Ground Improvement by Dynamic Replacement Within Intertidal Mangrove Mud Environment, Ichthys LNG, Darwin-Paper

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The Ichthys LNG project at Blaydin Point, Darwin, required ground improvement across extensive intertidal mangrove muds to support a raised flare pad and associated structures. The soft, highly organic soils were typically 2–5.5 m thick, with undrained shear strength of only 5–12 kPa. Daily tidal variation of up to 8 m, slope stability risks, and strict controls on acid sulphate soils and mangrove impacts made construction particularly challenging.<br /><br />Dynamic Replacement (DR) was selected for the onshore flare pad, while stone columns were used in the offshore module offload facility. DR uses repeated heavy tamping to drive granular rock into weak soil, forming large reinforcing pillars. For the flare pad, pillars averaging 2.7 m in diameter were installed on a 5 m grid, with some extending to 7.8 m. Finite-element settlement analyses and slope stability assessments informed the design; sensitive structures also received a temporary surcharge to meet tighter settlement limits.<br /><br />Construction planning addressed weak ground and tidal constraints through a carefully designed working platform, staged fill placement, and a leading stability berm that limited mud movement toward surrounding mangroves. Trials and monitoring procedures were used to manage heave and prevent exposure of acid sulphate soils. Verification included drilling, pressuremeter and plate-load testing, and monitoring of settlement and lateral movement.<br /><br />Approximately 350,000 m² was treated using about 330,000 m³ of rock. Testing confirmed the pillars reached competent ground, and post-construction monitoring found settlement within design predictions and horizontal movement below the 20 mm limit. The project demonstrated that DR could provide a rapid and effective solution while avoiding excavation and disposal of acid sulphate soils, reducing disturbance and limiting reliance on imported cementitious materials.

Keywords

Ichthys LNG

Blaydin Point

dynamic replacement

stone columns

ground improvement

mangrove muds

acid sulphate soils

flare pad

settlement monitoring

slope stability