Exposed Rock Face Research Articles

Rapid in situ assessment of luminescence-bleaching depths for deriving burial and exposure chronologies of rock surfaces

Recent developments in luminescence dating offer new ways to date exposure and burial durations of rocks. The new rock surface dating methods ideally require high-resolution data, faster sample preparation and measurement times, and field screening methods to select samples with appropriate luminescence characteristics and bleaching histories. Presented here is a demonstration of an EMCCD (electron multiplying charge coupled device) based system capable of imaging high-resolution infrared stimulated luminescence (IRSL) and infrared-photoluminescence (IRPL) from rock samples. The IRPL can be detected at both 880 nm and 955 nm. Using this instrument, the entire luminescence-depth profile can be reconstructed by imaging a single surface cut perpendicular to the exposed rock face. We demonstrate the possibility of reconstructing luminescence-depth profiles suitable for rock surface dating from large (cm-scale) rock samples, without using a regeneration dose for normalisation of the natural luminescence signals. Based on the different bleaching characteristics of the IRSL and IRPL emissions at 880 nm and 955 nm, we show that it is possible to gain reliable estimates of bleaching depths from measurement of as few as two images of the IRPL signal (one for each emission), or from measurement of the IRSL decay curve. We thus by-pass laborious sample preparations and the need for a gamma source to estimate the bleaching depth, thereby extending the 2D luminescence-depth profile imaging technique to other laboratories that lack access to ionising radiation facilities. This study also makes a significant progression towards development of a field instrument for in situ relative exposure dating, and sample screening for rock surface burial dating. • Direct imaging of luminescence-depth profiles in rocks using IRPL or IRSL. • New methods for normalising spatial variations in luminescence without irradiation. • Characterising bleachability of different OSL emissions from the principal trap. • Rock surface dating for rapid field analysis or laboratories lacking irradiation facilities.

Automated structural discontinuity mapping in a rock face occluded by vegetation using mobile laser scanning

Abstract Mapping of structures, such as discontinuities, in an exposed rock mass is fundamental for slope stability analysis. This study investigates mobile laser scanning technology to identify structural discontinuity in a complex environment where the exposed rock face is partially covered with vegetation. The conventional terrestrial laser scanning and photogrammetry based approaches for structure identification rely solely on coplanarity criteria or point normal vectors and often miss several prominent discontinuity planes in a complex environment with inherent noise. To enhance structural mapping in such environments, this study tests a mobile scanner which has multi-view data collection to reduce blind spots. A new automated and robust algorithm termed clustering on local point descriptors (CLPD) is developed for more accurate discontinuity identification. The algorithm involves a rigorous pre-processing step to remove erroneous and irrelevant points followed by computation of local point descriptors. Five descriptors (i.e. eigenvalue descriptor (EVD), radial surface descriptor (RSD), fast point feature histogram (FPFH), normal and curvature) were generated for each point to capture spatial distribution, geometrical relationships and local surface variations in the point cloud. Finally, a K-Medoids clustering was performed on the computed descriptors using a histogram of normals to identify discontinuity planes. Results indicate that the proposed CLPD algorithm outperforms existing approaches in terms of accuracy in discontinuity orientation estimate (3.50° in dip angle and 4.32° in dip direction) for the study site. The stereonet comparison also validated that the poles distribution from CLPD is on par with the ground truth as well as results from commercially available software. The study presents a comparative evaluation of various approaches for kinematic feasibility of planar, wedge and sliding failures.

Going Vertical

ABSTRACTMost archaeological practice involves horizontal excavations of ancient occupations and cemeteries, but the Chachapoya of Peru's eastern montane built tombs along narrow cliff ledges, which require innovative methods of investigation. Many of these sites are becoming exposed and threatened due to increased deforestation. The La Petaca mortuary complex includes 125 constructed platforms, modified ledges, mausoleums, and caves containing human remains across one section of an exposed rock face approximately 200 m across by approximately 80 m high. While the site has been looted and damaged due to various taphonomic processes (including ecological, geological, and cultural), we argue that there are recoverable details, especially in relation to how the ancient Chachapoya people created and accessed these vertical spaces. Through collaboration with technical professionals, we identified and documented many tombs using vertical progression techniques. This valuable partnership between spelunkers and archaeologists allowed us to develop techniques for “vertical archaeology,” including safe access in order to be able to document, sample, and make detailed observations of building methods and burial contents.

