This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
If ever there is a time when the Clerk of Works has to be more constantly upon the site than at any other, it is during the digging and laying of the foundations. These are generally shown upon the plans as of certain widths and depths, while the specification gives power to the Clerk of Works to order any further digging that may be necessary. Whether this power be definitely given him or not, however, he is obliged to exercise it if occasion demand, for in foundation work emergencies arise at very short notice, and must be dealt with at once, always bearing in mind that anything of a serious nature should be immediately communicated to the Architect.
If trial holes have been made in advance, and the trenches be found to conform to what was anticipated, there is rarely much difficulty; but, for all that, watchfulness is essential. It has been known, for instance, for new work on an open field, where it had been ascertained that there was a good subsoil of gravel demanding no extra precaution, to be suddenly interrupted by the excavators opening up an old Roman brick kiln, occurring just under the corner of the contemplated building. Frequently, too, streams of water are met with just as unexpectedly, and these, unless they are attended to at once, will often swamp the whole foundations and do a large amount of damage. Even the smallest trickle of moving water must always be viewed with suspicion, and carefully provided against by drainage into a larger stream if possible, in addition to temporary pumping to keep the water down while the foundations are laid. It must always be remembered that, however small a stream may be, it will eventually sap a foundation if it be continuous, and possibly the result will be settlement and destruction. Soft pockets, too, in an otherwise firm subsoil will frequently occur, and must in all cases be dug right out, and filled in either with concrete or hard core - preferably the former - while in serious cases piling may have to be resorted to. If these pockets occur on the sides, and not at the bottom of deep trenches, it often suffices to support the weak places by strutting and planking till the permanent work is completed; but this must be done promptly, else greater expense will be incurred in entire removal of the loose stuff, and its replacement by carefully rammed core or concrete.
When building upon old sites there are many other difficulties to be contended against. One may be on virgin soil one moment, and the next moment find that one is cutting across an old trench which has been filled in with rubbish. It may not be necessary to carry the foundation down to the bottom of this trench, but all the same it ought to be dug right out, so as to ascertain to what it leads. Soft earth such as is often found packed in in this way may occasionally be bridged by putting a good concrete block on either side and connecting these two blocks with steel joists round which concrete is packed, and this may eventually have to be done; but the wise course is to dig the trench out in any event, as there may be an old pipe or drain at the bottom which, with its open joints, is depriving the soil above of its necessary moisture. It is also by no means uncommon to come across old cesspools in this way, and, needless to say, they must be dug right out and their contents mixed with chloride of lime, or quicklime immediately, before being carted away, and the excavation similarly treated before it is filled in. Sometimes, where a trench has been filled in by the soil taken from it, there is considerable difficulty in discriminating by inspection between the original soil and the filling, though otherwise a distinct line can be seen between one and the other; but where this difficulty occurs the man with the spade can generally distinguish the one from the other by the ease with which he can cut into the filled-in work.
How to deal with all the cases that may arise it is impossible to explain in writing, as it is rare that two are alike. Only experience can lead to a sound decision in any case; but it is better to err upon the side of too great precaution than too little, keeping in mind the main points, - that moving water must always be given free means of escape, that soft places must be bridged or filled in, and that sandy banks must be held up so as not to fall into the trenches. Where a definite change of strata takes place, whether by what is known as a " fault" or by the ordinary running out on the side of a hill, it is well to consult the Architect, as it is just at such a point where a building may fail through unequal settlement; but the method of dealing with such a case is hardly a matter for a Clerk of Works to decide.
Concrete, whether intended for foundation work or for floors, needs careful watching, both while it is being mixed and while it is laid. So much depends upon accuracy of proportion that the Clerk of Works should see that proper measures are provided, and insist on their being used. It is also essential that the mixing should be done on a clean surface, preferably of boards, and that the materials should be turned over when in a dry state sufficiently to incorporate them thoroughly, then watered through the rose of a watering-can, and on no account douched with water from a bucket or hosepipe, and afterwards just turned over lightly again and gently shovelled into place. There are many mechanical concrete mixers on the market; but they are only economical when the concrete is to be used in great bulk, and are rarely employed upon building operations. Unless very carefully watched the workmen are likely to be slovenly in the mixing, and to tip the concrete instead of shovelling it. There is also a tendency on the part of many careless men to leave big lumps of stone amongst the aggregate without properly breaking it up, so that the screening to size specified should be watched.
When concrete is used for walls much attention must be paid to properly wetting each layer before the next is laid on top of it, so that there may be thorough adherence. When the walls are of what is known as armoured concrete - that is, consisting of concrete in which steel rods are embedded - it is of the very utmost importance that the steel and concrete should be intimately connected. It is consequently necessary that the aggregate should be very finely broken, and that the concrete should be lightly rammed, so as to expel all air-bubbles and make it perfectly homogeneous, bringing it into tight adherence to the steel.