Excavation Profile Research Articles

Drill and blast excavation forecasting using 3D laser scanning

AbstractThe application of 3D laser scanning technology in tunnelling has gained increasing significance in recent years. Laser scanning is an innovative holistic approach for data acquisition in tunnelling regarding the geometrical parameters. It is a distance‐based imaging technique for three‐dimensional and high‐resolution illustration (3D point cloud) of the surrounding rock that can be applied at various times to provide a variety of visualization and analysis possibilities. A new approach deals with excavation forecasting by linking the acquisition of the 3D geometry of previously excavated rounds with the geological documentation of the tunnel face. This approach enables the gathering of information about the expected overbreak or underprofile of subsequent excavation in similar rock conditions. Using this information, an optimization of the borehole positions (especially of the peripheral boreholes) can achieve the best possible excavation profile. This approach offers an improvement of excavation performance and saving potential with regard to excavation quantity and shotcrete consumption. The uniform excavation shape and consistent shotcrete thickness improve tunnel stability and finally increases the service life the tunnel structure.

Theoretical assumptions in development the undergroud facilities in different geomechanical conditions

The aim of this paper is to achieve a rational rates in development the underground facilities at the lowest costs of construction and possible maximum quality (in this case, under the quality is implied the satisfactory stability of contour of developed room and excavation profile as close to the shape and size of designed). The applied research methods include analyzing development the underground facilities as a complex system with a significant number of maximum-dependent elements that are within this complex system, conditioned by the work environment, organization, mechanization, energy and other factors for achievement of this objective. The results of these studies indicate that the construction process of development the underground facilities is functionally linked and depends on a whole range of interrelated factors. As the result, it is obtained that the corresponding mathematical models can define an optimal technology and organization of work for construction the underground facilities.

Experimental study on failure behaviour of deep tunnels under high in-situ stresses

Geomechanical model tests are conducted in this study to investigate the failure behaviour and instability of the “large, deep, long and in-group” tunnels constructed in the Jinping II hydropower station. On the basis of the experimental results, deformation, stress and failure analyses are conducted for the tunnels under high in-situ stresses. It is concluded that: (1) The tunnels should be stable and the majority of the rock mass remains intact even if the applied load is 1.3 P0, where P0 is the in-situ stress. The rock mass around the excavations starts to crack when the applied load is 0.5–0.6 P0, and presents nonlinear behaviour when the applied load is equal to the in-situ stress. (2) The displacement analysis indicates that the maximum displacement is 108mm at the crown along the excavation profile and 81.2mm in the rock mass 3m far from the excavations. The obtained maximum displacements from the physical model test are close to those later monitored in the field, although the physical model test is actually conducted well before the construction of the tunnels. (3) The stress and failure analyses reveal that local failures occur in the rock mass around the excavation forming local rockfalls but these local failures have not accumulated to result in failures in massive area of the rock mass. There are no obvious cracking and yield in the rock mass between the two tunnels, which indicates that the designed span of 60m may be feasible. (4) The geomechanical model test provides a means to understand the deformation, stress, cracking and potential failure of the tunnels constructed in the Jinping II hydropower station, which forms a basis for the subsequent 3D finite element modelling and construction process.

Geochemical and mineralogical control on the mobility of arsenic in a waste rock pile at Dlouhá Ves, Czech Republic

A waste rock pile with initial high sulfide (10–20 wt.%) and low carbonate content (1–2 wt.%) located at Dlouhá Ves in the Czech Republic has been investigated in two profiles (excavation and outcrop) using powder X-ray diffraction, electron microprobe analysis, bulk composition analysis and Mössbauer spectroscopy. The mobility of arsenic and other contaminants was evaluated by leaching experiments. The primary source of the arsenic was arsenopyrite, which was significantly oxidized in both profiles. The principal As-bearing phase at the excavation profile was goethite, located at the top of the profile, and minerals of the jarosite group which were found down to its base. Melanterite, rich in copper and zinc, was found in a sulfide-rich, lower part of the profile together with anglesite. At the outcrop profile, minerals of the jarosite–beudantite group, scorodite and kaňkite prevail and no Fe(II)-minerals were found. The paste pH was lower at the excavation profile (minimum about 1.9) than at the outcrop profile (minimum of about 2.8). Processes in the pile are affected by the pyrite/arsenopyrite ratio, where high pyrite content decreases the As/S ratio and results in the formation of jarosite group minerals and low pH conditions. Where arsenopyrite predominates, sulphides are coated by scorodite and other Fe–As phases like schwertmannite, which limit their further oxidation. Arsenic concentrations released during the leaching experiments were generally low; maximum amounts (up to 0.56 ppm) were released from horizons with jarosite and arsenopyrite. In contrast, minimum amounts of arsenic were released from horizons with beudantite and scorodite. Melanterite found at the excavation profile was fully soluble, releasing copper and zinc into the solution. Differences between both profiles are caused especially by the different degrees of water flow through the pile material and the limited penetration of oxygen into the deep parts of the excavation profile. It seems that beudantite and scorodite represent a long-term option for immobilization of arsenic, but arsenic stored in jarosite can be mobilized relatively easily. Highly soluble Fe(II) minerals such as melanterite may represent an environmental problem. However, potential mineralogical transformations and stability of arsenic in beudantite group minerals have yet to be evaluated.

