承压水减压引起的沉降分析

由下卧承压水层减压引起的固结沉降计算与一般的由堆载或潜水位下降引起的固结沉降计算不同,本文着重研究了在深厚弱透水层下卧强透水承压层的复杂地质条件下,下卧承压水层减压引起的土中应力变化及周围地表沉降的计算方法;在假定一维竖向固结的条件下推导了减压引起的沉降固结度计算公式,该公式与常规的双向排水固结公式相同,表明排水减压固结与常规的双向排水固结有着相同的效果.最后,将此分析方法应用于潮州供水枢纽工程西溪水闸沉降分析中,实践的结果表明,该方法简单可行,能较好地满足工程要求.     

Environmental impact of an urban landfill on a coastal aquifer (El Jadida, Morocco)

The El Jadida landfill is one among many uncontrolled dumping sites in Morocco with no bottom liner. About 150 tons/day of solid wastes from mixed urban and industrial origins are placed directly on the ground. At the site of this landfill, the groundwaters circulate deeply (10–15 m) in the Cenomanian rock (calcareous–marl), which is characterised by an important permeability from cracks. The soil is sand–clay characterized by a weak coefficient of retention.The phreatic water ascends to the bottom of three quarries, which are located within the landfill. These circumstances, along with the lack of a leachate collection system, worsen the risks for a potential deterioration of the aquifer.To evaluate groundwater pollution due to this urban landfill, piezometric level and geochemical analyses have been monitored since 1999 on 60 wells. The landfill leachate has been collected from the three quarries that are located within the landfill. The average results of geochemical analyses show an important polluant charge vehiculed by landfill leachate (chloride = 5680 mg l⁻¹, chemical oxygen demand = 1000 mg l⁻¹, iron = 23 000 μg l⁻¹). They show also an important qualitative degradation of the groundwater, especially in the parts situated in the down gradient area and in direct proximity to the landfill. In these polluted zones, we have observed the following values: higher than 4.5 mS cm⁻¹ in electric conductivity, 1620 and 1000 mg l⁻¹ respectively in chlorides and sulfate (), 15–25 μg l⁻¹ in cadmium, and 60–100 μg l⁻¹ in chromium. These concentrations widely exceed the standard values for potable water.Several determining factors in the evolution of groundwater contamination have been highlighted, such as (1) depth of the water table, (2) permeability of soil and unsaturated zone, (3) effective infiltration, (4) humidity and (5) absence of a system for leachate drainage. So, to reduce the pollution risks of the groundwater, it is necessary to set a system of collection, drainage and treatment of landfill leachates and to emplace an impermeable surface at the site of landfill, in order to limit the infiltration of leachate.     

Apports de la méthode sismique réflexion haute résolution à l'identification des structures profondes des formations tertiaires en Médoc (Gironde,France) : implications hydrogéologiques

Near Bordeaux (France), the Oligocene aquifer is a potential target for drinking water supply. A high-resolution seismic campaign and several exploration wells helped to clarify the geometry of this formation and, as a consequence, the Medoc Tertiary aquifers. The major information extracted from these new data is the existence of a soft deformation inducing two different deposit areas, showing different hydrogeological characteristics (thickness, type, hydraulic properties, etc.). The presence of an erosional gap area affecting the Oligocene formations lead us to propose a new image of groundwater flow in the area, in the context of a predicted intensive exploitation of this resource. To cite this article: F. Larroque, A. Dupuy, C. R. Geoscience 336 (2004).     

