Rock Excavation Introduction
The main purpose of this paper is to present rock excavation at an elementary level and provide key considerations and methods of removal when projects requiring rock excavation are encountered. The paper studies three aspects of rock excavation as it relates to building construction. First, rock-breaking processes are introduced and the most commonly used methods in building construction are identified. Second, the environmental impacts related to rock excavation are discussed to provide awareness of the various concerns contractors should consider when planning rock excavation operations. Third, cost implications and scheduling considerations associated with rock excavation are analyzed from an estimating perspective. As a supplement to the academic discussion, findings from an interview with General Excavation, Inc. are presented to provide an actual contractor’s perspective on the topics discussed throughout this paper. The following information will provide an understanding of rock excavation and the various considerations (i.e. cost, local regulations, safety, etc.) contractor’s should have in mind when estimating and planning projects that involve rock excavation.
The Rock Breaking Process
When determining the proper method for excavating rock there are a few things you should consider. Determine how much rock you anticipate needs excavating. Is the rock solid? Is the rock easily accessible, how far below grade? Do the aesthetics of the rock matter in the end product? And is there any local regulations restricting particular methods? These are all questions that need to be answered before any rock excavation method begins. The traditional methods are ripping, drilling, breaking and blasting.
Blasting is the controlled use of explosives to excavate rock and has been used for many years. When blasting a careful observation of the rock structure is performed. While blasting may seem like the simple solution to removing a large amount of rock, it can result in serious injury and major mistakes if proper precautions are not taken. Studying the discontinuities in the rock is very important when planning a rock failure. If these are not studied carefully you may be place explosives in the wrong place. Drilling is also a very important part in the blasting process because in order to use the explosives drilling is the most effective means of embedding them in the rock. The presentation of the finished product, the appearance of boreholes and the limitation of unwanted fracturing are all considered when deciding the proper method for the job.
The two general types of blasting are production blasting and controlled blasting. Production blasting uses large explosive charges, widely spaced, that are designed to fragment a large amount of burden. Production blasting is the most efficient way to remove large rock burdens, but it typically creates radial fractures around the blasthole and back break, fractures that extend into the final slope face, which reduce the strength of the remaining rock mass and increase its susceptibility to rockfall. Controlled blasting is used for removing material along the final slope face. In some cases, it’s also used before production blasting to create an artificial fracture along the final cut slope, which will prevent the radial cracks caused by production blasting from penetrating back into the finished face (Konya, 1985). Controlled blasting can also be used alone, without production blasting. Controlled blasting creates less backbreak than production blasting because it removes fewer burdens and uses more tightly spaced drill holes with lighter charges.
Presplit blasting, or presplitting, is used before production blasting to protect the final rock face from damage caused by the production blasting. Presplitting creates a fracture plane along the final slope face, which prevents the radial cracks created by production blasting from penetrating into the finished face. Presplitting also allows for steeper and more stable cuts than any other blasting procedure. In massively bedded, competent rock, a properly charged presplit blast will contain drill hole half cast for almost the entire length of the blast line and will have no backbreak because the energy from the blast will travel uniformly, thus creating a continuous fracture between holes (Andrew 2011).
Smooth blasting, also called contour blasting or perimeter blasting, can be used before production blasting as an alternative to presplitting. Smooth blasting uses drill holes with roughly the same diameter and depth as those used in presplitting, spaced slightly further apart and loaded with a slightly larger charge density. If the burden is adequately reduced, smooth blasting produces a more ragged slope face with minimal backbreak (Andrew 2011).
Cushion blasting, sometimes referred to as trim blasting, uses a row of lightly loaded “buffer” holes filled with crushed stone over the entire depth of the hole, which reduce the impact on the blasting holes and protect the surrounding rock mass from the shock caused by the blast, thus minimizing the stress and fractures in the finished slope face (Andrew 2011).
Drilling goes hand in hand with blasting as drilling is used to contain explosives. There are various orientations for drilling operations, from vertical through horizontal. To create vertical holes, which are used almost exclusively in production blasting, downhole and step drilling methods are used. Horizontal drilling is used for both production and controlled blasting because of limited drill rig access or geometry requirements. Angled drilling can be performed as determined by slope face angle.
Down hole drilling also known as vertical or production drilling, is a technique used in production blasting using a conventional rotary tri-cone blasthole rig or a rotary percussion rig if smaller blast holes are adequate. Tri-cone drill bit pictured to the right (Aboujaoude 1997).
