Deep Bore Soakaway

This section shows a typical deep bore soakaway before and after installation.

Borehole position before starting.

This image to the right shows the site of an existing drainage attenuation chamber. If you look closely you will see a manhole chamber in the grass.

The image to the left shows the cable percussion drilling rig moving over the borehole position.


The image to the right shows the drilling rig set up over the manhole chamber. The driller then removes the manhole chamber so that drilling can commence through the existing chamber.


The image to the left shows the trailer arriving with all of the drill casing and tooling.


The image to the left shows the inside of the attenuation chamber full of water.

The image to the right shows the attenuation chamber has now drained, the water now has a pathway through the impermeable clay layer and into the fractured chalk.


The image to the left shows the borehole position after the works have been completed. Notice how we have kept the site clean and tidy.

Deep bore soakaways are the most effective and low cost solution to drainage problems.

There is no need to have your existing soakaway replaced, we can recommission your existing soakaway to work perfectly.

Certain rural areas of Kent are high above the chalk aquifer making them unsuitable for water well drilling but perfect for deep bore soakaways. If you are not connected to the mains sewerage system you will be aware of the high cost of emptying your septic tank or Klargester and how quickly it fills up. Most septic tanks are also connected to soakaways to drain away the excess water. Sometimes the soakaways are situated in clays which are not suitable for the purpose and become ineffective very quickly. They also tend to silt up over time which stops the water from draining away. The problem can be exasperated when rainwater is also directed into the soakaway. The soakaway fills up very quickly and although 99 percent of the volume is rainwater, you have to pay the far more expensive rate for getting contaminated water disposed of.

Usually the solution is to have the soakaway emptied and then cleaned out, this can be extremely expensive - if you have a soakaway that isn't working correctly we are sure you are already aware of this!

Surface water drainage can be particularly problematic especially in rural areas when no mains drainage is present. Systems which require large scale excavation to provide sufficient soakaway drainage are not only very expensive and intrusive, they often fail through prolonged periods of heavy rainfall. The problem is that if the ground is clay, it doesn't matter how big the excavation is it will eventually fill with water, if the rainfall is more than the storage capacilty of the excavation the soakaway problem will still be there. In many areas of Kent there is a clay layer sometimes called head deposits which sits above the permeable chalk, this layer can be any depth from between a few centimetres to over 20 metres below ground level. Any soakaway constructed in the clay layer is doomed to failure as the water cannot permeate into the clay quick enough. By drilling through the  full extent of the impermeable clay and into the permeable chalk fractures, it is possible to drain massive volumes of water which will permeate through the fractures to eventually replenish the underground aquifers. Water can freely drain at rates in excess of 10 litres per second without any type of mechanical installation at all. The environment Agency should be consulted in the first instance to see if they have any reservations about discharging water into the ground. Generally speaking the Environment Agency only have a reservation if you are discharging foul water or are in very close proximity to an aquifer which may be used as a water supply. To find out what the geology is beneath your property just give us a call and we will do a FREE geological survey to determine if your property may be suitable for a deep bore soakaway.

If you already have a soakaway which is just filling with water and never soaking away, there is no need to have the soakaway emptied or cleaned out. We drill through the base of your existing soakaway and carry on drilling until we find the porous geology. We then install a well casing which will quickly drain all of the excess water. The installed well is positioned 1 metre above the base of the existing soakaway, this allows any silts which enter the soakaway to settle and not interfere with the drainage. No maintenance is required and you can look forward to trouble free drainage.

Please feel free to give us a call and receive a free no obligation quote, we are always happy to answer any questions a client may have.

For  those of  you who would like more detail please read on.

Technical detail

The deep bored soakaway consists primarily of two parts, a conventional chamber, and a narrow bore perforated liner usually 100mm to 150mm diameter. The chamber is constructed in more or less a similar manner as a conventional soakaway, but the chamber is used solely for attenuation.

The chamber is generally formed wholly from non-perforated rings (Milton Rings) which are constructed from preformed concrete. The size of the chamber is dependant on how much water flow will come into the chamber at peak flow and how quickly the water runs away down the deep bore soakaway pipe. Chamber depths do not usually extend beyond 7 metres and generally are 3 to 4 metres in depth.

In exceptional circumstances the lower part of the chamber may be perforated, provided the perforations commence a minimum of one metre into a suitable permeable strata. Passing through the base, which again can be concrete, is a plastic or steel liner usually 110 to 150mm in diameter in Kent, although in exceptional circumstances diameters of between 80 and 300mm have been used, these are used in exceptional circumstances such as for use by the local water companies. High volumes of water are pumped by the water companies to lower the turbidity (small particles suspended in water) of the water.

