SUBJECT CODE- CIV 107 ESTIMATING, COSTING AND SPECIFICATIONS
TOPIC:- TIMBRING in TReNCHES
Mrs. Mandeep Mam.
Rakesh Saini Roll no. 9 Civil 5th sem. Reg.no.4100070010
Trench is a type of excavation or depression in the ground. Trenc are generally deﬁned by being deeper than they are wide (as opposed to a gully or ditch), and by being narrow compared to their length (as opposed simple hole).
2. Timbering in Excavations:-
When earth has been excavated to a considerable depth the vertica of the excavations need supporting by means of timber, to preven soil from falling in and injuring the workmen or the work upon wh they are engaged.
The strength of the timbering used for this purpose necessarily de upon the nature of the soil, the depth of the excavations, and the l 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 which case short deal and batten "ends," varying from 1 by 4 1/2 in to 1 1/2 by 9 inches, and about 3 feet long, called Politics or Po Boards, are placed in pairs opposite one another against either side
trench, and are held in position by means of struts. These struts, w are usually short lengths of 4 inches diameter scaﬀold poles, or 4 b inch squared timbers. One end of the strut is placed against the mid one poling board, and the other end is swung vertically downw against the opposite poling board, and forced tightly against it by m of a few downward taps with a mallet.
The struts should not be closer together than 6 feet , otherwise the 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 c intervals than 6 feet the system of timbering shown in Fig.Shoul used. In this case it will be noted that the poling boards are suppor long horizontal members about 6 by 4 inches, called Walings or Wa 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 board placed against its sides, and strutted with a temporary strut placed 6 inches above the centre of the poling board. A little more of the t is excavated, and the next pair of poling boards inserted. This proc continued until the trench is long enough to receive the waling pi which are held in position until the struts are inserted.
2. The temporary struts are now knocked away, and one length timbering is complete.
3. If the trenches are deep the timbering is inserted in tiers unt required depth is reached, the struts being placed vertically unde another, and at a distance of about 6 feet apart, so that stages m supported upon them for the disposition of the excavated soil - 6 being the limit of depth from which an excavator can comfortably t soil out of a trench.
4.When poling boards longer than 3 feet are used they should supported by two rows of walings and struts, one row being placed the top end of the poling pieces and one row at the bottom, the wali that piece overlaps the end of the poling board by half its width. upper ends of the poling boards of the next tier of timbering are ins 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 sur 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 ab inches.
2. Two 9 by 1 1/2-inch boards - which when used for this purpose called Sheetings - are placed against the sides of the excavation strutted apart with temporary struts.
3. Another layer of soil or "spit" is excavated and another piece sheeting inserted, with its edges placed as closely as possible to edges of the ﬁrst pair of sheetings. This process is continued until or ﬁve boards have been inserted, when pairs of poling boards are p 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 la pressure of the soil.
The usual method of timbering excavations in such soils is as follow
1. Guide piles or Guide Runners, 9 by 9 inches, - as they are ca when used for timbering excavations, - are driven into the groun intervals of about 10 feet on either side of the piece of ground desired to excavate.
2.Stout waling pieces are bolted to these guide runners, and sheet p by 2 inches to 11 by 3 inches, and about 10 feet long, called Runn are driven a short distance into the ground behind the waling piec form a continuous wall between the guide runners.
3. The soil is now excavated between the two rows of runners, being taken not to excavate within a foot of the bottom of the run The runners are now driven a farther distance and another layer of excavated, this process being continued until the heads of the runne driven ﬂush with the ground, struts being placed at frequent interv prevent them bulging inwards. 4.If an excavation deeper than one set of piles be required, another piles is driven within the ﬁrst, and if the excavation be very wi vertical pieces are inserted between the wales; and to prevent horizontal struts from bending under the pressure from the e inclined struts, called Rakers, are inserted between them.
5. These rakers are ﬁxed at one end to cleats immediately und horizontal strut, while the other end is ﬁxed to a cleat on top of the 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 taken from the National Bureau of Standards (NBS) repo "Recommended Technical Provisions for Construction Practice Shoring and Sloping of Trenches and Excavations." In addition, w NBS did not recommend speciﬁc sizes of members, member sizes based on an analysis of the sizes required for use by existing codes on empirical practice.
(ii) The required dimensions of the members listed in below tabl These refer to actual dimensions and not nominal dimensions of 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 ______|_______|_______|_______|_______|_______|_______|_____________
(i) It is not intended that the timber shoring speciﬁcation apply to situation that may be experienced in the ﬁeld. These data w developed to apply to the situations that are most commonly experie in current trenching practice.
(ii) When any of the following conditions are present, the memb speciﬁed in the tables are not considered adequate.
(a) When loads imposed by structures or by stored material adjace the trench weigh in excess of the load imposed by a two-foot s surcharge. The term "adjacent" as used here means the area wit horizontal distance from the edge of the trench equal to the depth trench.
(b) When vertical loads imposed on cross braces exceed a 240-po gravity load distributed on a one-foot section of the center of the brace.
(c) When surcharge loads are present from equipment weighing excess of 20,000 pounds.
(d) When only the lower portion of a trench is shored and the rema portion of the trench is sloped or benched unless: The sloped porti sloped at an angle less steep than three horizontal to one vertical; members are selected from the tables for use at a depth which is s 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