Notes upon Slips of Earth, Subsidences and Movement in Foundations caused by “Boils” or an Upward Rush of Water in Loose Earths.—Consideration of some Precautionary and Remedial Operations.

With regard to “boils” in sandy soils and the general drainage of the site of dockworks, operations should be commenced as long as possible before the works of construction, so as to lessen the probability of the occurrence of slips, subsidences, and movement of earthwork. In order to reduce infiltration, it is well to make a trench round the area of any ground that has to be excavated, which may embrace the whole site. A system of drains and conduits should be established within it, and at the lowest level a sump at a convenient position, and to a depth a few feet below that of the work; it can then be made the chief pumping station of the dock; but all drawing away or flow of the earth must be prevented: and to obviate erosion of the drains, they may require to be rough lined with an impervious covering such as clay. The sump should consist of an iron cylinder with proper provision against a run of soil. In all loose earths the pumping station should be some distance from buildings or roads so as to avoid any settlement.

When a choice of sites exists and the position of a dock is not absolutely fixed, it may be possible to have the foundations of the whole work in one kind of earth; if otherwise, one portion may be stable and another unstable, always treacherous, and liable to slip and subside. In any case settlement is not likely to be equal, and therefore a foundation which is well able to sustain an evenly distributed load may yield from unequal strain and excess of lateral thrust. Consequent upon the situation of docks, the superficial beds upon which they have to be erected frequently vary in stability and reliability; and the location being altered in any direction may result most seriously; the earth upon one side of a comparatively narrow river being stable in character and on the opposite bank most treacherous. Difficulties often arise in foundations, especially in sandy soils, from making borings and trial pits too near the important parts of the work; they should be made as reasonably far away as is convenient. In boring, a sand flow may occur, when it may be necessary to fill the bore-hole and sustain the soil by a covering, or by consolidating the sand by means of Portland cement, and then, perhaps, reliable operations may be continued. Borings should be considered as unreliable if merely superficial; in any case of importance they should only be trusted for the place where they are made, and not as indicating the nature or condition of the soil over a considerable area. When pits cannot be sunk, it is desirable that in a suspectedly treacherous site the bore-holes should be at every 200 or 300 feet.

Excavating pits, using test-bars, and driving piles are some of the methods of determining the character of foundations, but care should be taken to ascertain in boring that boulders, or thin strata of hard gravel, are not mistaken for solid rock. In sand, mud, or soft clay they can be made by means of an iron pipe and the water-jet system. Experience has proved that boring with an auger is not so reliable as boring with a tube, such as is used for artesian wells. In the case of augers, when boulders are encountered, further boring is usually arrested in that place and another bore-hole has to be commenced. Trial pits, where practicable, should be preferred to boring, and they should be sunk to a depth considerably below the lowest level of the intended foundations, and then they may do for sump holes for pumping operations. In testing ground by borings, several should be made, as one hole might encounter a boulder or some hard soil, such as indurated clay, and the latter may adhere to the auger and arrest its progress; the specimens then brought up, being crushed and pressed together, will appear to be firmer than the actual condition of the ground.

Having briefly referred to the preliminary drainage, and some methods of ascertaining the nature of the ground, the former to lessen, and the latter to aid discovery of the character of any probable upward lushes of soil: “boils” in foundations are more specifically considered. It may be said that they generally proceed from an impervious top stratum being pierced, thereby tapping the water in a pervious bed which may be imposed upon another impervious layer; for the upper bed being excavated removes the weight upon the lower strata and induces a flow of the previously confined water.

The source of disturbance may be either from surface-water in the top soil or from deep underground springs, depending in great measure upon the extent of the catchment area and head-water level of the district, as its quantity and uplifting pressure will be principally governed by them. When the soil is in a delicate state of equilibrium it only requires a slight deteriorating alteration of the normal condition to initiate a movement. The disturbance of the ground may be merely superficial, nevertheless its effect may be sufficient to start a “boil,” although the chief cause potently exists at some considerable depth and distance.

“Boils” produced by simply surface-water require one system of treatment; if from underground waters, another remedy is necessary. Every effort should be made to know the reason, trace the origin, and to ascertain the power of the disturbing forces. It may be possible to determine them, although generally it can only be done by deductive reasoning and logical inference. In any event immediate action is invaluable when a “boil” appears. Weighting the ground around a “boil” with an impervious mass of clay, and the insertion of a stand-pipe will indicate the level to which the water will rise, varying according to the seasons, and will give an approximate idea of the head supply and pressure; care being taken that the pipe does not become obstructed and that the water has a free flow. It may also be ascertained by the insertion of pipe-rods to different depths, and by noticing the effects, such as the rapidity, the quantity and the character of the discharge. There may be an appreciable difference in the height to which the water rises in various stand-pipes; if so, its flow is obstructed or the source is not identical. An examination of the colour and nature of the suspended matter in the water and a comparison of it with the strata may show its source, but it is not always reliable unless the same colour is maintained for the lowest depths.

