Chasing a complete understanding of the triggering mechanisms of a large rapidly evolving rockslide

Rockslides in alpine areas can reach large volumes and, owing to their position along slopes, can either undergo large and rapid evolution originating large rock avalanches or can decelerate and stabilize. As a consequence, in particular when located within large deep-seated deformations, this type of instability requires accurate observation and monitoring. In this paper, the case study of the La Saxe rockslide (ca. 8 × 10⁶ m³), located within a deep-seated deformation, undergoing a major phase of acceleration in the last decade and exposing the valley bottom to a high risk, is discussed. To reach a more complete understanding of the process, in the last 3 years, an intense investigation program has been developed. Boreholes have been drilled, logged, and instrumented (open-pipe piezometers, borehole wire extensometers, inclinometric casings) to assess the landslide volume, the rate of displacement at depth, and the water pressure. Displacement monitoring has been undertaken with optical targets, a GPS network, a ground-based interferometer, and four differential multi-parametric borehole probes. A clear seasonal acceleration is observed related to snow melting periods. Deep displacements are clearly localized at specific depths. The analysis of the piezometric and snowmelt data and the calibration of a 1D block model allows the forecast of the expected displacements. To this purpose, a 1D pseudo-dynamic visco-plastic approach, based on Perzyna’s theory, has been developed. The viscous nucleus has been assumed to be bi-linear: in one case, irreversible deformations develop uniquely for positive yield function values; in a more general case, visco-plastic deformations develop even for negative values. The model has been calibrated and subsequently validated on a long temporal series of monitoring data, and it seems reliable for simulating the in situ data. A 3D simplified approach is suggested by subdividing the landslide mass into distinct interacting blocks.     

挡土墙填土边坡水文响应过程分析以赖屋山挡土墙填土边坡为例

深圳赖屋山的一挡墙表面出现数条裂缝,为了深入研究其变形破坏机制,在挡墙背后填土区布置自动监测仪器,主要包括渗压计、张力计与水分计及固定式测斜仪。通过对渗压计、张力计和水分计的监测数据分析表明:填土内地下水位埋深大,降雨对其影响小; 土体中孔隙压力和体积含水量对强降雨响应随着埋深存在不同程度的滞后性,强降雨入渗深度大于3m,并且在3m处形成瞬态饱和地下水,抗剪强度降低,易于引起边坡的浅层变形破坏。      

Approximate relationship between frequency-dependent skin depth resolved from geoelectromagnetic pedotransfer function and depth of investigation resolved from geoelectrical measurements: A case study of coastal formation,southern Nigeria

The task involved in the interpretation of Vertical Electrical Sounding (VES) data is how to get unique results in the absence/limited number of borehole information, which is usually limited to information on the spot. Geological and geochemical mapping of electrical properties are usually limited to direct observations on the surface and therefore, conclusions and extrapolations that can be drawn about the system electrical characteristics and possible underlying structures may be masked as geology changes with positions. The electrical resistivity study pedotransfer functions (PTFs) have been linked with the electromagnetic (EM) resolved PTFs at chosen frequencies of skin/penetration depth corresponding to the VES resolved investigation depth in order to determine the local geological attributes of hydrogeological repository in the coastal formation dominated with fine sand. The illustrative application of effective skin depth depicts that effective skin depth has direct relation with the EM response of the local source over the layered earth and thus, can be linked to the direct current earth response functions as an aid for estimating the optimum depth and electrical parameters through comparative analysis. Though the VES and EM resolved depths of investigation at appropriate effective and theoretical frequencies have wide gaps, diagnostic relations characterising the subsurface depth of interest have been established. The determining factors of skin effect have been found to include frequency/period, resistivity/conductivity, absorption/attenuation coefficient and energy loss factor. The novel diagnostic relations and their corresponding constants between 1-D resistivity data and EM skin depth are robust PTFs necessary for checking the accuracy associated with the non-unique interpretations that characterise the 1-D resistivity data, mostly when lithostratigraphic data are not available.     

