Marine territories in Qatar are the biggest gas fields in the world. The project is located in industrial region of Qatar – Ras Laffan, located 70 km north of Doha, and is rapidly becoming the hub of world LNG. In this respect a number of LNG tanks were under construction, including LNG T4 and T5. The capacity of both tank was 140,000 m3. The outer tank’s inner diameter was 76.3 m, and the inner tank’s height was 35 m. Maximum LNG height was 34 m.
The soil profile consisted of 4 meters of calcareous sand overlying 12 meters of limestone with different hardness. Cavities have been encountered in the siltstone layer between elevation -18.5 QNHD and -23.5 QNHD. The thickness of the upper soil layer was in average 4.2m and the Existing Ground Level (EGL) was in the range of +2.1m QNHD. The use of the concrete foundation directly lying on the original soil was not recommendable as the upper loose sand layer in particular would have been subject to: excessive settlement amplitude, which may be damageable to the structure.
Menard developed a unique foundation solution that combined a shear trench ring under the wall and dynamic replacement within the tank. In this process, the soil in the ring area was replaced by stones and dynamically compacted. Stones were also dynamically driven into the ground within the tank’s boundaries. Elasto-plastic Finite Element analyses using soil characteristics based on in-situ pressuremeter tests demonstrated the achievement of the performance criteria. This was confirmed by plate load tests and eventually the hydrotest (less than 20mm settlement).