DIRECTIONAL DRILLING FOR HIGH CAPACITY ANCHORS AT BLUESTONE DAM

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21st Century Dam Design —

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011



On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the region’s

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

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Directional Drilling 285

DIRECTIONAL DRILLING FOR HIGH CAPACITY ANCHORS AT

BLUESTONE DAM

Mark J. Rothbauer, P.E.1

Jeff R. Hopple, E.I.T.2

ABSTRACT

Bluestone Dam is located in Hinton, West Virginia and is owned and operated by theUnited States Army Corps of Engineers. Ongoing phased construction to correct

deficiencies identified in the Dam Safety Assurance Program includes the installation of

rock anchors containing up to 61 strands in 15-inch diameter holes. Due to embeddedstructures in the dam and close anchor spacing, conventional drilling tolerances would

increase the risk of intercepting structures or intersecting adjacent drill holes or tensioned

anchors. Directional drilling pilot holes is used to meet the specified tolerances, preventing interception and intersection issues. A real time, optical directional drilling

system is used to drill the holes. This system was selected to maximize the drillingcapability and limit site conditions from affecting the directional drilling. This system

has a number of unique components compared to conventional anchor hole drilling.After the pilot hole is completed, the hole is reamed to the final diameter. Each hole is

surveyed to ensure it meets the tolerance requirements. The system is able to drill

straight holes meeting a tolerance of 1:150 on holes as deep as 270-feet. Significantlessons learned from the project are discussed including a summary of the capabilities

and limitations of the directional drilling system used on the project.

INTRODUCTION

Bluestone Dam is located in Hinton, West Virginia, along the New River. It is owned

and operated by the United States Army Corps of Engineers. Construction of the dam

was completed in 1949. It is 165-feet tall, 2,048-feet wide, and encompasses a watershed that is 4,600-square miles.

The Bluestone Dam Safety Assurance (DSA) Program construction was started in 2001.

The program is a multiple-phase construction project to upgrade the capacity and stabilityof the structure to meet the probable maximum flood event. Failure of the dam during a

flood event would be devastating to the New and Kanawha River Valleys, located

downstream of the structure. The DSA project includes raising the dam by 8-feet,installing anchors to tie the dam to bedrock improving stability, adding emergency gates

to increase discharge capacity, scour protection, and miscellaneous site work. The first

phase of the project was completed in October 2004. The second phase, which includesinstalling rock anchors, is expected to be finished in 2011. Three additional phases

remain to be constructed.

1 Project Executive, Brayman Construction Corporation, 1000 John Roebling Way, Saxonburg, PA [email protected] Project Engineer, Brayman Construction Corporation, 1000 John Roebling Way, Saxonburg, PA 16056

[email protected]



286 21st Century Dam Design — Advances and Adaptations

The anchor construction phase includes 215 anchors, of which 57 were installed on top of

the dam at 8 or 9.5-degrees from vertical to resist overturning. The remaining 158anchors were installed on the face of the dam at 45-degrees to resist sliding. The anchors

range in size from 3 to 61 strands and have a design load up to 2,145-kips. Drill holes

range in size from 6.5 to 15-inches.

REASONS FOR TIGHT TOLERANCES

Due to the risk of intercepting adjacent drill holes or tensioned anchors, drill tolerances of1:150 are specified in holes as deep as 270-feet. The anchor spacing is typically 8-feet

center to center on the face of the dam. The holes located at the top of the dam are

located between the two face anchors as measured along the axis of the dam. At theintersection of the face and

crest anchors, the center to

center spacing is 4-feet. Theintersection point is over

200-feet deep for someanchors. The clear spacing

is less than 3-feet since thedrill hole has a 15-inch

diameter.

In addition to the risk of

intercepting adjacent

anchors, there are multipleobstructions within the dam.

