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Geotechnical Engineering

 

Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop

Abstract: A Few Examples Of Highway Design And Construction Complications
In Areas Of Prior Mining Operations In Arizona

Nicholas M. Priznar, Eng. Geologist,
Arizona Department of Transportation, Phoenix AZ

This presentation will summarize a few examples of the hidden complications that can effect the cost of highway design and construction in Arizona.

Example 1

Construction of SR89A, through Deception Canyon, was completed in December 1920. In December 2000, construction of a new retaining wall system to mitigate rock fall problems began in the vicinity of Station 672+00. Mining reports indicated that a tunnel was excavated, very near this location, as early as 1882. This work consisted of a 240-foot adit and a 100-foot crosscut. The (horizontal) location of the excavation was roughly identified by a mineral survey. Although the possibility of encountering mine workings appeared likely to occur during construction it was not intersected in any of the geotechnical borings. A search of the site revealed no surficial evidence of prior mining activity. However geologic mapping did identified an area of sulfide mineralization in a fault zone. A resistivity and induced polarization (IP) geophysical investigation, identified an anomaly at the site but it was not recognized as a mining feature.

During construction a disturbed fill area was encountered near the suspected location of the tunnel and crosscut. An additional 50 feet of tieback wall and mass concrete was necessary to complete the widening of the highway. This resulted in an increase cost of $300,000.00

Example 2

Portions of U.S. 191 north of Morenci had been constructed over mine dump material. Sections of highway have had been observed to subside on the order of several inches, in a matter of a few years after construction. A temporary realignment of the highway necessitated the construction of a new bridge across a small canyon partially filled with similar material. To control differential settlement of the bridge support structure a MSE mat was constructed. The mat consisted of 21 layers of geogrid with an overlying one-foot thickness of compacted-engineered fill. It was anticipated that some settlement of the mats would occur during construction as the load was placed. No long-term elastic settlement was anticipated. However long term subsidence of the dump material and the abutments could occur. The anticipated project life is 10 years. The mat foundation for the 220-foot bridge was predominately constructed from processed local material.


EXAMPLE 1

Complications Integrating, Geologic, Geotechnical, And Geophysical Data, With Mining Records To Interpolate The Location of Subsurface Mine Workings

Location

The project is located approximately ½ mile south of Jerome on State Highway 89A, Yavapai County, Arizona. The highway is cut in a rugged mountainous terrain on the north side of Deception Gulch. The project area is approximately 90 miles north of Phoenix Arizona.

Mining History

Copper mining in the vicinity started with the Spanish Conquistadors who were led to the area by Navajo Indians in the 18th century. Rediscovery of valuable mineral deposits occurred in the mid 1800's at the end of the Mexican American War. Following the Civil War and the adoption of the Mining Law of 1872 many of the properties in the area, were developed in a speculative way because of their close proximity with the known ore occurrences west of Jerome. Regional copper production began in 1883 and became a major local industry by 1888. Many of the mining claims were patented with questionable economic potential, but passed the "prudent man" test. At that time, the mining law required annual investments of capital and labor in excess of $100.00 per claim, to retain title to the mineral deposits. This often resulted in excavations placed in expedient areas not necessarily abundantly mineralized. Subsequently a government sanctioned mineral survey of the property often located the position of the discovery point and any shafts tunnels, drifts, adits, and trenches excavated on the mining claim. The surveys were state of the art for their day but they were often referenced to some local stone monument, or a distant typographical/hydrographic feature. Livestock or individuals that are unaware of their significance often obliterate these reference markers. Additionally the tape and chain methods often led to inaccuracies in horizontal distance, especially in steep terrain.

Highway History

SR 89A (formerly SR79) was one of the earliest highway construction projects to receive federal aid funding in the State of Arizona. Its completion led to a drastic reduction in the amount of time it took to travel from Prescott (the county seat) to Jerome. A trip that once took 5 hours now only takes 1.5 hours.