A perusal of Chapter II. will indicate some situations in which “boils” may be expected. The conditions under which they may appear are so numerous that it may be stated they generally occur in any situation when a layer of loose soil has a superimposed bed of more or less impervious boil upon it which is perforated by the excavation for a dock or other work; and especially should a water-bearing stratum, such as chalk, underlie a sand stratum; also when a water-bearing stratum is superimposed upon sand which lies upon an impervious bed such as clay, as water will percolate to the sand; and when sand overlies a water-charged stratum; or at the outcrop of chalk hills near the site; or where sand is below an impervious layer of clay and the latter is tapped. The “boils,” of course, become more serious as the head of water increases, and also when the strata dip towards the site.

An impervious bed may be of such thickness, that when the lowest foundation is excavated there may be no fear of water from any underground source being forced up through it; however, unless the thickness of the upper layer is known to be nearly uniform over the site of a dock, which is seldom the case, the water pressure, aided by the weight of the structure, may separate or loosen the soil at a weak place, and then a “boil” will be the result. The thickness of the crust required, which may be anything from about 15 feet upward, may be approximately ascertained by weighting the earth considerably above the load it will permanently have to sustain, and by watching the subsidence and general effect. Great care should be taken not to perforate the firm stratum, as danger will at once ensue should loose soil, such as sand, be tapped, for cavities will then be produced causing subsidence, probable fracture of the firm stratum, “boils,” and slips of serious extent. When a heavy structure has to be built upon such a soil, and it is impossible because of the expense to place the foundations except upon this superficial bed, experiments in the direction indicated should always be made, and any other means of proof that circumstances may allow other than by perforation of the stratum: in any case every effort should be made to reliably ascertain that the firm stratum is of uniform depth and character.

The discharge channel of any underground waters likely to disturb the foundations should be discovered, and when there are adjacent hills, by tracing their dampest part the probable direction of the surface flow may become known. Inquiry should be made to find out whether any borings have been made over the site in order to determine if the “boils” have been artificially created or are natural blow-wells, in which the water rises over the top and can, perhaps, be led away by gravitation. It is advisable to fill every bore-hole with Portland cement mortar before any excavation is commenced, or they will burst out when the surrounding ground is disturbed. Bags containing shot have been used to help to close a hole, but a preferable method is to insert a pipe down the bore, it having been previously cleared of all dirt; and to fill it with neat Portland cement mortar, the tube being raised as the hole is filled. Such a method, although successful with small holes, is useless in the case of “blows” of the ordinary size. A permanent shaft or cylinder is then necessary.

With regard to the treatment of “boils” in foundations, when in the possession of the information hereinbefore named it may indicate a remedy either for “boils” in which the cause of disturbance is at a considerable depth and the head and fall of water moderately large, or when the agitation is merely superficial, which may result from the range of the tidal waters or the want of surface drainage.

When a “boil” is of the first order, it is useless attempting to simply stop it by force and prevent the issue of water, as such a method of procedure would result in merely diverting the disturbance, and in addition leave a weak place in the foundations; however, if many “boils” appear, and the disintegrating agency of the water is removed either by conducting or diverting the flow from the site, they can then be filled, and it should be simultaneously effected, or any mere exudation in the whole of the “boils” may become increased in one or two to a flow sufficient to cause a slip or instability, and any concentration of the discharge may produce movement of the ground. Small springs or “boils” have been sufficiently arrested by depositing clay over them, excavating it, and putting in the foundations very promptly and before the “boil” burst out again; such treatment, however, may lead to the backing of a wall becoming saturated and the water being dammed up: on the other hand, the “boil” may be sealed and the water flow away in the original underground channels; also when a “boil” is tapped it may induce an increased flow of the underground waters, for they will find the course of least resistance; and should they reach a fine sand stratum will filter through and set it in motion, provided the head is sufficient to overcome friction; and the support rendered to any overlying stratum will therefore be destroyed.