Comparing electromagnetic and seismic geophysical methods: Estimating the depth to water in geologically simple and complex arid environments

An integrated electromagnetic (EM) and seismic geophysical study was performed to evaluate non-invasive approaches to estimate depth to shallow groundwater (i.e., < 5 m) in arid environments with elevated soil salinity, where the installation of piezometers would be limited or prohibited. Both methods were tested in two study areas, one serving as a control site with relatively simple hydrogeology and the other serving as the experimental site with complex hydrogeology. The control site is located near the shore of Utah Lake (Palmyra, Utah, USA) where groundwater is shallow and unconfined in relatively homogeneous lacustrine sediments. The experimental site is in Carson Slough, Nevada, USA near the Ash Meadows National Wildlife Refuge in Amargosa Valley. Carson Slough is underlain by valley fill, with variable shallow depths to water beneath an ephemeral braided stream system. The geophysical methods used include frequency domain electromagnetic induction with multiple antenna–receiver spacings. High-resolution P-wave seismic profiles using a short (0.305 m) geophone spacing for common depth-point reflection stacking and first arrival modeling were also acquired. Both methods were deployed over several profiles where shallow piezometer control was present. EM results at both sites show that water surfaces correspond with a drop in conductivity. This is due to elevated concentrations of evaporative salts in the vadose zone immediately above the water table. EM and seismic profiles at the Palmyra site accurately detected the depth to groundwater in monitoring wells, as well as interpolated depths between them. This demonstrates that an integrated approach is ideal for relatively homogeneous aquifers. On the other hand, interpreting the EM and seismic profiles at Carson Slough was challenging due to the laterally and vertically variable soil types, segmented perched water surfaces, and strong salinity variations. The high-resolution images and models provided by the geophysical profiles confirm the simple soil and hydrological structure at the Palmyra site as well as the laterally complex structure at Carson Slough. The integrated approach worked well for determining depth to water in the geologically simple site, but was less effective in the geologically complex site where multiple water tables appear to be present.     

Tracing groundwater flow and sources of organic carbon in sandstone aquifers using fluorescence properties of dissolved organic matter (DOM)

The fluorescence properties of groundwaters from sites in two UK aquifers, the Penrith Sandstone of Cumbria and the Sherwood Sandstone of South Yorkshire, were investigated using excitation–emission matrix (EEM) fluorescence spectroscopy. Both aquifers are regionally important sources of public supply water and are locally impacted by pollution. The Penrith Sandstone site is in a rural setting while the Sherwood Sandstone site is in suburban Doncaster. Fluorescence analysis of samples from discrete sample depths in the Penrith Sandstone shows decreasing fulvic-like intensities with depth and also shows a good correlation with CFC-12, an anthropogenic groundwater tracer. Tryptophan-like fluorescence centres in the depth profile may also provide evidence of rapid routing of relatively recent applications of organic slurry along fractures. Fluorescence analysis of groundwater sampled from multi-level piezometers installed within the Sherwood Sandstone aquifer also shows regions of tryptophan-like and relatively higher fulvic-like signatures. The fluorescence intensity profile in the piezometers shows tryptophan-like peaks at depths in excess of 50 m and mirrors the pattern exhibited by microbial species and CFCs highlighting the deep and rapid penetration of modern recharge due to rapid fracture flow. Fluorescence analysis has allowed the rapid assessment of different types and relative abundances of dissolved organic matter (DOM), and the fingerprinting of different sources of organic C within the groundwater system. The tryptophan:fulvic ratios found in the Penrith Sandstone were found to be between 0.5 and 3.0 and are characteristic of ratios from sheep waste sources. The Sherwood Sandstone has the lowest ratios (0.2–0.4) indicating a different source of DOM, most likely a mixture of terrestrial and microbial sources, although there is little evidence of pollution from leaking urban sewage systems. Results from these two studies suggest that intrinsic fluorescence may be used as a proxy for, or complementary tool to, other groundwater investigation methods in helping provide a conceptual model of groundwater flow and identifying different sources of DOM within the groundwater system.     

