Question:

In the context of the extension of a commercial port, a liquid natural gas (LNG) tank has to be built. The tank is 20 m in diameter and 30 m in height. It has been decided that a circular flexible concrete slab will be used as a foundation. A thin layer of permeable fill will be placed on the soil surface before the slab is constructed. The settlement of this thin permeable fill can be ignored.

Geotechnical investigations were conducted to characterise the soil profile, which consists of 8 m of a sandy clay (soil A), overlaying 4 m of gravelly sand (soil B) overlaying an incompressible and impervious rock substratum. The water table was found at a depth of 1m but for this assignment, the water table will be assumed to be at the ground surface.

Table 1 provides some parameters for soils A and B, as determined from laboratory testing. Table 2 provides the evolution of vertical strain with vertical stress under oedometric conditions for soil A. Finally, Table 3 summarises the settlement criteria of the project.

Table 1: Soil parameters.

Table 2: settlement criteria

1. Using the 1m discretization and the stress values calculated in assignment 1 (use stress values of assignment 1 solution), compute the immediate settlement under the centre and the edge of the footing. Present an example calculation for a depth of 0.5m and 8.5m, under the centre and under the edge. Then, tabulate the rest of the results. Clearly identify the value of total immediate settlement.
   
   
2. Using the 1m discretization and the stress values calculated in assignment 1 (use stress values of assignment 1 solution), compute the additional long term settlement under the centre and the edge of the footing associated with total dissipation of excess pore pressure. Present an example calculation for a depth of 0.5m, under the centre and under the edge. Then, tabulate the rest of the results. Clearly identify the value of additional settlement due to primary consolidation.
   
   
3. Estimate the amount of creep over the life of the structure (50 years) for soil A. For this question, use data of Table 2 and a 1m discretization to estimate how the in situ void ratio evolves with depth (before the tank is built). Assume that the cycles of filling and emptying the tank do not affect creep. Present an example calculation for a depth of 0.5m and tabulate the rest of the results. Clearly identify the total amount of creep.
   
   
4. Compute the time required to achieve 10%, 20%, 30%, 40%, 50%, 60%, 70% and 80% of average degree of consolidation. Then, estimate the primary consolidation settlement associated to each value of U%.
   
   
5. Tabulate and plot the evolution of the settlement under the centre and the edge of the tank from the moment it is filled (immediate settlement) to the end of service life (50 years). For this question, assume that the tank is filled once and never emptied (i.e. the soil is not unloaded).
   
   
6. Are the settlement criteria of the project met? Justify your answer.