19From a Quaternary science perspective, sand-sized quartz as well as silt-sized quartz is often 20 acknowledged as final products of glacial abrasion through different evolution mechanisms. This view 21 challenges the existence of any universal comminution process, which may relate the formation of detrital 22 quartz sand and silt. The contribution of grain size, energy input, and crystalline integrity in the scale of 23 quartz crushability has long been matter of much debate. The present empirical work examines the 24 micromechanics of sand-to-silt size reduction in the quartz material. A series of grinding experiments was 25 performed on Leighton Buzzard Lower Greensand using a high-energy disc mill. Analogous conditions to 26 glacial abrasion are provided due to the combined abrasion between grains' asperity tips, and also 27 between grains and rotating smooth tungsten carbide pestle. Discontinuous breakage approach allowed 28 a control on grains' crystalline defects. To enable an objective assessment of micromechanics of size 29 reduction, measurements of particle and mode size distribution, fractal indexes and micro-morphological 30 signatures were made. The crushing approach was probed through varied grinding time at a constant 31 energy input, as well as varied energy input at constant grinding time. Breakage pathway was inspected 32 via laser diffraction spectroscopy and transmission light microscopy. Results suggested that the grain 33 breakdown is not necessarily an energy-dependent process. Non-crystallographically pure quartz sand 34 and silt are inherently breakable materials through a fractal breakdown process. Results also revealed 35 that the internal defects in quartz are independent from size and energy input. 36

Although buried structures are generally believed to suffer a lesser degree of damage in the event of earthquake -than that of over-ground structures -significant damage has been widely reported to buried assets after major earthquakes, including the 1995 Kobe and the 2008 Wen-Chuan. Discontinuity is one key feature of rock as the supporting medium around subsurface excavated spaces. Joints, faults and bedding planes influence, by-and-large, the stability of structures made from/into rock. In particular, fault system around underground caverns such as hydropower house has a marked control on assets' seismic stability. This study builds on the current understanding through vigorous numerical modelling of fault-structure system under seismic excitation. A parametric approach is followed to determine the most critical layout of a single fault crossing a benchmark cavern. Fault system is systematically broken down into several combinations of dips and intersection points with cavern wall. For each case, a nonlinear dynamic analysis is conducted. To simulate the discontinuous medium, the hybrid finite differencediscrete element code CA2 (Continuum Analysis 2 dimensional) is implemented. The work showed that, similar to static conditions, fault influences the seismic stability of underground caverns through a tendency in extending the plastic zones and increasing displacements as well as asymmetric distribution of the latter and the former in rock medium. A 40° to 50° dip, single-point-intersection-on-crown k 0 =1 fault layout renders the most critical combination from both ultimate and serviceability limit states perspective. Under earthquake loading conditions however, the critical limit states condition took place for single fault intersected the cavern at heel and sidewall. The latter critical condition led to the tensile failure of cavern right sidewall. For faults intersecting the carven crown and having a k 0 =0.5, collapse would be more likely as fault dip increases. Collapse would be less likely with increasing dip for k 0 =0.5 fault crossing the bed and sidewall of caverns.

No abstract

ABSTRACT:The re-use of excavated loess in embankments needs a good understanding of its long-term mechanical response. Among the many collapse controlling factors, stress and hydraulic history are of significant importance as certain combination of these can alter the packing state to great extents. However, published works on stress path-packing state interaction is limited to clayey silts, underreporting the contribution of carbonates. Works on hydraulic path-packing state interaction lacks physical evidences for pore distribution, leading to disputes over the variation of air-volume over time and therefore fills' long-term behaviour. Identical artificial loess specimens were incrementally stressed on dry-, wet-, and wetting-surfaces, while microfabric, suction, particle and pore size distribution were recorded. The response of test material (moderately calcareous lightly clayey silt) showed the failure of dry-compaction in restricting the coefficient of consolidation. Wetting at any stress level improved the pore volumes and thus post-drying collapsibility. However, pre-loading to 25kPa before flooding provided the maximum degree of densification. Water-retention properties were deemed reproducible upon wetting-drying seasons for minimum content of 5-20µm size loess constituents. In short, controlled stress-hydraulic paths can guarantee the long-term response of site-won loess embankments.

No abstract