My Soil Report Has Come Back Class P – Problem Site. Should I Be Worried?
Opening your soil report to read ‘Class P – Problem Site’ can be unsettling. Fortunately, in most instances, a Class P site classification does not mean the site is a literal problem without solutions. In fact, most soil reports come back Class P.
In most instances, Class P sites require common solutions familiar to most builders. Sure, some sites are very problematic and may require costly design solutions that render a project unfeasible. However, such sites are generally rare. Note, this article is relevant to residential construction.
What is a Soil Report?
It’s good to know where a site class comes from. A Soil Report – formally referred to as a Soil Site Classification report – informs the structural engineer on what needs to be considered for a building’s slab and footing design.
The report outlines the key information the engineer needs to consider so the building’s footings are strong and stiff enough to resist factors such as soil reactivity, seasonal moisture changes, fill and/or other possible issues such as low soil strength. Key information outlined in the soil report include:
The Soil Report provides a site classification in accordance with ‘AS2870 - Residential slabs and footings’ and is to be prepared by a suitably qualified geotechnical consultant or engineer.
The soil report is an essential document for a project that involves footing works and is lodged as part of Building Permit documentation.
Soil Reactivity
Site soil classifications are based on soil reactivity. Fortunately, this is not ‘nuclear’ reactivity. Soils react to water. Reactivity is the shrink-swell potential of soil with changes in moisture content. Some soils such as gravel and sand have very low reactivity. Most silts lose their strength when significantly wet, turning from dry soil to a ‘soupy’ consistency. Clay soils shrink and swell with changes in moisture content and are the main culprit for footing movement and Class P sites.
To visualise clay reactivity, picture a sponge. A sponge shrinks when dry and swells/expands when wet. Clay acts much the same and has a diverse range in how much they can shrink / swell. Some are from slightly (S), moderately reactive (M), highly (H1), very highly (H2), even extremely reactive (E).
What is Class P?
Per AS2870, “Problem” means a soil’s ‘normal’ characteristics at a particular site are changed or altered in a way that needs to be considered at footing design. AS2870 outlines the normal soil characteristics based on the soil’s reactivity.
AS2870 requires the soil report preparer to provide a Class P classification on any site where the soil characteristics are not deemed normal. Such sites include those with:
Why are most sites classified as problem sites?
Unfortunately, what AS2870 defines as ‘normal’ site soil conditions are rare on most residential properties. A theoretical ‘normal’ site class would require a strip of land without significant vegetation and/or previous human activity. Note, the presence of shallow rock material in the soil profile can negate abnormal moisture conditions, given rock doesn’t react to moisture.
Properties in new estates often feature street trees close to the building envelope and likely subdivision fill. Any renovation or extension project will likely feature abnormal moisture conditions related to the existing building. Such sites usually come back Class P.
Conclusion
Most site classifications are Class P. The footings will need to be designed by an engineer, with some additional thought required when compared to a normal site classification.
If you are concerned, feel free to contact one of our engineers or your builder to discuss.
In most instances, Class P sites require common solutions familiar to most builders. Sure, some sites are very problematic and may require costly design solutions that render a project unfeasible. However, such sites are generally rare. Note, this article is relevant to residential construction.
What is a Soil Report?
It’s good to know where a site class comes from. A Soil Report – formally referred to as a Soil Site Classification report – informs the structural engineer on what needs to be considered for a building’s slab and footing design.
The report outlines the key information the engineer needs to consider so the building’s footings are strong and stiff enough to resist factors such as soil reactivity, seasonal moisture changes, fill and/or other possible issues such as low soil strength. Key information outlined in the soil report include:
- Soil make-up: clay, silt, gravel, sand, rock, fill etc.
- Soil properties: soil profile, reactivity, moisture, and strength etc.
- Any factors that are / may adversely affect the soil’s normal moisture characteristics
- Presence, type and characteristics of fill
- Suitable footing founding conditions – required footing depth and soil strength
The Soil Report provides a site classification in accordance with ‘AS2870 - Residential slabs and footings’ and is to be prepared by a suitably qualified geotechnical consultant or engineer.
The soil report is an essential document for a project that involves footing works and is lodged as part of Building Permit documentation.
Soil Reactivity
Site soil classifications are based on soil reactivity. Fortunately, this is not ‘nuclear’ reactivity. Soils react to water. Reactivity is the shrink-swell potential of soil with changes in moisture content. Some soils such as gravel and sand have very low reactivity. Most silts lose their strength when significantly wet, turning from dry soil to a ‘soupy’ consistency. Clay soils shrink and swell with changes in moisture content and are the main culprit for footing movement and Class P sites.
To visualise clay reactivity, picture a sponge. A sponge shrinks when dry and swells/expands when wet. Clay acts much the same and has a diverse range in how much they can shrink / swell. Some are from slightly (S), moderately reactive (M), highly (H1), very highly (H2), even extremely reactive (E).
What is Class P?
Per AS2870, “Problem” means a soil’s ‘normal’ characteristics at a particular site are changed or altered in a way that needs to be considered at footing design. AS2870 outlines the normal soil characteristics based on the soil’s reactivity.
AS2870 requires the soil report preparer to provide a Class P classification on any site where the soil characteristics are not deemed normal. Such sites include those with:
- Abnormal moisture conditions – As the name suggests, this is any change to a soil’s natural moisture, as deemed by the soil report preparer caused by one or a combination of:
- Current trees / vegetation – Trees ‘take’ moisture from the soil and can cause excessive drying of clay soils.
- Current buildings / infrastructure – Buildings cover the soil, causing the soil moisture to artificially vary from its natural state. Buildings can also cause wetting through poor drainage, leaking services etc.
- Pre-existing trees / vegetation / buildings / infrastructure – The previous drying action of the tree / building is removed, leaving a dry soil to absorb more moisture in the next rain event.
- Abnormal moisture can also be caused by septic tanks, dams, groundwater etc
- Fill – Cases where foreign soils / materials have been placed on natural soil.
- Mining activity – mine shafts, mullock material etc.
- Saturated and soft soils resulting in a loss of bearing strength
- Slope stability issues – steep slopes susceptible to land slips
Why are most sites classified as problem sites?
Unfortunately, what AS2870 defines as ‘normal’ site soil conditions are rare on most residential properties. A theoretical ‘normal’ site class would require a strip of land without significant vegetation and/or previous human activity. Note, the presence of shallow rock material in the soil profile can negate abnormal moisture conditions, given rock doesn’t react to moisture.
Properties in new estates often feature street trees close to the building envelope and likely subdivision fill. Any renovation or extension project will likely feature abnormal moisture conditions related to the existing building. Such sites usually come back Class P.
Conclusion
Most site classifications are Class P. The footings will need to be designed by an engineer, with some additional thought required when compared to a normal site classification.
If you are concerned, feel free to contact one of our engineers or your builder to discuss.