Case study

Glencoe Gorge Underslip

Overview

The underslip site on State Highway 50 is located within the steeply incised Glencoe Gorge, where the road traverses a gully with persistent groundwater seepage and a stream (Glencoe Stream) running below. The road has a ~6% longitudinal grade and is flanked by a 10 m high upslope cutting and a downslope embankment approximately 10m above the stream. During Cyclone Gabrielle in 2023, a significant underslip occurred, compromising the slope and threatening the serviceability of this critical transport corridor.

At the slip location, the stream bends sharply (~90°), scouring and undermining the fill slope. The geology comprises highly weathered, weak silty sandstone with sub-horizontal bedding.

Challenge

  • Design Life and Seismic Compliance: Required to meet a 100-year design life and seismic loads of 7.5Mw (DCLS) and 6.7Mw (SLS) per TREC Minimum Requirements and the National Seismic Hazard Model.
  • Geotechnical constraints:
    • MSE block wall ruled out due to space constraints, and traffic management requirements at the top of the slope.
    • Rock buttress not viable due to shallow slope angle (1.5H:1.0V) projecting into the streambed, increasing downstream scour risk.
  • In Stream Works: The base of the HEX-LOC® slope needed to be founded below the Glencoe Stream, requiring ground water management during formation of the foundation layers.
  • Private Land Impact: The underslip extended onto adjacent private property, requiring a solution that addressed both public and private land stability.
  • Guardrail support: The existing guardrail was undermined and needed to be incorporated into the new structure.
  • Maintenance Requirements: Solution required to be low-maintenance with no ongoing servicing.

Project:

Glencoe Gorge

Location:

State Highway 50 – Hastings

Size:

789 Modules

22 Tier’s High

Completion:

June 2025

Client:

TREC – Transport Rebuild East Coast

Alliance Members – NZ Transport Agency Waka Kotaki (NZTA), KiwiRail, Downer, Fulton Hogan & Higgins.

Contractor:

LHES Earthmoving

Solution

  • HEX-LOC® Modular System: Interlocking blocks filled with crushed aggregate and placed at a 35-degree slope from the toe of the slope. The unique castellated design enabled effective load transfer through the HEX-LOC® slope.
  • Embedded Units: To prevent undermining, the bottom HEX-LOC® units were embedded below the streambed, and a grade control rock trench was installed across the stream to stabilise the riverbed level.
  • Structural Reinforcement: HEX-LOC® units were horizontally tied together at intervals using galvanised M30 bolts to enhance structural integrity.
  • Construction Benefits: The HEX-LOC® design provided immediate toe protection from the river upon placement, and was comparatively simple and efficient to build using local non-specialised Contractors. The system allowed single-lane traffic to remain open during construction.
  • Stormwater Management: Integrated HEX-LOC® step modules were used to channel stormwater from the road, slowing its flow and directing it safely into the stream below.
  • Flood-Level Protection: The lower layers of the HEX-LOC®structure were filled with concrete up to flood level to prevent loss of granular infill during flood events.

Result

  • Resilient Infrastructure: The structure stabilised the slope and protected adjacent private property, effectively preventing further deterioration of SH50.
  • Structural Performance: The system accommodated the steep slope face of 35 degrees and provided robust scour protection at the toe of the slope.
  • Hydrological Function: The integrated step modules allow for the drainage of surface water from SH50, transferring it to the base of the slope without scour or slope infiltration.
  • Operational Success: Delivered a robust, long-term solution for a geotechnically constrained site.