Nearshore Works
Background
The Ichthys LNG Project nearshore works incorporated a temporary access embankment causeway, approximately 1 km in length, with an adjacent trench excavation in soft mangrove muds.
The trench was used to facilitate the pipe pull of the Gas Export Pipeline (GEP) from offshore to the onshore beach connection. Trench stability was maintained using a combination of battered slopes and steel sheet pile cofferdam with supporting steel structural frames.
Scope
CMW was subcontracted by Ertech to undertake the following scope of works;
- Detailed geotechnical design of the rockfill embankment access causeway, trench excavation, cofferdam structure and headwall
- Construction monitoring of the embankment causeway and trench including settlement analysis, strength gain and stability assessment
- Site supervision during causeway construction, pile driving and excavation stages.
Challenges
The nearshore geology consisted of very soft normally consolidated alluvial soils (mangrove muds) which were particularly challenging in terms of geotechnical design and constructability.
Excavated Trench inundated during high tide
In addition the project featured stringent restrictions regarding the environmental impact and soil disturbance in the mangrove area.
Solution
CMW’s design philosophy for the causeway was to surcharge the underlying material with the load from the causeway fill and incorporate the consolidation and associated strength gain of the underlying soils to provide long term stability.
The ‘hold period’ before the required strength gain was achieved was determined from a detailed analysis of the site investigation data. Strength gain verification was provided by on site cone penetration testing through the causeway by CMW site engineers, together with settlement and performance assessment on site.
A particular challenge associated with incorporating this design philosophy was the relative low factors of safety of the embankment stability in the short term condition prior to the consolidation and strength gain in the underlying soils. It was also crucial to ensure that the embankment construction could continue during the hold period.
Plaxis Model showing stress distribution during “hold period”
To improve the embankment performance the embankment included a basal and secondary layer of geofabric and geogrid. Topping up of the embankment was also carried out at localised areas of excessive deformation to maintain a level working platform.
Benefits
The solution adopted by CMW provided significant time and cost savings for the client while meeting strict environmental and safety considerations. The design solution avoided the need for extensive ground improvement and/or deep foundations.
CMW’s site presence during construction was crucial to the project meeting critical deadlines and an acceptable outcome.