In 1919 Federal Air Project No. 12, facilitated construction of the new state highway route 79, with a 20' wide roadway with 6 to 10% grades. The work on the two-mile segment just outside of Jerome was considered the heaviest roadway construction ever attempted by the state of Arizona. Its total cost was reported as $123,785.15, a rate of approximately $62,000.00 per mile. It was also reported in the Fourth Biennial Report of the State Engineer that, "Many mining claims lie adjacent to the new highway and prospects that heretofore lay inactive on account of the lack of a highway facilities will eventually be in process of development."

It is assumed that the route surveyors utilized access paths and pioneer trails previously constructed by the local mining interests. It is probably no coincidence that the present highway travels past many of the old mining structures and workings. The highway providing a direct down hill route to the smelter near present day Cottonwood Arizona, avoiding the steep uphill trail over Cleopatra Mountain.

In 1937 the highway was widened to 30 feet. At this time stone retaining walls and concrete culvert extensions were constructed with the help of the WPA. The present alignment with only minor surface improvements is essentially the same today.

Brief Geology

The local geology is composed of a Pre-Cambrian, sub-marine volcanic vent deposit. According to Linberg (1999), the volcanic rocks exposed in Deception Canyon were deposited beneath sea level and sulfide ore deposits were precipitated as conformable bodies on an ancient sea floor and buried by later volcanic rocks. As such, volcanic rocks, sulfide mineralization, and rock alteration are part of a single, conformable stratigraphic succession. Shortly after deposition, the entire crustal assemblage was intensely deformed within greenstone fold belts that displayed isoclinial folds and re-crystallized terranes. Schistositicy was later superimposed upon the rocks during this folding episode. Following this activity, faulting and dike emplacement took place in the area.

The area of interest occurs near the contact of two of the Precambrian volcanic deposits the Deception Rhyolite and the lower Cleopatra Rhyolite, and a mafic dike, which outcrops near station 671+50.

Deception Rhyolite

This formation is massive, isoclinally folded, and moderately jointed, undifferentiated cliff forming, fine-grained, rhyolite lava with a re-crystallized flow breccia. This rock hosts copper sulfide mineralization typically associated with the Verde Central Ore body and is the dominant cliff forming material in Deception Canyon.

Mineralization consisting of pyrite (1-5%), trace amounts of chalcopyrite, iron oxide and traces of green copper oxide outcrop with a small, high angle reverse fault are exposed in a road cut between stations 671+90 to 672+50.

Lower Cleopatra Rhyolite

This formation is a thick conformable rhyolite flow, lying over the Deception Rhyolite. This material is typographically expressed as a more moderate cliff former which grades into subdued slopes. However, locally the material is quite massive.

Mafic Dikes

A Precambrian lamprophyres, dike swarm, highly weathered, which probably controlled the erosion along the axis of Deception Canyon. The dikes cut through most of the Precambrian lithologies, which outcrop within the project area.

The Deception Rhyolite is composed of multiple lava flows with brecciated flow tops and is folded like an accordion. The extrusive Lower Cleopatra Rhyolite is made up of a few thick pulses of monotonously similar composition. The formations are oriented in such a way that is somewhat difficult to recognize their original mode of deposition.

The volcanic sequence hosts ore horizons that have been explored from the 1700's to the present. The Central Verde ore horizon, in the Deception Rhyolite, Mafic Dikes and associated jointing and faulting are the principal geologic features that impacts the highway project in the area under discussion.

Records Search

Since the project was located in the Verde Mining District a records search was conducted at the Arizona Department of Mines and Mineral Resources and the Phoenix office of the Bureau of Land Management. The search determined that major underground excavation work was not indicated within the limits of the proposed highway improvement.

However some exploration shafts tunnels and a crosscut were recorded on mineral survey plats in the area. These workings were recorded as evidence of due diligence of mine development which was a prerequisite to receiving ownership of the property from the federal government.

