Recently through conversations with builders, engineers and building surveyors it has become increasingly apparent that Geotechnical Engineers are taking a significantly more conservative view on soil classifications. It is common now to see Class P Due To Abnormal Moisture Conditions on soil reports submitted to us. This is a reaction to house slab movement in the upper levels of ground surface where tree roots can affect moisture levels in the soil. Trees have a capacity to affect the surrounding soils due to the large amount of moisture they remove. If there are trees on site, you can expect your Geotech Engineer to advise you of the potential dangers of trees in the proximity to your project.
With Pool engineering, we generally look beyond that classification, and look for the underlying classification – this could be class M, Class H1 or H2, or it could remain Class P due to other reasons such as fill on site. As a general rule, if there is more than 400 mm of fill on site, the Geotech Engineer will nominate Class P, but as our pool engineering usually is dealing with foundations below a metre we are again considering the underlying classification. Levels of fill are always noted on the Bore Hole Logs.
There are other complexities that arise from soil reports. For example we can be advised of poor bearing capacity of soils, therefore we need to found the pool beyond the expected foundation depth. Also, when rock is encountered we need to found the entire pool onto the rock foundation base.
Soil Reactivity is a recurring theme. The strong forces associated with swelling soils are countered by engineer-designed additional steel and concrete in the sides and base of the pool shell. Although these clays can appear very stiff and a good solid pool base, we still often need to take measures to ensure the pool you build will withstand these additional forces structurally.
As per consistent advice, always get a soil report. Read it. It will provide you with guidance in regards to engineering costs and construction costs. It will save you a potential headache if you have to go back to your client with an unexpected variation. It will protect you in the unlikely event that you have a VCAT contest with your client over their pool being out of level due to soil reactivity.