Groundwater Pressure Research Articles

Shallow groundwater flow and inverted fresh/saline-water interface in a hypersaline endorheic basin (Great Basin, USA)

Pilot Valley is an 828-km2 arid-region endorheic basin in western USA. Bounding mountain ranges rise as much as 1,900 m above the nearly flat 379-km2 playa floor. Up to 3.8 m of Pleistocene Lake Bonneville mud and thin oolitic sand layers form the surface layer of the basin floor. Groundwater conditions were evaluated using data from shallow monitoring wells and borings, springs, infiltrometer measurements, slug and dilution tests, geophysical transects, and precision elevation surveys. Alluvial fan groundwater discharges at fan/playa interface springs and underflows to the shallow basin sediments along the western side of the basin; the groundwater only underflows along the eastern side. Precision surveying established a Lake Bonneville shore-line break in slope as the cause of the spring discharges. Tectonic tilting causes groundwater to flow from east to west and to the topographic low. Monthly measured and pressure transducer data established seasonal pressure responses and upward groundwater gradients. All basin groundwater is lost to evapotranspiration at the topographic low, where a thin salt pan has developed. Groundwater evolves from fresh to hypersaline near the alluvial fan/playa interface where there is an inverted salinity gradient and a groundwater pressure ridge. The pressure ridge and inverted salinity interface are due to: (1) osmotic pressure established between the oolitic sand of high hydraulic conductivity and the overlying low-hydraulic-conductivity lake mud at the fan/playa interface, and (2) the collision between fresh groundwater flow driven by a steep hydraulic head and hypersaline groundwater flow driven by a nearly flat hydraulic head.

Hydromechanical Rock Slope Damage During Late Pleistocene and Holocene Glacial Cycles in an Alpine Valley

AbstractSubglacial water pressures influence groundwater conditions in proximal alpine valley rock slopes, varying with glacier advance and retreat in parallel with changing ice thickness. Fluctuating groundwater pressures in turn increase or reduce effective joint normal stresses, affecting the yield strength of discontinuities. Here we extend simplified assumptions of glacial debuttressing to investigate how glacier loading cycles together with changing groundwater pressures generate rock slope damage and prepare future slope instabilities. Using hydromechanical coupled numerical models closely based on the Aletsch Glacier valley in Switzerland, we simulate Late Pleistocene and Holocene glacier loading cycles including long‐term and annual groundwater fluctuations. Measurements of transient subglacial water pressures from ice boreholes in the Aletsch Glacier ablation area, as well as continuous monitoring of bedrock deformation from permanent Global Navigation Satellite Systems stations, help verify our model assumptions. While purely mechanical glacier loading cycles create only limited rock slope damage in our models, introducing a fluctuating groundwater table generates substantial new fracturing. Superposed annual groundwater cycles increase predicted damage. The cumulative effects are capable of destabilizing the eastern valley flank of our model in toppling‐mode failure, similar to field observations of active landslide geometry and kinematics. We find that hydromechanical fatigue is most effective acting in combination with long‐term loading and unloading of the slope during glacial cycles. Our results demonstrate that hydromechanical stresses associated with glacial cycles are capable of generating substantial rock slope damage and represent a key preparatory factor for paraglacial slope instabilities.

Influence of stagnation gradient for face support calculation in Slurry Shield Tunnelling