In the area of concern the records indicate that the Buncomb Lode Mining Claim was located on March 15th, 1882. The mineral survey plat indicates that the centerline of the claim closely intersects a 240' long tunnel and a 200' cross cut, which were described in the survey notes of 1898. The workings are cited as a small (4'X 6') tunnel, at the date of the survey. No other information has been discovered, regarding this feature. Therefore, continued unreported work or deterioration of the tunnel is not known.

Fortuitously after construction of this section of 89A, a right of way survey was conducted in 1935. Several boundary lines and corrected horizontal distances were established to adjoining mineral survey monuments. When the claim lines are carefully reconstructed onto the right of way map the axis of the tunnel and possibly the crosscut intersects the highway near station 672+36.

This orientation intersecting the previously referred to mineralization and fault reported between Stations 1671+90 and 672+50. It is suspected that the mineralization was the intended target of the early mining interests.

These records are helpful in indicating the general horizontal position and orientation, of these preliminary works in relation to the existing local cadastral surveys. However the records do not indicate the vertical position (elevation) of the improvements. Additionally, the horizontal techniques used to survey these improvements were not as precise as what is routinely used today with electronic and satellite systems. Therefore the relative accuracy of identifying the position of the workings is contingent on finding some existing trace of the original workings or original mineral survey monuments and correlating it to known vertical and horizontal positions.

A diligent ground search was conducted in the area of discussion, but unfortunately no evidence of the prior exploration work nor the mineral survey monuments were found. The area appears to be mantled with fill materials from highway maintenance work, rockfall, and vegetative growth.

Geotechnical Investigation

The principal objective of the geotechnical investigation was to provide subsurface information to redesign highway infrastructure along the old roadway alignment. In the area of the suspected mine workings several shallow borings were drilled to establish the depth to bedrock and to characterize the quality of the material for the foundation of a new retaining wall. The top of the bedrock contact was difficult to predict from the borehole data alone. Therefore, a geophysical investigation was conducted to try to further define the subsurface conditions in the vicinity of the proposed retaining wall.

Results of the Geophysical Investigation.

Both resistivity and induced polarization (IP) techniques were used to interpret the depth to bedrock in the vicinity of the retaining wall. In this regard the surveys were generally successful. However conflicting IP and resistively data for the mineralized zone in the Deception rhyolite and the un-mineralized Lower Cleopatra Rhyolite led to interpretation uncertainty. Because of this, only a preliminary survey was conducted. It is only after construction activities were well underway that it was recognized that the position of higher IP and lower resistively anomalies were closely associated with the mineralized zone and the interpolated position of the exploration tunnel and cross cut.

Conclusion

Generally construction activities in the area previously describe were very difficult. A temporary soil nail wall experienced several small distresses where the in place material failed and collapsed behind the gunnite and reinforcing members. Pyritic rock material was now directly in contact with the galvanized mesh of the temporary wall. Additional excavation work was needed to reinforce the subgrade of the material directly underneath the temporary and permanent retaining structures. In some locations, were bedrock was expected at shallow depth, none existed for the support of the retaining wall. In other locations the existing foundation materials were determined to need additional reinforcement or special treatment.

It can not be stated conclusively that the abandoned tunnel was directly responsible for all the construction difficulties but they may have been a contributing factor. Examination of photographs taken just after original construction and during the most recent attempts reveals that the area has been repaired in the past. There are retaining walls and thicker dump fills that were probably constructed by the 1935 reconstruction. Therefore it is probable that the earlier construction and subsequent maintenance repairs obscured the presence of the mine workings.

Overall the changed conditions resulted in an additional work that amounted to approximately $300,000.00. The project has been delayed several months. However a more detailed tunnel investigation and Stabilization program would probably cost an equal amount.

This project demonstrated that there are techniques and procedures that can roughly identify the position of abandoned underground workings. What is very difficult to predict is what is the present condition of the workings and the surrounding rock materials. Additionally it would appear very difficult to estimate where and how much effort would be required to mitigate a suspected area, without gaining access to it, or conducting extensive investigations far beyond routine geotechnical investigation work.

