SUBJECT CODE- CIV 107 ESTIMATING, COSTING AND SPECIFICATIONSTOPIC:- TIMBRING in TReNCHES SUBMITTED TO:Mrs. Mandeep Mam. SUBMITTED BY:Rakesh Saini Roll no. 9 Civil 5th sem. Reg.no.4100070010 1.Trench:Trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide (as opposed to a wider gully or ditch), and by being narrow compared to their length (as opposed to a simple hole). TIMBER TRENCH 2. Timbering in Excavations: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. 3 Trench used in different type of soil:3(1) Trenches In Firm Soil :Below fig. 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 4inch squared timbers. 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. The struts should not be closer together than 6 feet , otherwise they will prove a considerable inconvenience to the workmen in the trenches. 3(2) Trenches 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.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. The method of inserting the timbering shown in Fig. is as follows: 1. 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. 2. The temporary struts are now knocked away, and one length of timbering is complete. 3. 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. 4.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. 3(3) Trenches 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 method of inserting it being as follows: 1. The trench is excavated to the desired width, and to a depth of about 9 inches. 2. 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. 3. 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. 3(4) Trench in very Soft or Water-Logged Soil:Soil requires 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:1. 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. 2.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. 3. 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. 4.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. 5. 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. 4. Dimensions of timber members:- (i) The sizes of the timber members listed in below tables .These are taken from the National Bureau of Standards (NBS) report, "Recommended Technical Provisions for Construction Practice in Shoring and Sloping of Trenches and Excavations." In addition, where NBS did not recommend specific sizes of members, member sizes are based on an analysis of the sizes required for use by existing codes and on empirical practice. (ii) The required dimensions of the members listed in below tables . These refer to actual dimensions and not nominal dimensions of the timber. TABLE C-1 TIMBER TRENCH SHORING -- MINIMUM TIMBER REQUIREMENTS * SOIL TYPE A P(a) = 25 X H + 72 psf (2 ft Surcharge) ____________________________________________________________________ | | SIZE (ACTUAL) AND SPACING OF MEMBERS ** |_____________________________________________________________ DEPTH | | CROSS BRACES OF |_____________________________________________________________ | | | | HORIZ.| WIDTH OF TRENCH (FEET) | VERT. TRENCH|SPACING|_______________________________________| SPACING | | | | | | | | | UP TO | UP TO | UP TO | UP TO | UP TO | (FEET)| (FEET)| 4 | 6 | 9 | 12 | 15 | (FEET) ______|_______|_______|_______|_______|_______|_______|_____________ | | | | | | | | UP TO | | | | | | | 6 | 4X4 | 4X4 | 4X6 | 6X6 | 6X6 | 4 5 |_______|_______|_______|_______|_______|_______|_____________ | | | | | | | TO | UP TO | | | | | | | 8 | 4X4 | 4X4 | 4X6 | 6X6 | 6X6 | 4 |_______|_______|_______|_______|_______|_______|_____________ 10 | | | | | | | | UP TO | | | | | | | 10 | 4X6 | 4X6 | 4X6 | 6X6 | 6X6 | 4 |_______|_______|_______|_______|_______|_______|_____________ | | | | | | | | UP TO | | | | | | | 12 | 4X6 | 4X6 | 6X6 | 6X6 | 6X6 | 4 ______|_______|_______|_______|_______|_______|_______|_____________ | | | | | | | | UP TO | | | | | | | 6 | 4X4 | 4X4 | 4X6 | 6X6 | 6X6 | 4 |_______|_______|_______|_______|_______|_______|_____________ 10 | | | | | | | | UP TO | | | | | | | 8 | 4X6 | 4X6 | 6X6 | 6X6 | 6X6 | 4 |_______|_______|_______|_______|_______|_______|_____________ TO | | | | | | | | UP TO | | | | | | | 10 | 6X6 | 6X6 | 6X6 | 6X8 | 6X8 | 4 |_______|_______|_______|_______|_______|_______|_____________ 15 | | | | | | | | UP TO | | | | | | | 12 | 6X6 | 6X6 | 6X6 | 6X8 | 6X8 | 4 ______|_______|_______|_______|_______|_______|_______|_____________ | | | | | | | | UP TO | | | | | | | 6 | 6X6 | 6X6 | 6X6 | 6X8 | 6X8 | 4 |_______|_______|_______|_______|_______|_______|_____________ 15 | | | | | | | | UP TO | | | | | | | 8 | 6X6 | 6X6 | 6X6 | 6X8 | 6X8 | 4 |_______|_______|_______|_______|_______|_______|_____________ TO | | | | | | | | UP TO | | | | | | | 10 | 8X8 | 8X8 | 8X8 | 8X8 | 8X10 | 4 |_______|_______|_______|_______|_______|_______|_____________ 20 | | | | | | | | UP TO | | | | | | | 12 | 8X8 | 8X8 | 8X8 | 8X8 | 8X10 | 4 ______|_______|_______|_______|_______|_______|_______|_____________ 5. Limitation:(i) It is not intended that the timber shoring specification apply to every situation that may be experienced in the field. These data were developed to apply to the situations that are most commonly experienced in current trenching practice. (ii) When any of the following conditions are present, the members specified in the tables are not considered adequate. (a) When loads imposed by structures or by stored material adjacent to the trench weigh in excess of the load imposed by a two-foot soil surcharge. The term "adjacent" as used here means the area within a horizontal distance from the edge of the trench equal to the depth of the trench. (b) When vertical loads imposed on cross braces exceed a 240-pound gravity load distributed on a one-foot section of the center of the cross brace. (c) When surcharge loads are present from equipment weighing in excess of 20,000 pounds. (d) When only the lower portion of a trench is shored and the remaining portion of the trench is sloped or benched unless: The sloped portion is sloped at an angle less steep than three horizontal to one vertical; or the members are selected from the tables for use at a depth which is sloped portion. Reference: 1) Estimating, Costing and Specification by B.N. DUTTA 2) Estimating, Costing and Specification by Gurucharan Singh 3) Estimating, Costing and Specification by Chakravorti