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Home / Civil Engineering Methods / Method Of Statement For Structural Excavation & Backfilling

Method Of Statement For Structural Excavation & Backfilling

Excavation and Backfilling for Culverts, Storm Sewers and Ducts

Precast concrete and steel pipe culverts and pipe ducts will be installed in trenches cut into previously constructed and compacted embankment either from the subgrade or from an embankment height at least twice the nominal diameter of the pipe. Unless otherwise noted on the drawings, trenches will be no wider than the external diameter of the pipe plus 300 mm on each side. If trenches are dug too wide, the pipe culvert will be bedded in a concrete cradle.

Pipe bedding will be either graded aggregate 12 mm nominal size to Table 1 of BS 882, or concrete Class 12 shown on the drawings. Exposed concrete bedding will be cured for a minimum of 3 days.

Pipes will be laid so that the lower portion of each pipe is supported for its entire length to a depth at least equal to ¼ the external diameter. They will be fitted and matched so that when laid in the trench they form a pipeline with a smooth and uniform fall invert.

Granular material for pipe bedding will be compacted to the minimum bed thickness shown in below table. 50 mm less bedding may be provided under the sockets of spigot and socket pipes.

Sewer Pipe Bedding Thickness

Pipe Diameter (mm) Minimum Bedding (mm)
<300

300 to 500

>500

150

200

250

Granular bedding material will be placed and compacted in uniform layers on both sides simultaneously to 50 mm above the top of the pipes. Selected excavated material or approved granular fill will then be placed and compacted above the pipe for the full trench width to not less than 300 mm finished thickness.

Where rock, or similar material is encountered in the excavation bottom, it will be removed for a depth of 150 mm. The extra excavation will be backfilled with a properly compacted granular material.

In-Sity concrete pipe culverts shall be cast in trenches cut into previously constructed and compacted embankment from the subgrade or from an embankment height at least twice the nominal diameter of pipe. The trench width will be no greater than the external diameter of the pipe plus 300 mm each side when external forms are elected to be used or plus 80 mm each side when external forms are elected not to be used.

If forms are used and trench wider than specified is excavated, backfill to the height of the external haunch of pipe will consist of bedding concrete class 12. If specified, the side walls of the pipe will be increased to fill the channel width. Overdepth excavation will be compensated for by increasing the thickness of concrete base or as otherwise instructed by the Engineer.

Scheduling Excavation for Structures

Works will be scheduled so that no excavation is left in an exposed condition for a period greater than 30 days.

If earthworks progress is not integrated with the drainage structural work to the point where the road embankment dams the natural drainage, the Engineer may order the Contractor to open adequate temporary water structure through the roadway at locations where drainage structures are to be installed. Damage to the roadway caused by water passing through these openings will be repaired.

Backfilling For Structures

Only selected granular materials that will produce a stable, dense, backfill will be used for structures. In addition, backfill for metal pipe culverts will contain less than 1.5 % of combined chlorides and sulphates.

Selected backfill, which will be subject to the prior approval of the Engineer, will consist of well graded natural sands and gravels, crushed gravel and crushed rock (but excluding argillaceous rock types) with the properties shown in below table

Selected backfill properties

Particle size

Material passing 75 mm sieve

Uniform coefficient

10 % Fines value

75 mm maximum

15 % maximum

10 minumum

100 Kn minimum

No cast in place concrete structures will be subjected to pressures from backfilling until 3 days after expiration of the period designated for removal of forms. Backfill placed around the culverts, abutments and piers will be deposited on both sides to approximately the same elevation at the same time. Care will be taken to prevent any wedging action against the structure. Slopes bounding the excavation will be stepped to prevent such wedge action.

Materials will be placed in layers and compacted by means of rollers, vibrating plates or mechanical rammers. Each layer will be compacted to 95% of the MDD measured according to AASHTO T160 (method D). In no case will any layer be uniform and within the optimum moisture range.

At box culverts backfill will be completed to the level of the original ground line and to the full width of the excavation area. If the top of the culvert extends above the original ground line, Backfill operation will continue to the top of the culvert and for a width of 3 m on each side of the culvert to the full width of the roadway embankment.

At pipe culverts and storm sewers (including metal pipe and metal arch culverts) backfill will be completed around the culvert as specified to the level of original ground line and to the full width of the excavation area. If the top of the culvert extends above the original ground line, backfill operation will continue to the top of culvert for a width of 1 ½ times the maximum external width of the culvert on each side of the center line of the culvert to the full width of the roadway embankment.

At bridges backfill will be completed to piers and abutments to the level of original ground or to the top elevation of any adjacent embankment if appropriate.

Backfill around retaining walls will be compacted to the finished level shown on the Drawings. Catch pits, manholes, inlets and other structures will be backfilled in accordance with methods specified in the project specifications.

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