Step drilling is another type of vertical drilling that’s also used in production blasting, most often to produce relatively flat and benched slopes, usually shallower than 1:1. It’s similar to downhole drilling, but creates holes that gradually increase or decrease in depth to allow for a stepped slope "break" line. If done properly, step drilling can produce a slope face that shows minimal signs of blasting aside from drill holes entering the slope face, which are noticeable only by someone looking directly at the face. On projects involving step drilling, drillers are paid more for tightening the drill hole pattern and using lighter, distributed loading to avoid performing excessive blasting charges along the slope later (Aboujaoude 1997).
Horizontal drilling is an effective technique for starting new excavations and for small excavations with poor access at the top of the slope. There are two basic techniques used for horizontal drilling. The first uses blast holes drilled perpendicular to the final rock face, while the second uses holes drilled parallel to the rock face. When drilling perpendicular to the face, angled holes are typically required to mobilize and fragment the rock at the toe, these holes are called toe lifters, and the ditch, ditch lifters, to achieve the proper final slope configuration. In addition, horizontal drilling and blasting can produce badly fractured slope faces, as the production blastholes typically extend all the way to the final rock face. For this reason, a widened catch area is recommended for slopes excavated this way (Aboujaoude 1997).
Rock ripping is the process of breaking up rock and soil with a large tooth attached to the back of a bulldozer with a mounted ripper. Ripping costs are typically 50 to 65% less than blasting (Peurifoy 1996). Ripping is also significantly less dangerous than blasting and requires fewer permits and special precautions. Ripping can be done in close proximity to populated areas or other places where blasting noise and vibrations are restricted. However, ripping is limited to soft to moderately firm, fractured rock and construction of low-angle cut slopes and shallow, near vertical cuts. In dense rock formations, light blasting is sometimes performed before ripping. Once the material is loosened by ripping, an excavator can be used to remove it and perform slope sculpting.
Ripping is usually done in one direction, but in very tough materials ripping in a grid pattern will increase excavation efficiency. The pass spacing is determined by the end use of the material whether it’s fill, aggregate, or waste, all have different required pass spacing. The capacity of the excavating equipment also is a factor in pass spacing. In most cases, it is best to maximize ripping depth, but in stratified formations it may be best to rip along the natural layers(Peurifoy 1996).
There are basically two types of rippers: the pull-type ripper and the integral bulldozer mounted ripper. In rock excavation a bulldozer-mounted ripper works better than a pull-type ripper because it can exert greater downward pressure. Rippers also come in single- and multi-toothed configurations. Single-toothed rippers are used for difficult ripping work, where maximum ripping depth is required and/or the material is dense. Multi-toothed rippers, which can use up to five teeth, are used for softer ground or for secondary purposes such as breaking up already ripped ground.
Breaking is done with a hydraulic hammer also known as a breaker or hoe ram, a percussion hammer fitted to an excavator that is typically used for demolishing concrete structures. It is used to break up rock in areas where blasting is prohibited due to environmental or other constraints. Like a ripper, a hydraulic hammer can be used in most rock types, although when sculpting a slope face, it works best in soft or moderately to highly fractured rock; existing discontinuities in the rock act as presplit lines, minimizing hammer induced scars and fractures while creating a slope face that appears to be naturally weathered. Hammering locations are spaced evenly in a grid-like fashion so that the end rock product is fractured into pieces that can be loaded and hauled. For slope excavations, the hammering angle should not be parallel to the major the orientation of existing fractures, as this may cause more excessive fractures into the final slope face (Peurifoy 1996).
Environmental Impacts
Rock excavations, and particularly blasting operations, are under constant pressure and monitoring to reduce excavation related safety and environmental hazards. These “high consequence risk activities” carry strong environmental implications that have led to the establishment of increasingly strict regulations and specifications by government agencies. Combating these concerns requires an approach that incorporates best industry practices and current technological advancements in industry related tools and equipment. Simultaneously, advancements in the private sector have led to improvements in risk management through planning, good blast design, technological applications, accurate drilling and the correct choice of explosive materials.
One of the most obvious safety hazards during the blasting process is the Debris that is ejected or propelled through air at the time of detonation. “Flyrock” can not only cause significant injury to personnel and members of neighboring communities, but has also been responsible for significant damage to machinery and nearby buildings. Flyrock distances can range from zero for a well-controlled blast, to nearly 1.5 km for a poorly confined large, hard rock blast and many fatalities have occurred as a result (Richards).
