuary has caused a heaping up of the water on one side at low tide, and therefore it is unequally strained. This was found to be the case with an estuary embankment on the Scheldt, 2½ miles in length.

The form of the slope has also to be considered, but all the principles that determine the best profile in each case of a pier or breakwater do not necessarily apply to an embankment in an estuary. The recoil of the waves washing away the ground in front and at the toe is particularly to be guarded against, and the action of spray or a broken mass of water falling upon the formation, as also the direct action of the waves. A concave form should not be adopted throughout, as waves roll up until they approach the top portion, when they turn over and fall upon the flatter portion and often breach it, but when the face is straight, excepting for a few feet at the toe, the waves are diffused in travelling up, although they proceed to a higher point upon a flat slope than a steep one, and the recoil is greatly diminished, but with the view of avoiding direct wave action, when the ground is inclined in front of a sea or estuary embankment, it is well to curve the lower part of the slope and to make it cycloidal to the surface of the ground and the slope for a little distance, so as to reduce obstruction to a minimum. The method of making a level terrace or stepping the slope instead of a curved face has the advantage of checking the rising of the sea up the face, altering its direction and acting as a wave-breaker, and also combines these effects with giving as large an area of the base as can be obtained by a curved face, and in bringing the centre of gravity of the cross section of the embankment nearer to the seat, but the face must be securely protected. The width of the cess should increase according to the degree of exposure. If an embankment consists of earth the slope of the cess should be from 6 to 10 to 1, or the system is better avoided and a continuous face adopted.

Short groynes will often protect the toe of an embankment and prevent any longitudinal current undermining it. As a rule, in a tidal estuary the cost of protecting the slopes is considerable, and more above low-water level than below it. In adopting groynes formed of single or double rows of piles, in order to prevent erosion of the toe and a slip and subsidence in an estuary or sea embankment, the littoral currents must be considered before determining their direction and position, the object of their erection being to prevent the waves, and especially the prevailing waves, from scouring the shore, and also to cause a general deposition of shingle, and, therefore, they are usually placed at an angle to the set of the waves, so as to cause the latter to be diffused. The angle will vary; the best guide is to examine the effects of any that may be erected in a similar position to that to be built. An angle of 50° to 70° with the foreshore, in a leeward direction is frequently adopted, and with respect to the distance apart, this depends principally upon the direction of the current and prevailing wind, contour of the shore, degree of exposure, and the length of the groynes. When the whole of an open coast has to be protected, no natural defence existing, the line of the prevailing set of the current and wind on a straight shore can be set off at the end of a groyne, and before the point at which it meets the foreshore another can be erected. They are generally successful when properly placed, and are most frequently straight; if not, they have a concave face to the direction of maximum eroding force; a convex must be avoided, as it will not permanently retain the deposits; their practical effect being that the breach is heaped up or retained upon the prevailing wind, set of current, or wind-wave side to a height of some feet above the leeward side, therefore, a groyne should be so constructed that planks can be added to it as required. They have been proved to prevent the formation of bars when judiciously located, but their success greatly depends upon their proper position and direction, or in loose soils such as sand they may cause a deposit upon the windward and prevailing current side, but an erosion and falling away upon the other; and when placed in front of a reclamation embankment upon shifting sand until the ground at the back of the embankment becomes dry by the tidal waters being excluded, or the surface is impermeably coated, they may be of comparatively little use to prevent a slip or movement of the shore; for as the tide recedes below the level of the toe of the slope, a seaward flow of the tidal and land waters will be created in very porous soil under the seat of an embankment, and may continue until the tide returns and rises to the level of the ground, thus causing the shore to be constantly changing place and permanently established accumulation impossible. Short transverse spurs have been adopted to lessen this action but their effect is hardly noticeable. As failure of a groyne will probably cause erosion of the foot of an embankment and a slip, a cheap and effective means, attested by the experience of nearly half a century, of preserving the timber from the attacks of most marine worms may here be named. It consists in scorching the piles, thus preventing fermentation of the sap, and immediately tarring them: also in placing the wood in the opposite direction to that in which it grew; the latter operation has been found to increase the durability 50 per cent., the reason, it is believed, being that the capillary tubes in the trees are so adjusted as to oppose the rising of moisture when the wood is inverted.

In excavating for an enclosure embankment, the earth should not be disturbed nearer than is economically necessary, and a cess should be left of about 40 to 50 feet in loose permeable soil, the width being governed by the character of the earth, the depth of the excavation, and the height of the embankment. Should dredging have to be executed near an embankment, a considerable distance should be left between the toe of the slope and the line of operations. Before commencing an estuary or enclosure embankment, it is advisable to notice whether a deposit is left upon the shore by the incoming tide, and to ascertain whether it forms in some degree a protective covering, for if this should be the case, any increase of the velocity of the flowing water which might be caused by dredging or a concentration of the littoral currents should be avoided, or the tidal matter in a state of suspension will not gradually sink to the ground. The tidal deposit, although the earth forming the embankment should not be tipped upon it, may also be of importance as tending to prevent or lessen any percolation of water through the foreshore and under the seat of an embankment; for instance, on the Nile, the deposited slime is found to make a practically watertight covering on the loose sand. Mr. Thomas Stevenson has also stated that, according to the depth below the surface of low water that mud reposes, may be approximately judged the force of wave disturbance and degree of exposure; the less the depth, the less the power possessed by the waves.