SUBSOIL DRAINAGE BASIC AND CIVIL ENGINEERINGTUTORIALS

SUBSOIL DRAINAGE BASIC INFORMATION
What Are Subsoil Drainage System?


Subsoil Drainage ~ Building Regulation C2 requires that subsoil drainage shall be provided if it is needed to avoid:-

a) the passage of ground moisture into the interior of the building or
b) damage to the fabric of the building.

Subsoil drainage can also be used to improve the stability of the ground, lower the humidity of the site and enhance its horticultural properties. Subsoil drains consist of porous or perforated pipes laid dry jointed in a rubble filled trench.

Porous pipes allow the subsoil water to pass through the body of the pipe whereas perforated pipes which have a series of holes in the lower half allow the subsoil water to rise into the pipe.

This form of ground water control is only economic up to a depth of 1„500, if the water table needs to be lowered to a greater depth other methods of ground water control should be considered.

The water collected by a subsoil drainage system has to be conveyed to a suitable outfall such as a river, lake or surface water drain or sewer.

In all cases permission to discharge the subsoil water will be required from the authority or owner and in the case of streams, rivers and lakes, bank protection at the outfall may be required to prevent erosion.

Subsoil Drainage Systems ~ the lay out of subsoil drains will depend on whether it is necessary to drain the whole site or if it is only the substructure of the building which needs to be protected.

The latter is carried out by installing a cut off drain around the substructure to intercept the flow of water and divert it away from the site of the building. Junctions in a subsoil drainage system can be made using standard fittings or by placing the end of the branch drain onto the crown of the main drain.

NB. connections to surface water sewer can be made at inspection chamber or direct to the sewer using a saddle connector † it may be necessary to have a catchpit to trap any silt.

DRAINAGE FOR SUBGRADE STRUCTURES BASICS AND TUTORIALS

DRAINAGE FOR SUBGRADE STRUCTURES BASIC INFORMATION
How To Design Drainage Subgrade Structures?


Subgrade structures located above groundwater level in drained soil may be in contact with water and wet soil for periods of indefinite duration after long continued rains and spring thaws.

Drainage of surface and subsurface water, however, may greatly reduce the time during which the walls and floor of a structure are subjected to water, may prevent leakage through openings resulting from poor workmanship and reduce the capillary penetration of water into the structure.

If subsurface water cannot be removed by drainage, the structure must be made waterproof or highly water-resistant.

Surface water may be diverted by grading the ground surface away from the walls and by carrying the runoff from roofs away from the building. The slope of the ground surface should be at least 1⁄4 in / ft for a minimum distance of 10 ft from the walls.

Runoff from high ground adjacent to the structure should also be diverted. Proper subsurface drainage of ground water away from basement walls and floors requires a drain of adequate size, sloped continuously, and, where necessary, carried around corners of the building without breaking continuity.

The drain should lead to a storm sewer or to a lower elevation that will not be flooded and permit water to back up in the drain.

Drain tile should have a minimum diameter of 6 in and should be laid in gravel or other kind of porous bed at least 6 in below the basement floor. The open joints between the tile should be covered with a wire screen or building paper to prevent clogging of the drain with fine material.

Gravel should be laid above the tile, filling the excavation to an elevation well above the top of the footing. Where considerable water may be expected in heavy soil, the gravel fill should be carried up nearly to the ground surface and should extend from the wall a distance of at least 12 in (Fig. 3.7).

ROOF DRAINAGE BASICS AND TUTORIALS

ROOF DRAINAGE BASIC INFORMATION
How To Design Basic Roof Drainage?


Many roof failures have been caused by excessive water accumulation. In most cases, the overload that caused failure was not anticipated in design of those roofs, because the designers expected rainwater to run off the roof.

But because of inadequate drainage, the water ponded instead.

On flat roofs, ponding of rainwater causes structural members to deflect. The resulting bowing of the roof surface permits more rainwater to accumulate, and the additional weight of this water causes additional bowing and collection of even more water.

This process can lead to roof collapse. Similar conditions also can occur in the valleys of sloping roofs.

To avoid water accumulation, roofs should be sloped toward drains and pipes that have adequate capacity to conduct water away from the roofs, in accordance with local plumbing codes.

 Minimum roof slope for drainage should be at least 1⁄4 in / ft, but larger slopes are advisable.

