Causes of Overbreak in Tunneling: A case study of the Alborz Tunnel (original) (raw)
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Causes of Dynamic Overbreak and Control Measures Taken at the Alborz Tunnel, Iran
2015
Drilling and blasting is widely used in underground excavation projects, where the amount of damage to the surrounding rock mass is crucially important, due to its impact on the safety of working environment and operational costs,. The causes of overbreak in the Alborz Tunnel of Iran are evaluted. In this regard, ten rounds of presplitting and 11 rounds of smooth blasting methods were carried out to determine the dominancy of ground condition over the blasting pattern characteristics. Further study was undertaken to identify the most important parameters of ground condition affecting the overbreak area. These parameters include; joint condition, spacing, orientation, RQD and type of rock mass. As the characteristics of the blasting pattern have very little effect on the amount of overbreak, the smooth blasting technique was chosen for the future operations where the current ground condition is going to be dealt with for about 500 meters of length, based on the data acquired from the Alborz Exploratory Tunnel. Results of this investigation helped to solve disputes between contractors and clients over the issue of permissible overbreak.
Assessment of drilling inaccuracy and delineation of constructional and geological overbreak
Tunnelling and Underground Space Technology, 2021
Overbreak in underground excavation is one of the undesirable outcomes that may emerge from many qualitycompromising events. Among these, the drilling and blasting generally are one of the most important contributing parameters. This paper focuses on the study of drilling inaccuracy in manual drilling operations and the contribution of geological discontinuities to overbreak. A total of 855 blasts were conducted using conventional manual jumbo and computerized semi-automatic jumbo in different rock mass conditions wherein RMR varied from 45 to 75 and Q-value from 0.46 to 45. This was done to investigate the influence of rock mass quality on overbreak in underground drivage. From the study, it was found that 2.1% and 1.3% of overbreak tend to cause due to manual drilling inaccuracy of ramp and level drivages, respectively. Besides, the geological discontinuities have an effect on overbreak upto the Q-value 10 and this may be termed as geological and constructional overbreak. Above the critical point (Q-value 10) in the rock mass scale, the region may be termed as constructional overbreak due to the only presence of constructional error.
Causes, impact and control of overbreak in underground excavations
Tunnelling and underground space technology, 2005
Drill and blast system is used in hard rock excavation due to its economics and adaptability to changing rock mass conditions. Common question during mining and tunneling operations is 'whether overbreak has been caused by blasting practice or poor rock mass ...
Geotechnical and Geological Engineering, 2017
Determination of blast-induced overbreak is a very critical factor in underground excavations where drilling and blasting method is used. Overbreak, as an inevitable side effect, significantly affect the cost and safety of underground constructions. A lot of researches have been conducted by many scholars so as to minimize overbreak and excavation damage zone. There is a need for a contractor to quantitatively assess overbreak area. Unfortunately, as a result of complexity of the parameters influencing overbreak and little understanding of the phenomenon, there is no systematic and quantitative model for predicting overbreak considering the geological conditions, blast parameters, and geometry of the tunnels. At the present time, the constant magnitude of 10-15 cm is allowable in contracts, due largely to imprecise prediction about the quality and magnitude of overbreak. The main objective of this paper is to present a linear multiple regression analysis model for predicting the blast-induced overbreak of the tunnels driven in sedimentary rocks. This model is developed considering all the factors affecting the damage. The performance of the best model was evaluated showing the suitability of the model for assessing overbreak.
Detection of potential overbreak zones in tunnel blasting from MWD data
Tunnelling and Underground Space Technology
The damage from blasting to the remaining rock mass is analyzed with the purpose of developing a drilling index from measure while drilling (MWD) parameters, able to predict high risk of poten tia! over-and under-excavated zones produced by blasting in the con tour of a tunnel. A new methodology based on the comparison of scanner profiles of the excavated sections with the position of the contour blastholes, has been developed to obtain the excavated mean distance (EMD) between the blasthole and the excavated profiles at each MWD record position, which may be considered as a damage measure. MWD parameters, that describe the in-si tu rock mass properties befare the blast, are thoroughly normalized to remove externa! influences that may hide the actual response of the rig to rock mass properties and lead to wrong interpretations. 54 blasts, which comprise around 1700 contour blastholes, have been compared with more than 4000 excavated sections. A non-linear multiple-variable power-form model has been developed to predict the excavated mean distance as function of the normalized penetration rate, hammer pressure, rotation speed, rotation pressure and water flow parameters, and the lookout distance. These parameters combine the rotational, hydraulic and percussive mechanisms of the drill, and the confinement of the explosive charge with depth. Sources of uncertainty, unavoidable in the harsh condition in which the data were measured, such as drilling deviations, the scaling and primary support prior to scanning the excavated section, possible variations (unrecorded) in the explosive linear density, etc., have been assumed to be of random nature.
