Laying and Joining of Water Supply Pipes | Water Engineering

In this article we will discuss about the laying and joining of water supply pipes.

Laying of Water Supply Pipes:

Pipes are generally laid below the ground level, but sometimes when they pass in open areas, they may be laid over the ground.

The pipes are laid in the following way:

(a) First of all detailed map showing all roads, steels, lanes etc. is prepared. On this map the proposed pipe line with as sizes and length will be marked. The position of existing pipe lines, curb lines, sewer lines etc. will also be marked on it. In addition to this position of valves and other pipe specials, stand posts etc. will also be made so that at the time of laying there should be no difficulty in this connection.

(b) After the general planning the centre line of the pipe line will be transferred on the ground from the detailed plan. The centre line will be marked by means of stakes driven at 30 m interval on straight lines. On curves the stakes will be driven at 7 m to 15 m spacing. If the roads or streets have curbs, the distance of centre of pipe line from the curb will be marked.

(c) When the centre line has been marked on the ground the excavation for the trenches will be started. The width of the trends will be 30 cm to 45 cm more than the external diameter of the pipe. At every joint the depth of excavation will be 15-20 cm more for one meter length for easy joining of the pipes. The excavation of the trends is done in such a way that only pipe should be supported and its joint portion should remain over-hanging.

The pipe line should be laid more than 90 cm below the ground so that pipe may not break due to impact of heavy traffic moving over the road or ground if the excavation is to be done in soft soils, the sides of the trenches should be protected by means of timber planks, so that it may not fall or collapse.

(d) After the excavation of trenches the pipes are lowered in it. Generally, in practice, the pipes are stacked on the opposite side fixed on derricks, but lighter pipes are lowered manually. The pipe laying should be started from the lower level and proceed towards higher level with socket end towards higher side. The jointing of pipes should also be done along with the laying of pipes.

(e) After laying the pipes in position, they are tested for water leakage and pressure.

(f) When the pipe line is tested, the back filling of the excavated material will be done.

The soil which was excavated is filled back in the trenches all around the pipes and should be well rammed. All the surplus soil will be disposed of and the site should be cleaned.

Joining of Pipes:

(a) Unloading:

Unloading (except where mechanical handling facilities are available) of pipes up to 60 kg should be done by two persons. Heavier pipes should be unloaded from the wagon or truck by holding them in loops, formed with ropes and sliding over planks set not steeper than 45°. One pipe should be unloaded at a time. Under no circumstances pipes should be thrown down from the carriers or be dragged or rolled along hard surfaces.

(b) Storing:

The pipes and specials should be handled with sufficient care to avoid damage to them. These shall be lines up on one side of the alignment to the trench, socket facing upgrade when line runs uphill and upstream when line runs on level ground. Each stack should contain pipes of the same class and size. Consignment or batch number and particulars of suppliers, where possible, should be marked.

(c) Cutting:

While cutting the pipes in required lengths, a line should be marked around the pipe with a chalk at the point of cut. The line shall be so marked that the cut is truly at right angle to the longitudinal axis of the pipe. The pipe shall be rightly held on two parallel rafters nailed to cross beams, taking care that the portion to be cut does not overhang and the cut mark is between the two rafters.

The pipe shall be neatly cut at the chalk mark with carpenter’s saw or hacksaw having a long blade. Cutting of the pipe at the overhang should, as far as possible, be avoided as an overhanging is liable to tear off due to its weight before the cutting is complete.

(d) Trenches:

The trenches shall be so dug that the pipe may be laid to the required alignment at required depth. The bed of the trench, if in soft or made up earth, shall be well watered and rammed before laying the pipes and the depressions, if any, shall be properly filled with earth and consolidated in 20 cm layers.

When the trench bottom is extremely hard or rocky or loose stony soil, the trench shall be excavated at least 15 cm below the trench grade. Rocks, stone or other hard substances from the bottom of the trench shall be removed and the trench brought back to the required grade by filling with selected line earth or sand (or fine moorum if fine soil or sand is not available locally).

The filled material shall be compacted so as to provide a smooth bedding for the pipe. In case excavation required blasting operation, it shall be checked that no pipes have been stacked in the vicinity and completed pipe line in the vicinity has already been covered before starting of blasting operation. This is necessary to prevent damage to the exposed pipes in the vicinity by falling stones during blasting.

After completing the excavation of the trench, hollows shall be cut at the required position to receive the socket of the pipes and these hollows shall be of sufficient depth to ensure that the barrels of the pipes shall rest throughout their entire length on the solid ground and that sufficient spaces are left for jointing the underside of the pipe joint. These socket holes shall be refilled with sand after jointing the pipes.

The trench shall be kept free from water shoring and timbering shall be provided wherever required. After dewatering, the excavation shall be done below the water table.

(e) Laying:

The pipes shall be lowered into the trench by means of suitable pulley blocks, sheer legs chains, ropes etc. In no case the pipes shall be rolled and dropped into the trench. One end of each rope may be tied to a wooden or steel peg driven into the ground and the other end held by men which when slowly release, will lower the pipe.

After lowering, the pipes shall be arranged so that the spigot of one pipe is carefully centered into the socket of the next pipe, and pushed to the full distance, that it can go. The pipe line shall be laid to the level required.

Specials shall also be laid in their proper position as stated above. If so directed, the pipes and specials may be laid on masonry or concrete pillars. All the pipes shall rest continuously on the bottom of the trench. The pipes shall not rest on lumps of earth or on the joints.

Some clayey soils like black cotton soil are drastically affected by extremes of saturation and dryness. While changing from saturation to dry condition, these soils are subjected to extraordinary shrinkage which is usually seen in the form of wide and deep cracks and may damage the underground structures.

At such places an envelope of 10 cm minimum of tamped sand shall be made around the pipe line. At slopy bed of pipe lines having more than 30° slope, it is necessary to anchor few pipes against sliding downward.

(f) Installation of Trust Blocks:

Thrust Blocks are required to transfer the resulting hydraulic thrust from the fitting or pipe on to a larger load bearing soil section. Thrust blocks shall be installed wherever there is a change or when the pipe line ends at a dead end. These may be constructed at valves also.

(g) Back Filling and Tamping:

Back filling of trenches shall follow pipe installation as closely as possible to protect pipe from falling boulders, eliminating possibilities of lifting of the pipe due to flooding of open trench and shifting pipe out of line by caved in soil.

The soil under the pipe and around it shall be properly tamped to provide a firm and continuous support for the pipe line. Tamping shall be done either by tamping bars or by using water to consolidate the back fill materials. The initial back fill material used shall be free from large stones and dry lumps.

In bogs and marshes, the excavated, material is usually little more than vegetable matter and it should not be used for bedding purposes. At such places, gravel or crushed stone shall be used. The initial back fill shall be placed evenly in a layer of about 10 cm thick, which shall be properly consolidated. This shall be continued till there is a cushion of at least 30 cm over the pipe.

While doing the back filling of balanced work, it need not be done so carefully. Care shall be taken to avoid back filling with large stones which may damage the pipe line.

Extra care shall be taken while doing back filling on a slope, because until backfill becomes totally consolidated, there is a tendency for ground or surface water to move along this looser soil resulting in a loss of support to the pipe. At such places the soil shall be filled at least 10 cm above the ground level.