Mapping subvertical discontinuities in rock cuts using a 400-MHz ground penetrating radar antenna

Hidden subvertical discontinuities oriented parallel to subparallel to the exposed faces of outcropping sandstone were effectively mapped at three different study sites in central Missouri using a ground-penetrating radar system (GPR) equipped with a 400-MHz monostatic antenna and a survey wheel. At each site, a suite of 2-D ground-penetrating radar profiles were acquired along multiple closely spaced traverses on relatively smooth exposed rock surfaces. Time-zero correction was applied to the raw GPR data which were then processed using band-pass filtering, range and display gain, color transformation, and deconvolution techniques. Pseudo 3D images of each identified discontinuity at each site were constructed based on the interpretation of the nonmigrated ground-penetrating radar profiles. These pseudo 3D images were hand-migrated and transformed into true 3D images which depict variable depths at “perpendicular horizontal distance” to each discontinuity relative to the exposed rock face. The results demonstrate that GPR can be used to detect and map hidden discontinuities. This information can then be used for rock slope stability analysis and rock engineering purposes.

Lake and catchment response to Holocene environmental change: spatial variability along a climate gradient in southwest Greenland

The Kangerlussuaq area of southwest Greenland is a lake-rich landscape that covers a climate gradient: a more maritime, cooler and wetter coastal zone contrasts with a dry, continental interior. Radiocarbon-dated sediment sequences (covering ~11,200–8,300 cal year) from paired lakes at the coast and the head of the fjord were analysed for lithostratigraphic variables (organic-matter content, bulk density, Ti, Ca). Minerogenic and carbon accumulation rates from the four lakes were compared to determine catchment and lake response to Holocene climatic variability. Catchment erosion at the coast was dominated by cryonival processes, with considerable sediment production due to the limited vegetation cover and exposed rock faces. Input of minerogenic sediment at one site (AT4) was high (>1 gDW cm−2 year−1) during the period 5,800–4,000 cal year BP, perhaps reflecting intensification of cryogenic processes on northeast-facing slopes and rapid delivery to the lake. This period of erosional activity was not observed at the nearby, higher elevation site (AT1) due to the lower catchment relief; instead, there was an abrupt decline in carbon and minerogenic accumulation rates at ~5,800 cal year BP. Sediment accumulation rates at the inland sites were much lower (<0.005 gDW cm−2 year−1) reflecting greater catchment stability (more extensive vegetation cover), lower relief and substantially lower precipitation, but synchronous increases in mineral accumulation rates from ~1,200 to 1,000 cal year BP may reflect wind erosion associated with regional cooling and local aridity. Carbon-accumulation-rate profiles were similar at the two inland sites, with higher-than-average accumulation (~6–8 g C m−2 year−1) during the early Holocene and a subsequent decline after ~6,000 cal year BP. At the inland lakes, both mineral and carbon accumulation rates exhibited a stronger link to climate, driven by trends in effective precipitation and regional aeolian activity. Catchment differences (relief, altitude) lead to more individualistic records in both erosion history and lake productivity at the coast.

Integrated geophysical and geological investigations applied to sedimentary rock mass characterization

The Salento Peninsula (south-eastern Italy) is characterized by sedimentary rocks. The carbonatic nature of the rocks means they are affected by karst phenomena, forming such features as sinkholes, collapsed dolines and caverns, as a result of chemical leaching of carbonates by percolating water. The instability of these phenomena often produces land subsidence problems. The importance of these events is increasing due to growing urbanization, numerous quarries affecting both the subsoil and the surface, and an important coastline characterized by cliffs. This paper focuses on geological and geophysical methods for the characterization of soft sedimentary rock, and presents the results of a study carried out in an urban area of Salento. Taking the Q system derived by Barton (2002) as the starting point for the rock mass classification, a new approach and a modification of the Barton method are proposed. The new equation proposed for the classification of sedimentary rock mass (Qsrm) takes account of the permeability of the rock masses, the geometry of the exposed rock face and their types (for example, quarry face, coastal cliff or cavity), the nature of the lithotypes that constitute the exposed sequence, and their structure and texture. This study revises the correlation between Vp and Q derived by Barton (2002), deriving a new empirical equation correlating P-wave velocities and Qsrm values in soft sedimentary rock. We also present a case history in which stratigraphical surveys, Electrical Resistivity Tomography (ERT), and seismic surveys were applied to in situ investigations of subsidence phenomena in an urban area to estimate rock mass quality. Our work shows that in the analysis of ground safety it is important to establish the rock mass quality of the subsurface structures; geophysical exploration can thus play a key role in the assessment of subsidence risk.