EXAMPLE 2

Utilizing Modern Mining Records and Recycling Mine Waste Rock Materials To Optimize Costs For Bridge Foundation Design and Construction

Location

This project is located near the east central border of Arizona in Greenlee County, on SR 191 approximately 3 miles north of the mining community of Morenci.

History

Open Pit mining in the Morenci area had been an ongoing since 1937. Several expansions of the mine have necessitated the relocation of U.S. 191. In much of this area the highway has been constructed over mine waste rock material. These sections of highway have been observed to subside on the order of several inches, in a matter of a few years after construction. Another temporary realignment of the highway necessitated the construction of a new bridge across a small canyon partially filled with similar material.

Geology and Site Conditions

Information provided by the mining firm indicated that pre mining topography had been excavated to form a relatively flat mine bench excavated in a drainage channel. This surface was subsequently covered with waste rock material in the footprint of the bridge and ranged from 24 to 86 feet deep. Geotechnical borings consequently confirmed these depths.

The in situ rock material is composed of the waste rock overlying two igneous intrusive stocks that are described below.

Precambrian Granite

This material is a homogeneous, equiganular, medium grained material that locally has graphic texture. Regionally finely disseminated ferric oxide tints the prevailing red color with yellowish red to a deep brownish red shading.

Larmide Granite Porphyry

This material has a felsic composition, with gray color. The material typically consisting of phenocrysts of quarts and feldspars, in a fine-grained ground mass. This younger stock intrudes the older Precambrian granite essentially dominating the subsurface of the bridge foundation area. The material is commonly hydrothermally altered, with abundant fracturing (jointing) oriented 45 degrees from horizontal. RQD values ranging from 0 to 33 percent.

Waste Rock Material

The waste rock material can generally be described as poorly compacted deposit of poly-lithologic, angular, boulders, which is poorly graded, with a relatively small percentage of fines. Compaction of the existing material was sufficient to allow egress of mine equipment through the area, however it was never designed to resist differential settlement.

Geotechnical Design Considerations

The primary geotechnical concerns for the site were: 1) Develop stable and reliable foundation support for the bridge. 2) Control differential movement due to potentially variable foundation conditions. A decision by the project stakeholders, to utilize a mechanically stabilized earth mat (MSE) treatment of the foundation to support installation of the MSE abutments and the center pile of the bridge. This option was attractive because of the temporary nature of this structure, the reduction in cost, and the availability of local materials and short duration of construction.

The mat foundation has several beneficial attributes. (Abstracted from Pavlichek and Clark)

  1. Removal of a portion of the dump material and the replacement in a controlled manner provides more uniform bearing conditions for the MSE abutments and bridge pier
  2. Reduction in contact pressure of the untreated material from the embankment and the pier loads by distributing stresses over a larger area.
  3. Provide some rigidity below the abutments and pier and between the piers and abutments, allowing the MSE mat to span over localized loose zones.
  4. Reduce the likelihood of possible subsidence in the underlying mine dump material from reaching the ground surface.
  5. Provide dampening and resistance to settlement from vibrations by future mining activities.

Construction

Construction initially consisted of over excavating approximately 25 feet of mine waste rock material. This material was crushed and screened to achieve a specified a dense gradation. The mat was constructed by installing 21 layers of Tensar BX1200 biaxial reinforcement geogrid with an overlying one-foot thickness of compacted processed fill.

It was anticipated that some settlement of the mats would occur during construction as the load was placed. Some hand placed waste rock fill in the abutments area required higher labor costs than anticipated, but the bridge construction schedule was generally maintained. No long-term elastic settlement is anticipated. However long term subsidence of the dump material and the abutments could still occur. The anticipated project life is 10 years.

