The purpose of this Stonebloc information sheet is to provide a plain english explanation of the requirements for a bridging design where a Stonebloc wall is in close proximity to an underground utility pipe.
Standards related to underground services are governed by 55 territorial authorities and a myriad of water service providers, local lines companies, and gas distributors. As such standards and requirements are different in your local area. This document is based on Auckland and National standards and is intended to provide a general guide only.
Stonebloc is a residential gravity retaining system. At the heart of the system are a series of interlocking mass concrete blocks ranging in weight from 300kg to 900kg. In general, the blocks are stacked on top of eachother in a running bond pattern. In good ground conditions simple foundations of leveling fines are required.
One of the unique benefits Stonebloc offers is the ability to non-destructive dismantle walls to access utilities for maintenance purposes, and then reconstruct the walls with ease.
Some councils do not require segmental retaining walls to have a bridging design as the wall can be non-destructively dismantled to access the underground pipe in the event maintenance is required. This should be discussed with council before undertaking structural design work.
Bridging, in a technical sense, is required when a retaining wall is within a defined ‘Zone of Influence’ in relation to an underground [water] pipe. The assumption is that the retaining will exert pressure on the pipe and may damage it. Therefore a ‘bridging’ design is required to ensure the force of the wall is directed to a point outside the Zone of Influence.
There are two core scenarios where bridging may need to be considered:
Pipe runs underwall. Pipe runs parallel to the wall.
The Zone of Influence is a wedge of area emanating from the pipe. The exact parameters of the zone will be specified by the local authority, however in Auckland it equates to 3V, or 45 degrees from the pipe.
The calculation for this is the horizontal distance of the wall from the pipe divided by the depth of the pipe. Where the answer is less than 3, the wall is within the zone of influence.
Where the wall falls within the zone of influence a ‘bridging’ design is required for the foundation.
The geotechnical engineer will specific a foundation which distributes the load in a way where it will fall outside the zone of influence.
Where the pipe runs under the wall a foundation design is required to achieve the same aim, example below.