Stereonet Data from Terrestrial Laser Scanner Point Clouds

In order to assess the stability of a rock slope, it is essential to characterise rock discontinuities at the exposed rock face. Conventionally, a field engineer will use simple instruments (compass and clinometer) to take a limited number of measurements of dip angle (DA) and dip direction (DD) directly on the rock face. This data is then entered into an analysis system and is presented as a stereonet. The accuracy of the analysis is limited by how representative of the whole slope the DA and DD data is. If there are inaccessible areas of the slope it cannot be used in the analysis. Through the generation of a “point cloud”, terrestrial laser scanning (TLS) has the ability to map entire rock faces without contact. This paper presents a strategy for extracting DA and DD data from a point cloud using Cyclone software. The strategy was tested by comparing the stereonet of a slope produced by both conventional (manual) and TLS survey. It was found that the TLS derived data was as accurate as, and more precise than, the manual measurements and provided more information and improved the stereonet analysis compared to that of the conventional survey.

Assessment of physical disintegration characteristics of clay-bearing rocks: Disintegration index test and a new durability classification chart

Clay-bearing rocks cover large areas on the earth surface and are highly sensitive to changes in their water content. It is well known that strength and deformability properties of these rocks also deteriorate due to wetting and drying processes, causing numerous engineering problems. Due to the importance of disintegration of the clay-bearing rocks in geo-engineering practice, several simple test methods and classification systems have been proposed to assess durability The slake durability index test is one of the most widely used of these tests. However, due to mechanical breaks occurring during the test, immersion of samples in water for only 10 min and the assumption that fragments larger than 2 mm are durable, the slake durability test has some limitations. In addition, the field behaviour of clay-bearing rocks and the associated disintegration have not been fully considered in previous studies. In this study, a comprehensive research program was conducted on different clay-bearing rocks selected from Turkey to assess their disintegration characteristics under both field and laboratory conditions. Based on field and laboratory investigations, new approaches and a durability classification system are recommended to assess disintegration characteristics of clay-bearing rocks. The disintegration index test suggested in this study can minimize some of the limitations of the slake durability test. The comparison between the disintegration index values of the laboratory specimens and those of the samples from the same outcrops exposed to atmospheric conditions for 1 year showed close agreement. Based on these observations, a durability classification system consisting of six classes is suggested. The proposed classification system uses a new parameter called slake durability rating to determine the degree of disintegration of any clay-bearing rock. In addition, general information related to physical and mechanical properties of any class can be obtained from the explanation part of each class. This system may be useful to practitioners for identifying the type and degree of disintegration that may occur on a freshly exposed rock face.

HOW MITES COPE WITH LIFE IN THE FREEZER

While a cold and exposed rock face in Antarctica is the last place many animals would choose to live, Antarctic lichen mites, Halozetes belgicae , are quite at home. They are often found near gull \`rubbish dumps', or middens, grazing on the lichens encrusting the rocks. How the mites survive this

Landform evolution in the Marine Plain region, Vestfold Hills, East Antarctica

Landforms of Marine Plain in the Vestfold Hills contrast with those of most in the Vestfold Hills. They include consistent land surfaces at 40+ m and 25 m, characteristics controlled by orientation of rock features, and imprints of phases of glaciation, deglaciation and marine and freshwater inundation. The 40+ m surface is widespread in the Vestfold Hills and has been noted previously. The 25 m level was an earlier coastline, is more localized and marked by water rounded boulders; it serves to differentiate clearly between two terrains of different relief and erratic distribution. The Pliocene sedimentary rocks below the 25 m level have been dislocated, probably during an interval of glaciotectonism caused by northward movement of sediments under an ice load due to northern extension of the Sørsdal Glacier or expansion of the East Antarctic Ice Sheet some time after the mid-Pliocene. Soil development is active. These features are accompanied by the normal aspects of a glaciated landscape such as glacial striations, sand wedges, erratics, and patterned ground. Wind has been important in transporting sand and developing honeycomb weathering on exposed rock faces.

Determination of Discontinuity Size Distributions from Scanline Data

The paper outlines a new method for estimating the distribution form, and mean size, of discontinuities from scanline data. By assuming that the discontinuities are circular discs it has been possible to apply existing solutions for the distributions of censored semi-trace lengths sampled by a scanline at an exposed rock face. These solutions have been implemented by numerical quadrature in a standard Excel spreadsheet, with solution optimisation achieved with Solver. Problems caused by a singularity in the integration were overcome by applying an integration offset parameter. A trigonometrical substitution for removing this singularity is also outlined. The numerical quadrature strategy was validated by comparison with the analytical solution for the uniform distribution, and by comparison with the results of an extensive geometrical simulation of the stereological process. A new distribution, here named the Wicksell distribution, has been identified. This distribution is characterised by the fact that the distribution of disc diameters and the distribution of complete traces on a cutting plane are identical. Two examples, based on real scanline data, are presented to illustrate the practical application of the new methods.