AbstractIn slurry shield tunnelling, a reliable face support is presumed if a sufficient support pressure is transferred and the slurry excess pressure provides efficient support pressure on the soil skeleton. The required support pressure is calculated in reference to the groundwater pressure and the earth pressure. In this paper, the sliding wedge mechanism is applied. To ensure face stability, the support pressure transfer needs to be provided within the theoretical sliding wedge at the tunnel face. Therefore, the penetration depth of the bentonite suspension as a support medium has to be kept within limits. Research shows that certain penetration is required.The stagnation gradient defines the relation of the support pressure and the penetration depth of the suspension. The gradient has a described minimum, but also an undefined but existing maximum. The German standard DIN 4126 describes two cases for the minimum at each trench depth: Either the pressure losses due to the pressure acting outside the sliding wedge are less than 5 % or the stagnation gradient is at least 200 kN/m3. If the supporting pressure gradient is below 200 kN/m3, DIN 4126 defines rough reduction factors. For a more precise observation of the stagnation gradient, this paper presents a detailed supplement to the DIN 4126 approach taking the tunnel diameter and the angle of the sliding wedge into account. The aim is to determine the minimum recommended stagnation gradient for which no increase of the slurry excess pressure is necessary due to too deep penetration.

Experimental study on the acoustic emission response and permeability evolution of tunnel lining concrete during deformation and failure

Due to long-term coupling of ground stress and water pressure during operation processes, lining concrete will experience different degrees of cracking and spalling, which seriously affect the safety of tunnels during service. In this paper, lining concrete seepage tests under different confining pressures are performed; the mechanical properties and acoustic emission (AE) responses during the loading process and the relationship between load-induced plastic deformation and lining concrete specimen permeability are studied. The plastic internal variable definition is introduced to characterize concrete specimen plastic deformation and is calculated after unloading. Different initial stress states and stress levels during loading and unloading processes affect pore and fissure development in lining concrete. The peak intensity increases with increasing confining pressures, and deformation parameter variation has elastoplastic coupling characteristics. AE parameters and axial stress show good agreement. The AE signal of the samples shows a transition from the Kaiser effect to the Felicity effect before the peak, indicating that AE can be an effective means of monitoring internal sample damage. A power function relationship can represent the relationship between the initial permeability and confining pressure, and the permeability evolution model of concrete samples in the plastic phase is established after the initial yielding.

Estimation of fault models for short-term slow\xa0slip events from groundwater pressure in soft sedimentary layers

Deep low-frequency tremors and short-term slow slip events (SSEs) are known to occur along the Nankai Trough in Japan. Although short-term SSEs are thought to occur repeatedly in the northern part of Ise Bay, deep low-frequency tremors are not observed there. Because the area around the northern part of Ise Bay is characterized by soft sedimentary layers not suitable for strainmeter or tiltmeter installation, there are no high-quality crustal deformation observation sites in the area. Accordingly, the local extent of slip of short-term SSEs has not been confirmed. We significantly improved the crustal strain sensitivity of the groundwater level (pressure) in the inner pipe of the observation well at the Hokusei observation site (HKSi, northwest of Ise Bay) by sealing it with a packer. Consequently, we were able to observe groundwater pressure changes related to slow slip at the plate boundary. We estimate that slip of short-term SSEs during July 2016, November 2017, and April 2018 extended to the northern part of Ise Bay. Furthermore, groundwater pressure fluctuations at HKSi in April 2018 greatly improved the SSE fault model, confirming that slow slip occurs in the area, despite the absence of deep low-frequency tremor. The slips without tremors are a characteristic of the northern area of Ise Bay, differing from other areas of the Nankai Trough. Our results demonstrate that groundwater level/pressure fluctuations are a powerful tool for crustal deformation observations in areas characterized by soft sedimentary layers.

Effects of Seismic Waves in Water Level Changes in a Well: Empirical Data and Models

