Wellington retaining walls get bigger

A windy, wet, and shaky Wellington

Wellington is built on hills. It also has some of the windiest and wettest weather in the country. Couple this with high earthquake risk, and you have a recipe for soil slips all over town. The 2022 winter was the wettest on record, with rainfall almost twice the average (744mm in Kelburn) since records began nearly a hundred years ago.

What does this mean?

Slips, slips, and more slips!

We have noticed a massive increase in slips, retaining wall designs, and assessments over this winter, with an enquiry almost every day. But the designs we have produced in the past to remedy the slips will need more structure than before because of an update to the Ministry of Business Innovation and Employment (MBIE) Module 6.

In addition to soil slip responses, new construction projects including commercial, medium-density, and residential buildings, often need new retaining walls. These new retaining walls must be properly engineered for static, earthquake, hydrostatic, and temporary load cases. To help you calculate the load cases and navigate new updates in best practice, new retaining walls should be designed by a Structural and/or Geotechnical engineer.

MBIE Module 6 update

In November 2021, the MBIE 'Earthquake resistant retaining wall design' guide was updated to reflect a higher level of ground shaking during a design-level earthquake event for all regions in New Zealand.

Wellington had an average increase of 82% for earthquake loads on retaining walls compared to earthquake loads before the November 2021 update.

What does this mean?

Designs will need to get bigger!

This means the earthquake load on any new retaining wall has become significantly higher than before. As a result, new retaining wall designs will need more structure.

The technical stuff

We compared some standard retaining wall design cases with the old guidance vs the new to highlight some of the structural changes that can be expected. The retained height of the retaining wall in the example is 2.4m; for easy reference, this is also the general ceiling height of a residential house. The below example covers two main types of retaining walls:

Cantilever timber pole retaining wall

Cantilever reinforced concrete masonry retaining wall

And all design cases consider the following information:

Click on the images to see a higher resolution detail

Summarised findings for cantilever timber pole wall:

For the same retained height, soil properties and pole spacing, you can expect to see:

1. Deeper foundation embedment depth (approximately ~20% deeper)

2. Larger pole size in some cases. (In this case, the bending of the poles was not the governing factor and therefore did not lead to an increase in pole size.)

3. Closer pole spacing in some cases

4. More double lagging at the bottom of the wall

5. Larger diameter foundation holes for poles to achieve the back angle (approximately ~10% larger)

Summarised findings for cantilever concrete block wall:

For the same retained height and soil properties, you can expect to see:

1. Bigger wall section seize (Approximately ~25% bigger)

2. More wall reinforcing in some cases

3. Longer footing length (Approximately ~15% longer)

4. Deeper shear key (Approximately ~60% deeper)

5. Longer heel in some cases (where the overturning governs the design)

6. More steel for the foundation

What can you do?

With hills, wind, and rain, Wellington will always need retaining walls. Add earthquakes to that factor, and you have a recipe for disaster if your retaining wall is not designed correctly. If your retaining wall needs building consent, a Structural or Geotechnical engineer can help consult on what type of wall suits your needs and design it to the latest MBIE guidance.

Latest news

Our director, Marlo Bromley, has recently been interviewed about the problems wellingtonians face with old retaining walls. Have a listen here: https://www.rnz.co.nz/national/programmes/morningreport/audio/2018870846/hundreds-of-wellington-retaining-walls-need-fast-attention If you want to learn more about when you need a building consent for your retaining wall, have a read of our article here: https://www.dtce.co.nz/post/when-does-your-retaining-wall-need-building-cons If you want to learn more about what type of retaining wall you can build, have a read of our article here: https://www.dtce.co.nz/post/what-types-of-retaining-walls-can-i-build