Drainage fill is the material most often overlooked on site. On the face of it the job is simple: lay the pipe, tip crushed stone over it and close the trench. Yet the material placed there has two jobs at once, and those jobs conflict. It must pass water quickly and at the same time hold back the fine particles of the surrounding soil so that they are not carried into the pipe. Choose the grading for permeability alone and the system fills with fine material in the first winter; choose it for retention alone and no water passes at all. This guide sets out the criteria that strike that balance and how they are applied on site.
The two jobs of drainage aggregate: passing water and retaining soil
The job of a drainage system is to collect the water that would damage a structure and carry it to a controlled outlet. Water building up behind a retaining wall applies a horizontal pressure the wall was never designed for; water entering a road body reduces its bearing capacity; water bearing on a basement wall puts the waterproofing under strain. In all these cases the solution is to capture the water in a permeable body before it reaches the structure and direct it into a pipe. The material that forms that permeable body is drainage aggregate.
Water does not travel alone, however. As it percolates, it drags along the fine particles of the soil it passes through. If those particles enter the voids of the drainage material and stay there, the voids gradually fill and permeability falls; this is called clogging. If the particles reach the pipe, deposits build up inside it and the system loses its function entirely. Worse still, the continuous removal of fine material creates voids in the soil; this internal erosion leads to settlement at the surface and, at an advanced stage, to collapse.
From this follow the two jobs of drainage aggregate: to be permeable and to act as a filter. These two conflict with each other through the particle size distribution. If the material is coarse and single-sized, water passes very well but no fine particles are retained. If it is fine and continuously graded, it retains the particles but does not pass water. The right answer lies somewhere in between, and you find that point with the filter criterion rather than by guesswork.
Permeability is set by the particle size distribution
The capacity of a granular material to pass water depends less on the volume of its voids than on their size. What resists the flow of water is the friction generated as it passes through narrow channels. The size of the voids is set by the smallest particles in the material, because small particles settle into the channels between the large ones and narrow the passage. Permeability is therefore about the fine end of the curve, not about the average particle size.
The practical consequence on site is this: a small amount of fine material mixed into a drainage material lowers its permeability disproportionately. A fines content of even a few per cent can reduce the water flow capacity of the material many times over. In drainage and filter material, therefore, the proportion below 0.063 mm is the most tightly controlled property and the material is produced washed as a matter of rule. Unwashed crushed stone looks like the same material; but the film of dust on its surface is carried down by the first rain and clogs the voids.
The width of the particle size distribution is decisive too. In a material with a high uniformity coefficient, that is, a wide distribution, the small particles fill the voids and permeability falls; in exchange, packing density and stability increase. In a narrowly graded material close to single-sized, the voids are large and interconnected and permeability is high, but the particles can slide against one another under load. A narrow grading is generally preferred in the drainage body and a wider one in the transition layers. We explained how these indicators are read off the curve in the article on sieve analysis and the grading curve.
The filter criterion: why fine material blocks the pipe
The filter criterion is a set of rules that relates the particle size distribution of the drainage material to that of the surrounding soil. Its logic is this: the voids of the filter material must be small enough not to let the coarse particles of the soil being protected pass through. Those coarse particles bridge over the mouth of the void, finer particles lodge behind them and the soil forms a natural, self-filtering layer. If this bridging cannot form, fine material is carried away continuously.
The classical and widely accepted rule rests on two ratios. The first is the retention criterion: the size below which fifteen per cent of the filter material falls must not exceed four to five times the size below which eighty-five per cent of the soil being protected falls. This condition keeps the voids of the filter small enough to hold back the coarse particles of the soil. The second is the permeability criterion: the size below which fifteen per cent of the filter material falls must be at least four to five times the size corresponding to the same percentage in the soil. This condition makes the filter appreciably more permeable than the soil, so that water does not accumulate within it.
The two criteria together define a range. Permeability sets its lower bound and retention its upper one; the particle size distribution of the material must fall within that range. Some applications add the condition that the two curves should be roughly parallel to each other; when the filter curve has a similar slope to the soil curve, the transition is more stable. The filter material must also not segregate within itself, that is, its fine and coarse fractions must not separate during haulage and placing; filters with an excessively wide grading are problematic for this reason.