The high-precision water level measurements with a sampling interval of 5–10 min were carried out in 1996–2017 in the YuZ-5 well, Kamchatka. In the obtained time series, water level variations caused by the passage of seismic waves (hydrogeoseismic variations—HGSV) during 19 earthquakes with Мw = 6.8–9.1 which occurred at epicentral distances of 80–14.6 thousand km are revealed. Based on the HGSV morphological features, four main types of these variations are distinguished: oscillations (I); short (up to tens of hours) water level rises superimposed on oscillations (II); short rises (III); and long (1.5–3 months) drawdowns (IV). The dependence of the occurrence of the revealed GHSV types on earthquake parameters (magnitude and distance), specific energy density and maximum seismic wave velocity, and the amplitude-frequency content of ground motion is analyzed based on the records at a nearest seismic station. Based on several case studies, hydrogeodynamic processes of HGSV formation are investigated using numerical modeling. It is shown that the forced and free amplitude fluctuations in the water level (types I and II) can arise due to the enhancement of groundwater pressure variations in the well–water-bearing rock system during the passage of surface seismic waves with periods corresponding to the resonant frequency of the well (τ = 44.6 s). The rise in the water level in well lasting for tens minutes to hours (types II and III of HGSV variation) is caused by the short increase in pressure under violation of the steady water flow in the direct vicinity of the well; strong local earthquakes accompanied by ground shaking with intensity Imsk-64 ≥ 5 cause sustained drawdowns (type IV) due to pressure drop with the amplitudes up to 0.1 bar within a radius of up to a few hundred meters from the well.

Quantifying sediment mass redistribution from joint time-lapse gravimetry and photogrammetry surveys

Abstract. The accurate quantification of sediment mass redistribution is central to the study of surface processes, yet it remains a challenging task. Here we test a new combination of terrestrial gravity and drone photogrammetry methods to quantify sediment mass redistribution over a 1 km2 area. Gravity and photogrammetry are complementary methods. Indeed, gravity changes are sensitive to mass changes and to their location. Thus, by using photogrammetry data to constrain this location, the sediment mass can be properly estimated from the gravity data. We carried out three joint gravimetry–photogrammetry surveys, once a year in 2015, 2016 and 2017, over a 1 km2 area in southern Taiwan, featuring both a wide meander of the Laonong River and a slow landslide. We first removed the gravity changes from non-sediment effects, such as tides, groundwater, surface displacements and air pressure variations. Then, we inverted the density of the sediment with an attempt to distinguish the density of the landslide from the density of the river sediments. We eventually estimate an average loss of 3.7 ± 0.4 × 109 kg of sediment from 2015 to 2017 mostly due to the slow landslide. Although the gravity devices used in this study are expensive and need week-long surveys, new instrumentation currently being developed will enable dense and continuous measurements at lower cost, making the method that has been developed and tested in this study well-suited for the estimation of erosion, sediment transfer and deposition in landscapes.

Discussion on The Design of Water Blocking and Drainage Limit of Water-rich Mountain Tunnel in Ecologically Fragile Area

In mountainous areas with abundant groundwater and high water pressure, the design of waterproof and drainage is the most important part of the whole tunnel construction process. Improper treatment of groundwater will not only damage the local ecological environment, but also bring hidden danger to the safety of tunnel structure, especially in the ecologically fragile area. The tunnel project under construction in the East Tianshan Mountain is located in the high-altitude mountain area. The surface is the national forest protection area, and the vegetation is perennial. Once a large amount of groundwater is lost, the surface ecology will be damaged and hard to recover in a short time. In this paper, the existing design of mountain tunnel waterproof and drainage is summarized, and its advantages and disadvantages are analyzed. Based on the previous research, the concept of waterproof and drainage design suitable for the East Tianshan tunnel is proposed: “one block, two rows and Three Preventions”. “One block” is a circle of grouting outside the tunnel to block most of the groundwater. “Two rows” is to set two layers of drainage pipe network between shotcrete and waterproof layer and between waterproof layer and lining. “Three proofing” is to use three layers of water proofing: shotcrete layer, special waterproof layer and lining concrete. Layers of fortification can minimize the interference to groundwater and ensure the safety of tunnel structure.

