What is a temporary access road?
Temporary access roads are regularly required on construction sites. They provide access for what can be heavy construction vehicles carrying materials and equipment needed to complete the project at hand.
Because these roads are required only temporarily, they tend to be constructed from a granular fill or aggregate placed on existing ground. The granular fill or aggregate acts as the road surface itself.
These roads can be as short as a few tens of metres or long enough to extend several kilometres (for example, to allow access for the construction of wind farms).
How are temporary access roads designed and built?
Although an access road may only be needed for a short period—say six months—it should be designed based on the anticipated traffic load it will carry over its operational life.
An access road design assessment will take several factors into account for the site under consideration:
• Existing ground strength—normally expressed as a California Bearing Ratio (CBR) , undrained shear strength or stiffness modulus
• Anticipated traffic load—expressed as a number of vehicle visits of differing type and configuration (e.g. concrete mixers, steel delivery vehicles) from which a number of Equivalent Standard Axle Loads (ESAL’s) can be calculated*
• Granular fill grading—this refers to the quality of the aggregate used to build the road and would typically be classified using a particle size distribution curve
• Target surface deformation or rut depth—a serviceability target for the road surface after trafficking
*It’s important to recognise that before the traffic for the main construction project uses these temporary roads, vehicles delivering the granular fill to build the access road itself will need to use them too. This means the critical time for building an access road is the construction phase itself, where the vehicles delivering the granular fill are travelling at their closest point to the subgrade soils.
The design procedure identifies what thickness of granular fill is needed to carry the traffic load. The traffic load is based on a surface deformation target established using a method of calculation derived from empirical data.
In the UK, the granular foundation to a surfaced road is designed separately to the asphalt layer laid over it. The methods adopted can be traced back to the Transport Road Research Laboratory (TRRL) and the UK Highways Agency (Figure 1).
The design of roads is inherently empirical…..
Figure 1: Confirmation from UK Highways that road design is empirical
What are the benefits of using geogrids in temporary access roads?
Many temporary access roads are needed where existing ground conditions are poor as well as variable. Often, this means that using “traditional methods”, these roads could require a significantly thick layer of stone to cater for the anticipated traffic load and will often need regular maintenance to address surface deformation and rutting.
Stabilisation geogrids can give road designers and users significant benefits in terms of reducing the thickness of the granular layer needed to build the road. This results in:
Tensar geogrids have been used in temporary access roads for more than 35 years. These performance-based products are developed and applied to thousands of kilometres of roadway constructed over many different ground conditions to deal with a large variation of loads.
However, it’s important to recognise that including any geogrid in a granular roadway construction doesn’t mean that savings will be available in every case.
The measurable benefits from including geogrids in temporary access roads can’t be identified simply by assessing their suitability based on parameters on a product specification sheet.
As mentioned earlier, the design of granular access roads is typically based on empirical data from testing and research. This is also the most effective way of quantifying the benefits of a geogrid in a granular road.
To this end, Tensar International in the UK have carried out a programme of nine full-scale trafficking trials at the Transport Research Laboratory (TRL), formerly the TRRL, over a 30-year period and have demonstrated the following:
• No one geogrid parameter has been identified as a good indicator of expected performance.
• The ultimate tensile strength of a geogrid is a poor indicator of expected trafficking performance.
• It’s the effect of the geogrid in combination with the granular aggregate that gives the greatest benefit to the road. This effect is known as “stabilisation”.
• Geogrid-specific “stabilisation factors” can be derived to allow the effect of a geogrid to be modelled in the road design assessment.
Incorporating a stabilisation geogrid within a granular roadway forms what’s known as a mechanically stabilised layer (MSL). The interaction between the granular particles and the stiff geogrid structure is critical (Figures 2 and 3).
Figure 2: Granular particles confined within the stiff geogrid apertures
Figure 3: A Tensar mechanically stabilised layer consisting of TriAx geogrid and granular fill
How can the benefits of a geogrid be quantified?
Reducing the roadway’s thickness while maintaining trafficking performance is made possible by using the data gathered from the programme of full-scale trials to determine geogrid-specific “stabilisation factors”. These factors can then be incorporated in bespoke roadway design software (Figure 4).
Figure 4: TensarPave 7 roadway design software comparing a non-stabilised section on the left with a mechanically stabilised layer on the right
Using this software, designs with and without stabilisation geogrids can be compared and assessed and savings can then be quantified. Including a stabilisation geogrid in a granular access road will reduce the non-stabilised thickness by up to 50%.
While other mechanisms are available, they rely on tensile strength. This means deformation is likely to develop in the roadway, not just at the surface but at the level at which of the reinforcement geogrid which will remain in the roadway permanently.
A stabilisation geogrid, however, doesn’t deform like a “hammock” but remains level as part of the MSL. This protects the subgrade from permanent deformation (Figure 5).
Figure 5: The difference between stabilisation and reinforcement mechanisms with geogrids
How do geogrids match project conditions?
Tensar has developed a range of stabilisation geogrids to give engineers and road designers flexibility in catering for conditions specific to their projects. Its criteria for selecting geogrids is based on 35 years of practical experience, covering:
• existing ground conditions
• anticipated traffic load
• grading of granular fill to be used in the access road
• the temporary nature of the access road
The different grades of stabilisation geogrid aren’t based on varying levels of strength, but different sizes of aperture and different product stiffness to suit specific project conditions. This means that a wide range of fill types can be used to form the MSL, from coarse fill all the way down to a sandy material (Figure 6). They can also be used on a wide variety of subgrade soils.
Figure 6: Range of stabilisation geogrids to allow flexibility in the type of granular fill
Some of the most extreme conditions in which temporary access roads are needed come in the construction of wind farms. Often, these roads:
- are in remote locations
- extend over existing ground consisting of low-strength soils (such as peat) or clays
- must be able to carry numerous heavy construction vehicle
- Once built, they are then needed for future maintenance and decommissioning traffic and so should remain stabilised for many years.
Great Eppleton wind farm, UK
The photos above show the geogrid being installed over low-strength soils (left image) and the finished access road once the granular fill has been placed and compacted on top of the geogrid (right image).
The contractor confirmed that including the stabilisation geogrid “….allowed the construction thickness (of the access roads) to be significantly reduced and our installation was rapid and straightforward”.
Whitelee wind farm, UK
Around 45 km of access roads were required to build a large wind farm just outside Glasgow in Scotland. The ground conditions across the site consisted of low-strength and variable soils and initial site traffic would have a total weight of around 50 tonnes.
Stabilisation geogrids were incorporated into granular fill with the resulting MSL assessed to cater for the type of stone-delivery vehicle shown in the above photograph. The access roads reduced the thickness of the roadway, saving the contractor both time and money.