GEOTECHNICAL ENGINEERING DIFFICULTIES AND SOIL CONDITIONS IN SOUTHEAST LOUISIANA

摘要

Geotechnical engineering in Southeast Louisiana is far more complex than the “let's drive piles to rock” approach common in more geologically fortunate regions. This area is the delta of the Mississippi River, where Pleistocene-age erosion up to hundreds of meters deep has been refilled with normally- or even underconsolidated deposits, predominately clays. It thus shares geotechnical difficulties with similar deltaic areas around the world. The groundwater levels are high. Away from major rivers, the normal groundwater level is essentially at the ground surface. Near rivers or flooded canals, the potentiometric level can approach river/canal level, I.e., be well above the ground surface. Geologic subsidence is a long established phenomenon throughout the region, with subsidence rates of 0.5 to 1.5m per century. A big geotechnical problem here is evaluation of shear strength; sample disturbance in soft materials or those from great depths affects laboratory test results. Therefore, common practice today is to avoid unconfined compression tests for such materials in favor of triaxial or direct simple shear tests in the laboratory. In-situ vane shear testing has been used here for decades, but the cone penetrometer is becoming the preferred field strength determination procedure. Many projects such as levees, landfills, or large storage tanks are feasible only by using strength gain with consolidation techniques. This procedure was used by the Corps of Engineers to attain the levee heights they did achieve along the MRGO, Atchafalaya River, etc. Several landfills in the New Orleans area have reached heights twice those which would have caused failure without strength gain over time. Excavating can be difficult because of the combination of soft soils and high groundwater; hydrostatic bottom heave is often a concern. For levees adjacent to hurricane- or river-impacted water bodies, hydrostatic heave at the land side is a problem.