Forecasting the long-term activity of deep-seated landslides via groundwater flow and slope stability modelling

Large (deep-seated) landslides present complex geometries, rock/soil properties, and kinematical behavior. Complex geometries are due to the presence of several sliding zones, while complex properties typically result from the dilation, compression, or fatigue of geologic materials. Kinematical behavior is often episodic, with periods of stability followed by periods of enhanced slope movements owing to shear strength reduction in response to groundwater pressure changes. These mechanisms complicate our capacity in forecasting the long-term activity and thus, the choice of a strategy for hazard management. This technical note introduces a method for predicting the long-term activity of deep-seated landslides based on one-way coupled hydromechanical numerical modelling. The method is applied to analyse the long-term stability of a deep-seated compound slide in the Swiss Jura Mountains. Results indicate that, under natural groundwater pressure changes, the analysed compound slide will continue to move in an episodic fashion in response to groundwater levels in the slope, without developing velocities greater than several centimeters per year. This example demonstrates how one-way coupled hydromechanical modelling constrained by field data is a reliable tool for assessing the long-term activity of deep-seated landslides and helping the management of associated hazards.

Numerical modelling of slope damage in large, slowly moving rockslides: Insights from the Downie Slide, British Columbia, Canada

The deformation of rock slopes is associated with the formation and accumulation of internal and external features (such as tension cracks, rock mass bulging and dilation, rockfall, etc.) that can be comprehensively referred to as “slope damage”. In this paper, we use a 3D distinct element numerical modelling approach to investigate the role of shear zone morphology and groundwater pressure on the displacement and slope damage accumulation at the Downie Slide, a large, extremely slowly moving rockslide in British Columbia (Canada). First, we briefly review the external slope damage features that can be observed in an airborne laser scanner (ALS) dataset, allowing four slope damage domains (upper, central, northern, and southern domain) to be interpreted within the slide area, based on the orientation and type of features. Using the same ALS dataset we construct a 3D model of the present-day slope, explicitly including the two shear zones along which displacements occur, in order to investigate their role in the later stage of the landslide evolution. We assign a strain-softening constitutive model to the slide body, in order to account for the decrease in material properties due to damage accumulation. Virtual inclinometers are also implemented in the model, allowing for the comparison of simulated and observed displacement direction along the shear zones. The progressive deformation and failure of the slope is then simulated both assuming dry and wet conditions, in order to examine the role of pore water pressure, and the morphology of the upper and lower shear zones on the magnitude, orientation, and distribution of displacements. For each numerical model, the simulated slope damage features are recorded, by analyzing the zone volumetric strain and failure state, and compared with the type and orientation of features observed in each of the interpreted slope damage domains, thus allowing the numerical results to be better constrained and validated. It is clearly demonstrated that the orientation and distribution of slope damage and displacements observed in both the surface ALS and the subsurface borehole inclinometer data can be well reproduced in the 3D numerical models. Numerical modelling results show that the principal factor controlling the spatial distribution of slope damage at the Downie Slide is the morphology of the lower shear zone, whereas a negligible role is played by the upper shear zone morphology. We also observe that models incorporating a groundwater table display larger displacements, without significant effects on the orientation and distribution of simulated slope damage. This paper demonstrates that an analysis of slope damage is very important for understanding the mechanisms underlying the behavior of large landslides and should be a fundamental step in the comprehensive characterization of any major slope failure.

Seasonal deformations under a road embankment on degrading permafrost in Northern Canada

Permafrost degradation is a major concern in cold regions that are warming because of climate change. To assist in understanding the process, ground temperatures, lateral and vertical deformations and groundwater pressures were measured for 6 years under a road embankment in northern Manitoba, Canada. The road surface requires ongoing maintenance due to irregular settlements. The field data allowed the calibration of numerical modelling that related deformations to ground temperatures. It confirmed the presence of remnant permafrost under the centre of the embankment, but none under the midslope or toe. This ‘frost bulb’ plays an important role in the ground thermal regime, the groundwater pressures, the observed deformations and, ultimately, necessary road maintenance.