As a result, it is critical to predict a ‘safe’ blasting area dependent on the knowledge of how the flyrock will propel. Software applications for predicting such distances has been developed where the operator inputs the charge mass, the distance from the blast hole to the nearest free face or burden, and a site constant calibrated based on existing site conditions. The output is the distance that rock will be thrown, and these quantities can be used to establish both safe clearance distances, and the critical range of burdens and stemming heights (Bhandari).
Ground vibration and transient air pressure generated by explosions known as “Air blast” must also be factored in as a major component of any environmental controls program. Regulating agencies have applied restrictions and standards to limit the effects of blasting on nearby buildings, rock slopes, utility wells and aquifers, however; recent advancements in bore design and utilization of comprehensive blasting analysis software supports and improves compliance with blasting plans and contributes to blast performance. Newest generation software has the capacity to update and evaluate original design studies while providing real time modifications to the operator.
Blasting operations can also generate large quantities of dust. Fine particulates, when released in an uncontrolled manner, may cause a widespread nuisance and potential health concerns for on-site personnel and surrounding communities. Depending on meteorological conditions the dust dispersal can travel to substantial distances endangering health of communities. The Generation of fines and dust are influenced by several blasting and rock parameters including bench height, blast design information and rock characteristics. This, once again, highlights the need for qualified contractors to follow a comprehensive blasting plan. Wind speed and direction, temperature, cloud cover and humidity will affect the dispersion of airborne dust. Atmospheric stability effects dispersion of the emitted plume, determining the extent of the vertical and horizontal, transverse and axial spread of the emitted particulates (Bhandari). Once again, a computer model has been developed to simulate the dispersal of dust during blasting operations (Kumar and Bhandari, 2002).
Meteorological conditions are also utilized to combat the growing concern over fumes produced during the discharge of explosives. Unlike the dispersing of fines however, the use of potentially harmful oxides has raised concerns in the past related to groundwater contamination as well. As a result, the implementation of water resistant explosive and proactive sheathing similar to a liner can mitigate leaching concerns when applied in accordance with a comprehensive blasting plan.
In short, governmental oversight paired with advancements in the private sector have led to improvements in environmental risk management through planning, good blast design, technological applications, accurate drilling and the correct choice of explosive materials. Today’s low impact, safety conscious blasting, applies basic blast engineering practices under tightly controlled conditions. First, qualified contractors and engineers design the blast to the rock face conditions and required results with environmental considerations in mind. When executing the blast, the focus is on controlling the drilling pattern and then appropriately loading explosive materials, and finally, monitoring several parameters during the blast event and evaluating the outcome, storing results for future data analytics (Bhandari). Conformance to these standard practices has allowed engineers and technicians to develop technology to aid in each step of the process making blasting operations safer than ever before.
Cost and Schedule Considerations
In every project there is a point at which the owner must choose whether to define and explore the subsurface which he/she intends to build said project, or not define it at all. With this exploration comes the discovery of buried geological information in the form of soil borings, test pits, or sonic analysis. Using this information an owner and/or contractor can decide how to move forward on a project as it relates to rock excavation. In the case of an unclassified project, a contractor may not have a luxury of the aforementioned testing. This is why it is imperative that time and effort be spent completing due diligence on the site, creating a schedule that reflects the geography, and has the appropriate contingency built into the schedule in case unexpected discoveries are made.
In classified bids, the contractor is paid for excavation and then again for any subsequent rock found during excavation. The owner and contractor will reach an agreement as to the unit price per boulder, cost per cubic yard using a caterpillar dozer, or by truckload of haul away rock, etc. In this type of contract it behooves the owner to have an accurate geological report for the site in order to plan excavation contingency funds in the case that the contractor finds lots of rock. In classified projects contractors should build a certain amount of time into the schedule for unknown buried hazards. Rock excavation isn’t the only thing that can affect the schedule during the excavation process. The discovery of buried Native American remains, historical build sites, or other geologic rarities can halt construction in seconds; a schedule should be prepared for the worst while still remaining competitive. Besides earthen and human remains, the discovery of an endangered species during site work or rock excavation can also bring construction to a standstill; although unlikely these scenarios have and will happen.
Contracts that are written so that site conditions are “unclassified,” tend to be your most complex and possibly risky projects. With little to nothing known about the substructures and geologic sub-surfaces, it leaves one wondering “how will these unknown conditions affect the schedule?” Without the presence of geological reports, soil borings, or test pits to make educated guesses from, it’s impossible to tell exactly what type of machinery is needed for the rock that may or may not be encountered. The contractor is responsible for any rock found and there are many ways of removing it including bulldozers, rock scrapers, explosive blasting, pneumatic fracturing, hydraulic fracturing, mechanical fracturing, rock wheel grinding, water blasting etc. Various forms of rock, various onsite conditions, and various contractors will choose different equipment and methods. A project that requires blasting will take considerably more time, money, and planning/executing than a project that requires the removal of a few large stones or no rock at all.
