utilization of detonation cord to pre

8/4/2019 Utilization of Detonation Cord to Pre

1/7

Utilization of Detonation Cord to Pre-split Pennsylvanian Aged

Sandstone and Shale, Grundy, Virginia

Steven S. Spagna, L.G., Project Geologist

U.S. Army Corps of Engineers, Huntington District

Figure 1. Upstream end of the Grundy Redevelopment Site

Project Summary

During Summer 2001, the U.S Army Corps of Engineers awarded a contract to theconstruction firm of Bush and Burchett of Allen, Kentucky, for the purpose of developing

a 13 acre redevelopment site by removing approximately 2.5 million cubic yards of rock

(fig. 1). The Redevelopment Site will be the future home for a large portion of the Cityof Grundy, Virginia. Additional work items include the construction and relocation of

3,000 feet of the Norfolk Southern railroad bed, the placement of 95,000 cubic yards of

fill and the placement of 16,000 cubic yards of stone slope protection along the Levisa

Fork River. Bush and Burchett received a notice to proceed with construction in July2001. Currently, the contract is near completion. Current activities include: hauling of

material from the Redevelopment Site to the disposal area; placing fill material on the

Redevelopment Site, and placing Stone Slope Protection (SSP) along the Levisa Forkriver. Approximately one year into the construction highly weathered rock, degraded to

near soil-like condition, was encountered in the upstream portion of the excavation. Over

one-third of the original cutslope was adjusted and the blasting specifications had to be

amended to provide solutions for the material that was encountered in this area.


2/7

Grundy, Virginia

Figure 2. Project Location Map

Authorization of Project

Located along the banks of the Levisa Fork River, below the 100-year flood

elevation, the town of Grundy has been plagued with flooding for years. The Flood of

Record occurred during April 1977 and devastated the town, causing deaths and millionsin damages (figs. 3 and 4). Shortly after the 1977 event, Congress recognized the need

for flood protection measures at Grundy and authorized Section 202 of the Energy and

Water Development Appropriations Act of 1981, which provided the town of Grundywith specific authorization for flood protection.

Figures 3 and 4. April 1977 Flood of Record

Site Geology

Grundy is situated at the confluence of Slate Creek into the Levisa Fork River insouthwestern Virginia (fig. 2). The valley bottom is narrow, ranging from approximately

100 to 230 meters wide. The Levisa Fork riverbank is steep from the river bottom at

approximate elevation 313 meters (msl) to the floodplain at elevation 317 meters. Thefloodplain is relatively flat, ranging in elevation from 317 to 321 meters. The angle of

the valley walls rise steeply (average from 35 to 40 degrees) from the floodplain to


3/7

narrow serrated ridges at approximate elevation 660 meters, providing a 339 meter relief.The valley wall slopes rise non-uniformly and are broken at intervals by near-vertical

cliffs of resistant sandstone. In general, the uppermost soil stratum found on the valley

walls consists of a relatively thin layer of unconsolidated residual-colluvial material.The top of rock surface lies from 0 to 2 meters below the top of ground, which generally

rises at a 35 to 40 degree slope. Bedrock is exposed along this slope where massiveweather-resistant sandstone members outcrop in near vertical cliffs. Bedrock is alsoexposed where the Norfolk Southern railroad was excavated into a massive sandstone

member at the base of the slope.

Bedrock at the project site consists of sedimentary rocks of the Pennsylvanian-

aged Norton Formation. For simplicity, the bedrock as sampled from elevation 318 to438 meters is divided into six members. Included in these six divisions are four

identified coal seams. The members, in descending order, include: upper shale,

interbedded, McClure Sandstone, intermediate sandstone, lower shale, and the lowersandstone members. The identified coal beds within these members include: Kennedy,

Aily, Raven Number 2, and Raven coal beds. See Figure 5 for a generalized geologic

column.

Figure 5. Generalized Geologic Column

The lower shale member is approximately 22.4-meters thick and is characterized with

low RQD and high core loss during drilling. Most of this member downstream of station6+25 consists of a gray to dark gray shale that is soft to moderately hard, broken and


4/7

occasionally clayey or carbonaceous. During drilling for blasting operations, it wasdiscovered that this member transitions to a highly weathered brown shale at

approximately station 6+25 and extends to the upstream end of the project, approximately

station 9+12. Weathering of this unit throughout the upstream zone of the excavationwas much worse than envisioned in the original design; therefore alterations to the

original design were necessary. The original design of two lifts with 18 intermediatebenches configured on a 2v:1h geometry was not achievable. A new design for theupstream end of the project was required to construct a stable cutslope above the railroad.