Tuen Mun–Chek Lap Kok Link: an outstanding subsea tunnel project in Hong Kong

The 4·5 km long northern subsea tunnel section of the new Tuen Mun–Chek Lap Kok Link in Hong Kong had to overcome challenging ground conditions, not least groundwater pressures of over 5 bar. Hard ground meant some 2000 disc cutters on the two 14 m diameter slurry tunnel boring machines needed changing during the drives, initially by professional divers kept in saturation conditions. Two robotic systems were later developed and installed on the tunnelling machines to do the job. Other innovations included pipe-jacking the 41 subsea cross-passages and creating a 500 m long, 43 m deep caterpillar-shaped cofferdam for a cut-and-cover section. This paper presents details of the innovative solutions used and lessons learned.

Empirical relationships of groundwater head\u2013salinity response to variations of sea level and vertical recharge in coastal confined karst aquifers

Coastal aquifers are characterized by a mixing zone with freshwater–saltwater interactions, which have a strong relationship with hydrological forcings such as astronomical and storm tides, aquifer recharge and pumping effects. These forcings govern the aquifer hydraulic head, the spatial distribution of groundwater salinity and the saline interface position. This work is an empirical evaluation through time-series analysis between aquifer head and groundwater salinity associated with the sea-level dynamics and the aquifer recharge. Groundwater pressure, temperature and salinity were measured in a confined aquifer in the northwest coast of Yucatan (México) during May 2017–May 2018, along with precipitation. Cross-correlation and linear Pearson correlation (r) analyses were performed with the data time series, separating astronomical and meteorological tides and vertical recharge effects. The results show that the astronomical and meteorological tides are directly correlated with the aquifer head response (0.71 < r < 0.99). Salinity has a direct and strong relationship with the astronomical tide (0.76 < r < 0.98), while the meteorological tide does not (r < 0.5). The vertical recharge showed a moderate correlation with the aquifer head (0.5 < r < 0.7) and a nonsignificant correlation with the groundwater salinity (r < 0.5). In this study, the sea level (r > 0.7) is a more important forcing than the vertical recharge (with 0.5 < r < 0.7). Empirical relationships through time-series analysis and the separation of individual hydrological forcings in the analysis are powerful tools to study, define and validate the conceptual model of the aquifer.

Quantifying Resistance of Reinforced Concrete Pipe Joints to Water Infiltration

AbstractHydrostatic joint performance for infiltration is a key design requirement for a reinforced concrete pipe (RCP) to withstand in situ groundwater pressure. However, current standards and tes...

Borehole monitoring of thermo-hydro-mechanical rock slope processes adjacent to an actively retreating glacier

Most natural rock slope failures result from long-term strength degradation. It is often hypothesized that glacial retreat leads to enhanced progressive damage accumulation in adjacent rock slopes, due to mechanical unloading and changes to the thermal and hydraulic boundary conditions. However, direct observations of subsurface processes in a rock slope subject to glacial retreat are rare. In this paper, we present the design, implementation, and performance of a new borehole monitoring system installed on a rock slope in fractured crystalline rock located beside the glacier tongue of the retreating Great Aletsch Glacier (Valais, Switzerland). The three vertical, 50 m deep boreholes were instrumented to continuously monitor groundwater pressure, temperature and deformation at high resolution, in order to investigate thermo-hydro-mechanical coupled slope processes that drive progressive rock mass damage. We show that the system is capable of measuring both reversible and irreversible displacements along single fractures at magnitudes ranging between 0.001 mm to >2 mm in the studied rock slope, and that it is often possible to identify drivers of these deformation signals. The transient subsurface temperature field shows clear indications of former ice occupation in form of cold temperatures preserved at depth and superimposed annual temperature cycles penetrating down to a depth of about 17 m. The variability of the pressure head in the slope is driven by annual snowmelt infiltration cycles, rainfall events, and the assumed connection to englacial water of the temperate glacier. The new and continuously growing dataset presented here will enable us to relate the changing boundary conditions caused by glacial retreat and fatigue from daily to annual thermal and hydraulic loading cycles to progressive rock mass weakening, which may ultimately result in rock slope failure.