Land subsidence and caprock dolines caused by subsurface gypsum dissolution and the effect of subsidence on the fluvial system in the Upper Tigris Basin (between Bismil–Batman, Turkey)

Karstification-based land subsidence was found in the Upper Tigris Basin with dimensions not seen anywhere else in Turkey. The area of land subsidence, where there are secondary and tertiary subsidence developments, reaches 140 km². Subsidence depth ranges between 40 and 70 m. The subsidence was formed as a result of subsurface gypsum dissolution in Lower Miocene formation. Although there are limestones together with gypsum and Eocene limestone below them in the area, a subsidence with such a large area is indicative of karstification in the gypsum. The stratigraphical cross-sections taken from the wells and the water analyses also verify this fact. The Lower Miocene gypsum, which shows confined aquifer features, was completely dissolved by the aggressive waters injected from the top and discharged through by Zellek Fault. This resulted in the development of subsidence and formation of caprock dolines on loosely textured Upper Miocene–Pliocene cover formations. The Tigris River runs through the subsidence area between Batman and Bismil. There are four terrace levels as T1 (40 m), T2 (30 m), T3 (10 m) and T4 (4–5 m) in the Tigris River valley. It was also found that there were some movements of the levels of the terraces in the valley by subsidence. The subsidence developed gradually throughout the Quaternary; however no terrace was formed purely because of subsidence.     

From the geological to the numerical model in the analysis of gravity-induced slope deformations: An example from the Central Apennines (Italy)

This paper presents the findings from a study on gravity-induced slope deformations along the northern slope of Mt. Nuria (Rieti-Italy). The slope extends from the village of Pendenza to the San Vittorino plain and hosts the Peschiera River springs, i.e. the most important springs of the Central Apennines (average discharge: about 18 m³/s).

Detailed geological-geomorphological and geomechanical surveys, supported by a site stress-strain monitoring system and laboratory tests, led us to define the main evolutionary features of the studied phenomena. Based on the collected data, a “geological-evolutionary model” was developed with a view to identifying a spatio-temporal correlation between relief forms, jointing of the rock mass and its stress conditions. The geological-evolutionary model was expected to improve numerical simulations and to test our assumptions.

The numerical model also allowed us to simulate changes in the stress-strain conditions of the rock mass and correlate them with jointing, seepage, as well as with site-detected and site-monitored forms and deformations. In particular, significant relations between seepage, tensile stresses within the rock mass, karst solution and collapse of cavities were identified.     

锚杆支护及在高承压水土层基坑中的应用

在邻海地区富含高承压水的土层中施工预应力锚杆支护工程的难度很大.介绍了锚杆的构造、作用机理、施工工艺以及决定锚杆施工质量的关键技术.以深圳某商住楼基坑施工为例,探讨了在承压水土层中采用锚杆支护的方法,创造性地提出了"先堵水后注浆"的施工工艺,可供沿海、沿江地区的类似工程借鉴.     

Salinisation des nappes côtières : cas de la nappe nord du Sahel de Sfax,Tunisie

The intensive agricultural and economic activities induce the increase of the risk of groundwater quality degradation through high groundwater pumping rates. The salinization and contamination are the main sources of this pollution, especially in coastal aquifers. The explanation of the origin of salinity for the shallow aquifer of Northern Sahel of Sfax was analysed by a chemical study of the groundwater main compounds. The partitioning of groundwaters into homogenous groups is undertaken by graphical techniques, including a Stiff pattern diagram, an expanded Durov diagram and several binary diagrams. The study indicates the presence of various salinization processes. In the recharge area, salinization is the result of dissolution/precipitation of the aquifer formation material (group I). The irrigation water return and the intensive pumping have been identified as major sources of salinization in the south by direct cation exchange and mixing reactions (groups II and III). The anomaly of high groundwater salinity observed near the Hazeg zone was explained by the presence of a seawater intrusion in this area. This hypothesis is related to the high chloride concentration, to the presence of inverse cation exchange reactions (group IV), and to the piezometric level inferior to sea level. To cite this article: R. Trabelsi et al., C. R. Geoscience 337 (2005).     

