Subsurface Drainage


The purpose of subsurface drainage is to control the moisture content of the pavement and the underlying material in order to assist in maintaining pavement strength and serviceability throughout the design life.

Subsurface or subsoil drains are required to intercept and drain excessive moisture or groundwater flow in order to avoid premature pavement failures. This moisture can come from:

  1. seepage or capillary rise from the water table (when in close proximity to subgrade/pavement)
  2. seepage from ponded stormwater into embankment/pavement
  3. seepage from an aquifer or other groundwater flow (e.g. springs)

The design and installation of subsurface or subsoil drains beneath/adjacent to road pavements is essential where groundwater or seepage is known or considered to be present.

It is important to note that the construction of an underground stormwater drainage system with associated granular pipe bedding can result in the interception of seepage and the concentration of this intercepted water at drainage structures. The installation of subsoil drains should be considered in conjunction with the drainage pipes to allow seepage water to be collected and discharged into the drainage system.

Subsurface or subsoil drains are provided in order to avoid the following types of premature failures:

  • loss of subgrade strength and shape due to an increase in moisture content in moisture susceptible materials
  • overload of the subgrade due to hydrostatic transmission of live load through a saturated pavement
  • layer separation and potholing

In general, type D subsoil drains is being used and cross-section is shown below taken from MTRS03

For latest copy of MRTS03, go to TMR website or for the copy click this link Transport and Main Roads Specifications MRTS03 Drainage, Retaining Structures and Protective Treatments

Strip filter drains:

 Strip filter drains shall comprise of a plastic core of nominal thickness not less than 40 mm, encased by a non-woven geotextile

The strip filter drain shall permit the passage of high water flows along the drain, and shall have a crush strength not less than 200 kPa.

Geotextiles shall comply with MRTS27 Geotextiles (Separation and Filtration).

Minimum cover for various compactors unless approved otherwise are as below:

Hand-held units: 100 mm

Units < 15 tonnes: 200 mm

Units > 15 tonnes: 300 mm

Trench backfill material shall be crushed or granular material which conforms to the grading requirements stated in this clause.

For Type D subsoil drains, backfill material shall consist of coarse sand with 100% of material passing the AS 4.75 mm sieve and not greater than 10% passing the AS 0.3 mm sieve.

Construction of subsoil drains

Subsoil drains shall be constructed in the locations and in accordance with the drawings.

Where the grades are not specified, the bottom of trenches shall be trimmed to provide not less than 0.5% longitudinal fall towards outlets for pipe-type drains, and not less than 1% longitudinal fall towards outlets for strip filter-type drains.

Clean-outs shall be located at the heads of all subsoil drains and at subsequent spacings not exceeding 50 m for strip filter-type drains. Clean-outs shall be constructed in accordance with the details shown on Standard Drawing 1116.

Trench backfill material shall be damp when placed in the trench where strip filter drain is employed and shall be compacted to achieve effective mechanical interlock between particles.

Where subsoil drains are constructed within the subgrade, they shall extend to the underside of the lowest pavement layer unless specified otherwise in the drawings.

Where subsoil drains are constructed outside the subgrade edge point, the trench backfill material shall be placed to within 150 mm of the ground or finished surface and the remaining 150 mm filled with impervious material compacted as for embankment material in accordance with MRTS04 General Earthworks.

After the drains are constructed, they shall be flushed out. Flushing shall continue until the outlet water is clean and flows consistently. It is a witness point.

No construction equipment shall be permitted to stand or travel directly on completed subsoil drains.

Concrete outlets, concrete surrounds and marker posts shall be constructed and/or installed in accordance with Standard drawing SD1116


Pavement drain and subsoil drains are different things. Pavement drains that are used in trafficked areas where new pavements abut old, and in other areas considered appropriate for their use. These drains shall consist of a strip filter and backfill (filter) material.

For pavement drains you need to refer to Technical Specification Transport and Main Roads Specifications MRTS38 Pavement Drains

Materials for pavement drains: 

  1. No fines concrete
  2. Strip filter
  3. Cleanout and outlet pipes
  4. Marker posts
  1. No fines concrete

No-fines concrete shall consist of cement, water and coarse aggregate. The quantity of cement used and grading shall be as specified in Table 26 below.

No-fines concrete shall be moist cured for at least four days by covering with wet hessian, polythene sheet or other similar material. The use of wet sand or any other material which can enter the voids shall not be permitted for curing purposes.

  1. Strip filter

There are many requirements listed for the strip filter in MRTS38 but the few major ones are below:

  • Strip filter minimum dimensions are 300 mm height and 25 mm thickness.
  • The strip filter shall be designed to allow the drainage of water along the drain and encased by a non-woven geotextile.
  1. Cleanout and outlet pipes

 The cleanout and outlet pipes shall, as a minimum, be 100 mm PVC Class 18 pressure pipe to AS 1477 Series 1.

  1. Marker posts

 Marker posts shall be tubular steel and comply with the requirements shown on the Standard Drawings.  Concrete used for the installation of the marker posts shall be 20 MPa/20.

Construction of pavement drains:

Where typical cross-sections for pavement drains are not included in the Drawings, the following requirements shall also apply:

  1. a) dimensions shall be in accordance with the requirements detailed in Figure 7.1 below:
  2. b) the bottom of the pavement drain shall be not less than 100 mm below the bottom of the working platform, or the bottom of the lowest pavement layer where a working platform is not provided

Minimum grade for construction:

The bottom of the trench is to be constructed at the same grade as the roadway where the grade of the roadway is equal to or greater than 0.5%, and in such a manner that localised ponding of water does not occur. Where the grade of the roadway is less than 0.5%, the trench shall be excavated and trimmed to provide a minimum grade of 0.5%.