Decision strategy for landslide event based on 4 parameters

Landslides are typically caused by heavy rain in some locations that have a certain slope of land compounded by deforestation or earthquakes. For instance, in 2018 there were 148 reported landslides in the Banjarnegara highlands, Indonesia. By using that fact, some research has been done. The research is about the indicator parameters of landslides by combining rain gauge sensors, soil moisture sensors, ground shift sensors and groundwater pressure sensors using Arduino Promini. The data from sensor is sent through LoRa Bee to the database server using NodeMCU ESP8266 which is connected to the Internet in real-time through access points. The data that has been sent to the server will be processed and displayed on the website. In this paper, the author devises an algorithm for analysing the data from each sensor to a decision-making strategy. The decision-making method for an early warning of landslides uses a decision tree that compares the measured parameters with a predetermined threshold of each sensor. After analysis and testing, this system is expected to provide early warning of the landslides quickly and accurately.

Groundwater response to oil storage in large-scale rock caverns with a water curtain system: Site monitoring and statistical analysis

Site monitoring provides not only critical information for the safe operation of underground facilities but also important data for characterizing the groundwater flow within heterogeneous rock masses. Site monitoring and statistical analysis were performed to ascertain the groundwater response to oil storage in the Huangdao pilot large-scale underground oil storage facility. An all-sided monitoring scheme was designed and implemented for the assessment of the performance of the facility. The groundwater level and pressure along with rainfall, storage pressure, water curtain level were obtained with the monitoring system. The containment property was assessed according to the monitoring results, showing that the containment property of the facility can be maintained under operational conditions. Autocorrelation and cross-correlation analyses of the monitored data were performed to investigate the quantitative relations of the groundwater response to the rainfall amount, water curtain level and storage pressure. The correlation coefficients and time delays of the groundwater responses to the rainfall amount, water curtain level and storage pressure were obtained through statistical analysis. Evidently, the influence of rainfall on both the groundwater level and the groundwater pressure is insignificant. The groundwater pressures in the upper cavern pillars are influenced mainly by the water curtain system, while the groundwater pressures in the lower cavern pillars are influenced by the storage pressure. The hydrogeological conditions determine the time delays of the groundwater level and groundwater pressure responses to the water curtain level and storage pressure. Eventually, it was concluded that the groundwater response to oil storage in rock caverns with a water curtain system is related to both the geometrical layout and the hydrogeological conditions of the facility.

Early Pleistocene River Terraces of the Gediz River, Turkey: The role of faulting, fracturing, volcanism and travertines in their genesis

Reconstructing and interpreting past hydrological system changes from the Quaternary sediment-landform record presents many challenges especially when trying to establish causality. In such circumstances attempting to isolate individual drivers is perhaps unrealistic and thus inferences are made with respect to the combined influence of multiple controls and constraints. In this paper we seek to interpret the landscape responses recorded in the sediment-landform record of the Early Pleistocene Gediz valley, western Turkey, within the context of interacting geodynamic and hydrodynamic system changes.Our earlier work on the Gediz sequence established the presence of a rich fluvial archive constrained by a comprehensive geochronology from overlying lava flows. Repeated damming of the Gediz valley floor and consequent re-routing of water flow, testifies to a direct connection between fluvial activity and volcanism. This volcanism has been linked to regional tectonic extension with regional uplift promoted as a major driver of the progressive river incision of the Gediz River over the Quaternary. This incision has been punctuated by periods of deposition dictated by sediment and water budgets controlled principally by fluctuating climate and consequent vegetation change. Recently however, the significance of more local fault movements and fracturing has been recognised and here we examine and attempt to quantify these geodynamics in greater detail. In addition, we examine and describe, for the first time, extensive associated travertine deposits. Isotopic analysis of these travertines confirms their thermogenic origin and directly links their creation to the faulting/fracturing, volcanism and the recycling of calcium-rich groundwater. The formation of mounds and sheets extend onto their contemporary valley floors, thus directly connecting the fluvial and travertine archives and allowing linkage between surface and subsurface hydrological change.By utilising both the fluvial and travertine archives a more complex picture of landscape evolution emerges. The youngest terraces (<GT6) appear to be driven by local fault movement associated with the onset of volcanism and thus the link with fluctuating climates, as demonstrated in the higher terraces (GT7 +), is unclear. Detailed observations suggest formation of these lower terraces is also conditional on the local substrates. Here it appears that GT5-GT1 form in response to the creation of a knickpoint where the river intersects the faults in areas of hard basement rock. Hence, this response is highly localized, dependent on the immediate geometry and structure of the underlying strata. The presence of complex basement topography also plays a key role in generating ground water pressure and thus the creation of travertine mounds.The Gediz sequence demonstrates increasing complexity. There is no reason to suggest this is unusual. Although climate and tectonics remain as key drivers the local landscape response is also conditioned by many additional factors. While it is tempting to search for global patterns (“the big picture”), such reduced complexity may be over simplistic. Recognising complexity requires detailed observation and therefore more emphasis on “the smaller picture” which can only be obtained through extensive field observation.

