A numerical simulation study on the evolutionary characteristics of the damage process of karst soil cavity under positive pressure effect

Positive pressure effect is the result of rapid urban development in recent years,which leads to over-exploitation of karst groundwater,coupled with the emergence of extreme weather(such as heavy rainfall,the increase in the duration of heavy rainfall),leading to the sharp rise of karst water level,the gas in the karst cavity can not be discharged in time to form a high positive pressure in the soil cavity and promote the destruction of the soil layer,which then induces a series of phenomena,such as karst subsidence.As a new collapse mechanism,the positive pressure effect causes geological disasters that seriously affect the safety of people's lives and properties in karst regions.In order to study the damage characteristics and evolution characteristics of karst soil cavities under positive pressure effect,this study is based on the ASCII text provided by FLAC itself,and the orthotropic simulation computation compilation program,which realizes the finite element analysis in FLAC3D,and focuses on the inhomogeneous change of soil displacement,redistribution of stresses,and plastic damage of karst soil cavities in different evolution stages under the action of positive pressure,and summarizes the characteristics and laws of stress,strain and damage in plastic zone of karst soil cave at different stages of evolution.The results will play a positive role in further investigating the potential mechanical effects,development mechanism and critical warning conditions during the evolution of covered karst soil caves,and also have important scientific research value in deepening the theory of prevention and control of collapse disasters in covered karst soil caves.

Alterations in renal function in patients with obstructive sleep apnea syndrome and effects of continuous positive airway pressure

Objective To assess renal function in patients with obstructive sleep apnea syndrome (OSAS) during two nights when they were untreated and treated with continuous positive airway pressure (CPAP). Methods Tubular function was estimated on the basis of the renal lithium clearance and its derived formulae, urinary osmolality, osmolal clearance and free water clearance; while glomerular function was evaluated by endogenous creatinine clearance. Twenty patients with OSAS and sixteen normal controls were studied. Results The untreated patients with OSAS had significantly lower fractional proximal tubular sodium reabsorption (59.1%±7.1%,P<0.001 vs controls), lower fractional distal tubular sodium reabsorption (93.8%±1.7%, P<0.05 vs controls) and urinary osmolality (573.0±107.9 mosm·kg 1 , P<0.05 vs controls). The absolute distal tubular reabsorption rate of sodium and osmolal clearance in the untreated patients were higher 47.0±26.0 mmol·L 1 and 25.0±4.1 ml·L 1 ; P< 0.01 and P<0.05 respectively vs controls). During CPAP treated nights all abnormal renal function indexes in the OSAS patients restored to normal control values (P>0.05). Conclusions The natriuresis and diuresis of OSAS patients were due to the decrease in proximal and distal tubular sodium reabsorption and in tubular concentration ability during their nocturnal sleep and were returned to normal by CPAP therapy.