Assessment of geological overbreak for tunnel design and contractual claims
International Journal of Rock Mechanics and Mining Sciences, 1997
Overbreak of rock beyond the designed periphery of a tunnel is a structural risk which more than occasionally results in filing of a claim by the contractor. It is difficult to estimate the overbreak risk at the time of tender with sufficient degree of accuracy because of the uncertainty associated with the geology, construction technique, and the interaction of these two domains. The key variables that influence the extent of overbreak, and their interactions, are illustrated through the proposed interaction matrix. The assessment of overbreak for a tunnel excavated using the conventional, drill-and-blast technique, is performed using an approach that integrates simulation and sensitivity analysis. The stability of wedges formed by two joint sets is examined using a two-dimensional limit-equilibrium analysis incorporating the negative influence of blasting. The results of the analysis can be used to define overbreak thresholds at a given level of confidence.
Geotechnical Investigation of Fracture Patterns in a Rock Mass during Excavation by Blasting
Overcharging in rocks (wall faces) during blasting and excavation usually causes damage to rock mass in most mining and quarry industries. This creates blast-induced fractures which can relates with pre-existing fracture pattern thereby increasing sliding and rockfall from the crest and body of an excavated wall. The spacing and orientation of pre-existing fractures are predominant at a small-scale mining (galamsey) site at ‘Atta ne Atta’, a town near Beposo, in the Western Region of Ghana. Geotechnical field studies were carried out to investigate the possibility of any instability within the area to eradicate the occurrence of an unexpected future wall failure (rockfall). The geotechnical mapping conducted was focused on fracture distribution and spacing. Mean spacing (Sm) of existing fractures was calculated and corrections were made to obtain calculated spacing (Sc). The scanlines of wall face 001 and wall face 002 intersect with their corresponding strike and dip at 78° and 80° respectively creating a slightly favourable fracture pattern and rock wall stability. The fracture pattern created at Wall Face 003 and Wall Face 004 were unfavourable for rock stability with their corresponding scanlines having a strike and sip of 67° and 73° respectively. The instability of these wall faces (003 and 004) is as a result of parallel orientation of the induced fractures to the strike of the pre-existing fractures. Observations made from the stereographic projections and rose diagram indicate a cluster of fracture patterns with a general strike of NNE-SSW. Hence, the fracture patterns in the study area are composed of favourable (stable) rock mass at some walls and unfavourable (unstable) rock mass at other wall faces due to overcharging of blast holes.
Classification of fault zones in mechanized tunneling projects
CRC Press eBooks, 2023
The crossing of fault zones by underground works is often coupled with serious technical problems for tunnel boring machines. Fault zones may sometimes act as highly conductive groundwater channels or natural flow barriers. Such barriers can develop veryhigh water pressure behind or inside them. Crossing through them, especially with deep galleries, there is a considerable risk of meeting a sudden flow of water and crushed rock material under high pressure. Squeezing problem and collapse of tunnel surrounding rocks may also cause long delays in tunnel advance due to jamming of shield or cutterhead. So, to avoid difficulties in tunneling projects, it is very important to identify the main characteristics of fault zones and predict material behavior in such complex zones before crossing them by TBM. Knowledge gained from the application of TBM in different types of fault zones in various geological conditions can be utilized as a tool for predicting tunneling conditions in a given geological situation. In this paper, based on the experiences gained from mechanized tunneling in Iran and considering the most important influencing factors, a preliminary proposal for the classification of rock mass behavior and geological hazards in fault zones has been presented.
Blast induced rock mass damage around tunnels
2018
Drilling and blasting is a preferred method of rock excavation world-wide due to low initial investment, cheap explosive energy, easy acceptability among the blasting engineers and, possibility to deal with different shapes and sizes of openings. Although, drill and blast method has witnessed significant technological advancements, it has inherent disadvantage of deteriorating surrounding rock mass due to development of network of fine cracks in it leading to safety and stability problems. The damage in the peripheral rock mass culminates in the form of overbreak and damaged zone beyond overbreak. In some cases the projects cost has increased more than 15% because of overbreak. Although significant efforts have been made to assess damage to the surrounding rock mass using different methods, the solution based on easily available site parameters is still lacking. Authors have carried out field investigations at five different tunnels located in Himalaya, India to study blast induced ...
Frattura ed Integrità Strutturale, 2021
Rocks are frequently host materials for underground structures, particularly for deep Tunnels. Their behavior plays a fundamental role in the overall stability of these structures. In fact, the erection of deep tunnels imposes rocks excavations around the defined routes. These excavations are generally carried out by various methods of which the most used are Drill-and-Blast (DB) and Tunnel Boring Machine (TBM). However, regardless of the tunnelling method used, the impacts such as the perturbation of the initial stress field in rocks and the release of the stored energy are always significant. The impacts produce damage, fractures and deformations which are generally time-dependent and influence the long-term stability of deep tunnels built in rocks. Thus, by considering the aforementioned excavation methods, this paper identifies, reviews and describes the relevant factors generated during and after rock excavations. Interestingly, such factors directly or indirectly influence th...