Evaluating extent and causes of overbreak in tunnels

Damage or overbreak not only endangers safety of structure but also increases cost of construction and time of completion. Drilling and blasting being cost-effective for excavation of any underground structure should strictly adhere into specialized controlled blasting pattern to minimize the unacceptable impact on peripheral in situ rock mass. The paper reveals that in addition to geo-technical properties of rock mass, in situ stress condition plays an important role in enhancing the magnitude of overbreak. Implementation of same blast pattern throughout the length of tunnel results into different magnitudes of overbreak and the magnitude increases in highly stressed zone. Furthermore, implementation of same controlled perimeter blast pattern along the tunnel cross-section may result into different magnitudes of overbreak. Different magnitude of overbreak along the tunnel cross-section i.e., in left and right wall and crown has been observed even with implementation of same controlled blast pattern throughout the tunnel cross-section. Feasibility and compatibility of drilling equipment with respect to tunnel cross-section also adds to the quality and magnitude of overbreak. Undersize drilling equipment leads to angular drilling on either walls or crown and enhances the magnitude of overbreak. The paper with the help of statistical and graphical analysis revealed that blast pattern for peripheral rock mass should consider geo-technical properties and in situ stress condition of rock mass to minimize the magnitude of overbreak. The authors also emphasized that to contain magnitude of overbreak within allowable limit; the implemented blast pattern should be different for different sections viz., right wall, left wall and crown of tunnel. Furthermore, in poor rock mass condition or in highly stressed zone, drivage of tunnel should be carried out in small sections and in different phases until the excavation reaches the required excavation profile in that area. Excavation in small sections and in different phases would lead to proper excavated profile and minimize overbreak and damage of peripheral rock mass.

Two-dimensional theoretical model for design of burn-cut pattern

Excavation methods for underground drivage should be cost effective and minimise damage to the periphery rock wall. Among the various methods of excavating systems the 'drill and blast' method is the most commonly used because of the low capital investment required, its easy manoeuvring and excavating capability and the flexibility of changing the excavation profile if ground conditions necessitate. The burn-cut method of blasting is cost effective and has a good advance rate compared to other conventional drilling patterns. This paper investigates how errors in the burn-cut method can be minimised and how the parameters influencing the progress per round of blast can be optimised. The importance of drilling accuracy, low burden and heavy charge concentration for the initial holes of the burn-cut method of blasting is discussed. This paper proposes a conceptual mathematical model to enable a better understanding of the burn-cut method of blasting. The model assumes that the process of breakage for the initial holes is a slabbing action and uses the concept of the deflection of a cantilevered beam (rock strata between charge and relief hole) to explain the importance of deeper relief holes. The results of an experimental study conducted in one of the tunnels in the Dul Hasti hydroelectric project in the Himalayan region are also discussed.

Optimal excavation profile for a pipeline freely resting on the sea floor

A submarine pipeline resting on a rigid, frictionless sea bed assumes an equilibrium configuration which can be determined by solving a unilateral contact problem, i.e. a quadratic program or a variational inequality. Since the sea bed profile is usually irregularly hilly, its regularization is often carried out, in present offshore technology, by costly trench excavations, both in order to avoid excessive bending moments in the pipe and to bury it for protection. Thus, the problem arises of determining profile changes of minimum cost under the condition that an assigned curvature be nowhere exceeded. This optimal design problem is tackled in the paper with reference to a discrete model of the mechanical system, as the minimization of a linear cost function under linear constraints and a single, nonlinear and non-convex, complementarity constraint. A theory is developed which reduces this nonlinear programming problem to a sequence of linear programs, the optimal solutions of which are shown to converge to the original NLP solution. Upper and lower bounds on the absolute minimum cost and optimality conditions are established, on the basis of duality theory in linear programming. An algorithm suitable for solving the problem in a finite number of steps is proposed. Generalizations obtained by relaxing some of the simplifying hypotheses are considered.