Borehole water and hydrologic model around the Nojima fault, SW Japan

The active fault drilling at Nojima Hirabayashi after the 1995 Hyogoken-nanbu (Kobe) earthquake (M_JMA = 7.2) provides us with a unique opportunity to investigate subsurface fault structure and the in-situ properties of fault and fluid. The borehole intersected the fault gouge of the Nojima fault at a depth interval of 623 m to 625 m. The lithology is mostly Cretaceous granodiorite with some porphyry dikes.The fault core is highly permeable due to fracturing. The borehole water was sampled in 1996 and 2000 from the depth interval between 630 and 650 m, just below the fault core. The chemical and isotopic compositions were analyzed. Carbon and oxygen isotope ratios of carbonates from the fault core were analyzed to estimate the origin of fluid.The following conclusions were obtained. (1) The ionic and isotopic compositions of borehole water did not change from 1996 to 2000. They are mostly derived from local ground water as mentioned by Sato and Takahashi [Sato, T., Takahashi, M., 2000. Chemical and isotopic compositions of groundwater obtained from the Hirabayashi well. Geological Survey of Japan Interim Report No. EQ/00/1, 187–192.]. (2) Geochemical speciation revealed that the borehole water was derived from a relatively deep reservoir, which may be situated at a depth of 3 to 4 km where the temperature is about 80–90 °C. (3) The shallower part of the Nojima fault (shallower than the reservoir depth) has not been healed from the hydrological viewpoints 5 years after the event, in contrast to the rapid healing detected by S wave splitting [Tadokoro, K., Ando, M., 2002. Evidence for rapid fault healing derived from temporal changes in S wave splitting, Geophys. Res. Lett., 29, 10.1029/2001GL013644.]. (4) Precipitation of calcite from the present borehole water since drilling supports the idea of precipitation of some calcite in coseismic hydraulic fractures in the fault core [Boullier, A-M., Fujimoto, K., Ohtani, T., Roman-Ross, G., Lewin, E., Ito, H., Pezard, P., Ildefonse, B., 2004. Textural evidence for recent co-seismic circulation of fluids in the Nojima fault zone, Awaji Island, Japan., Tectonophysics, 378, 165–181.]. (5) Carbon and oxygen isotope ratios of calcite indicated that the meteoric water flux had been localized at the fault core. (6) A difference in the carbon isotope ratio between the footwall and the hanging wall suggests that the fault has been acted as a hydrologic barrier, although the permeability along the fault is still high.     

福建悠远矿区破碎地层绳索取心技术分析

以悠远矿区ZK303孔为研究对象,运用回归分析法对该孔的钻探进尺和井身质量数据进行了分析,认为该孔的极限完孔日期应在59d内,全井段的最大弯曲出现在780m;1 200m以上拟采用孕镶金刚石梯齿性钻头,以下采用PDC锯齿型钻头。依据扩散双电层理论,采用提高泥浆中Na+浓度来抑制水敏地层的水化分散,并结合适当次数的冲孔再钻进,这样既保证了井壁的稳定,又可大大提高钻速,从而获得良好的钻探效果和经济效益。此结果可为矿区其它钻孔的施工提供参考。     

The heat transfer in the region of the Mauna Kea (Hawaii)—constraints from borehole temperature measurements and coupled thermo-hydraulic modeling

The objectives of this paper are an understanding of the thermal and hydraulic field because of a negative temperature gradient and cold temperatures in the 1-km-deep borehole of the Hawaiian Scientific Drilling Project (HSDP), located near the coast line. The temperature pattern is attributed to a superposition of thermal and hydraulic processes. In the deeper borehole (HSDP-2, depth 3.1 km) detailed temperature monitoring was performed. Temperature measurements reveal two different thermal regimes. The upper part is characterised by cold temperatures and a negative temperature gradient similar to those observed in the shallow pilot borehole. Below 1100 m, increasing temperatures are observed. Different processes, such as topographically driven groundwater flow, ingress of salt water and conductive heat flow are investigated by numerical modeling. A pure conductive scenario fails to match the temperature measurements, implying that both borehole sections are overprinted by advective conditions. Coupled fluid and heat flow modeling with solute transport yield results that agree with observed temperatures. The results of these simulations suggest that meteoric water flow from the mountain range controls the thermal conditions in the upper part of the borehole. Below this level, the thermal regime is additionally affected by circulation of salt water from the nearby ocean. Each of these flow systems has been observed at other locations: topographically driven fresh water at locations with pronounced topography and ingress of salt water is typical for islands or coastal areas. At Hawaii, they coincide and influence each other, resulting in a salt water interface occurring at greater depth than expected.     

