When trenches have to be dug in loose ground it is necessary to support the sides of the excavation by timbering and shoring.
In moderately firm ground, after a depth of 3 or 4 feet has been excavated, a few rough planks or "poling hoards" P P (Fig. 383) are placed at intervals varying with the nature of the soil against the sides of the trench, and kept up by jamming or wedging in between them struts (S) of rough scantling from 4 to 6 inches square.
In looser ground it is necessary to place the poling boards closer together, and so support them (Fig. 384) by 3-inch planks W W called "walings." The struts must be made thick, in proportion to the width of the trench and the pressure upon them, and their distance apart will depend upon the strength of the walings and the nature of the soil.
The poling boards P P are often in short lengths of about 3 feet, so that no greater depth has to be excavated before they can be inserted.
In very loose soils, such as running sands or slipping clays, it is evident that the sides would fall in if an attempt was made to excavate the trenches to a depth of 3 or 4 feet before supporting them (Fig. 385).
To prevent this the poling boards are sometimes put in horizontally - as "sheeting " - one at a time. A portion 9 inches or a foot deep is excavated, and at once supported by planks placed longitudinally on both sides and kept apart by struts, then another depth of 9 inches is taken out and another plank placed on each side below those already in position, and these last also strutted. When five or six planks have been thus inserted on each side walings may be added, and some of the struts dispensed with.
The timber used for shoring important excavations should be hard and tough - seasoned, - barked before use - so placed as to receive the stress on its end grain, and as large a bearing surface as possible should be allowed, especially when the end of one timber bears upon the side of another.
All shores should be driven from above, not sideways or horizontally. The planks or walings at the sides of an excavation should be at a slight inclination, as in Fig. 385, the upper edge sloping toward the earth they support, so that when the shore, whose ends are cut to the proper angle, is driven down from above, it will take a fair bearing.1
Fig. 385 shows round shores, which are sometimes made by cutting up old fir scaffold-poles. Half-round walings are also often used.
Sometimes in very bad soil long planks called " runners," having sharp ends shod with iron, are substituted for the poling boards; these are driven in as the trench is dug, their points being kept a foot or so below the bottom of the portion excavated. In very deep excavations platforms are required at vertical intervals of about 5 feet to receive the earth thrown up by the men from stage to stage.
1 Transactions, Society of Engineers, 1873.
In this case these stages may rest upon the struts of the timbering, which should be made particularly firm to ensure safety.