Horizontal drainage excavation procedure

Designing of drainage systems for prevention or liquidation of waterlogging must meet the requirements of the Construction Standards and Regulations SNiP 2.06.14-85 and SNiP II-52-74. At that, drainage systems with gravity water diversion are more preferable. Drainage systems with pumped water evacuation will require additional expenditure.

Depending on hydrogeological conditions, horizontal, vertical and combined drainages are laid.

Construction of horizontal drainage by the surface trench and trenchless way is dictated by financial viability.

In all cases of use of vertical drainage its water intake section needs to be placed in highly permeable soils.

In closed drainage systems, sand & gravel, claydite, clinker, polymeric and other materials are used as filter and filter package. The thickness and grading of sand and gravel is selected according to the requirements of the Construction Standards and Regulations SNiP 2.06.14-85. Filters in the form of sand-gravel packages or covers made from artificial fiber materials should be provided for around drainage pipes.

When constructing any type of drainage, the following primary and secondary processes and procedures are to be carried out. For horizontal subsurface drainage, the primary processes include the following: installation of a trench or a slit trench of target depth and designed slope gradient; installation of drainage pipes; arrangement of filter protective layer; connection of the drains with a collector; primary padding of drainage pipes; and control of the quality of the construction works. Secondary production processes can be divided into preparatory and final ones. Preparatory processes include setting out of drainage system components; preparation of building easements; and delivery of construction equipment and required materials. At the final stage of construction, the following works are carried out: refilling of trenches; restoring of topsoil on the building easement and its leveling; building of surface inlets; installation of drains. Under high groundwater level, additional measures for lowering it in the zone of laying of drainage pipes often needs to be implemented. It can be a preliminary lowering of groundwater levels by using a system of open canals, pilot drains; unwatering installations of different types. Light and vacuum wellpoint systems are applied with required water lowering depth up to 5.5 m. Ejector wellpoint sytems can help performing preparatory lowering of groundwater levels down to 10-20 m, which allows addressing almost all the problems occurring at industrial and urban construction in areas requiring protection against waterlogging.

Horizontal subsurface drainage can be built by wide-trench, trench, narrow-trench and trenchless methods.

Wide-trench method of construction is applied with high groundwater level. A trench of compound trapezoidal form with stick is dug by single-bucket excavators. Pipes depending on diameter are laid manually or by means of hoisting cranes. Such a scheme does not require preliminary water lowering. Groundwater accumulates in a trench situated lower than flange and flows towards the outfall drain.

Trench method consists in excavation of a trench with bottom width of 0.5 m and more, into which pipes or tubes of any suitable material can be laid by specialized layers or manually.

With narrow-trench method they arrange a trench 25-40 cm wide, which much reduces the volume of excavated ground, but makes impossible for workers to descend into the trench and requires use of special drain-laying excavators.

With trenchless drainage construction method, an unpowered operating element of a special drainage machine cuts a narrow slit 2 m wide, while tubes are laid and packed with depth filtering material. This method is used mainly for plastic tubes. Building of trenchless drainage is accomplished by using machines the design of which can be broken down into the following four types:

    • machines that lay factory-built pipes into a slit trench; • machines that pull factory-built pipes into “mole’s cavity”; • machines that lay pipes formed during building of drainage belts into a slit trench; • machines that make pipes during building process by means of delivering different materials forming hard permeable structure into “mole’s cavity”.

Machines of the first and third groups are the most capable. Machines of the second group are not reliable enough because pipes get damaged by stone and other inclusions and the water intakes holes of the pipes get closed. Machines of the fourth group have not become widespread because of technical difficulties with even feed of the material from which pipe is formed and its compaction in “mole’s cavity”.

When laying a factory-built drainage pipe, it is laid on the bottom of the slit trench cut by a drainage machine from above through a skid conveyor (or pipe). When the pipe end is fastened in drain opening, working motion of the machine starts for laying a drainage line.

To connect a drain with closed drainage collector, an outside drainage pit is made, into which the movable object of an excavator-drainage machine is brought down together with a bunker of sand and gravel mixture. In the collector pipe, a hole is made, through which water is received through drains. The drain pipe is fastened in lead-in outside drainage pit with a hold-down fork. If the collecting drain flows into an open collector, there is no need for a lead-in outside drainage pit, because the movable object of the excavator-drainage machine can be descended directly into a canal. During cutting-pass of the excavator-drainage machine a pipe goes down from a reel stand, comes through the bunker by guiding devices to the slit trench cut by the movable object of the machine and is descended to the bottom packed by a layer of protective filtering material, which pours out through the hole of the bunker. The drainage pipeline source is formed till rising of the drainage machine’s movable object. The pipe is cut and shut by a solid plug. The process of drain building by the narrow-trench method is executed in similar manner.

The building is to be carried out beginning from collectors and high-order drains. Trenching and laying of drainage pipes should be carried out from the outlet to the head (from the bottom upwards).

Refilling of the drainage trenches should be carried out in two stages: padding of drains and final filling. Padding of drains is to be implemented mechanically immediately after laying the pipes. The padding material must not contain stones more than 5 cm in diameter and frozen soil clods more than 10 cm in diameter. Final refilling of the drainage trench is to be carried out by bulldozer when it moves along the trench center line or at an angle of 30° at most. Refilling is to be carried out from its head to outlet. When drain laying by trenchless drainage machine is over, the slit trench is to be closed up. Drainage outlets and wells are to be installed simultaneously with laying of drainage pipes.

In the course of the in-process inspection of the quality of the works being performed, it is necessary to examine for compliance with the design and requirements these regulations: position of drain center lines; thickness of topsoil stripping (when digging a trench by a single- bucket excavator); slope of a drainage trench; elevation of outside drainage pit for embedding of trenchless drainage machine’s movable object; diameter of drainage pipes and their protection by filtering material.

Selected bibliography