Comment on “Short-lived pause in Central California subsidence after heavy winter precipitation of 2017” by K. D. Murray and R. B. Lohman

In a study by Murray and Lohman (M&L), the authors suggest that remote sensing data are useful for monitoring land subsidence due to aquifer system compaction. We agree. To infer aquifer dynamics, we provide a more detailed and joint analysis of deformation and groundwater data. Investigating well data in the Tulare Basin, we find that groundwater levels stabilized before 2015 and show that M&L’s observed continued subsidence through July 2016 is likely caused by the delayed compaction of the aquitard. Our analysis suggests the observed 2017 transient uplift is not due to recharge of the aquifer system after heavy winter rainfall because it requires an unrealistic vertical hydraulic gradient nearly five orders of magnitude larger than that typical of Tulare Basin. We find that, regardless of the amount of rainfall, transient annual uplifts of ~3 cm occur in May to June. Using an elastic skeletal storage coefficient of 5 × 10−3, we link this ground uplift to annual groundwater level changes.

Aluminum Sulfate Hydration Retarders for High-Calcium Fly Ash Used in Highway Construction

ABSTRACTThe rapid hydration and setting associated with the use of high-calcium fly ash as an additive in soil and aggregate base stabilization in highway construction imposes certain limitations in regards to operational time and volume of work executed. Aluminum sulfate and its ammonium salt were evaluated as hydration reaction retarders. Mixtures of Ottawa sand and Class C high lime fly ash in a 1:1 weight ratio were used for the evaluations. These additives minimized the adverse effects of delayed compaction by recovering some of the compressive strength lost to the rapid hydration, although in all cases the density of the mixes decreased. The recovery of strength was related to the heat of hydration, wherein the peak temperature was reduced from 90°F to the range of 86–78°F at 2 hours; further temperature decreases were observed as reaction time increased. The availability of the sulfate ions, as manifested by the presence of ettringite, helps the hydration process continue, minimizes the adverse effects of delayed compaction and assists positively in the reduction of the void area of mixes and in stratlingite formation, which contributes to a strong crystalline framework.