The primary drainage system should be supplemented by a secondary drainage system at a higher level to prevent ponding on the roof above that level.

The overflow drains should be at least as large as the primary drains and should be connected to drain pipes independent of the primary system or scuppers through the parapets.

The roof and its structural members should be capable of sustaining the weight of all rainwater that could accumulate on the roof if part or all of the primary drainage system should become blocked.

CONSTRUCTION SITE DRAINAGE TIPS AND TECHNIQUES TUTORIALS

CONSTRUCTION SITE DRAINAGE BASIC INFORMATION
What Are Construction Site Drainage? How To Create Site Drainage For Construction?


Difficulty often occurs in draining a site where large scale earthmoving is taking place. The excavations disturb the natural drainage of the land and large quantities of mud may be discharged to local watercourses during wet weather.

Complaints then arise from riparian owners and water abstractors downstream. If this possibility should occur the resident engineer should advise the contractor to approach the appropriate drainage authority (the Environment Agency in England and Wales) to seek advice on the best course of action to alleviate the problem, such as arranging some form of stank to pond the runoff and allow the heaviest suspended solids to settle out.

It is the contractor’s responsibility to dewater the site, and this includes the obligation to do so without
causing harm or damage to others. Dewatering can range from simple diversion or piping to ditches, to fullscale 24 h pumping and groundwater table lowering. It is usual to cut perimeter drains on high ground around all extensive excavations.

In dry weather this may seem a waste of time, but once wet weather ensues and the ground becomes saturated, further rain may bring a storm runoff of surprising magnitude. If no protection exists for these occasions extensive damage can be caused to both temporary and permanent works.

The resident engineer should assist the contractor to appreciate the danger of flood damage by providing him with data showing possible flood magnitudes. A frequently used precaution is to assume that a flood of magnitude 1 year in 10 (i.e. 10 per cent probability) will occur during the course of construction.

The need to dewater an excavation in the British Isles is the rule rather than the exception. Once dewatered an excavation should be kept dewatered. To repeatedly dewater an excavation during the day and let it fill up overnight can cause ground instability, and timbering to excavations may be rendered unsafe.

The need for 24 h pumping should be insisted upon by the resident engineer if he thinks damage or danger could occur from intermittent dewatering. The electric self-priming centrifugal pump is the most reliable for continuous dewatering, having the advantage that it is relatively silent for night operation as compared with petrol or diesel engine driven pumps.

For groundwater lowering, pointed and screened suction pipes are jetted into the ground at intervals around a proposed excavation and are connected to a common header suction pipe leading to a vacuum pump. It may take a week or more before the groundwater is lowered sufficiently, but when the process works well (as in silt or running sand) the effect is quite remarkable.

It permits excavation to proceed with ease in ground that, prior to dewatering, may be semi-liquid. However, it can be difficult to get the well points jetted down into ground containing cobbles and boulders; and in clays the well points need to be protected by carefully graded filters, or the withdrawal of water may eventually diminish because the well point screens become sealed by clay.

Special precautions must be taken to avoid damage to any adjacent structures when dewatering any excavation or groundwater lowering. In some soils groundwater lowering may cause building foundations to settle, causing considerable damage.

The contractor may have to provide an impermeable barrier between the pumped area and nearby structures, monitor water levels and perhaps provide for re-charge of groundwater under structures. Avital precaution is for the resident engineer to record in detail all signs of distress (cracks, tilts, etc.) in adjacent structures and take photographs of them, dated and sized, before work starts, in order to provide evidence of the extent of any damage which may occur.

The drainage of clay or clay and silt can present difficulty. The problem is not so much that it cannot be done, but that it can take a long time, perhaps many weeks. Sand drains (i.e. bored holes filled with fine sand), can be satisfactory as part of the permanent design of the works, but they usually operate too slowly to be of use during construction. If ground is too soft, any attempt to start excavating it by machine may make matters considerably worse, and end with the machine having to be hauled out.

The act of removing overburden may make a soft area even softer as springs and streams, otherwise restrained by the overburden material, break out and change the area to a semi-liquid state. If the resident engineer sees the contractor moving towards these difficulties he should advise him of the possible consequences, and endeavour to give assistance in devising a better approach.

A paramount need may be to call in an experienced geotechnical engineer to investigate the problem and give advice as to the best policy to handle the situation.