Inclusive in the massconcrete of the dam are two

galleries that run along the

axis of the dam, hundreds offoundation relief drains,

mechanical controls for gate

operation, and 18-foot-

diameter penstocks. Figure 1shows a typical cross section of the dam with some dimensions added to identify the

distance from the top of the hole to the potential interference item. Holes drilled from the

face pass within 4-feet of the inspection gallery. Drill holes do intersect existingfoundation drains, but the drill accuracy limits the number of drains lost and enables the

owner to predict which drains will be affected.

The tight tolerance in conjunction with the survey data of the drill-hole location will

allow additional anchors to be added in future phases of work. Phase 4 includes

installing additional anchors in the spillway section of the dam. Future engineering mayalso identify the need to add additional anchors.

Figure 1. Drill Hole Interference




FIELD STUDY

It was unknown if conventional drilling methods could produce the precision necessary

for the anchor construction. A field study was conducted to prove whether conventional

drilling methods could produce drill holes with tolerances of 1:150. In 2002, four 61-

strand anchors were installed at Bluestone Dam. Conventional drilling methods wereused to install two anchors inclined at 45-degrees on the face of the dam and two anchors

inclined at 8-degrees on the crest of the dam.

Using conventional drilling methods, the tolerance is affected by gravity, drill rod

rotation, drill rod flex, drill setup, and variation in the layers of rock. Drill hole survey

results showed that the 45-degree holes would not meet the necessary tolerance. Theseanchors were most susceptible to gravity which pulled the drill string to a more vertical

position. At the bottom of the 170-foot-deep hole, an angle change of nearly 2-degrees

was observed which resulted in tolerances of 1:40. This tolerance represents anapproximate 4-foot deviation from the target line. For a deeper hole this would be

significantly larger as the deviation is likely to increase exponentially with depth ratherthan linearly.

DIRECTIONAL DRILL SYSTEM AND COMPONENTS

The directional system was built with job specificconditions in mind. The system needed to provide

real time feedback in order to efficiently drill the

quantity of holes required. Pulling drill rods andsurveying at specified intervals would significantly

slow the drilling process. Since reinforcing steel is present throughout the dam concrete, magnetic

surveying devices would not be reliable. Because

the drill holes will encounter water at depths asshallow as 25-feet, the system had to be watertight

for optical readings. Vibration from the

construction activities could affect the data. To

provide the accuracy required, the system needed to be independent of the drill and any surfaces affected

by operating equipment.

The following are the components of the system:

-Optical Survey Instrument w/camera (Figure 2)-Tablet PC/Video Monitor (Figure 2)

-LED Light Target (Figure 3)

-Slant Face Bit (Figure 4)

-Dual Wall Drill Rods

-Independent Drill Stand (Figure 5)

Figure 2. Monitor and Camera

Figure 3. LED Target




The system’s main component is an optical survey

instrument fitted with a camera. The cameramonitors the LED target, which is located at the top

of the down-the-hole hammer. The camera is

connected to a video monitor and gives real time

video footage of the target. Dual-wall drill rodsallow air to flow through the outer rod to operate the

hammer, leaving the inner rod free of water and

debris. This makes viewing the target at depths upto 270-feet possible. The slant-face bit is used in

lieu of a standard button bit and steers the drill

string. The independent drill stand holds the survey

instrument and camera above the drill where it is isolatedfrom vibrations by the mass of the dam.

DIRECTIONAL DRILL SETUP AND PROCEDURE

A small hydraulic crawler drill is

used to directionally drill a small-

diameter pilot hole. For thisapplication, the pilot hole had a

5.75-inch diameter. The hole is

then reamed to the desired finaldiameter.

The first step in setting up thedirectional drill is placing the

independent reference frame tosupport the optical instrument.

With the frame in place, the

optical instrument is located in-line

with the theoretical drill line. Thedrill is then set at the appropriate

angle and azimuth along the drill

line between the optical instrumentand a survey nail located at the

entry point on the concrete.

