The higher the walls, the greater the pressure on the wall. A common question asked by customers is how high Envirocon can build walls. The height of the wall, along with the parameters, determine the type of wall built.
The table below provides an indicative maximum retaining height by product and wall type
Gravity | MSE | Cantilever | |
Interbloc | 2.4m | 8.4m | 3.2m |
Stonebloc | 2.0m | NA | NA |
The higher the slope of the backfill extending from the top of the wall, the move load will be exerted on the wall. Higher slopes have a disproportionately higher load on the wall.
The slope behind the wall affects the overall stability of the wall. The slope behind the wall should be specified by an Engineer prior to the commencement of the design process.
Slope can be specified in percentages, degrees or gradients.
e.g.
These can be easily converted using online tools such as https://www.archtoolbox.com/calculating-slope/
Where a wall is to be built close to underground pipes there may be extra design requirements which may need to be met. This is referred to as bridging designs.
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. The following is based on Auckland and National standards and is intended to provide a general guide only.
Bridging, in a technical sense, is required when a retaining wall is within a defined ‘Zone of Influence’ in relation to an underground 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:
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.
Generally a structural engineer should be involved in specifying the solution, of which there are two;
It is important to consider the possibility of bridging requirements at an early stage of the process to avoid technical difficulties later in the process. Bridging designs (if required) can be easily obtained from the projects Structural Engineer. In some cases, project engineers will be able to justify bridging exemption and supplement relevant calculations with BC application.
As previously stated a surcharge is defined as any vertical pressure applied to the ground surface in the vicinity of a retaining wall is a surcharge. This surcharge load will result in an additional horizontal pressure on the retaining wall.
There are two types of surcharges on a retaining wall:
Taking the height of the retaining wall (from ground level) and measuring horizontally back from the top of the retaining wall, there is a zone of influence. This is represented by the ratio H1:1.
If a live or dead load falls within this zone then the design of the retaining wall needs to accommodate the load.
Tip: Tiered Retaining
A potential option for avoiding the consent process is to suggest a tier retaining wall configuration. Providing all rules are followed, this can be a cost effective and aesthetically pleasing solution.
The surcharge load should be specified by an Engineer prior to the commencement of the design process.
Simplification for reference below:
Backfill parameters & Base soil parameters
The above parameters should be found in the geotechnical report as per the example snippets further in the manual (Geotech reports section). If the parameters have not been specified, additional geotechnical input may be required to confirm these.
This is discussed at more length in the next section of this manual.
The configuration of Envirocon blocks and the subsequent profile of the wall can have positive impacts on the design.
The weight of the fill behind the wall can be utilised to provide additional overturning resistance. With Interbloc this means turning the base row perpendicular to the wall. With Stonebloc this means utilising the 800 or 1200 base block.
This approach can be applied to both gravity and cantilever walls.
When deciding on a trade off between adding extra blocks, which can increase costs for the customer, or increasing the length of the toe of the wall (i.e. using an 1800 instead of 1200 block for Interbloc, or 1200 instead of 800 block for Stonebloc) often increasing the length of the toe delivers a superior design outcome.
Tip: Backfill = 18 kN/m3 v Concrete 24 kN/m3.
It is a regulatory requirement that all walls with a fall of 1m or greater must have fall protection [balustrade] at the top of the wall. As a result the sales person will often need to take this into consideration.
There are three options for fall protection with Envirocon retaining walls:
Fixing the fence to the wall creates a new surcharge - the potential live load of people pushing against the fence can be significant. This requires additional inputs to the design which create both benefits and negatives.
Benefits | Negatives | |
Top Fixed | Maximises space utilisation Better aesthetic | Requires stainless steel brackets to be added to the back of the wall to tie blocks together. |
Rear Fixed | Cheaper than top fixing | Slight loss of space Required to be fixed to multiple blocks |
Not Fixed | Cheapest option Off the shelf designs | Significant loss of useable space |
Affects the short-term sliding of the wall - the earthquake zone of the project can be verified using the map below. Zone 1- least risk & more liberal design; Zone 4- most risk & most conservative design.
Gravity wall designs in the Auckland and Northland (Zone 1) region may allow walls up to 2.4m high without additional need for vertical or horizontal (geogrid) reinforcing.
Gravity wall design in the Otago (Zone 3 & 4) region will make the retaining wall design more conservative and the maximum height of the wall utilising this approach lower. Specific engineering will often be required when the wall height exceeds 1.6/1.8m high.
