Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
APPENDIX G Rock Excavation Methods Tectonic Engineering and Surveying Consultants, PC (Tectonic) has prepared this summary for rock excavation methods for the specific application to rock removal at the proposed Hudson Steppe residential housing project located in the Village of Ossining, NY. Rock Excavation The excavation of highly weathered and fractured bedrock is most readily performed using heavy duty soil excavation equipment such as larger excavators and dozers. These machines can be equipped with ripping bars to extend their ability to break up more competent bedrock. Where the bedrock is only slightly fractured to massive, and especially when the fracture spacing exceed 2 to 3 feet, and the intact rock is competent, rock excavation by ripping is typically considered ineffective. Therefore, other means need to be employed. The excavation of competent bedrock will require the use of specialized rock excavation techniques such as mechanical breakage, drilling and splitting, or controlled blasting. Mechanical breakage is generally accomplished by hydraulic hoe rams mounted to excavator arms which fracture the rock to a size small enough to fit within an excavator’s bucket. Drilling and splitting requires drilling multiple open holes into the rock in line with the desired fracture plane, which are then either filled with expansive compounds capable of fracturing the rock, or the rock is fractured by utilizing mechanical methods such as hydraulic splitters. Controlled blasting involves drilling holes into the rock in a coordinated pattern and then inserting a designated quantity of explosive material capable of fracturing the rock to smaller-sized particles. The end result of each of these techniques is fractured rock blocks small enough to be removed by conventional excavation equipment. The primary advantages of drilling and blasting is that it typically is the quickest and least expensive means of fracturing rock. It also eliminates the need to bring in a large dozer, as the otherwise rippable rock is loosened during the blasting process and can subsequently be excavated with smaller equipment. The primary disadvantage of drilling and blasting is the perception by both the general public and some town administrators that the use of blasting has inherent unacceptable risks. These include excessive ground vibrations, air-overpressure (annoyance noise or causing window breakage) and flyrock when there is improper stemming, no blast mats and/or inadequate soil cover. It is emphasized that all of these concerns are addressed with a reasonable margin of safety with properly designed blasting. Blasting has been successfully and safely conducted within several feet of existing buildings and high pressure gas lines under Tectonic’s purview, and historically, blasting has been performed very commonly in urban environments. Problems associated with air overpressure and flyrock are relatively easy to address through the use of limiting the amount of explosive detonated at a single instant, by maintaining proper spacing of the drill holes, and by making sure there is adequate stemming, soil ballast and blast matting to confine the blast. Therefore, within urban environments, safe blasting focuses on maintaining vibration levels at magnitudes that are safe for the different structures in the area of the blast. Buried structures such as utilities and retaining walls can withstand relatively large vibration magnitudes as they are restrained from free vibrations by the surrounding soil. Consequently, there is no amplification of the vibration induced displacements that can occur in above ground structures when the vibration frequency is near the natural frequency of the structure. Similarly, above ground utilities, as well as the structural components of buildings and other structures typically have sufficient strength to withstand high vibration magnitudes without damage. The limiting factor has been found to be the weaker, brittle items within buildings, of which the drywall joint compound at joints within walls or ceilings, and plaster wall and ceiling coverings, are the most limiting. Although many factors come into play, based on both empirical observation as well as some limited theoretical analysis, safe vibration magnitudes of 4 inches per second (ips) peak particle velocity (PPV) are commonly assumed for buried utilities, over 12 ips PPV for above ground concrete (and much higher for steel structural components), ¾ ips PPV for drywall joint compound, and ½ ips PPV for plaster on lath wall coverings. It is noted that 2 ips PPV was commonly used for residential structures with very few instances of cosmetic damage resulting within the drywall or plaster of the structures. However, the safe vibration limit was reduced due to consideration of the impacts of lower frequency vibrations resulting from larger mine blasts, frequencies that were closer to the natural frequency of residential buildings. Occasionally, cosmetic damage was observed at vibration magnitudes as low as ¾ ips PPV for drywall construction and ½ ips PPV for plaster on lath construction. Vibrations from construction related blasting generally results in vibration frequencies significantly above the natural frequency of residential structures, and 2 ips PPV is still occasionally used because of this. The vibration magnitude that results at a given location has been found to be primarily dependent upon the distance of the blast from that location and the weight of explosive detonated at a single instant during the blast. Specifically, the vibration magnitude has been found to be proportional to the distance to the blast divided by the square root of the weight of explosive detonated in a delay interval (a time period in excess of 8 milliseconds, as this has been shown to prevent subsequent blast delays from generating constructive interference). With this relationship, the maximum amount of weight that can be detonated instantaneously by a single detonator can be evaluated based on the proximity of the neighboring structures. From the site specific structures, a safe blast can be designed using these scaled distance relationships. If necessary, the charges within a single blast hole can be separated with stemming and detonated at different times to keep the required explosive weight per delay within the required limit. Typically, blasting would begin at the point furthest from the critical structures, and a site specific scaled distance energy attenuation relationship would be developed and the blasting plan modified as required as the blasting approaches the critical structures. A detailed blasting plan or rock removal plan shall be developed by the excavation subcontractor to show that the rock removal can be conducted in a manner that will minimize ground vibrations at adjacent structures and also limit the amount of air overpressure. The plans shall provide detailed descriptions of typical production rounds including specifics such as the proposed diameter, spacing, burden, depth and orientation of each drill hole; the type of detonators and delay patterns proposed; the type of explosive to be used; the weight and distribution of charge to be used within each hole; as well as the total weight of explosive charge for each delay and the total weight for the blast round. The plan shall also include the type and distribution of stemming to be used in each hole and the methods of matting or covering of the blast area to prevent flyrock and excessive air overpressure. The predicted vibration magnitudes at the neighboring buildings and the magnitude of air-overpressure shall also be provided. It is understood that many of the identified parameters will be preliminary estimates and may require adjusting during actual blasting operations. This plan shall be reviewed by the Owner’s engineer. As discussed, the maximum permissible vibration level shall be 0.75 ips PPV at neighboring buildings with drywall interior wall coverings or 0.5 ips PPV if the homes have plaster on lath construction. The air overpressure shall be limited to 134 dB when measuring with a 0.1 Hertz high pass system. Measuring equipment using other frequency responses shall conform to the United States Bureau of Mines (USBM) recommendations from RI 8485. Vibration and air overpressure monitoring shall be implemented to verify that the vibration and air overpressure limits are being met. Vibration monitoring shall also be performed if rock excavation is performed with hoe-rams or by ripping. Pre-blast surveys will also be conducted to identify the existing condition of surrounding buildings and other structures in the area. These surveys will include both a written and photographic documentation of the existing conditions of both the interior and exterior of buildings and other structures. The pre-blast survey will be performed by a firm specializing in this type of work as the ability of the survey to mitigate false claims is highly dependent upon the quality of the survey. Surveys will also be performed if rock excavation is to be performed through the use hoe-rams and rippers.