Development of unconfined conditions in multi-aquifer flow systems: a case study in the Rajshahi Barind,Bangladesh

Identifying flow processes in multi-aquifer flow systems is a considerable challenge, especially if substantial abstraction occurs. The Rajshahi Barind groundwater flow system in Bangladesh provides an example of the manner in which flow processes can change with time. At some locations there has been a decrease with time in groundwater heads and also in the magnitude of the seasonal fluctuations. This report describes the important stages in a detailed field and modelling study at a specific location in this groundwater flow system. To understand more about the changing conditions, piezometers were constructed in 2015 at different depths but the same location; water levels in these piezometers indicate the formation of an additional water table. Conceptual models are described which show how conditions have changed between the years 2000 and 2015. Following the formation of the additional water table, the aquifer system is conceptualised as two units. A pumping test is described with data collected during both the pumping and recovery phases. Pumping test data for the Lower Unit are analysed using a computational model with estimates of the aquifer parameters; the model also provided estimates of the quantity of water moving from the ground surface, through the Upper Unit, to provide an input to the Lower Unit. The reasons for the substantial changes in the groundwater heads are identified; monitoring of the recently formed additional water table provides a means of testing whether over-abstraction is occurring.     

空间坐标系下地下岩体结构面数学描述方法初探

钻孔摄像技术能够采集到大量地下岩体内的结构面数据,分析其数据有助于进一步揭示结构面的空间分布规律和组合关系。针对钻孔内获得的结构面几何参数的特殊性,提出了综合倾角、倾向和位置深度等参数的特征点描述方法,在局部坐标系下对结构面特征点的描述方法及计算过程进行了阐述,推导出了结构面几何参数与特征点坐标之间的转换公式,在全局坐标系下对特征点坐标进行了归一化处理,最后对空间坐标系下特征点的分布规律进行了探索。研究结果表明,（1）空间坐标系下的结构面特征点是结构面几何参数和空间位置的综合反映;（2）特征点在空间的分布反映了结构面的空间组合关系。为结构面的统计分析提供新的思路和方法,有助于开展结构面的空间分布规律和交切关系研究,对钻孔摄像技术的发展和以结构面几何特征为基础的岩体稳定性评价起到积极的推动作用。     

不同基准面上的速度场及时深转换

速度场及时深转换基准面的选取与速度分析基准面、静校正基准面密切相关。鉴于目前常用的两步法静校正来讲,速度建场和时深转换时,有2个面可供选择：一是速度谱分析面,即CMP面;一是统一的固定水平基准面,即最终成果剖面的零时间起始面。选取不同的基准面进行速度场研究及时深转换,所得最终构造图精度不同。笔者分别在这2个不同基准面上进行了速度场及时深转换研究,并提出使用在一个排列长度范围内,对检波点高程和炮点高程进行统计平均的方法,作为对应时间域CMP面的深度域时深转换面。对比研究表明,在此面上进行速度场研究和时深转换,在不加任何井资料约束前提下,所得平均速度场比常用的在统一水平基准面求取的平均速度场精度高,时深转换后所得最终构造图的对井误差,小于在统一水平基准面上进行的时深转换结果。     

管桩挤土效应的现场试验和数值模拟

以PHC管桩加固某物流加工二期标准仓库、厂房项目为依托,进行了吹填土地区PHC管桩的挤土效应研究。利用预先埋设的孔压计和测斜计,借助沉桩过程的现场测试手段,研究了PHC管桩在沉桩过程中桩周孔压变化和土体位移分布特征,发现沉桩过程中超孔隙水压力随距桩心距离的增加而近似呈线性规律衰减,影响范围约为10倍桩径,土体水平位移不仅与地基土性质有关,而且其分布特征也与深度有很大关系,表现为0.2～0.4倍桩长处土体水平位移较大。采用数值模拟有效地模拟了现场测试结果,进而将结果推广,得到了沉桩引起的桩周土体位移随深度和距桩心距离的变化规律。数值模拟结果表明：桩-土界面摩擦特性和桩径对土体位移场的影响较大。     

