Nu-Guard® 31 posts should be installed in soil conditions that meet the requirements of AS/NZS 3845.1:2015, Specification TNZ B/2 - Construction of unbound granular pavement layers and TNZ M/4 - Basecourse aggregate.
However, due to its smaller footprint and no blockout requirement, Nu-Guard® 31 may need to be installed in hard surfaces. This has previously been defined as areas that have thick asphaltic layers or a concrete footpath. But rigid surfaces also include structural asphalt and asphaltic concrete (AC) and therefore this installation detail does need to be used with these types of surfaces.
A hard surface surrounding the Nu-Guard® 31 post has no flexibility, unlike soil, so it can create a pinch point. In a vehicle impact this may cause the post installed in the hard surface to snap very quickly and, as a result, not provide enough support to the highway rail to perform as designed. The recommended treatment involves creating a square hole in the hard surface around the post to remove the possibility of any pinch point and filling it with a low strength concrete/grout or flexible asphalt. This is detailed on page 12 of the Nu-Guard® 31 Product and Installation manual - click here to view.
An alternative method of providing the weakened area around the posts, in addition to what is shown in the manual, is to install a circular hole cored to a depth of 75mm, with the post driven and then the weakened grout added. The circular hole must be 300mm diameter (or 450mm diameter for median installation) with the post driven against the nearside of the hole for a roadside (non-median) application.