Construction of two road underpasses at Ma On Shan, Hong Kong SAR

Two road underpasses each some 200 m long, 16 m wide and 12 m high were constructed in tunnel during 2002-2003 to serve as slip roads as part of a new major trunk road (Road T7) in Ma On Shan, Hong Kong Special Administration Region. The tunnel alignments are curved horizontally with gentle vertical gradients and encounter weak, altered and metamorphosed sedimentary rock and strong granite rock. Power tools and drill and blast techniques were employed for the tunnel excavation. Due to the presence of heavily sheared stratified rocks and an inclined principal joint set system, the influence of groundwater seepage on tunnel stability was critical. The initial temporary support system adopted for the weak rock excavation comprised a combination of support canopy (forepoling), a lightweight steel set and fibre-reinforced shotcrete. Detailed rock mapping and monitoring of excavated rock faces were undertaken to optimise temporary tunnel supports. The initial temporary support design was re-assessed by numerical analysis (FLAC model) with a range of geological input parameters based on detailed face mapping carried out during construction. This led to the subsequent omission of the invert strut of the steel set. The effects of blasting vibration on the closely spaced large span tunnels was monitored using seismographs and the results fed back into the design and procedures associated with production blasting. The vibration as detected at ground surface was monitored to be all within the allowable maximum peak particle velocity of 25 mm/s. Permanent lining construction was facilitated with the use of movable lining shutters and the underpass construction was able to be completed one month ahead of programme. Prior to and during construction there was significant interaction between the contractor and a geotechnical specialist acting as an advisor to the tunneling contractor. The full-time input of the specialist advisor has proven to be worthwhile and cost effective leading to optimization in temporary support systems in poor quality rock and blasting patterns in more competent rock. The specialist geotechnical advisor was engaged in regular and detailed mapping of exposed rock faces, collection and interpretation of monitoring data and high level analyses of construction options. This led to an efficient and productive communication and problem solving relationship with the engineer's staff on site and in their design office. The regular input from the advisor made a significant contribution to the completion of the tunnel excavation works one month ahead of programme, despite a delayed start. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

A New Method for In-situ Non-contact Roughness Measurement of Large Rock Fracture Surfaces

This paper presents a new method for in-situ non-contact measurements of fracture roughness by using a total station (TS). The TS is a non-reflector geodetic instrument usually used for measuring control points in surveying and mapping. By using a special-developed program, the TS can be used as a point-sensor laser scanner to scan a defined area of the fracture surface automatically, in field or in laboratory, at a distance away from the target surface. A large fracture surface can be automatically scanned with a constant interval of the sampling points, both within a defined area or along a cross-section of the exposed rock face. To quantify fracture roughness at different scales and obtain different densities of the scanned points, the point interval can be selected with the minimum interval of 1 mm. A local Cartesian co-ordinate system needs to be established first by the TS in front of the target rock face to define the true North or link the measurements to a known spatial co-ordinate system for both quantitative and spatial analysis of fracture roughness. To validate the method, fracture roughness data recorded with a non-reflector TS was compared with the data captured by a high-accuracy 3D-laser scanner. Results of this study revealed that both primary roughness and waviness of fracture surfaces can be quantified by the TS in the same accuracy level as that of the high accuracy laser scanner. Roughness of a natural fracture surface can be sampled without physical contact in a maximum distance of tens of meters from the rock faces.

Estimating three-dimensional rock discontinuity orientation from digital images of fracture traces

This paper describes a computer approach that has been developed for estimating three-dimensional fracture orientations from two-dimensional fracture trace information gathered from digital images of exposed rock faces. The approach assumes that the fractures occur in sets, and that each set can be described by a mean orientation and a measure of the scatter about the mean. Mathematical relationships are developed that relate the 3D fracture properties with the trace angles that would be measured on one or more rock faces. These algorithms are used in conjunction with a genetic algorithm to invert the trace angles to estimate 3D joint orientation. A number of case studies have been conducted indicating a great potential for the technique.

