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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.