The framework of the sheeting should be placed with a cross brace for each end of each ranger and a cross brace for the middle of each ranger. If the ends of two rangers rest on the same cross brace an accident displacing one ranger will be passed on to the next and might cause a progressive collapse of a length of trench, whereas the movement of an independently supported ranger should have no effect on another ranger. The cross braces should have horizontal cleats nailed on top of them as shown in Fig. 107 to prevent the braces from being knocked out of place by falling objects. In driving vertical sheeting a vacant place will be left behind each cross brace corresponding to the original block placed to hold the ranger away from the bank. This is an undesirable feature in the use of vertical sheeting. It is ordinarily remedied by slipping in planks the width of the slot and wedging or nailing them against the convenient cross bracing. In extremely wet trenches, after all other pieces of vertical sheeting are in place, the original cleat behind the cross brace can be knocked out and a piece of sheeting slipped into this opening and driven. Care must be taken in this event not to drive the rangers down when driving the sheeting. If the bracing begins to drop, it should be supported by vertical pieces between the rangers and resting on a sill at the bottom of the trench.
Fig. 110.—Steel Clamp for Pulling Wood Sheeting.
155. Pulling Wood Sheeting.—Wood sheeting is pulled after the completion of the trench by a device shown in Fig. 110. In wet trenches where the removal of the sheeting would permit a movement of the banks, resulting in danger to the sewer or other structures, the sheeting should be left in place in the trench. If sufficient saving can be made the sheeting is cut off in the trench immediately above the danger line, usually the ground water line. The cutting is done with an axe or by a power driven saw devised for the purpose.
156. Earth Pressures.[[92]]—The various theories of earth pressure are so conflicting in their conclusions as to be confusing. Rankine’s theory, the most frequently used, assumes that the pressure increases with the depth, whereas Meem’s theory[[93]] leads to an opposite conclusion. The discussion following Meem’s article is very illuminating. It indicates that no matter how good the theory, practical experience together with the use of generous sizes and close spacing are the best guides for bracing trenches and coffer dams. All are not possessed with the desired practical experience and some basis on which to commence work is essential. Another factor affecting computations of sizes based on theory is the tendency in practice to use the same size material for rangers and braces on any one job for all except very deep trenches and other special cases. Occasionally where there is an independent brace for each end of each ranger, the brace is made thinner, but is of the same depth as the ranger.
The application of Rankine’s theory of earth pressure to the computation of the sizes of rangers and braces will be shown. His formula for the active earth pressure against a retaining wall is:
P = wh cosθ cos θ − √cos2 θ − cos2 φ
cos θ + √cos2 θ − cos2 φ
in which w = the weight of earth in pounds per cubic foot; h = depth in feet at point at which pressure is to be determined; θ = the angle of surcharge, or the angle which the surface makes with the horizontal; φ = the angle of repose of the earth. Usually taken as 33°–41′ = 1½ horizontal to 1 vertical; P = the intensity of pressure in pounds per square foot on a vertical plane in a direction parallel to the surface of the ground.
In studying the pressures for trenches the surface of the ground will be assumed as horizontal and the formula reduces to
P = 1 − sin φ
1 + sin φwh.