High resolution GPR imaging and joint characterization in limestone

ABSTRACTWe focus on the application of Ground Penetrating Radar (GPR) to evaluate limestone characteristics of interest in environmental and engineering studies, and in particular: a) to image joints, bedding planes and cavities; b) to improve accuracy and resolution of the method; c) to evaluate joint/bedding planes characteristics which affect the radar response with particular reference to thickness, sedimentary infilling, water/clay content and spatial frequency. The work is based on experiments carried out close to road cuts and cavities, where the exposed rock face allows calibration and validation of results. The test‐sites are located in NE‐Italy and are part of the Peri‐Adriatic carbonate Platform. The rocks in the area of study date back to the Paleocene and are mainly peritidal regressive limestone sequences consisting of more than 90% carbonates. Joints and bedding planes surfaces, as results from direct inspection at the rock face, are sufficiently smooth to neglect roughness effects. We demonstrate that imaging can be improved by non‐conventional data acquisition paradigms, and in particular by multi‐offset (linear multi‐fold, LMF) methods. Multi‐offset/multi‐azimuth (azimuthal multi‐fold, AMF) techniques were exploited to select optimum grid orientation and offset range for LMF application. Multi‐fold velocity analysis on CMP gathers shows rather constant wave propagation velocities. They slightly vary between different test‐sites (between 10 cm/ns and 12 cm/ns) primarily due to macro/microscopic characteristics of limestone, such as spatial frequency of joints and porosity respectively, which affect water content in the rock mass. Constant velocities allowed application of post‐stack time migration algorithms. Such algorithms attained imaging accuracy below 3% in the reconstruction of the detectable discontinuities at most test sites. A Kirchhoff algorithm proved to be the optimum solution as it effectively handled the steep dips (up to 70°) that characterize most of the examined sets of joints. Enhanced data quality further results from the application of original processing techniques, such as, in particular, Hough Transform based coherent noise/background removal and Wavelet Transform based instantaneous parameters computation and analysis. Maximum penetration depth at the examined test sites ranges between 15 m for 250 MHz (central frequency) bow‐tie shielded antennas and 23 m for 50 MHz unshielded resistively loaded linear dipoles. As for resolution, antennas in the range of 200‐250 MHz apparently provide effective discrimination of joints/bedding planes spaced not less than 40 cm, and seem therefore the adequate choice at the examined test sites. Low frequency antennas, in the range of 50 to 100 MHz (central frequency), provide a maximum 55% increment in penetration depth at the expenses of a substantially diminished resolution (around 20% of that provided by 200‐250 MHz antennas). The comparison of modelling results to multi‐fold data allows discrimination of radar response from joints filled by air and clayey deposits. Such results were validated by geological evidence at the exposed rock face.

Measuring fracture orientation at exposed rock faces by using a non-reflector total station

Measurements of fracture orientation are usually taken by using a compass-inclinometer device on exposed rock faces. The drawbacks when using this method is that it is time-consuming if many fractures are measured and that measurement might be impossible if the rock face cannot be safely reached physically. To improve field mapping of rock fractures, a method for applying a non-reflector total station to measuring fracture orientation is presented in this paper. A non-reflector total station is a geodetic device that captures three-dimensional co-ordinates of target points without using a reflector. Therefore, physical touching the rock surfaces is no longer required. To determine a fracture orientation, co-ordinates of a set of points on the exposed fracture surface are captured at a distance from the rock face. The best-fit plane of the exposed fracture surface is defined by the co-ordinates of the target points, and the orientation (e.g. dip angle and dip direction) of the fracture surface is determined as that of its best-fit plane. This paper presents the technical procedure and a portable system designed for the field mapping of fracture orientation. Results of a case study performed at an exposed rock face are also included.

Mechanisms of desiccation tolerance in cyanobacteria

Drying of cells leads to damage resulting from crowding of cytoplasmic components, condensation of the nucleoid, increases in the Tm of membrane phase transitions, and imposition of stress upon cell walls. Prolonged desiccation leads to oxidation of proteins, DNA and membrane components through metal-dependent Fenton reactions, while Maillard reactions generate cross-linked products between the carbonyl groups of reducing sugars and the primary amines of nucleic acids and proteins. Although such damage restricts many organisms to aqueous environments, some, including many cyanobacteria, can tolerate the air-dried state for prolonged periods. Cyanobacteria in the Tintenstrich communities of exposed rock faces, Microcoleus and Lyngbya spp. in intertidal mats, chasmoendolithic Chroococcidiopsis spp. in the rocks of hot and cold deserts, and terrestrial epilithic crusts of Tolypothrix and Nostoc are examples that show a marked capacity to withstand the removal of their cellular water. For Nostoc commune, the mechanisms of desiccation tolerance reflect both simple and complex interactions at the structural, physiological and molecular levels.