Portland Slope Instability Assessment

Expert Geotechnical Support for Slope Instability on a Historic Landslide Complex

Ground Projects were engaged to support a site affected by significant ground movement, following the identification of post-construction damage to infrastructure.

The site is currently used to store imported grain from the local port within several purpose built warehouses, prior to distribution across the UK via road freight.

Ground movement and associated structural damage have been observed at the site, which is situated within a known historic landslide complex.

In 2023, a significant full-depth crack developed within the floor slab of one of the warehouses, extending across almost the full length of the building approximately 12 months post-construction. The crack reached widths of up to 80 mm, with a vertical displacement of up to 40 mm. Reinforcement across the crack had failed in tension, confirming structural distress within the superstructure rather than superficial or cosmetic cracking.

Ongoing damage progression has been identified through both surface and subsurface monitoring between August 2023 and January 2026, indicating continued ground movement.

Site topography and instability are controlled by complex layered geology, interpreted to be:

• The Portland Stone Member forms steep cliffs upslope
• Portland Sand Formation, which comprises colluvial scree slopes at approximately 25°.
• Beneath this, the Kimmeridge Clay Formation presents weaker strata, forming a gentler slope of approximately 12°.

Under current groundwater conditions, the slope has been assessed as marginal to unstable.

Ground Projects has been closely involved in the investigation and ongoing assessment of the site, providing geotechnical expertise to help understand the underlying causes of the instability and inform appropriate mitigation strategies.

Our role has included the design and supervision of ground investigations, interpretation of complex geological and geotechnical data, and the development of slope stability assessments to evaluate current and future risk.

We hae also assessed the feasability of a range of ground and structural mitigation measures, including: Piled solutions (including anchored piles), a toe buttress, horizontal drainage bores and vertical pumped drainage bores.

We have also implemented and managed a programme of instrumentation and monitoring to track ground movement over time, supported by regular reporting.

Through this ongoing involvement, we have worked collaboratively with the wider project team to develop practical, risk-based solutions and provide clear guidance to support decision-making as the project progresses.