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 Bruce Snook by e-mail or on (08) 9323 4634.
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.
MRWA Supplement to Austroads Guide to Road DesignPart 7 - Geotechnical Investigation and Design
This Supplement has been developed to be read as a supplement to the Austroads Guide to Road Design, Part 1: Introduction to Road Design (2009), a copy of which can be purchased via the Austroads website.
In Western Australia, state-based information, in this website and elsewhere, takes precedence over Austroads Guides and Standards Australia Standards. National Guides and Standards take precedence over international Guides and Standards, unless specifically stated otherwise.
This Supplement has the same structure as the equivalent Austroads document and only additional requirements, clarifications, or practices different from the Austroads Guide appear. Where appropriate, this Main Roads Guide may also contain additional sections and figures not covered in the Austroads document, but the numbering sequence found in the Austroads document remains.
The following further background information is aimed at Road Designers and Project Managers when considering the need for geotechnical investigations.
Topsoil stripping is undertaken to remove and/or stockpile any vegetable matter or seed bearing soil prior to the earthworks phase. The depth of required stripping may not always be apparent and accordingly, it may be appropriate to seek geotechnical advice regarding the most suitable depth for a particular project.
Embankment foundations are typically assessed from test pit or borehole information taken at about 100m to 500m intervals along the route, depending on ground conditions
Proposed cuttings are typically evaluated to determine the presence of rock and/or subsurface water. A variety of equipment is used and may include dozers to carry out excavation trials in rock cuttings, or the taking of bore samples. Cuttings in rock are typically assessed by diamond cored boreholes or solid auger drilling. Solid auger borehole spacing may be as close as 20 meters. An evaluation of the dip and strike of the rock, and any discontinuities, may be important in determining the most effective cut slope angle for initial and long term stability. Seismic refraction (acoustic) surveys are sometimes used to determine the indicative rippability of insitu materials.
The stability of embankments may be an issue where the design profile is well above natural surface, and/or where unavoidable constraints require batter slopes steeper than 1V:3H (in sand), and/or where the toe of fill may be subject to erosion due to the presence of a moving water body (ie toe scour) The settlement potential under high fills may also need to be considered including a requirement for pre-loading. Where soft or wet soils are encountered, additional boreholes and test pitting may be required at a relatively close spacing of 50 meters.
Where sheeting or widening of an existing pavement is proposed, the relative permeability of different pavement layers and shoulder materials can be critical. For example, widening an existing pavement with a less permeable pavement material may well lead to water entrapment unless the original pavement is extremely well drained. In addition, tests on the residual strength of the existing sealed pavement may assist with a decision to widen alone, or in combination with sheeting.
Insitu materials found along the alignment will normally be tested to confirm the quality of locally available road building materials. A detailed evaluation of these materials is integral to decisions on how they can best be used. For pavement design it is also important to know the soaked and unsoaked CBR (California Bearing Ratio) of the material that will form the subgrade and the amount of traffic the road is required to carry over its design life. For pavement design purposes Main Roads has adopted a 40 year design life. The design procedure is described in Engineering Road Note 9.
Bridge foundation investigations are normally carried out in accordance with the Austroads Bridge Design Code, and borehole logging is done by an experienced engineering geologist. The information so derived assists with decisions of whether, for example, the structure should be on piled foundations or spread footings. Load-bearing reinforced soil structures also require an evaluation of foundation bearing capacity. Large culverts - particularly those that will be located on wet or soft soils - may also require a foundation investigation.
Water quality can be important when corrugated steel pipes are being considered or where salt water is likely to be encountered. A resistivity analysis will assist in determining the propensity for accelerated corrosion of the steel pipes. For concrete pipes in a saline environment, water quality testing will allow an analysis of the risk to reinforcing steel. Consideration should also be given to the erosive power of stormwater flowing from culverts. The information can be used to determine the design velocity that best suits local soils and thereby minimise downstream erosion potential.
Whilst some of these issues can be addressed by knowledge of past performance of existing roads in similar geological and climatic conditions, they should be carefully considered for all projects. Given the time involved, investigations should be initiated as early as possible in the design phase. (The Materials Engineering section of the Main Roads website contains a complete list of all Test Methods).