How Important Are Those Fracture Zones? Scale Dependent Characteristics Revealed Through Field Studies and an Integrated Hydrological Model of a Mountain Headwater Catchment

Modeling groundwater flow in bedrock can be particularly challenging due to heterogeneities associated with fracture zones. However, fracture zones can be difficult to map, particularly in forested areas where tree cover obscures land surface features. This study presents the evidence of fracture zones in a small, snowmelt-dominated mountain headwater catchment and explores the significance of these fracture zones on groundwater flow in the catchment. A newly acquired bare earth image acquired using LiDAR identifies a previously undetected linear erosion zone that passes near a deep bedrock well at low elevation in the catchment. Borehole geophysical logs indicate more intense fracturing in this well compared to two wells at higher elevation. The well also exhibited a linear flow response during a pumping test, which is interpreted to reflect the influence of a nearby vertical fracture zone. The major ion chemistry and stable isotope composition reveal a slightly different chemical composition and a more depleted isotopic signature for this well compared to other groundwaters and surface waters sampled throughout the catchment. With this evidence of fracturing at the well scale, an integrated land surface – subsurface hydrologic model is used to explore four different model structures at the catchment scale. The model is refined in steps, beginning with a single homogeneous bedrock layer, and progressively adding 1) a network of large-scale fracture zones within the bedrock, 2) a weathered bedrock zone, and 3) an updated LiDAR-derived digital elevation model, to gain insight into how increasing subsurface geological complexity and land surface topography influence model fit to observed data and the various water balance components. Ultimately, all of the models are considered plausible, with similar overall fit to observed data (snow, streamflow, pressure heads in piezometers, and groundwater levels) and water balance results. However, the models with fracture zones and a weathered zone had better fits for the low elevation well. These models contributed slightly more baseflow (~14% of streamflow) compared to models without a weathered zone (~1%). Thus, in the watershed scale model, including a weathered bedrock zone appears to more strongly influence the hydrology than only including fracture zones.

THE EFFECT OF SPHEROIDAL WEATHERING ON THE PYSICO-MECHANICAL PROPERTIES OF TROPICAL GRANITE BOULDER

The aim of this research is to investigate the effect of spheroidal weathering on the physico-mechanical properties of in situ granite boulder in tropical environment. This research was conducted to gain more understanding due to lack of study related to this topic especially on tropical boulder in completely weathered zone. A number of 34 in situ granite boulders that naturally formed in completely weathered zone were selected and investigated from two quarries located in Southern Johor, Malaysia. Classification of weathering grade was conducted on the material surrounding boulder based on discolouration, degree of friability and texture characteristics as suggested by ISRM(1). The colour of the weathered material were classified based on Rock Colour Chart (2). Petrographic analysis and scanning electron microscopy (SEM) were carried out to analyze the mineralogical and microstructure alterations, respectively due to weathering effect. The changes of physico-mechanical properties at the surrounding of boulder due to weathering were also analyzed including dry density, porosity, point load strength, uniaxial compressive strength and permeability. The dry density and porosity were conducted based on ASTM (3) and Brown (4), respectively. The point load uniaxial compressive strength were carried out according to ISRM (1) while permeability test was undertaken based on Head (5). The tropical granite boulder possessing rindlets with significant different of colours, degree of friability and texture. This differences were classified into four primary zones: corestone (C), inner rindlets (IR), outer rindlets (OR), and saprolites (S) with weathering grade of II, III/IV, IV/V and V, respectively. Due to spheroidal weathering, three to six concentric sheets of rindlets with whitish grey to light brown colour were formed at the surrounding of corestone's boulder. Each sheets of rindlets has 2.0 cm to 3.0 cm thick with total thickness up to 80 cm. The microcracks aperture of IR and OR is ranging from 150 to 200 µm and 200 µm to 1 mm, respectively. Analysis revealed that the higher the weathering grade, the higher and wider the fracture opening formed in the rindlets up to 2 mm width. Analysis exibited that the increase of weathering grade from Grade II to IV along C to S were significantly reduced the plagioclase, K-feldspar, and biotite with reduction of 74%, 74.2% and 87.5%, respectively. The alteration of minerals and microstructure due to the weathering were significantly reduced the dry density, point load strength, uniaxial compressive strength and permeability with 32%, 99.5%, 98.6% and 84.8%, respectively.The reaction of spheroidal weathering is significantly affecting the physico-mechanical properties of the tropical granite boulders by alteration of its minerals and microstructure characteristics at the surrounding of the boulder.

Safety Mining Technology of Coal Seams in Weathered Zone Based on Ground J-Type Pregrouting Reinforcement

A thin basement coal seam mining model under different overlying strata conditions was developed using the discrete element software UDEC. This approach is used to discuss the safety mining of the thin basement coal seams. Fracture development in overlying and rock strata movement law in the stope was discussed. The relationship between support and surrounding rocks under different overlying strata conditions was analyzed. Lastly, a field industrial test was conducted based on the research results. A few major conclusions could be drawn. Under load transmission in loose water-bearing strata, causing a large-scaled rock strata movement to advance into the working face is easy when only one bearing stratum exists in the overlying strata. Meanwhile, the support bears strong loads, which can easily be collapsed. When two bearing strata exist in the overlying one, the upper bearing stratum can form a voussoir beam structure. Loads on the support decreased substantially compared with those under single bearing stratum, whilst the probability of pressing frame was reduced accordingly. A weathered zone above the stope was reinforced by ground J-type drilling pregrouting, thereby improving the physical and mechanical properties and increasing the bearing capacity of the rock strata in the grouting range and safety mining of the working face in the lower coal seams. Research results provide important references for the safety mining of thin basement coal seams under similar conditions.

Superficial structures cartography of the Essaouira basin under ground (Morocco), by small refraction seismic: contribution of the static corrections in the reinterpretation of the speeds variations.

The static corrections are a necessary step in the sequence of the seismic processing. This paper presents a study of these corrections in the Essaouira basin. The main objective of this study is to calculate the static corrections by exploiting the seismic data acquired in the field to improve the deep structures imaging. It is to determine the roof and the basis of the superficial layers which constitute the weathered zone while calculating the delays of seismic wave’s arrivals in these layers. The purpose is to cancel the effect of the topography and the weathered zone, in order to avoid any confusion when the seismic and geological interpretation. The results obtained show the average values of the static corrections varying between - 127 and 282 ms (double time), with existence of high values by location, particularly in the Eastern and North-Eastern of the basin, which meant the presence of altered zone with irregular topography and whose thickness and speeds vary laterally. In effect the variations of velocities in the fifty meters from the surface may introduce significant anomalies in seismic refraction, with heavy consequences when the interpretation or the drilling establishment. These variations are mainly due to lateral changes in facies and variations in the formations thickness. The calculation of the static corrections, revealed high values at certain areas (East and North-East), which will enable us to better orient the future campaigns in these zones. It is therefore necessary to concentrate the seismic cores drillings and the small refraction seismic profiles by tightening the seismic lines meshes in order to have the maximum values of static corrections and thereafter a better imaging of the reflectors.