If the soil is very fine-grained, a silty or clayey soil for example, it may not be possible to satisfy both the retention and the permeability condition with a single filter layer. A reverse filter is used in this case: two or more layers, becoming progressively coarser from the soil towards the pipe, are laid. Each layer protects the one before it and the last layer is in contact with the pipe. The graded transition divides a size difference that cannot be bridged in one step into several steps.
Pipe perforations, slot width and surround fill
The filter criterion has a pipe side as well. The perforations or slots in a drainage pipe must be sized so that they do not admit the particles of the surrounding aggregate. The generally accepted rule is that the coarse fraction of the material around the pipe should be appreciably larger than the perforation or slot opening; the size below which eighty-five per cent of the material falls is kept somewhat above the diameter of a round perforation, or above the slot width in a slotted pipe. The particles then bridge over the opening instead of pouring in.
This criterion is the point most often neglected on site. When a perforated corrugated pipe is used together with a fine sand bed, the sand pours in through the perforations and the pipe blocks within a few seasons. With the same pipe and washed crushed stone of the right size, the system works for years. Pipe selection and aggregate selection are therefore not separate decisions; the size of the surround fill cannot be decided without knowing the perforation geometry of the pipe.
The thickness of the fill around the pipe also matters. The material must be laid not only beneath and beside the pipe but also above it in sufficient thickness; water usually reaches the pipe from above, and if the cover is thin the fine material of the fill above will sit directly on the pipe. At the sides, the material must be placed beneath the pipe, and in flexible pipes especially into the haunch zone, so that no voids remain; a void underneath loads the pipe at isolated points and deforms it.
Use together with geotextile
A geotextile is a permeable fabric laid between the drainage aggregate and the surrounding soil, and it takes on part of the filtering job. It has two basic properties: opening size and water flow capacity. The opening size determines the largest particle the fabric will pass and, exactly as with an aggregate filter, is chosen according to the particle size distribution of the soil being protected. The water flow capacity ensures that the fabric can carry the flow coming from the soil; if it is inadequate, water builds up behind the fabric and the drainage loses its function.
A geotextile is not a material that replaces an aggregate filter but a component that completes it. Where building reverse filter layers is expensive or impractical, wrapping a coarse single-sized drainage material in geotextile achieves both high permeability and retention at once. Drainage blankets behind retaining walls and pipe surround fills inside trenches are mostly formed this way.
The geotextile itself can also clog. In fine-grained soils, especially silt-dominated ones, particles can lodge in the pores of the fabric and obstruct the passage of water. To reduce this risk the opening size is chosen carefully to suit the soil and, where necessary, a thin sand transition layer is placed between the geotextile and the soil. Non-woven needle-punched types offer higher water flow capacity thanks to their thickness, while woven types give higher strength; the choice depends on whether the filtering function or the separation and reinforcement function takes priority.
Three details decide the outcome in practice. The first is the overlap; the strips of fabric must overlap one another by an adequate width, otherwise fine material passes through the joints. The second is avoiding tears during laying; tipping sharp-edged coarse material directly onto the fabric and manoeuvring plant on top of it will punch holes in it. The third is the direction of the wrap; the fabric must close around the drainage body completely and its top must not be left open.
Behind a retaining wall: drainage or fill?
Backfill behind a retaining wall has to answer two different demands at once. On the one hand it must drain well and generate no water pressure, so as to reduce the horizontal thrust on the wall; on the other hand, being a fill itself, it is expected to compact and to have an adequate angle of internal friction. Meeting both demands with a single material is usually possible, but the design has to be made deliberately.
The common solution is to form a drainage blanket of washed crushed stone of a given width immediately behind the wall. This zone runs down to the collector pipe at the toe of the wall and carries the water straight down. Ordinary fill material is used behind the blanket. A geotextile is laid between the two zones to prevent the fine material of the fill behind from migrating into the blanket. Weep holes are left in the body of the wall so that the collected water can be discharged, and the mouths of the weep holes are also protected with filter material.
Washed crushed stone of a single size or a narrow grading is generally used as blanket material; the medium and coarse crushed stone fractions are typical for this work. Compacting this material differs from compacting a continuously graded fill: it beds in under the vibration of the roller rather than its static weight, and compaction is controlled not by classical density tests but by layer thickness and the number of passes. Since a heavy roller cannot be used in a narrow zone, the strip close to the wall is worked with small vibrating equipment in thin layers.
