Subsoil Drainage Detailed Design

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Detailed Design

Subsoil Drainage - Detailed DesignDocument No: D09#90695 Revision: 3A Date amended: 25-Apr-2012

The information below is intended to reflect the preferred practice of Main Roads Western

Australia ("Main Roads"). Main Roads reserves the right to update this information at any

time without notice. If you have any questions or comments please contact Cameron

Smith by e-mail or on (08) 9323 4916.

To the extent permitted by law, Main Roads, its employees, agents, authors and

contributors are not liable for any loss resulting from any action taken or reliance made by

you on the information herein displayed.

Revision Register

Ed/Version Number Clause Number Description of Revision Date

1 All Guideline Developed 13-May-2002

1A 1.6Main Roads Technical

Specifications updated.10-Oct-2006

1B 1.6Relinked to Updated

Specification 403.11-Sep-2007

2 All Guideline Revised and Approved. 08-May-2009

3 Header Contact updated. 11-Nov-2011

3A HeaderTelephone number of contact

person updated.25-Apr-2012

Table of Content2. Chapter 2 of 3. DETAILED DESIGN

2.1 General

2.2 Location of Subsoil Drainage

2.3 Design Information

2.4 Flushing Out Points

2.5 Subsoil Drainage Outlets

2.6 Geotextile for Subsoil Drains

2.7 Geocomposite Drains

2. Chapter 2 of 3. DETAILED DESIGN

2.1 General

The design of subsoil drainage should be undertaken as part of the geotechnical

investigation for the site by a suitably qualified engineer. Specialised guidance may be

found in Gerke (1987) and NAASRA (1983).

Subsoil drainage may be required:

to ensure adequate drainage of the pavement and subgrade

to lower the water table below the road formation

to intercept groundwater that could infiltrate the pavement.

Generally subsoil drainage should be designed to the Average Annual Maximum

Groundwater Level (AAMGL) which can be obtained from Waters & Rivers Commission.

https://www.mainroads.wa.gov.au/BuildingRoads/StandardsTechnical...

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Waters & Rivers Commission may impose limits to the lowering of ground water levels.

This needs to be confirmed on a project by project basis. Subsoil drainage should be

designed to provide the required capacity. To avoid the system failing due to partial

blocking of the pipe, the pipe should be designed to carry at least three times the

expected flow. However, the minimum slotted pipe to be provided is 100 mm diameter for

maintenance purposes.

The design shall take into account maintenance requirements to ensure the effective long

term operation of permanent subsoil drainage installations.

The minimum desirable grade of subsoil drainage is 1%, however in some instances this

may not be practical to achieve.

For specific design details of subsoil drain components such as geotextile, filter material

and drainage pipes, reference should be made to the manufacturer's product catalogues

which provide information to assist in this area.

2.2 Location of Subsoil Drainage

Figure 2.1 indicates examples where subsoil drainage should be placed with respect to

the carriageway and the pavement.

Figure 2.1 Typical examples of subsoil drainage

construction

2.3 Design Information

The subsoil drainage design information shown on the drawings shall be in accordance

with the requirements of Main Roads' Design and Drawing Presentation guideline.

2.4 Flushing Out Points

Typical details are shown in Figure 2.2 including cleanout openings which should be made

easily accessible for maintenance operations, and should be provided at intervals of not

more than 60 metres and at abrupt changes of grade and alignment.


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Figure 2.2 Subsoil Drain Flushout Points

2.5 Subsoil Drainage Outlets

The outlets of sub-soil drains not discharging into gully-pits, manholes etc. should be

constructed together with a small area of stone pitching to prevent grass growth and

blocking of the outlets. To aid finding the outlet a post marker shall be provided. The

preferred marker is a 80x40x1200mm steel box section capped, painted white.

If a subsoil drain discharges into a solid pipe then manholes for this pipe shall be provided

in accordance with the requirements described in Main Roads' Guide to Piped Systems.

If a subsoil drain does not terminate in a gully pit or man hole but rather discharges by a

surface outlet, the last 6.0m should be made impermeable by encasement in concrete or

PVC.

2.6 Geotextile for Subsoil Drains

Before specifying geotextiles, it is important to undertake a soil analysis to determine the

required geotextile properties. Past experience has shown that soils with silt content and

a low plasticity index may clog certain types of geotextiles. A proper design of geotextile

filter should be carried out based on the application, soil boundary and hydraulic

conditions. It may also be warranted to undertake analysis of the groundwater as certain

elements such as iron oxide can be detrimental to the performance of certain geotextiles.

The use of geotextiles shall be in accordance with Main Roads' Specifications 403

Sub-soil Drains. The Specification covers the supply and installation of subsoil drains

only. It is the designer's responsibility to ensure that the geotextile specified is adequate

to perform not only its design function but also withstand any construction loadings.

2.7 Geocomposite Drains

Geocomposite drains comprise a prefabricated polymeric drainage core with an outer

geotextile filter. They are generally referred to as Prefabricated Geocomposite Edge Drain

(PGED). PGED is a cost effective and easy to install solution for subsoil drainage works

including; roadside edge drains, under kerb drainage, cross drainage across highways,

under drains for road widening etc. Advantages of PGED include:

Geocomposite drains have a larger surface area than a round pipe, so they

generally collect infiltrating water more quickly.

These drains have a fabric filter fully supported by an internal core so it will not

intrude or burst.

These drains may clog due to the intrusion of fines and to buckling during or following

construction. Because it is nearly impossible to clean a geocomposite drain once it

becomes clogged, the presence of erodible fines and the potential for migration should be

investigated before such a drain is selected. In considering their use, risks associated with

these drains becoming blocked and the adverse effect on pavement performance should

be considered. For more details on geocomposite edge drains, refer to Austroads

Pavement Rehabilitation - A Guide to the Design of Rehabilitation Treatments for Road

Pavements (2004).

References:


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Gerke, R.J., 1987, Australian Road Research Board's Special Report No. 35,

Subsurface Drainage of Road Structures.

1.

National Association of Australian State Road Authorities, 1983, Guide to the Control of

Moisture in Roads. Sydney. NAASRA.

2.

Austroads 2004, AP-G78/04 : Pavement Rehabilitation - A Guide to the Design of

Rehabilitation Treatments for Road Pavements. Sydney. Austroads.

3.


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Subsoil Drainage Detailed Design