SUBJECT CODE- CIV 107

ESTIMATING, COSTING AND

SPECIFICATIONS

TOPIC:- TIMBRING in TReNCHES

SUBMITTED TO:- SUBMITTED BY:-

Mrs. Mandeep Mam. 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 4-

inch 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