As the hole advances, the operatorcan watch in real time the

theoretical drill line versus the

actual drill line (Figure 6). Thetheoretical drill line is shown as a

cross hair on the screen. Deviation

from the cross hair can be corrected by stopping the rotation of the drill

Figure 4. Slant Face Bit

Figure 5. Directional Drill Setup

Figure 6. Actual vs. Theoretical Alignment




string and chiseling, or steering, the slant face bit back into the theoretical drill line. The

target has an extra LED light that aligns with the slant face of the bit allowing theoperator to know its orientation at all times. For quality control purposes, optical three-

dimensional observations are made at intervals as the hole progresses. The specifications

require down-hole instrumentation surveys to verify the location of the hole. The optical

shots are more accurate than the instrument surveys; however, since they are taken beforethe final drill operation has been completed, they do not meet the contract specifications,

but are used as reference in the final verification survey.

DIRECTIONAL VERSUS CONVENTIONAL DRILLING

The equipment used for directional drilling in rock and conventional down-the-holehammer drilling is generally the same except for the key components that were outlined

previously. These items are not readily available and many of the components have

undergone modifications to adapt to the rock and concrete drilling at Bluestone Dam.

The main difference between the two types of drilling is the extra time required fordirectional drilling. Despite knowing in real time the location of the directional drill

string, care needs to be taken while drilling to stay close to the theoretical alignment.Dedicated drill operators are beneficial for this operation. Different steering techniques

are needed depending on the rock formation, the hole angle, and other factors. The

steering process is slow. The lengths of the drill hole where corrections are made cancause problems for successive drill operations and anchor installation. Surveying the

location of the drill bit is limited by the line-of-site from the optical instrument. If the

hole begins to drift too far, the light target above the hammer can be eclipsed by the bendin the drill rods. This is known as “losing light”. Once the lights are lost, the hole is being

drilled blind. Over-steering can cause the loss of sight of the light target also.

Another step that adds time to the overall drilling process is the setup and alignment of

the drill. Special care needs to be taken to assure the drill starts in the correct locationand orientation. The survey instrument setup is also crucial to the alignment of the drill

hole. The instrument must be backsighted throughout the drilling process to ensure it has

not moved.

The final time restraint is the borehole diameter. Tooling and hole size must be

optimized for the project specifications. There are trade-offs in the selection of the pilot

borehole size, tooling, and steering ability. The larger the final diameter of the hole, themore passes required to ream the hole to the final diameter. This will add additional

drilling and setup time. The reaming diameter for the additional passes is limited. An

estimate to the reaming limit is approximately double the pilot hole size. Depending onthe required diameter and the initial pilot hole size, multiple reaming passes can be

required to complete a hole.

The ability to view the light target and steer the drill bit can become a limiting factor.

Even in a straight hole, viewing the light target becomes more difficult with depth. The

targets used on this project may be difficult to observe at depths beyond 270-feet.




Reflections of light in the drill rods can also cause survey error if it is not detected

properly. In cases where the line-of-sight is lost, steering can be performed on a limited basis and the hole can be advanced without real-time survey observations. This is only

recommended for holes that are near completion and beyond any interference risks. For

holes that can not be completed because they are out of tolerance or the tolerance can not

be verified in real time, grouting must occur. Once the hole is grouted, the hole can bedirectionally drilled again after the grout has achieved equal strength to the surrounding

material. Even with a proper grout cure, there is still a tendency for the drill bit to follow

the original drill hole. Embedded steel in the concrete and voids or very soft layers ofrock are the primary causes to lose the tolerance of a drill hole.

VERIFICATION OF ALIGNMENT

The Reflex Maxibor II is used for verification of the borehole location in addition to the

optical survey data obtained while drilling the pilot hole. Two downhole surveys arecompleted for each hole. The first survey is run after directional drilling is complete.

The second survey is run upon completion of the final reaming of the hole. For eachsurvey, data is obtained at 10-foot intervals and the average from the survey, going into

and out of the hole, is used. The surveys are performed with the instrument centralizedinside the drill rods. All of the survey data is compiled to make a final survey report.