In the event of there being only one “boil,” but that of important extent, perhaps the best way to proceed is to place clay upon the surface for a few feet around it, the weight of which should not be less than the normal pressure of the earth removed by excavation, vide Chapter V.: and by the insertion of a cylinder, with a properly designed bottom, to prevent the issue of soil to a level some feet below the lowest intended excavation upon the site, or to the depth at which the greatest flow of water is obtained; care being taken that no run of sand or soil is allowed and that the water is not charged with earth, either by a thick gravel layer until the sand ceases to pass through it, or by other means, remembering that the motive power of running sand is that of the pressure of water. The flow should be gently led away, if necessary, to a sump, and discharged by pumping, thus draining the works. The sump should always be lined, not only to prevent any flow upon or into the surface causing saturation, but so as to control and regulate the water some feet below the lowest level of the foundations, the great point being to draw downwards the water from the earth and then to carry it away. An iron or wooden cylinder sump is to be preferred to one constructed of sheet piles or timber-lined, as the soil may be so loose that any shaking or vibration should not be allowed, and a close joint is not easy to attain in timber pile work. It is well to deposit some clay puddle for a little distance round the sump after sinking it in order to weight the soil and steady the cylinder. When a discharging outlet has been made it should be maintained in perfect condition, or a slip or subsidence may occur from leakage or diversion of the flow. It may also happen that no discharge is necessary, for if the “boil” is only, as it were, of local extent and caused by a weak vein in otherwise stable soil, the water may rise to the head level and there remain; the equilibrium being restored which had been destroyed by reason of the upper strata of an impervious character being excavated. The removal of the normal pressure upon the loose soil to some extent, liberates the water, and the head ceasing to be balanced by it, the earth cannot do otherwise than allow water to upraise it. A calculation of the weight of the earth removed may give an indication within which will be the probable head level of the supply, and usually it is much below a head of water that would equal the normal pressure of the soil. Supposing the weight of the earth to be twice that of water, when 10 feet of soil is excavated before a “boil” began, its removal would approximately be equivalent to a 20 feet head of water.

The great point is to confine a “boil” within a certain space, and there to treat it and so prevent the appearance of others and interruption to the prosecution of the works. All agitation of the surface of sand, especially when a “boil” appears, must be prevented, as inducing a disturbed and more porous condition.

When the seat of a “boil” must be built over, the well system of foundations can be used as not only affording reliable support, but also preventing a “boil” extending, and allowing the flow of water to be discharged, care being taken that complete connection is made with a wall constructed upon any other principle. If “boils” appear over a considerable area, it is evident that the disturbance is general, and may either be because the site or the foundations of the works are below the level at which water is usually found in the district, or be produced by the rise and fall of tidal waters. When by the former, drains are required a few feet below the general water-bearing level, and they should intercept the percolation of the land waters and conduct them from the site of the works, or so gently localize them that they can be treated as before described. The land waters are the most troublesome because their volume, time of appearance, and duration of flow will vary, whereas the range of the tidal waters is known, and therefore the effects may be ascertained with some approach to accuracy.

It may be impossible to erect walls in very loose soils without encircling the site with sheet piles to prevent a run of sand, to lessen “blows” and to reduce the disturbance, as placing them upon an impervious stratum maybe prohibitory because of the expense: but the adoption of the well system of foundations down to a firm stratum, or at a sufficient depth to be stable, and the building walls thereon may cause the temporary pile support to be unnecessary and prevent a slip or subsidence. Quick erection and the least exposure or disturbance of earth are essentials in such cases.

By the use of Portland cement concrete, foundations can now be cheaply obtained with the ground in a wet condition, which in the days when it was considered only suitable for secondary works, would have required the employment of expensive methods of construction. It may be unwise to arrest the upward flow of the tidal waters in loose soil except by intercepting, lessening, or preventing it by shore protection, for the upward pressure of the head of water may “blow” up the earth and cause disturbance; on the other hand, should the “boils” be merely superficial and the head very little, a thick watertight covering of Portland cement concrete, when accompanied by the necessary drainage operations, may suffice; the surface water blows being covered, and the pressure of the water being overbalanced by the superimposed weight.

All works to prevent “boils” or to drain waters from the site should, if possible, be executed prior to the excavation, as the effect of draining can then be observed and whether any “boils” arise from land waters or from a tidal flow, and the earth will have had some time in which to subside and become consolidated. A month’s test will afford some criterion in a variable climate and when changes of weather are frequent; but should a long period of drought prevail it may be no indication of the state in which the earth would be in a wet season, or under ordinary meteorological influences; and further, the land waters may take a considerable time in flowing to the shore, depending upon the catchment area, character of the soil, position of the beds, whether the district is drained or not, and many other local conditions.

When the soil is firm clay, the locality of any leakage or water seam in it will usually be indicated by surface exudation over the water-charged vein; also in nearly all compact earth, particularly if of a clayey or loamy nature, and therefore the seat of disturbance may not have to be discovered as in looser or sandy soils.

Should a water-bearing fissure occur in rock and it be necessary to stop it, there being no danger of diverting the flow to another place upon the site of the works, it can be done by means of neat Portland cement mortar pressed down and kept in place by timber strips, weights or other means until it has set; the face of the fissure should be made as clean as possible, and the water-bearing seam be filled and weighted directly a leak appears, or the flow may become much increased and accelerated and a current of water be induced which may become of serious extent.

To avoid slips and movement of the ground when it is necessary that a water-charged bed of sand be excavated to enable a structure to be built upon a firm impervious stratum below it, sheet piles can be used or corrugated iron sheets, strutted at intervals, so as to prevent a run and slip of sand; and the space between them can be excavated, a Portland cement concrete foundation being inserted upon the firm soil, the wall being erected in lengths, and the piles or case being practically three-sided so as to leave no unsupported surface of sand, care being taken to prevent any run between the front and the back of the last built length of wall and the earth, so that each length is erected in a contained area.