Conclusion

The estimated costs for the MSE Mat included $270,520.00 for geogrid delivered and placed. An additional $513,000.00 for excavation, processing, and replacing 57,0000 cubic yards of material. These total $783,520.00 for the mat foundation alone. The total bridge construction costs were approximately $2,000,000.00

Construction activities were dominantly uneventful. Useful information provided by the mining firm and their consultant enable accurate plans and estimates to be made. The structure has been in place for two years without any reported abnormalities.

Acknowledgement

The present mine owner has provided financial and physical resources to facilitate the design and construction of the bridge.

REFERENCES

Although not specifically cited in the text body, information was compiled from the following sources:

Anderson, C., A., Creasely, S., C., 1958, Geology and Ore Deposits of the Jerome Area, Yavapai County Arizona, Geologic Survey Professional Paper 308

Benedict, P., C., 1923, Geology of Deception Gulch and the Verde Central Mine, MS Thesis, Massachusetts Institute of Technology, unpublished

Call and Nicholas, 1998, US-191 Phase II Relocation Design, Geotechnical status and Slope Design Strategy, Prepared for Coronado Pit, Tucson Arizona,unpublished reportt

Lindberg, P. A., 1999, Geology Report: Highway 89 A Widening Project, Jerome South, Arizona, Background Information, Part A: Highway 89a, Station 665+00 to 676+00,For Arizonaa Department of Transportation, Materials Division, 1221n21st AV, Phoenix, Arizona. unpublished report

Lindgren, W., 1905, The Copper Deposits of the Clifton-Morenci District, Arizona, Department of the Interior, U.S. Geological Survey, Professional Paper No. 43

Lindgren, W., 1926,Ore Deposits of the Jerome and Bradshaw Mountains, Quadrangles, Arizona, Department of the Interior, U.S. Geological Survey, Bulletin 787,

Lundwall, D., 1998, US 191 Overpass at Phelps Dodge Haul Road, Bridge Structure Report, Structural Concepts Inc., Construction Engineers, 8230 E. Broadway Blvd. Suite W-7 Tucson, AZ 85710

Lundwall, D., 2000, ACI-Arizona Chapter, 2000 Excellence in Concrete Awards Program Entry, Submitted by: Structural Concepts Inc. Consulting Engineers

Miller C., W., 1991, Stake Your Claim!, The tale of America's enduring Mining Laws, Western lore Press, Tucson, Arizona

Mayerle C., M., Carlson N., R., 1999, Geophysical Report Resistivity and IP Surveys, State Route 89 A Yavapai County, Arizona, for Dames and Moore, Zong Job # 9952, unpublished report

Pavlicheck, R., W., Clark D., R., 1998, Geotechnical Engineering and Pavement Report, Proposed U.S. Highway 191 (Temporary Alignment), East Coronado Alignment, Morenci, Arizona, Prepared for MMLA, 800 East Wetmore Road, Suite 110, Tucson, Arizona 85705MMLA Job No. 97-79-01, Terracon Project No. 63975130, unpublished report

Other References

Arizona State Highway Department, Right of Way Map through Located Mining Claims, Prescott-Jerome Highway, Station 600+94 to Station 697+37, Scale 1"=200'November 1935

Fourth Biennial Report of the State Engineer to the Governor of the State of Arizona, For the period July 1, 1918 to December 31, 1920, pp.55-56, Arizona State Capital Library Archive's

Mineral Survey No.1285, Plat of the claim of the Jerome Copper Co, Surveyed April 25 through May 4, 1898. Patent Date 4-10-1916, Patent No. 31018, and Serial No. 04645.

Available at the Phoenix office of Bureau of Land Management Public Records Room

Manual of Instructions for the Survey of the Public Lands of the United States, 1973, Prepared by the Bureau of Land Management, Technical Bulletin 6, U.S. Department of the Interior

State of Arizona, State Highway Department, 1935-36, Plan and Profile of Proposed State Highway, State Route 79, Prescott to Flagstaff, Yavapai County, National Recovery Highway Project No. WPMH 12

Phelps Dodge Morenci Inc. Morenci Fact Sheet, Revised January 1994

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