Protection of Adjoining Structures

PROTECTION OF ADJOINING STRUCTURES When inserting foundations for new buildings the foundations of adjoining structures must not be disturbed. Most laws recog nize the rights of adjoining owners, and impose penalties for damages to existing property. The upper portions of existing structures must be supported by shoring while adjacent excava tions are in progress (see SHORING).

If the new foundations are to be lower, to admit of deeper basements, then the old structure must be re-supported at the lower level. The process of inserting new supports is known as under-pinning.

Under-pinning.

The problem in under-pinning is to place solid supports underneath the old foundations and to avoid settle ment of the existing building in the process. Methods of under pinning cannot be standardized ; fig. 3 shows one method in which short lengths of foundation are exposed by pits sunk to the new level. In heavy structures, dead shoring is required which consists of "needles" passed beneath the foundations and supported on short uprights or from the adjacent ground. The excavations are then extended and portions of the new foundations erected. For brickwork, cement mortar is used and the last joint wedged with slate. If in concrete the top clearance is grouted solid with cement. When the new work may receive load the intervening portions of foundation are inserted. For small build ings, on firm ground, under-pinning can sometimes be done with out temporary support.

Pre-test Piles.

A method of pile under-pinning has been patented in America (1917) and in England known as "pre-test" piling. Pre-test piles may also be used for new buildings. For the purpose of under-pinning, short lengths of the old foundations are exposed to allow of the insertion of a short length of steel tube, 14 in. to 20 in. diameter and in. thick, which becomes the casing to a complete pile—afterwards concrete filled. The short tube is forced into the ground by a pair of powerful hydraulic jacks operated by a force pump working at high pressure. The tube is cleared of earth by grabbing or otherwise. Another length is added and the process repeated, additions being made by sleeve jointed sections until the hard substratum is reached. The interior of the steel tube is filled with concrete and the pile completed. In the space between the old foundation and the head of the pile, a steel joist packing is inserted and wedged tight while two jacks keep the full load on the pile (see fig. 4).

By wedging tight while the pressure is maintained on the pile, a great advantage in supporting power is gained. When the pres sure on such a pile is being increased, a "bulb of pressure" develops in the earth beneath the base of the pile and opposes its further penetration. If the load be released this "bulb" disap pears and will only be re-formed if the pile be once more forced forward into the ground. Hence, if the under-pinning be com pleted while the pile is not under pressure, the full resistance can only be developed by further settlement. The pre-test method eliminates this settlement and may also ensure a known factor of safety, since the jacks can be made to develop a definite and known thrust. Care is required in packing the bearings of the jacks on the concrete both of the pile and the old foundation and in wedging up the new support to avoid local damage to the con crete. The steel support and the heads of the piles are finally encased in concrete.

Papers referred to in preparing this article:—Hoole and Kinne, Foundations, Abutments and Footings; Fowler, Engineering and Build ing Foundations, vol. i. ; W. M. Patton, Practical Treatise on Founda tions; White and Prentis, Modern Under-pinning (1929) ; M. J. McCarthy, "Piling in the Service of Structural Engineering," Struc tural Engineer (1927). (F. E. D.; J. L. M.)