16-Civ-A4 Geotechnical Materials and Analysis · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Das & Sobhan, Principles of Geotechnical Engineering (Cengage); Craig, Soil Mechanics; Holtz, Kovacs & Sheahan, An Introduction to Geotechnical Engineering. Canadian practice: effective-stress, consolidation and Rankine methods as summarised in the Canadian Foundation Engineering Manual (CFEM, 4th ed.).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Controlling drainage condition. The key is the combination of a saturated, low-permeability clay with rapid filling and emptying. Because the clay cannot drain in the time taken to fill or empty the tank, each load (and unload) cycle is applied under undrained conditions. The critical design case for bearing capacity and stability is therefore the short-term, end-of-filling undrained state, analysed with a total-stress ($\phi = 0$) approach using the undrained shear strength $c_u$.
Recommended tests. The primary tests should measure undrained strength: unconsolidated–undrained (UU) triaxial tests on the intact clay (supplemented by unconfined-compression tests and, in the field, a vane shear test for a strength profile with depth) to obtain $c_u$ for the rapid-loading case. Because the tank is loaded and unloaded repeatedly, the effective-stress history matters too, so consolidated–undrained (CU) triaxial tests with pore-pressure measurement should also be run to recover the effective-stress parameters $c'$ and $\phi'$ and to check the long-term (drained) condition once the clay has consolidated under the mean load. Where the repeated fill/empty cycles are frequent, a cyclic (repeated-loading) CU triaxial programme is warranted to detect strength degradation and pore-pressure build-up under many cycles.
Sample type. The tests must be run on high-quality undisturbed samples — thin-walled (Shelby) tube or, better, piston samples pushed (not driven) and sealed to preserve the in-situ water content, fabric and stress state. The reason is that the undrained strength of clay is strongly dependent on its natural structure and water content; a disturbed or remoulded sample loses this structure and returns an unrepresentative (usually unsafe, lower) strength. Samples should be taken at several depths to define the $c_u$ profile beneath the tank.