Characterization of hydrogeologic properties for a multi-layered alluvial aquifer using hydraulic and tracer tests and electrical resistivity survey

A multi-layered aquifer, typical of riverbank alluvial deposits in Korea, was studied to determine the hydrologic properties. The geologic logging showed that the subsurface of the study site was comprised of four distinctive hydrogeologic units: silt, sand, highly weathered and fresh bedrock layers. The electrical resistivity survey supplied information on lateral extension of hydrogeologic strata only partially identified by a limited number of the geologic loggings. The laboratory column tracer test for the recovered core of the sand layer resulted in a hydraulic conductivity of 5.00×10⁻² cm/s. The slug tests performed in the weathered rock layer yielded hydraulic conductivities of 4.32–7.72×10⁻⁴ cm/s. Hydraulic conductivities for the sand layer calculated from the breakthrough curves of bromide ranged between 2.08×10⁻³ and 2.44×10⁻² cm/s with a geometric mean of 6.89×10⁻³ cm/s, which is 7 times smaller than that from the laboratory column experiment. The trend of increasing hydraulic conductivity with an increase in tracer travel length is likely a result of the increased likelihood of encountering a high conductivity zone as more of the aquifer is tested. The combined hydrogeologic site characterization using hydraulic tests, tracer tests, and column test with geologic loggings and geophysical survey greatly enhanced the understanding of the hydrologic properties of the multi-layered alluvial aquifer.     

Assessing regional‐scale spatio‐temporal patterns of groundwater–surface water interactions using a coupled SWAT‐MODFLOW model

Interaction between groundwater and surface water in watersheds has significant impacts on water management and water rights, nutrient loading from aquifers to streams, and in‐stream flow requirements for aquatic species. Of particular importance are the spatial patterns of these interactions. This study explores the spatio‐temporal patterns of groundwater discharge to a river system in a semi‐arid region, with methods applied to the Sprague River Watershed (4100 km²) within the Upper Klamath Basin in Oregon, USA. Patterns of groundwater–surface water interaction are explored throughout the watershed during the 1970–2003 time period using a coupled SWAT‐MODFLOW model tested against streamflow, groundwater level and field‐estimated reach‐specific groundwater discharge rates. Daily time steps and coupling are used, with groundwater discharge rates calculated for each model computational point along the stream. Model results also are averaged by month and by year to determine seasonal and decadal trends in groundwater discharge rates. Results show high spatial variability in groundwater discharge, with several locations showing no groundwater/surface water interaction. Average annual groundwater discharge is 20.5 m³/s, with maximum and minimum rates occurring in September–October and March–April, respectively. Annual average rates increase by approximately 0.02 m³/s per year over the 34‐year period, negligible compared with the average annual rate, although 70% of the stream network experiences an increase in groundwater discharge rate between 1970 and 2003. Results can assist with water management, identifying potential locations of heavy nutrient mass loading from the aquifer to streams and ecological assessment and planning focused on locations of high groundwater discharge. Copyright © 2016 John Wiley & Sons, Ltd.     

Is there a superior conceptual groundwater model structure for baseflow simulation?

Previous work has shown that streamflow response during baseflow conditions is a function of storage, but also that this functional relationship varies among seasons and catchments. Traditionally, hydrological models incorporate conceptual groundwater models consisting of linear or non‐linear storage–outflow functions. Identification of the right model structure and model parameterization however is challenging. The aim of this paper is to systematically test different model structures in a set of catchments where different aquifer types govern baseflow generation processes. Nine different two‐parameter conceptual groundwater models are applied with multi‐objective calibration to transform two different groundwater recharge series derived from a soil‐atmosphere‐vegetation transfer model into baseflow separated from streamflow data. The relative performance differences of the model structures allow to systematically improve the understanding of baseflow generation processes and to identify most appropriate model structures for different aquifer types. We found more versatile and more aquifer‐specific optimal model structures and elucidate the role of interflow, flow paths, recharge regimes and partially contributing storages. Aquifer‐specific recommendations of storage models were found for fractured and karstic aquifers, whereas large storage capacities blur the identification of superior model structures for complex and porous aquifers. A model performance matrix is presented, which highlights the joint effects of different recharge inputs, calibration criteria, model structures and aquifer types. The matrix is a guidance to improve groundwater model structures towards their representation of the dominant baseflow generation processes of specific aquifer types. Copyright © 2014 John Wiley & Sons, Ltd.