The most common mistake is to leave the drainage blanket open right up to the top of the wall. Rain and surface water then enter the blanket directly from above and the system runs permanently loaded. The correct practice is to seal the top of the blanket with an impermeable clay or compacted fill plug and to remove surface water through a separate channel. The second common mistake is forgetting the outlet of the collector pipe at the toe, or giving it insufficient fall; if the collected water cannot be discharged, the blanket turns into a reservoir and creates precisely the pressure you set out to prevent.
Trench fill and pipe bedding: the grading decision
In utility trenches, aggregate is used in three separate zones, and each zone expects something different. At the bottom is the pipe bed: the layer the pipe rests on, giving it even and continuous support. The job of the bed is to support the pipe along its whole length rather than at isolated points, so the material must be easy to place, free of large stones and workable by hand. In crushed stone, the medium and fine crushed stone fractions are suitable for this.
The second zone is the side fill on either side of the pipe, and it is the most critical zone for flexible pipes. A flexible pipe carries the load above it not through its own stiffness but through the reaction of the soil at its sides; as the pipe begins to flatten it spreads sideways and takes support from the side fill. Without that support the pipe goes oval and eventually collapses. The side fill must be placed without voids right into the haunch zone beneath the pipe and compacted in equal thicknesses on both sides; compacting one side only will push the pipe out of line.
The third zone is the protective cover over the pipe and the main backfill above it. The protective cover shields the pipe from impacts and from coarse material coming from above, so it must contain no sharp-edged large stones. In the main backfill the expectation changes: what is wanted here is no longer drainage but bearing capacity and freedom from settlement. In trenches beneath a road, this zone must be brought to the same compaction density as the surrounding body; otherwise settlement occurs along the line of the trench and a longitudinal mark appears on the asphalt surface. For the material logic of this layer, see the article on what sub-base aggregate is.
A common mistake in trench backfill is to use a single material in every zone. If single-sized drainage material is used to fill the whole trench, the trench turns into a long drainage channel inside the road body, collecting water from the surroundings and carrying it into the road. Conversely, if fill containing fine material is placed around the pipe, the drainage function is lost and the pipe perforations fill up. The correct practice is to use different materials zone by zone and to separate the zones with geotextile where necessary.
Material quality and the mistakes made on site
What is expected of drainage and filter material differs from what is expected of concrete or asphalt aggregate, but it is no less demanding. The first requirement is cleanliness: the material must be washed and the proportion below 0.063 mm kept to a minimum. The second is mechanical durability. That expectation may seem surprising, since the material carries no structural load; but if the particles break during compaction and under the weight of the fill above, new fine material is created and this dust clogs the voids of the system from within. A drainage layer built with a weak aggregate can work perfectly in its first year and lose its function within a few.
The third is particle shape. Cubical, angular particles interlock under load and preserve the void structure; flaky particles stack on top of one another, closing the voids and breaking easily. The fourth is long-term chemical durability. Limestone aggregate is widely and successfully used in drainage and filter applications; but in the particular settings where acidic groundwater is present it should be remembered that calcium carbonate can dissolve, and a siliceous material should be used instead if necessary. For the general character of limestone, see the article on what limestone is.
Most of the problems on site come not from the material but from the workmanship. The most common is the use of unwashed or dusty material; the second is laying drainage material straight onto the soil with no filter layer or geotextile in between. The third is fine material falling from the trench slopes during excavation and mixing into the drainage body; this is the quietest way of contaminating clean material. The fourth is inadequate overlapping of the geotextile, or leaving its top open. The fifth is failing to check the fall and the outlet of the collector pipe.
A properly built drainage system protects a structure for years without ever drawing attention to itself; a badly built one usually stays invisible, and the problem only comes to light as a crack in a wall, settlement in a road or damp in a basement. Material selection is therefore one of the decisions taken on day one that are most expensive to put right later. At its limestone quarry in Milas, İTS Maden produces washed drainage and filter fractions, pipe bedding and permeable fill materials with these requirements in mind. To determine the particle size distribution suited to the ground conditions and the pipe type on your project, you can review our products and contact us.