The new cut template had to be designed behind the weathered materials and tie into the

existing portion of the cut already constructed downstream.

Development of the Construction Oversight Team

To provide real-time design corrections for problems associated with theconstruction of the upstream portion of the project, the Huntington District formed a

Construction Oversight team comprised of Huntington District Project Geologist Steven

Spagna and QA inspector Mark Wheeler, geotechnical engineer Greg Yankey fromFuller, Mossbarger, Scott, and May Engineers (FMSM), blasting consultants Dr. Calvin

Konya (ISEE member) from Precision Blasting and Ed Smith (ISEE member) from

D.B.A. Locator Group, and geotechnical representatives Ron Maynard and Dick

Zimmerman representing Norfolk Southern. The oversight team met regularly to discussproject challenges, milestones, and to implement changes as needed during construction.

Oversight personnel were essentially on-call for the District: personnel on the team

convened rapidly to evaluate problems and recommend solutions.

Implementation of the Test Blasting Program

Once the new design was completed and the contractor began excavating the

upstream area, problems were initially encountered with the pre-split blasting. The firstchallenge of the oversight team was to develop a blasting program that would minimize

the damage to the pre-split wall. The faces being exposed were damaged with the

conventional pre-split products being used. Excessive crest overbreak, backbreak,cracking in the half-casts, and the inability to maintain specified tolerances warranted

change to the blasting specifications. Numerous geologic discontinuities present within

this reach of the project were dominating the final face, and the excess amount of energy

being used for the pre-splitting was causing damage to the final wall. Cracks in the half-casts (fig. 6) extending to planar discontinuities weakened the final face thus requiring

extensive scaling. The extensive scaling resulted in irregular faces, decreased designed

bench widths, and presented future maintenance and safety problems if not performedadequately. Working together with Dr. Calvin Konya of Precision Blasting, it was

determined that the pre-splitting procedures needed to be changed. Changes included the

implementation of a test program where explosive loads and center to center holespacings were adjusted. Since the conventional 7/8 pre-split product was delivering too

much energy, the team recommended the use of multiple strands of 100 grain detonating

cord to form the pre-split walls. The amount of energy required for a successful shearplane to propagate between the pre-split holes varied. Test blasts were performed in both


5/7

the sandstone and shale members. Typical test blasts enabled oversight personnel to view25 meter reaches of pre-split holes spaced on each recommended spacing interval.

Initially the spacing intervals included 18, 24, 30, and 36-inch center to center hole

spacing. Results were evaluated for each type of spacing and whether or not theexplosive loads needed to be adjusted on subsequent blasts. Explosive loads were

adjusted by reducing or increasing the number of strands of 100 grain detonating cord.The oversight team members were able to inspect a side by side comparison of resultsobtained in both sandstone and shale using two different pre-split products (figs. 7 and 8).

In figure 7 heavier loading left of red line caused breaking into a parallel discontinuity.

In figure 8 overbreakage occurred in areas above the red line.

Figure 6. Pre-split shot on 30 centers with 7/8 pre-split product


6/7

Figure 7. Upper Shale Station 7+75

36 center to

center spacing

using 7/8 pre-

split product

30 center to center

spacing using 3

strands of 100 grain

detonating cord

Figure 8. Lower Sandstone Station 7+25

24 center to center spacing using 7/8

pre-split product

24 center to center spacing using 4

stands of 100 grain detonating cord

Conclusions

Detonation cord was used to form the pre-split faces on over 37,800 linear meters of theGrundy Redevelopment Site cut. The use of detonation cord proved to be a successful

method of pre-splitting sedimentary rock dominated by geologic discontinuities. Insandstone, 4 strands of detonation cord with pre-split holes spaced 24-inchs center to

center yielded the best results. In shale, 2 strands of detonation cord with pre-split holes

spaced 24-inches center to center yielded the best results. The lighter loads minimized

problems with crest overbreak, backbreak, and eliminated cracking in the half-casts.Controlled blasting utilizing lighter loads and closer spaced pre-split holes prevented


7/7

excessive overbreak. Figure 9 shows results obtained in the upper shale unit utilizing 2strands of 100 grain detonating cord in holes spaced 24-inches center to center.

Figure 9. Upper Shale pre-split face (Sta 7+25)

Shot on 24 centers with 2 strands of detonating cord

utilization of detonation cord to pre

Documents