The developer proposed to demolish an existing house and construct three detached houses.
The land buying department of the developer had identified a site at 11 Brownswood Road, Beaconsfield. One of the main worries that the developer had when deciding to purchase the site was the unknown factors that could push up the cost of the development. This is because “most project delays and cost over-runs are due to ‘unforeseen ground conditions' being encountered once construction works have commenced on site.” To mitigate this risk a desk study was carried out in the early stages.
A desk study is a relatively inexpensive way of gaining a large amount of data about a site and allowing potential ground-related hazards to be identified at an early stage.
A desk study usually includes information on the:
- Site history
- Geology and geomorphology
- Topography and drainage
- Vegetation and land use
- Underground features such as existing foundations, services or tunnels
- Archaeological potential
The geological information from the desk study showed that the site was overlying chalk; however this was only encountered at depth. From this information the land buying department assumed that the storm water for the proposed development could be disposed of by using soakaways in the strata's above the chalk and that the foundation were to be traditional strip footings.
As a result of the desk study the land buying department for the developer went ahead and purchased the site. Now that the developer had full access to the site, they were able to commission a full Site Investigation for the use of the Consulting Engineer. The Site Investigation's aims were to confirm the geological findings from the desk study as well as providing ground bearing information for the Consulting Engineer for foundation design and permeability tests for soakaway design.
The Site Investigation generally confirmed the desk studies findings by identifying that the site was on chalk, except that in lower areas of the site the chalk outcropped at a higher than anticipated level. Due to their findings the Site Investigation Company advised against the use of soakaways due to the possible presence of solution features within the chalk.
After consulting further with the Site Investigation Company the developer commissioned cone penetration tests. The cone penetration test was chosen due to its increased accuracy, speed of deployment and reduced cost compared to other soil testing methods.
A Cone penetration test is an in-situ method used to determine the geotechnical properties of the soil and delineating soil stratigraphy. The test consists of pushing an instrumented cone tip into the ground at a controlled rate. The resolution of the cone penetrating test in delineating stratigraphic layers is related to the diameter of the cone tip.
The cone penetration test found a large bell shaped void in the chalk filled with drift material, hence a large solution features. This solution feature was found in the rear gardens of the proposed properties. The Site Investigation Company again stated that “no soakaways shall be permitted and water concentrations should be avoided as water can activate movement of the metastable material.”
Solution features are also commonly known as a swallow holes. They are naturally occurring features created over time by the gradual removal of the soluble bedrock (chalk in this case) by percolating water. In this case the solution features became inactive and became filled with drift material. When filled with drift material the solution feature is structurally sound, which makes it so difficult to detect.
If the developer chose to use soakaways in the rear gardens of the properties where solution feature were present it could have had potentially devastating consequences. The presence of soakaways would have reactivated the solution feature because of the change in water flows within the upper strata's. The concentration of water coming from the soakaway would wash out the drift material potentially causing settlement/collapse. Solution features are so dangerous because they are very difficult to detect because they cannot be seen from the surface.
The developer was now able to instruct the Infrastructure Engineer to design an alternative offsite disposal of the storm water. Although doing this was more expensive, it was a known cost prior to commencement on site. Similarly “Piled foundations will need to be adopted due to the presence of dissolution features”. This is a redesign from traditional strip footings.
The Site Investigation Company visited during excavation but found no evidence to arouse concern for the other areas of the site.