The schedule depends a lot on the rock excavation and site work to run smoothly in order to meet final completion within the specified project duration. A poorly quantified rock excavation estimate could lead to the demise of the entire project. In addition to time contingency for known rock excavation, it is most important to complete due diligence and estimate unclassified jobs as precisely and accurately as possible. This ensures no mistakes are made during equipment mobilization, excavation, and haul-off. Unlike classified work, unclassified jobs may not be able to gain lost time from the A/E or owner like they would in a classified project; additional crews, equipment, and overtime may have to be employed to meet the schedule demand and remain on the critical path.
General Excavation, Inc. Interview
In addition to the general research presented throughout this paper, our team was afforded an opportunity to discuss rock excavation directly with a contactor in the field of expertise. Jack Tucker with General Excavation, Inc. (GEI) located in Warrenton, VA provided general insight into rock excavation operations from a contractor’s perspective. The following paragraphs outline the discussions had with Jack Tucker.
GEI specializes in commercial site development, roadway projects, and utility construction. Their commercial site development projects are typically anywhere from 10-40 acres. For these projects, excavations vary from 5-10 feet in depth for relatively flat topography and 10-20 feet in depth for more rolling topography. In the Northern Virginia area, GEI has found that rock is often found between 10-12 feet below grade. Approximately 75 miles northwest in Winchester, VA, GEI has found that rock is frequently encountered only 5 feet below grade. Most often, limestone is what is found below grade, but in areas of Culpepper, VA, a reddish volcanic rock has been unearthed. Depending on the location and required depth of excavation for a building construction project, rock excavation may or may not be factor for contractors to consider.
When bidding a potential project, GEI usually has a geotechnical engineering report available to assist with identification of subsurface conditions. More often than not, the geotechnical engineering reports are accurate. Also, there have been rare cases where GEI has performed their own soil exploration activities. In regard to classified or unclassified projects, GEI pursues both. In reviewing potential projects, a contact being classified or unclassified is not the greatest concern. Classified projects are the most attractive to excavation contractors because it puts all the risk on the Owner, however the contractor still is paid a unit price for what is excavated whether it is rock, clay, or sand. For estimating schedule durations, estimators at GEI have in house production rates that are used to schedule and price a given project. When bidding a project, GEI will consider the most practical method of rock excavation for a give project size and location.
For rock excavation methods, there is no one method that is the most cost effective in all cases. For every project, there will be a different set of criteria contributing to the selection of a rock removal method. The dominant criteria driving the selection of a rock removal method for a project is safety and local regulations. For the contractor performing rock excavation on a 10-acre site, blasting would be the most cost effective method, but close proximity to built up areas, public rights of way, and utilities may result in local regulations mandating a safer more contained method of rock removal such as breaking, drilling, or ripping. When blasting is not permitted, GEI typically uses drilling and rock breaking methods (i.e. hoe ram attached to an excavator). In the case blasting is permitted, GEI typically uses dynamite with blasting fertilizer (ammonium nitrate). The contractor will drill a 2 ½ - 3 inch hole in the rock approximately 2 feet below the rubble of where they will be grading, load the hole with dynamite and/or blasting fertilizer, put a blasting mat on the hole to prevent rockfly, and set the charge.
Conclusion
For contractors bidding and working on construction projects involving rock excavation have a difficult situation to evaluate. The contractor must identify the appropriate method of exaction whether it is blasting, breaking, drilling, ripping, or a combination of the methods. The contractor must also examine the environmental implications and safety factors involves with a given project where rock excavation is required. The rock excavation method, environmental implications, and safety factors all tie in to the construction contractors estimating approach. The estimating and planning process that contractors go through is governed first and foremost by safety. Local laws and regulations must be researched and identified by contractors during the bid stage to ensure the appropriate method or rock excavation is proposed in their estimate. Secondly, the proximity of built up areas, public rights of way, and utilities must be considered as not to cause harm to the public or damage to existing structures. At this point, the most cost effective method allowable, considering laws and regulations and proximity to a project’s surroundings, may be selected and included in the estimate. During all stages of rock excavation projects, contractor’s must be pay close attention to the bid documents describing subsurface conditions, local laws and regulations, environmental implications, and primarily public safety.
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