Limitations in depth of the liner as a result of groundwater protection concerns are increasingly resulting in the use of 200mm and 250mm diameter liners. However for domestic use a 110mm, deep bore soakaway pipe should be more than adequate. A call to your local buiding control or environment agency office should be able to advise if there are any special requirements.

The liners are inserted into a borehole drilled normally by a cable percussion shell and auger or rotary auger rig depending upon the underlying geology, with the diameter of the borehole a minimum of 50mm greater than the outside diameter of the liner. The plastic deep bore soakaway liner is perforated over the permeable layer to facilitate discharge of the surface water run-off into the surrounding soil and as such its length is dependent upon the required soakage rate although it may be limited in depth by the requirements of the environment Agency to avoid contaminating a water aquifer. The usual requirement of the Environment Agency is that the deep bore soakaway must terminate 10 to 20 metres above the water table or aquifer.

A section of non perforated liner is normally installed to below the interface with the soakage medium (usually gravel or shingle) and the overlying relatively impermeable soils. It is recommended that the non-perforated liner penetrates at least 2 metres into the soakage medium. Between the perforated liner and the side of the borehole is a gap which is backfilled with 'pea-gravel' or similar typically 10mm in size and up to 20mm maximum size. Like the granular material surrounding a conventional chamber this helps to disperse the water evenly from the liner perforations into the soil and backfills the void between the borehole and the liner to allow the installation of the bentonite seals above the perforated section of the pipe.

As the diameter of the borehole is more controlled than the dimensions of the conventional chamber excavation, the soakage area of the liner is taken as the borehole diameter. Above this annulus of granular material lies a bentonite seal which restrains the water to below this level even when the run off backs up into the chamber. For particularly sensitive sites it may be appropriate to introduce a reducing coupling local to the grouted area to enable a thicker annulus of grout to be obtained (eg reduce 150mm perforated pipe to 110mm plain pipe at or below the grouted zone and extend 110mm pipe into the chamber) and/or increase the depth of the seal above the standard depth increment of 2 metres. Capping the liner which protrudes one metre above the base of the chamber is a syphon head placed to prevent the blockage of the deep bore. This also helps to inhibit the ingress of silt or foreign objects into the liner. Access into the soakaway chamber to install the syphon head must be treated as a confined space if at any significant depth and can only be undertaken with the necessary safety precautions (Gas detector, safety harness, tripod winch, backup team etc). Borehole-Driller uses a custom made syphon head which can be lowered over the soakaway pipe which negates the need to enter the chamber. The deep bored soakaway intercepts far fewer joints than the conventional soakaway and therefore relies upon the driving head developed by the water column to force the run-off through the available fissures that are encountered in the sides of the borehole.

This installation is prone to smearing the fissures in the borehole which further reduces the soakage rate. Use of Muds and Polymers to stabilise the borehole can also be detrimental to the permeabilty of the fractures. Cable percussion rigs suffer far less smearing due to the nature of the process of drilling and are therefore preferable. We undertake a series of soakage tests as the borehole is deepened and then  carry out a permability test prior to and after installing the deep bore soakaway liner to confirm the soakage rates. The field tests do, however, satisfactorily model the capacity of the deep bored soakaway generally being of a similar diameter to the proposed liner. Field tests on the installed liners are possible and less water is generally required to demonstrate the liner capacity compared to a conventional soakaway although tankers will often be required to fully justify the capacity of the deeper liner systems in highly permeable strata. These tests would be needed if the deep bore soakaway was for perhaps a housing estate, but for domestic use a water bowser and pump test would suffice.

In general terms drilling within the Alluvial gravels and chalk can be undertaken by conventional cable percussion (shell and auger) techniques. The 'rock-like' structure of the Lower Tunbridge Wells Sands and especially the Hythe Beds often precludes the use of the light percussion rig methods. In such deposits it may be necessary to utilise rotary coring drilling techniques. The costs of this specialist drilling is however quite high, typically 2 to 4 times that of cable percussion rig work. In some circumstances it may be advantageous to employ a light cable percussion rig to drill through the overburden materials and prove the position of the underlying 'rock' strata, the rotary rig can then move onto the position and carry on with the rest of the drilling. This may be particularly useful on sites where a number of deep bore soakaways are required, such as a large housing estate.

Once this has been undertaken the more expensive specialist rotary drilling rig can be brought onto site to complete the works. For brown field sites, whilst the deep bore soakaway liners themselves may be in suitable strata, the chambers and/or inlet pipes may be in loose made ground. Settlement of the chamber and/or opening of joints in the inlet pipes will stop some of the water reaching the soakaway liner which may result in surface settlements and/or washouts. It is therefore important to consider the need for ground improvement local to the soakaway chamber and greater flexibility in the inlet pipes on larger scale soakaways.

Whether it’s a water well or a deep bore soakaway, contact us to discuss your requirements. We guarantee to beat any genuine drilling quote!!!