Feature by Emanuele Pellichero, LHG and Bade Sozer, PhD, PE
Water inflows can be one of the most critical challenges in underground projects, causing unsafe and difficult conditions during construction, impacting schedule, and resulting in ground settlement or undesirable impacts on water resources. The risks posed by the water inflows in underground construction can be recognized during the design phase via a comprehensive hydrogeological analysis that takes into account the specific characteristics of the local geological and hydrogeological system, the underground construction activities, and their impact on the distribution and movement of groundwater in the soil and/or rock.
McMillen Jacobs Associates recently expanded its in-house capabilities in the field of hydrogeology. We offer comprehensive services in every application of hydrogeology in construction works solving complex hydrogeological problems to support clients in risk-based decision making. We optimize planning and execution of field geotechnical investigations in soil and rock by using our cloud-based borehole data storage software to immediately process and query new information as it is collected, update our geological/hydrogeological models, and make real-time field adjustments. The data are used to conceptualize complex aquifer systems, and efficiently build analytical and numerical groundwater flow models to simulate the response of the local groundwater regime to future construction activities.
We offer a wide array of hydrogeological services, including:
Estimates of inflow magnitudes at different phases of construction in complex geological systems. With the help of numerical modeling, inflows into drill-and-blast or tunnel bored excavations can be estimated during excavation, before and after pre-excavation grouting, and after lining installation. Numerical models allow simulation of complex hydrogeological systems with irregular feature geometries, multiple aquifer and aquitards, fractured bedrock systems, preferential fluid flow conduits such as faults or shear zones, reduction of hydraulic conductivity (and inflows) after grouting, and how inflows may vary with time.
Assessment of the effectiveness of groundwater control systems—e.g., ground support, dewatering, grouting, ground freezing. Groundwater control systems are often not readily interchangeable, and each system has a relatively narrow range of application based on the site-specific hydrogeological conditions. With numerical modeling it is possible to simulate different groundwater control techniques applied to a site-specific excavation plan, assess their effectiveness, and determine the most feasible and cost-effective method.
Support of excavation design. The design and construction of support of excavation (SOE) systems can be complex because of the soil type, depth of cut, and presence of a high groundwater table. We utilize numerical models to evaluate the optimal depth of watertight SOEs and to estimate water table drawdown and related settlement. The models are a tool to allow clients make informed decisions on the SOE depth based on permit requirements or acceptable means and methods for water cut-off and handling.
Dewatering system design. Numerical analysis is an essential tool for optimizing the design of a groundwater control system and thereby increasing its efficiency. For instance, with numerical models it is possible to estimate the optimal configuration of a deep-well dewatering system in terms of number of wells, distance between wells, and screen depth to obtain the desired drawdown within the excavation.
Estimate of groundwater / surface water depletion. Numerical analyses are beneficial for assessing the quantitative impact caused by a dewatering system in terms of reduction in water availability. Such reduction could negatively affect nearby water-dependent features, such as surface water bodies or drinking-water wells, and cause major environmental and social problems.
Assessment of contaminated groundwater mobilization or saline intrusion. Numerical modeling is used for assessing the impact of dewatering operations on water quality. This is particularly important in coastal areas where dewatering operations can cause a potential increase in saline intrusion, or in urban areas in proximity to industrial sites where dewatering operations are often at risk of mobilizing nearby groundwater contaminant plumes.
Settlement in compressible soil layers and hydraulic conductivity change with depressurization. Hydromechanical processes can be incorporated into numerical groundwater flow simulations. This makes it possible to model the reduction in hydraulic conductivity as a function of compaction caused by the change in effective stress during a dewatering effort. The ability to correctly simulate change in hydraulic conductivity allows for a more accurate analysis of groundwater inflows, land subsidence, and well productivity reduction due to compaction.
These are a few of the many hydrogeological services McMillen Jacobs Associates offers. Future Segments issues will spotlight project-specific applications of hydrogeology in more depth.