三峡工程地下电站主厂房围岩渗透性研究

利用压水试验资料分析了主厂房区岩体在水平方向和垂直方向的渗透性。在水平方向,主厂房区岩体的渗透性以微透水-弱透水为主,部分甚至不透水,个别岩体中等透水,厂房位于比周边岩体渗透性弱的区域上。在垂直方向,主厂房区大部分岩体的渗透性随深度的增加而递减,但也有小部分岩体的渗透性变化不明显,并非随深度的增加而递减。导致这种现象的原因是这部分岩体中高倾角的裂隙和断层比较发育。在同一高程上,断层端点对岩体透水率有较大的影响,断层端部发育聚集的地方,岩体的透水率大。     

Water depth modifies relative predation risk for a motile fish taxon in Bahamian tidal creeks

We evaluated the influence of water depth on relative predation risk for mojarra (Eucinostomus spp.) in six tidal creeks on Andros Island, Bahamas. Relative predation risk was determined using a tethering protocol combined with underwater visual census. In one experiment, we found that relative predation risk increased predictably with water depth (r² = 0.83), and survival of tethered mojarra decreased with water depth (r² = 0.71). We identified three depth zones containing differing levels of predation threat: refugia (0–19 cm), transition (20–69 cm), and predation (> 70 cm). Predation on mojarra rarely occurred within the refugia zone (2% eaten) and always in the predation zone (100% eaten). Additional factors not examined in this study (e.g., mangrove complexity, predator density) likely drive variability of relative predation risk within the transition zone. In a second experiment, we directly examined influence of water depth on relative predation risk at fixed locations from high tide to low tide in a single creek. Mean relative predation risk was significantly higher during higher tides at deeper water depths. Results provide experimental evidence that exploitation of shallow water refugia by motile prey can significantly reduce predation risk. We expect the distribution of motile fishes is at least partially influenced by spatially-dynamic shallow water refugia.     

Mapping of the hyporheic zone around a tidal creek using a combination of borehole logging,borehole electrical tomography and cross-creek electrical imaging,New South Wales,Australia

A combination of electrical imaging carried out across a tidal creek and borehole electrical tomography between strings of electrodes installed in bores adjacent to the creek is used to establish the shape of the hyporheic zone beneath a tidal creek in a sand aquifer. The interpretations provided by the data inversions are checked against fluid electrical conductivity (EC) samples collected from bundled piezometers installed in the creek banks and from bulk EC measurements made using induction logs in bores on the creek banks. Extensive seepage into the base of the creek is identified from the electrical image and verified by independent measurements of hydraulic head and fluid EC. The Effective Medium Theory is used to derive values of fluid EC from bulk resistivity measurements obtained from the inversion process. The data show an extensive zone of mixing between seawater and rainwater recharge into the aquifer, with the shape of the hyporheic zone strongly influenced by regional groundwater discharge and the presence of a thin layer of cemented sands at a depth of 10 m. The combined interpretation demonstrates the importance of borehole control in the interpretation of electrical images measured from the surface over complex EC distributions. Electronic Publication     

近水平岩层滑坡启动的临界水柱高度分析

从岩性特点和外部诱发因素等方面分析了近水平岩层滑坡的形成机制,提出此类滑坡启动的主要诱发因素是暴雨或持续降雨.从力学角度讨论了滑坡启动的判别依据,推导了滑坡启动时的临界水柱高度.研究表明,裂隙水对近水平岩层滑坡启动的影响是有条件的,在滑体厚度d一定的情况下,当滑面长度L小于安全长度LS时,滑坡就会启动,否则裂隙水不足以引起滑坡的启动.同时对应于一定的滑面长度,分析了浅层、中层、厚层滑坡的临界水柱高度值.     