RESULTS

For the purpose of this paper, the 100 longest drill holes out of the 185 completed drill

holes were analyzed: 32 of the anchors are 8-degree, “top-of-dam anchors” (located at thetop of the dam) and the remaining 68 are 45-degree, “face-of-dam anchors” (located at

the downstream face of the dam). The following is an explanation of the terms used forsurveying the hole. The target line and deviation from the target are evaluated in three-

dimensional space; however, for reporting and analysis it is helpful to use two-

dimensional planes.

Upstream Deviation – Upstream is a reference to the direction perpendicular to the axis

of the dam going upstream along the river. Deviations are

measured in the vertical plane perpendicular to the dam axisrelative to the theoretical drill line. Deviations in this plane would

represent changes in the drill hole inclination. Gravity and drill

medium changes have the greatest influence on changes in thisdirection.

Dam Axis Deviation – All drill holes are installed at the azimuth perpendicular to the

dam in the upstream direction. Deviations measured by thisreference are measured in the horizontal plane relative to the

theoretical drill line. Deviations in this plane would represent

changes in the drill hole azimuth. Drill rod rotation and drillmedium changes have the greatest influence on changes in this

direction.




The following two plots are typical of what would be found in the final alignment survey

report. The first plot depicts the upstream deviation (Figure 7). Three sets of survey dataare shown for evaluation.

The three-dimensional

tolerance cone is shown in

two-dimensions for a perspective reference.

The data confirms that the

reaming passes followedthe pilot hole and the

accuracy of the downhole

instrument compared wellto the optical survey shots.

The second plot depictsthe dam axis deviations

(Figure 8). The same setsof data are shown as in the

upstream deviation graph.Similarly, the deviations

measured by the different

sets of survey data overlay

closely.

The scale of the graphs is

notable. The vertical axis

of each graph is elevation

with a 200-foot range.The horizontal axis

represents the deviation in

inches with a range of 4-inches. If the graphs were

shown on a 1:1 scale the

deviation would not bevisible for evaluation.

For evaluation, the finalsurvey for the hole shown

in these examples has adeviation of less than 1-

inch until a depth of 206-feet and a maximum deviation of roughly 2-inches to the west at the bottom of the survey

along the dam axis. Perpendicular to the dam, the maximum deviation is 4-inches near

the top of the hole and 4.5-inches at the bottom of the hole. The deviation for the totalhole shown based on the last survey reading is 4.5-inches. The tolerance at the bottom of

the survey is 1:608 compared to the required 1:150.

Figure 7

Figure 8




It is common and acceptable for the top of the hole to be out of tolerance as shown in the

perpendicular axis at the top of the hole. The initial dip can be seen in every survey andis attributed to rods lying on the bottom of the borehole instead of in the center.

TOP-OF-DAM ANCHORS VERSUS FACE-OF-DAM ANCHORS

The following data (Figures 9 and 10) illustrate the differences, in the drill accuracy

results, between the top-of-dam anchors and the face-of-dam anchors. These anchors

have been separated in the analysis to show the influence gravity has on the results as theangle from vertical increases in the drill hole. The pie charts are used to count the

number of anchors which land within the tolerances listed on the chart. As illustrated in

the chart, the face-of-dam anchors were prone to lower tolerances than the top-of-damanchors.

FACE-OF-DAM ANCHORS — TOLERANCE BY DEPTH

As depth increases, the ability to stay on the theoretical drill line becomes more difficult.In the test program, the upstream tolerance on the face-of-dam anchors was a larger

problem than the dam axis deviation. The upstream tolerance was less than 1:41 in one

hole. To show the improvement of directional drilling, only the upstream deviations are

evaluated in further detail. This is illustrated through a comparison of the tolerance of theface-of dam anchors at 100-feet depth in the drill hole and at the bottom of the hole.

