By Troy Schultz (Principal Engineer) & Kerry Saron (Senior Urban Designer)
It’s hard to believe that there was once a time in SEQ when developers could readily buy flat and serviceable land. Now we see that development sites tend to be steeper and harder to service. Despite these sites being more challenging, with some careful planning and smart engineering and urban design, it is possible to deliver attractive sites that work and that achieve good commercial returns.
Having recent experience delivering several development projects with natural slopes in excess of 20% we thought it would be useful to share some key steps in the approach to developing steep and sloping land.
Step 1. Review site topography and natural slopes
Think about different land uses and where the most appropriate location for the specific use might work best. If rock or difficult geotechnical conditions are not influencing this, then there’s obviously more flexibility in the urban design and earthworks options. However putting terrace housing product, sports parks or schools on land with natural slopes of 5% or more (with hard rock underlying the surface) generally means significant and costly earthworks and retaining outcomes. Inappropriate land use designations are decretive to feasibilities with the cost to deliver the land use often higher than the profit being realised.
Step 2. Engage with a geotechnical engineer to undertake a desktop review of likely sub surface conditions
This step allows you to approach the urban design and earthworks strategy from an informed perspective. For example, is the site better suited to a ‘more earthworks / less retaining strategy’, or a ‘ less earthworks / more retaining’ strategy. With hard rock present, adopting an earthworks design involves less cut and better matches the natural surface (less earthworks / more retaining) can save considerable costs in blasting, crushing and processing of rock. If no rock is present, the site might be best designed as an export site with spoil material going to another nearby site.
Step 3. Work with the urban designer to ‘custom fit’ the layout to overcome the constraints
Avoid too many curves and irregular shaped road patterns. Align roads perpendicular to contours so side boundaries of allotments can step up / down the terrain. This can avoid having large double tiered walls in rear of allotments which add to cost and adversely impact the usability of the allotment. In the case of sportsfields, perhaps a strategically located tier will optimise earthworks, better match natural surface and reduce costs. Consider a built form / split level housing product in select locations to ‘break the back’ of the slope and to achieve smaller walls in other areas of the site.
Step 4. Undertake a high level earthworks design
Grade the roads and establish likely lot levels. Take account of drainage pipes and sewerage reticulation being another 2-2.5m below the design surface level. Major roads often have maximum grade limits (8%) so ensuring these are located around high points rather than going straight through then often reduces cut considerably and makes for a more cost effective project solution.
Step 5. Arrange for a targeted geotechnical testing/test pits on site in select locations.
It is important to validate the design in the field. We like having the contractor dig test pits using a 30T excavator fitted with a ripper. It enables a more accurate assessment of what actual site conditions and constructability will be like ‘on the ground’ when civils commence. A borehole is useful (as is seismic testing) but nothing beats excavating a few test pits on site in select location to better understand how easy or hard the excavation will be. This information is critical in understanding accurate earthworks costs.
Step 6. Review urban design and earthworks philosophy based on site investigations.
Armed with useful information, optimise the urban design and earthworks solution. Many Council’s are now pushing for tiered walls when the wall height is greater than 1.5m. Consider changing the grid to include deeper lots where tiered walls are present in order to fit them in and maximise building area. Modify alignments of major roads to avoid high points, change allotment access locations, lift the design in key locations, shortened cul de sac heads etc. To achieve minimum wall heights in rear of allotments, use the house set back line to kick up at 1 in 6 on the high side of the road and 1% on the low side of the road.
Whilst most of the value lies in layout and design optimisation, we have found a few things to be useful at tender time in order to maximise savings and get the most competitive, informed tender price:
- Include as much geotechnical information in the tender as possible (for information only);
- Provide sufficient time for the tender response including making the site available to Contractors to dig their own test pits and inform themselves; and
- Obtain a range of pricing options based on lump sum (fixed price) or shared risk (Provisional Quantity) to suit the risk profile of the project.
Essentially, more information and more time results in a lower risk price.
As can be seen, a one size fits all approach is too simplistic when considering steeper sites. (The same is true for any site but particularly steep ones). Tailoring a solution to overcome site specific challenges can save time, reduce both risk and cost while presenting a highly desirable product to market.