ASSESSMENT OF DESILTING QUANTITIES

Work methodology adopted for desilting of stormwater drains in an industrial estate:

Drain Geometry

To ascertain the invert levels of drains in dry areas, leveling was adopted using dumpy levels at two or three locations on each drain. The locations were cleaned using manual labour to make invert visible for leveling. For wet locations and locations with sludge, sounding was adopted using sounding rods after removal of silt/sludge manually at two or three locations per drain.

The drain cross sections were measured using tape. Bottom width is measured at typical locations were invert levels are measured after removal of silt.

Drain linings have been classified by visual inspection into five categories:

A: Good Lining

B: Mild Damage, needs replastering

C: Moderate Damage, needs refurbishment

D: Completely Damage, needs reconstruction

E: No Drain

Drain length covered by cover slabs, culverts and entrances of industrial units have been individually measured.

In some areas it has been noted that existing drains have been destroyed by previous desilting excavation using excavators. In these cases, it is difficult to locate the invert levels of the drains. In such cases, invert levels are assumed to be the hardest levels that have been achieved by sounding. This approach may be subject to errors if the sediments have consolidated considerably. In these areas, execution of contract should be based on cost-plus-fee basis rather than lumpsum basis.

Similarly some areas are such that grades have been blocked by on going development. Flow gradient has also been reversed due to deposition of construction debris at many locations.

Sediment Geometry

The profile of silt deposited in the drains is assessed by leveling at approximately 10m intervals in dry areas. In wet locations and sludge locations, profile of silt is assessed by sounding rods at 10m intervals. The height of deposit is calculated from the depth of invert levels measured previously.

Type of sludge has been classified as follows:

D: Dry

W: Wet

S: Sludge

E: Effluent

In most areas it is found that quantity of silt is about 0.8m3 per running meter of drain. Silt in areas already destroyed by excavators is nominal.

Desilting Quantities

Based on the drain width and silt height, the quantity of silt is calculated for every 10m length of drain and summarized

Technical Specifications for Desilting

1. The contractor shall visit the site and assess the adequacy of the data regarding drains and silt quantities provided.

2. All unit holders shall be notified to take necessary precautions to prevent damage to services or utilities during the desilting work.

3. All unit holders shall be notified to remove cover slabs over drains wherever necessary.

4. Not more than 10m of drain shall be continuously covered during desilting.

5. Vegetation on the drains if any shall be removed by the contractors using his own equipment and labour.

6. If found necessary traffic on road shall be diverted during desilting. This may be necessary on narrow roads.

7. The contractor shall not pump water from drains onto roads. Care shall be taken to prevent flooding of roads with water during desilting.

8. While working from upstream side of drain care should be taken to ensure that water flow is not blocked down stream.

9. While working from downstream, care should be taken that silt is not washed again into already cleaned areas of downstream from upstream.

10. Silt deposits on the existing drains are classified into following types: Dry earth, Wet earth and Earth mixed with sludge/solid waste.

11. The material may include earth, debris of construction materials, vegetation, dead vegetation, organic matter, effluent, sludge and non-decomposable solid wastes.

12. Equipments such as excavators with maximum bucket size 300mm shall be permitted only on drains wider and deeper than 1m. Even on these drains only mass excavation shall be permitted mechanically. Thickness of silt about 15-20cm in contact with the lining shall be removed by manual excavation only.

13. Material that does not require dewatering arrangements or water stop arrangements for removal is considered dry earth.

14. Material that require dewatering arrangements using ordinary dewatering pumps for removal shall be classified as wet earth.

15. It may be necessary to make arrangements to stop flow of water into the area by bunds and to employ dewatering pumps to remove water. After removal of water, the residual material may be treated similar to removal of dry earth.

16. Material that require sewage/sludge pumping arrangements for removal shall be classified as sludge/sewage mixed earth.

17. Sullage may be removed using sewage/sludge pumps or sewage loading trucks. Residual material may be removed by ordinary excavation. In this case compulsory precaution shall be taken for labourers working in sewers.

18. If necessary the contractor may be permitted to deposit material from within drain to the banks for drying for one or two days. Later the material can be loaded into trucks for disposal.

19. Contractor should notify wherever debris is left for drying on roads, so that dispute regarding transportation may not arise.

20. Site for deposition of excavated silt shall be arranged.

21. The material removed from drain shall be removed as immediately as possible without creating hindrance to road traffic and construction work in progress.

22. Arrangement shall be made to deposit the transported material uniformly at the deposit sites. If any agency deposits heaps of materials at entrance of spoil areas, subsequent deposition becomes difficult.