CAN I GET A DIFFERENT SITE CLASSIFICATIONS FOR THE SAME SITE?
What is a “site Classification”?
A site classification according to AS2870 Residential Slabs and Footings applies to residential dwellings, however it has become a nominal standard for assessing sites for larger projects.
How is it assessed?
To establish the site classification, a value called the characteristic surface movement (ys) needs to be calculated using the following factors (including where they come from):-
A. The zone of moisture variation, depth of crack zone and soil suction variation – from the code or published data
B. The subsurface profile and how it varies across a site – the site investigation
C. The properties of the soil with respect to how the soil will behave when subject to soil suction (note NOT soil moisture content) changes – testing of samples collected from the site investigation
D. Proposed cuts and fills across a site – usually from the client or project engineer
How does a number become a classification letter?
Using the information above a value for the characteristic surface movement (ys) is calculated and depending upon the number, the site can be classified as S, M, H1, H2 or E (listed in increasing value of ys).
There are an additional two classifications, P and A class sites. The former has to be adopted in a number of situations and whilst the P stands for “Problem” the reason it is adopted would possibly be better described as “special cases” where the adoption of a simple classification due to a calculated ys value may not be appropriate (or even misleading) e.g. deep fill, slope stability issues, trees on the site etc.
So how can you get different site classifications for the same site?
Let’s consider an example, the following four cases could result from different site investigations on the same site, depending on where holes were drilled, material tested:-
• Investigation 1 – two holes are drilled and both intersect 0.3m of existing sand fill over reactive clay soils to the total depth of the borehole of 2.5m. The clay soil is tested and the Iss value of 3.3% results. From the above information an M class site applies in its current state.
• Investigation 2 – two holes are drilled in a different section of the site both intersect 0.3m of existing sand fill over reactive clay soils to the total depth of the borehole of 2.5m. The clay soil is tested and this time a slightly higher Iss value of 3.9% results. From the above information a H1 class site applies in its current state. So a small change in the Iss value jumps the site class up a level.
• Investigation 3 – two holes are drilled, one intersects 0.3m and the other 0.9m of old sand fill over reactive clay soils to the total depth of the borehole of 2.5m. As the depth of fill exceeds 0.8m (sand fill) the site has to be classified as a P class site. Slightly deeper fill changes the class.
• Investigation 4 – Some earthworks are undertaken on the site and the development calls for cuts of 0.9m across the site. Yet another site investigation is undertaken and we have two boreholes with reactive clay for the total depth of the boreholes (2.5m). A test is undertaken and an Iss value of 3.1% is calculated (the lowest so far). But because the crack zone has now been removed a H2 class would apply to the site. Being provided with proposed site works changes the class again.
The above example therefore shows by a few small changes, four different site classifications can be assessed for the same site. The aim of any investigation is to assess the site classification so that any domestic structure built on the site is designed for the correct site conditions.
So, which one is right?
They are all right for the situation, results of drilling and testing and the assumptions made.
This illustrates the need for experienced site assessors to be engaged on a project and for engineers to understand how a change to the development can significantly alter the site classification on a site.
Site classifiers need information about the proposed development to make an accurate assessment. Changes to those assumptions can also mean a change to the site classification.