When earth has been excavated to a considerable depth the vertical faces of the excavations need supporting by means of timber, to prevent the soil from falling in and injuring the workmen or the work upon which they are engaged.

The strength of the timbering used for this purpose necessarily depends upon the nature of the soil, the depth of the excavations, and the length of time it is likely to be kept open.

Excavation In Firm Soil

Fig. 74 shows the method of timbering a trench in firm ground, in which case short deal and batten "ends," varying from 1 by 4 1/2 inches to 1 1/2 by 9 inches, and about 3 feet long, called Politics or Poling Boards, are placed in pairs opposite one another against either side of the trench, and are held in position by means of struts. These struts, which are usually short lengths of 4 inches diameter scaffold poles, or 4 by 4-inch squared timbers, and cut about inch longer than the clear distance between a pair of poling boards, are fixed in position in the following manner: One end of the strut is placed against the middle of one poling board, and the other end is swung vertically downwards against the opposite poling board, and forced tightly against it by means of a few downward taps with a mallet.

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Fig. 74.

The struts should not be closer together than 6 feet, otherwise they will prove a considerable inconvenience to the workmen in the trenches.

Excavations In Loose Soil

Should the looseness of the soil necessitate supports being fixed at closer intervals than 6 feet the system of timbering shown in Fig. 75 should be used. In this case it will be noted that the poling boards are supported by long horizontal members about 6 by 4 inches, called Walings or Waling Pieces, which in turn are supported by struts at intervals of 6 feet.

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Fig. 75.

The method of inserting the timbering shown in Fig. 75 is as follows: A short length of trench is excavated, and a pair of poling boards are placed against its sides, and strutted with a temporary strut placed about 6 inches above the centre of the poling board. A little more of the trench is excavated, and the next pair of poling boards inserted. This process is continued until the trench is long enough to receive the waling pieces, which are held in position until the struts are inserted. The temporary struts are now knocked away, and one length of timbering is complete.

If the trenches are deep the timbering is inserted in tiers until the required depth is reached, the struts being placed vertically under one another, and at a distance of about 6 feet apart, so that stages may be supported upon them for the disposition of the excavated soil - 6 feet being the limit of depth from which an excavator can comfortably throw soil out of a trench.

When poling boards longer than 3 feet are used they should be supported by two rows of walings and struts, one row being placed near the top end of the poling pieces and one row at the bottom, the waling so that piece overlaps the end of the poling board by half its width. The upper ends of the poling boards of the next tier of timbering are inserted behind the lower waling pieces of the first tier.

Excavations In Bad Soil

When the ground is so bad that it will not stand to a vertical surface while the poling boards are inserted, the system of timbering shown in Fig. 76 is generally used, the method of inserting it being as follows: The trench is excavated to the desired width, and to a depth of about 9 inches. Two 9 by 1 1/2-inch boards - which when used for this purpose are called Sheetings - are placed against the sides of the excavation and strutted apart with temporary struts. Another layer of soil or "spit" is excavated and another piece of sheeting inserted, with its edges placed as closely as possible to the edges of the first pair of sheetings. This process is continued until four or five boards have been inserted, when pairs of poling boards are placed vertically, and strutted against them, after which the temporary struts are knocked away. This whole operation may be repeated until the desired depth has been reached.

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Fig. 76.

In shallow trenches the sides are battered or sloped to prevent the timbering from collapsing when the soil behind the sheeting sinks - as it frequently does owing to the water being withdrawn by gravitating to the bottom of the trench. Another advantage of a battered trench is that when narrow concrete foundations are to be placed therein it can be kept at the bottom to the calculated width of the concrete, the batter giving ample room for the men to work.

Excavations in very Soft or Water-Logged Soil require to be heavily timbered to resist the considerable lateral pressure of the soil. The usual method of timbering excavations in such soils is as follows: Guide piles or Guide Runners, 9 by 9 inches, - as they are called when used for timbering excavations, - are driven into the ground at intervals of about 10 feet on either side of the piece of ground it is desired to excavate. Stout waling pieces are bolted to these guide runners, and sheet piles, 9 by 2 inches to 11 by 3 inches, and about 10 feet long, called Runners, are driven a short distance into the ground behind the waling pieces to form a continuous wall between the guide runners. The soil is now excavated between the two rows of runners, care being taken not to excavate within a foot of the bottom of the runners. The runners are now driven a farther distance and another layer of soil is excavated, this process being continued until the heads of the runners are driven flush with the ground, struts being placed at frequent intervals to prevent them bulging inwards. If an excavation deeper than one set of piles be required, another set of piles is driven within the first, and if the excavation be very wide, vertical pieces are inserted between the wales; and to prevent the horizontal struts from bending under the pressure from the earth, inclined struts, called Rakers, are inserted between them, as shown in Fig. 77. These rakers are fixed at one end to cleats immediately under a horizontal strut, while the other end is fixed to a cleat on top of the next lower horizontal strut on the opposite side of the excavation. It will be noticed that the runners are pointed in a similar manner to the piles described in Chapter I., save that here a greater splay is given to the edges facing towards the excavation, in order to cause the runner to press against the retained earth on being driven.