Design of Sub Surface Drainage System of Roads
McAdam one of the fathers of designed pavements, stated “it is the native soil which really supports the weight of traffic; that whilst it is preserved in a dry state it will carry any weight without sinking, if water passes through a road to fill the native soil, the road whatever may be its thickness, loses support and goes to pieces.”
The thickness of the road pavement is primarily a function of the subgrade CBR and CBR values for soils are function of their soil type, moisture content and density.
The road pavement is susceptible to moisture ingress from beneath the structure as well as from infiltration from above or from the sides.
When the pavement becomes wet, there is significant risk of the pressure from passing wheel loads forcing water deeper into the sub-grade. If the granular layers are saturated, then the vehicular tyre pressure is passed downwards to the subgrade in a concentrated hydraulic pulse through the incompressible water. This considerable increase the stress imposed on the sub grade and can lead to a very deterioration of road condition.
If the sub grade is allowed to be saturated, then fine particulates can be forced into the subbase. This is often called ‘pumping’ and will eventually result in the permanent deformation of the road pavement.
For these reasons, sub surface drainage is installed to prevent moisture from entering the road structure in both cut and fill situations, or to prevent the upward movement of groundwater from the water table. In the UK, this is achieved by installing longitudinal sub surface drains. Typically, a narrow filter drains or a fin drain. These are installed immediately adjacent to the road structure and at a depth that is sufficient to drain the road structure and sub grade.
Generally, the sub surface drain should be constructed either; 600mm below the compacted and shaped formation, or that the invert of the drainage pipe should be set at least 50mm plus the nominal pipe diameter below the bottom of the capping.
The depth is the deepest of these two criteria but may in some specific cases be deeper.
Design of Longitudinal Sub Soil (Trench) Drains
Traditional granular material trench drain?– The traditional design of a sub soil drain is trench below the bottom of pavement level. This is backfilled with a graded granular material which will be considerably more permeable than its adjacent soils to allow free drainage. The purpose of the grading is to provide:
The grading of this material therefore must neither be too coarse, which allows fines to migrate into the large pore spaces of the filter material which will cause blocking, nor too fine which will reduce flow discharge to an unacceptable level.
Although many formulae are available for the design of aggregate drains, the most commonly used is that developed by the US Waterways Experiment Station, and it has particular application for sands and silts.
The following criterial should be achieved.
The mechanism for clays is more complex but coarse sand is usually satisfactory
(*F stands for filter and S stands for excavated/protected soil)
During construction of these drains it is important to ensure that segregation and/or crushing does not occur in placing the aggregate.
Particularly when high flows are expected a pipe is usually included at the bottom of the trench. However, this increases the cost of construction and is more difficult to construct.
Note that the slots or holes are best placed in the bottom semi-circle of the pipe to prevent filter material blocking the holes. The size of these holes or slots is approximately linked to half the size of the 85% passing of the filter material.
A nominal size pipe is normally chosen and is adequate for run-off purposes.
If the filter material is too fine, then situation can arise when the permeability of the trench drain is less than that of the pavement layers. Typically, in the UK, the granular subbase is expected to drain pavement water towards the trench drain.
The longitudinal fall of the drain may also be important as this will affect the hydraulic gradient and on long flat grades the free water surface in the drain may be higher than the pavement. When a filter trench is backfilled with granular material it is usually ‘capped’ with an impermeable material such as clay. This will prevent surface flows entering the drain, which may seem to be an unnecessary treatment and indeed an inefficient one. However, surface run off will carry fines capable of blocking the filter material. In addition, the surface water could enter the lower pavement layers via the trench drain.
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Composite of Fin Drains?– These are the latest in subsoil drainage development. This type of drain requires very little trench digging and hence very little filter aggregate backfill.
The usual mechanism by which fine material is washed into a filter or drain (or borehole) is as follows. Prior to the advent of filter fabrics, the pipe (or screen) contained relatively small and few slots through which the water had to pass to get into the pipe. As the flow approached these holes it must speed up. In doing it is able to erode the filter material and sweep it into the pipe where it may settle, of block some of the slots. This process can migrate into the filter material for a considerable distance. To avoid this a suitably graded material must be interposed between the soil and drain.
Because a geotextile allows flow over all its surface the flow concentration does not occur, and less erosion takes place.
A polyethylene core with layers of fabric fastened on either side is then wrapped round a plastic perforated of slotted pipe which acts as a carrier. The system can be assembled on or off site. An important factor to be considered is the permeability of the layers and the effect of confining pressures. Care must be exercised again in the choice of the fabric pore size.
These cores are known as thin cores, transmitting water to a carrier pipe. They are typically up to 10mm thick, and usually incompressible.
Thick cores claim to transmit water longitudinal without the need of a carrier pipe.
Example Calculation for Fin Drain Size:
Based on HR Wallingford 5-year storm 60 min duration, the rainfall event is between 16 and 20mm/hour depending on the site location.
For assessment of how much water can theoretically get into the drain, it is assumed that a poorly maintained road could allow up to 20-30% of the water to pass into the road structure.
For example, for 8m wide carriageway, 2m verge, 20% passing through road structure, 30% passing through verge. Rainfall (M5-60) 20mm/hour.
E = R.W/3600) * (F/100) = 1.2 litres/sec/100m of carriageway.
Upward Flow
Flows of water from springs or aquifers are not common but can be dealt with either with a horizontal drainage layer underneath the pavement or with deep trench drains at the side of the roads. In many cases a combination of both is required. More commonly encountered is a recharged water table which has been depressed by a cutting. This may require a similar treatment to that above. Modified standpipes can be sunk into aquifers along the side of the road to relieve pressure and carry flow upwards to a side trench.
Lowering Water Tables
Shallow water tables are likely to have a strong influence on not only the performance of a pavement but also the construction.
Deep trench drain may be appropriate to lower shallow water tables. Deep side drains in all but permeable materials are unlikely to be effective for construction purposes as far as lowering the water table is concerned. This will of course be useful for preventing run-off and seepage into the site.
If lowering of the water table can be achieved in the construction stage then a stronger sub grade could be expected, and if this could be protected from future wetting up this would allow the laying of a thinner pavement on top.
Suction (under capillary action etc.)
Soil suction can be defined as the ability of a soil mass because of its structure to attract water above the water table through absorption and surface tension. Such water is held above the water in tension and hence can be considered a negative pore water pressure. The suction force increases with distance above the water table. The finer the material and hence the voids, the greater the suction forces and the ability to raise water to a higher level above the water table.
The strength of soil is increased by suction, the negative pore water pressure i.e. (suction) pulling the soil particles together. This is a reason why greater strengths are achieved for a soil dry of optimum moisture content. Optimum moisture content is achieved when the soil is almost saturated and nearly developing positive pore water pressures. Drying back will provide suction and hence greater strength.
In the UK the water table is usually near the surface and changes little seasonally. Therefore, formation strengths are not so greatly affected by the suction phenomenon.
In the warmer countries the water table often is lower in the soil profile and more importantly moves up and down seasonally. The sub grade therefore can have high strengths because of this suction effect but can then drop drastically if the water table rises. Careful site investigation is required in such circumstances.
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11 个月Thank you for the great article. I am trying to understand matric suction more for pavement subsurface design. I noticed in the article you were referring to the subgrade. What about the subbase? Do you consider this parameter in how much flow happens in the subsurface which will ultimately need a subsurface drainage?