陆相深水沉积层序和体系域边界识别方法初探——以东营凹陷牛38井为例

浅水环境中 ,在水体由深变浅或者由浅变深变化过程中 ,沉积岩的特性会发生明显的变化。但在深水环境中 ,沉积岩的特征对水体深度的小范围变动的反应并不明显。如何进行偏深水陆相沉积地层的沉积旋回划分 ,是层序地层学研究中一个薄弱的环节。以位于东营凹陷牛庄洼陷内资料较为齐全的牛 38井为研究对象 ,选用对水深变化较敏感的指标 ,如总有机碳含量、稀土元素、声波时差等 ,并结合Fischer曲线法 ,对深水沉积地层的层序地层单元划分进行了初步探讨 ,取得了较好的对应效果。研究结果表明 ,在层序和体系域边界上 ,总有机碳含量较低 ,声波时差曲线的斜率会发生明显的变化 ,稀土元素的相对含量较低 ,Fischer曲线的斜率也会发生变化     

A semi-analytical solution for swelling around a borehole

Previous field and laboratory studies have shown that the creation of a borehole in a saturated cohesive soil mass induces significant pore water suctions in the vicinity of the hole. The dissipation of these pore water suctions (swelling) leads to local increases in the moisture content and hence a softening of the soil around the hole. This softening may have important consequences for the stability of the hole and also for the ultimate load capacity of any foundation elements (bored piles or drilled shafts) constructed in these holes. This paper presents a semi-analytical solution for the radial dissipation of pore water pressure around a freshly created, vertical hole. It is assumed the soil deforms elastically during the swelling process. The solutions are presented in the form of isochrones of excess pore water pressure and may be used to obtain estimates of the time required for the soil around the hole to swell and therefore to soften. Both permeable and impermeable borehole interfaces have been considered, together with either no support for the hole or partial support provided by hydrostatic pressure within the hole.     

利用测井数据解析含煤地层渗透特性

研究受采动影响含煤地层渗透特性的变化规律,结合电视钻孔成像技术综合分析覆岩破坏高度,对煤矿开采瓦斯抽放系统钻孔位置合理布置具有重要意义。根据测井资料（密度、伽马）,解析地层边界、泥质含量及纯岩段夹层、弱层的存在及其对含煤地层渗透特性的影响。理论分析与现场实测表明:原始地层伽马曲线和泥质含量可以解析泥岩、粘土层和夹层、弱层的位置;含煤地层渗透率受采动影响,渗透率增高区由采前泥岩、粘土与其他岩体层交界和原始裂隙区向采后纯岩夹层、弱层区演化;被研究含煤地层冒落带顶点深度433 m,裂隙带顶点深度382 m。      

Fluids’ dynamics in transient air sparging of a heterogeneous unconfined aquifer

Water table dynamics, dissolved oxygen (DO) content, electrical resistivity (ER) in monitoring wells and air pressure in the vadose zone are monitored in air sparging (AS) accompanied by soil vapor extraction (SVE) at a hydrocarbon-contaminated groundwater site in Oman, where a diesel spillover affected a heterogeneous unconfined aquifer. The formation of a groundwater mound at the early stage of air injection and potential lateral migration of contaminants from the mound apex called for an additional hydrodynamic barrier constructed as a pair of pump-and-treat (P&T) wells whose recirculation zone encompassed the AS and SVE wells. In all monitored piezometers the phreatic surface showed a rapid and distinct peak, which is attributed to the time of air breakthrough from the injection point to the vadose zone and a relatively mild recession limb interpreted as a decay of the mound. Tracer tests showed a layer of a relatively low hydraulic conductivity at an intermediate depth of the screened interval of the wells. Increased levels of DO and borehole air pressure that have been observed (as far as 50 m away) are likely mitigated by SVE and P&T. Radius of influence can be indirectly inferred from ER and DO changes in the AS operation zone. Salt tracer tests have shown that groundwater velocity within the AS zone decreases with the increase of air injection rate.