New Preconsolidation Heads Following the Long-Term Hydraulic-Head Decline and Recovery in Houston, Texas.

The Houston area in Texas, United States, has been experiencing land subsidence for a century from the 1920s to 2010s. A substantial portion of the Houston area had finished a consolidation cycle following the long-term hydraulic head decline and recovery. A new "maximum effective stress" (preconsolidation stress) was preserved in the memory of the aquitards. For an aquifer system comprising aquifers and aquitards, the preconsolidation stress is corresponding to the lowest hydraulic head in the aquitards, not in the aquifers. Preconsolidation head is generally regarded as a groundwater-level threshold below which inelastic compaction begins. The preconsolidation head finalized after the long-term hydraulic head decline and recovery is called new preconsolidation head. This study has developed an empirical equation for projecting the new preconsolidation head. According to this study, the new preconsolidation heads in the primary aquifers (lower Chicot and Evangeline) are local specific: varying from about 30 m below land surface (-30 m) in the south to -50 m in the north of the Harris-Galveston Subsidence District (HGSD) Regulatory Area 1, from -60 m in the east to -80 m in the west of Area 2, and from -70 m in the south to -100 m in the center of Area 3. In Areas 1 and 2, the current hydraulic heads are about 10 m to 20 m higher than the local new preconsolidation heads; thus, remarkable land subsidence (>1 cm/year) would not be reinitiated unless the hydraulic heads are to fall below the local new preconsolidation head.