Soakage in Accordance With E1/VM1: That’s Just a Start

There are many stories of stormwater soakage disposal systems failing soon after construction or perhaps performance is seen to slowly deteriorate over time. Many of the issues that lead to this can be traced back to the design.

When it comes to designing soakage systems the Ministry of Business, Innovation and Employment’s Verification Method E1/VM1 (VM1), Section 9, Disposal to Soak Pit is often quoted by designers as a means to demonstrate compliance with the Building Code. As such, Councils are then bound to accept these calculations.

However, VM1 comes with limitations. For example, it clearly states that determining the suitability of the ground for soakage and the overall stability of the ground are outside its scope. These two factors are critical to designing soakage systems, particularly if the term “ground” is interpreted to mean the groundwater table beneath it as well as the geology. The warnings and limitations noted in VM1 are often not addressed, or perhaps not well understood, by designers when proposing soakage disposal of stormwater. Some Councils, recognising this, have prepared more comprehensive design guidelines. Others have not.

While soakage designed using VM1 is appropriate for some situations it is not for others, like subdivision scale disposal, without it being supplemented by more comprehensive investigations and expert advice. This might require a hydro-geotechnical professional, and may require carrying out groundwater modelling to understand seasonal highs or effects of soakage on a shallow water table. Like all areas of engineering, the level, complexity and detail of any particular design needs to consider site specific conditions as well as addressing, and where possible mitigating, the consequences of design failure.

Over many projects, Beca’s stormwater and hydro-geotechnical professionals have identified several key factors that must be accounted for when soakage is being proposed. Some of these seek to address VM1’s limitations and its perceived gaps. This paper discusses what we have learnt, including the significance of:

i. site geology

ii. the groundwater table (including its seasonal and inter-annual variability)

iii. the nature of the surrounding topography and proximity to steep slopes, structures and streams

iv. the difference between field and design percolation rates and selecting an appropriate factor of safety

v. selecting an appropriate percolation test methodology (different to that required by VM1)

vi. site testing in the location of the proposed soakage device, at varying depths, with more than one test and

vii. including pre-treatment in the design to protect the soakage device from blinding.

The implications are that the soakage disposal section in VM1 should be revised or expanded, even if just to draw attention to some of the critical wider issues and