Engineered Retaining Wall

Precision stabilisation against cyclone damage

 

The Challenge: A residential property in Mahia faced a critical threat following Cyclone Gabrielle. The intense rainfall overwhelmed the existing, non-engineered retaining wall, causing it to fail and allowing a substantial volume of slip material to cascade onto the site. The need was immediate: construct a robust, certified Timber Pole and Lagging Wall to permanently secure the hillside.
 

Extreme Ground Conditions Demand Precision: The weak, saturated ground combined with a highly constrained jobsite and steep batters presented particular design and execution difficulties. The structural solution required deep, heavy piling:

• Wall Type: Timber Pole and Lagging Wall set into a concrete foundation.

• Hole Specification: The design required critically tight pole spacing, with 600mm diameter holes placed just 400mm apart (edge-to-edge), drilled to a depth of 4m.

• The "Blow Out" Risk: The minimal spacing between the planned 450mm poles created a high risk of "blow out"—where the wall of earth between the drilled holes could collapse before the poles were set.
   

The Pryde Solution: Staged Precision Piling. To overcome the instability and mitigate the blow-out risk, our team executed a strategy that showcased exceptional technical skill:

1. Staged Construction: We performed a two-stage piling process, drilling and concreting every second pole hole first. This allowed the newly set concrete to cure and provide temporary shoring before we returned to drill and concrete the intermediate, or 'in-between,' holes.

2. Specialised Machinery: This critical work was executed using a 13-tonne excavator fitted with an auger attachment and extension bar (due to the deep holes).
    

Millimetre Accuracy Under Duress: The wall design further required the poles to be installed with a precise 5∘ lean into the wall to ensure maximum stability against the slope. Achieving this angle while contending with solid rock clasts that constantly pushed the auger off-line demanded world-class operator skill:

• Operator Finesse: Our operator made skilled manoeuvres to control how the auger bit into specific sides of the hole, constantly correcting the trajectory against the resistance of the rock.

• Manual Guidance: Precision was verified in real-time by a ground crew member using a spirit level, providing constant communication to the operator to maintain the exact 5∘ angle required.
   

The Outcome: Through careful logistical management to navigate the restricted access and exceptional operator skill, we successfully delivered a large-scale, engineered Timber Pole and Lagging Wall. The result is a permanently stabilised hillside that protects the residential property and paved access against future erosion events.