Theoretical Basis for Designing Seepage-Control Walls in Dam Foundations

The structures arranged using the method of «slurry trench cutoff wall» or using secant piles are frequently used as seepage-control elements in dam foundations. However, they are designed based on empirical approaches without theoretical validation. With the aid of theoretical modeling the author revealed main specific features of seepage-control walls stress-strain state. They work not only for cross bending but also are subject to bending in their own plane, as well as they perceive considerable vertical forces. These forces appear due to friction between the wall and soil. The author obtained empirical formulae which at preliminary design stages permit predicting maximum values of compressive stresses in the wall at the moment when vertical forces have maximum values. The study made it possible to verify the recommendations for selection of material for arrangement of seepage-control walls. As a rule, deformation modulus of the wall material should not exceed the deformation modulus of the weakest soil by more than two times. Clay-cement concrete or plastic concrete is recommended to be used for the wall material. Its compressive strength is provided due to lateral compression of the wall by lateral compression of soil and groundwater pressure. For not deep «hanging» walls it is possible to use concrete. The assessment was given of foundation deformation impact on the stress-strain state of seepage-control walls. Occurrence of weak interlayers in the foundation causes in the wall appearance of zones concentrating compressive stresses and additional bending deformations. In high dam foundations there is a danger of sharp decrease of the wall compression pressure due to deformations of the foundation lateral expansion. It is not recommended to use the cantilever pattern of the wall conjugation with the dam body, but arrange a concrete gallery above the wall head. Conjugation by a cantilever is possible only for the walls arranged at the edge of the dam profile.

Hydraulic fracturing effect on punching-induced cover-collapse sinkholes: a case study in Guangzhou, China

Punching-induced cover-collapse sinkholes occur rapidly, and multiple collapses occur simultaneously compared with those caused by other common factors, such as pumping, rainstorms and mining. Erosion cannot explain the formation of this type of collapse entirely. We studied the hydraulic fracturing effect from geological analysis, field groundwater monitoring, laboratory testing, and pressure calculation. Four stages can be established for the development of cover-collapse sinkhole based on a case study. Hydraulic fracturing and the suction effect cause sinkhole opening with the percussion bit falling and pulling repeatedly when the pile foundation is being built. The behavior can be likened figuratively to a piston action. The pressure of the calculated hydraulic fracturing effect is 6.05 × 104 Pa, which is equivalent to a 5.93-m-high water column and is much bigger than the critical pressure of 0.7 m at which hydraulic fracturing occurs in laboratory testing. The significance of this study from a practical perspective is to provide a preventative, early warning method for sinkhole opening by groundwater monitoring during pile foundation construction.