The pie charts are used to count the number of anchors that land within the toleranceslisted on the chart (Figures 11 and 12). This set of data describes the difference in

tolerance at different depths in the hole. Only three face-of dam anchors (4%) had a

tolerance of less than 1:150 in the upstream direction at full depth. These three anchorswere within tolerance at 100-feet. The anchors went out of tolerance beyond the depth of

any obstructions and were accepted in this manner. At 100-feet, all of the anchors were

within tolerance with the majority more than triple the required tolerance. Distinguishing

Figure 9 Figure 10




a varying tolerance acceptance criteria based on known obstructions could allow stricter

tolerances in critical zones while allowing larger tolerances in less critical zones. Thisoffset can lead to a time savings in the drilling process and reduce the expectancy of

performing re-work due to out-of-tolerance drilling.

LESSONS LEARNED

The directional drill system has undergone modifications throughout the length of the

Bluestone Dam Safety Assurance project. The diameter of the directional drill tooling

has been modified to balance stiffness to keep the hole aligned, with flexibility to steerand correct the hole when it deviates from the target alignment. The directional drill pilot

hole size was increased early in the project to allow design changes to the reaming bits.Multiple down-the-hole hammer reaming bits, of various sizes, were fractured whileothers were capable of drilling a unique hole outside of the pilot hole prior the pilot hole

size changes being made. Material changes were made to the directional drill tooling to

withstand the wear of hammer drilling in hard rock. This ranged from metallurgy

changes in the tooling manufacturing to O-ring upgrades to keep rod seals lasting longer.All of the added parts to the drilling system become additional sources of error or

problems in the drilling process. Through experience, the drillers are able to develop

sensitivity to detect problems early and fix them quickly.

The sensitivity of the survey instrument and the effects of the sun had to be monitored

closely since the instrument was mounted on a steel frame. Throughout the day, thelattice of the stand would expand and contract moving the optical instrument with the

camera. This has the effect of moving the directional drill target that appears on the

driller’s screen. Since this was a slow change, it would appear to the driller that the hole

was moving off target. To prevent this problem, the surveyor would block sunlight fromthe stand when possible, along with back sighting and adjusting the instrument more

frequently when the weather conditions warranted it.

Figure 11 Figure 12




Most of the holes on the project have three sets of survey data: the optical data, the

directional downhole instrument survey, and the final downhole instrument survey. Theability to capture the optical data was not understood at the time the specifications were

written or when the initial construction plans were established. As the data became

available, it became evident that the optical data was the most reliable and accurate

survey information. The downhole instrument survey, after completing the directionaldrill hole, was eliminated from the process at many hole locations. With proper design of

the reaming bits and observing drill penetration rates, the reaming process can be

guaranteed to follow the pilot hole. Consequently, the downhole instrument survey could be eliminated or reduced to a small percentage of verification tests.

The directional drilling system developed at Bluestone Dam is ideal for drilling straightholes where accuracy is vital to the object being anchored. It has been adapted for very

extreme rock drilling. It would be easy to adapt to less severe rock drilling but there may

be better alternatives for sands, gravels, or clays. Drilling through obstructions isdifficult but can be accomplished with this system. The bit locating system is not

affected by embedded steel in the concrete. The location of the bit is known to a highdegree of accuracy in real-time. The system limitations are discussed in detail with the

comparison to conventional down-the-hole drilling, but primarily it is extra time thatequals extra cost to the project.

REFERENCES

"U.S. Army Corps of Engineers, Huntington District - Bluestone Dam." U.S. Army Corps

of Engineers, Huntington District - Welcome to the Huntington District . Web. 02 Nov.2010. www.lrh.usace.army.mil/about/history/bluestone.

Fuller, Mossbarger, Scott, and May Engineers Inc.. Directional Drilling Report Dam

Safety Assurance Program Bluestone Dam Hinton, WV . Rep. Print.

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DIRECTIONAL DRILLING FOR HIGH CAPACITY ANCHORS AT BLUESTONE DAM