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Rock Fill 

 
Authors: Jack Caldwell

Summary

This review discusses the use of rock fill in the construction of mine facilities. Rock fill is defined, and links to literature and descriptions of rock fill embankments at mines are given. Topics discussed include lift thickness, gradation, moisture conditioning, test pad size, and rock fill density.

INTRODUCTION

This technology review discusses the use of rock fill in the construction of mine facilities, including foundations for plant buildings, heap leach pad foundations, mine haul roads, and embankments for tailings impoundments.

This review is based on work done by the senior author on the Cannon Mine tailings embankment, a 470-ft high rock fill embankment at the mine adjacent to Wenatchee, Washington. The second, junior, author was the original design engineer on the first phase of the embankment and tailings impoundment reservoir.

In this review, we provide links to mine facilities involving the use of rock fill and to general literature on the placement of rock fill for civil engineering purposes.

BACKGROUND

Large quantities of mine waste rock material (non-ore overburden) are generally available from open pit mine cuts. The mine waste rock, already drilled, blasted or ripped and loaded into rock haul trucks, can be economically hauled and placed in embankment fills several miles from the open pit mine for less cost than local borrow development within the planned impoundment or leach pad grading fill limits.

HISTORY

The earliest writings on rock fill dams we have found is the 1888 book The Design and Construction of Dams, all of which is available at this link.

Richard Wiltshire in a seminal paper 100 years of Embankment Design and Construction in the U.S. Bureau of Reclamation surveys the history of earth and rock fill dam construction in the United States.

The largest earth and rock fill dam in the world is the Tarbela dam. Both authors have been privileged to work with Syd Hillis who was the chief geotechnical engineer on this dam.

Rock dump construction in thick and dry loose rock fill lifts has been successful in providing stable water storage embankments for large and small dams for more than 100 years after this type of rock fill construction began in the 1850's. However, several larger dams constructed between 1910 and 1940, and exceeding 100 feet (30 m) in height, experienced excessive post-construction settlement and movement of the upstream facing materials during first reservoir filling.

Thorough wetting or flooding of the dry rock dump lifts during lift placement in the 1940's to 1950's, as well as a reduction of the loose lift thickness in the mid to late 1950's, minimized post-construction settlement movements in the larger dams during reservoir filling.

From the 1960's to the present, the accepted practice for modern-day rock fill dam construction is moisture conditioning and compaction in thinner controlled lifts with vibratory steel drum roller compactors. Self-propelled rollers are more efficient over tractor-pulled rollers, particularly at dam abutments. Rock fill moisture conditioning is generally site-specific with wetting of rock materials in the borrow area or on the fill surface.

In comparison to the earlier thick rock dump techniques, the placement and compaction of rock fills in thinner controlled lifts significantly reduced post-construction settlement, reduced material segregation due to high lift rock fall on the slopes, increased the rock fill density and related strength, and allowed lateral and vertical placement of transitional zones for fine to coarse rock materials, as well as better tie-in to internal earth fill core and drain filter systems within the dam embankment.

SOME MODERN ROCK FILL EMBANKMENTS FOR MINES

The design and construction of the rock fill embankments and dikes for the Canadian diamond mines is described at these links:

  • Diavik diamond mine are described at this link.
  • Snap Lake at this link.
  • Cannon Mine at this link,
  • Ekati Diamond Mine at this link.

THE LITERATURE

This Technical Review is augmented and expanded on in an e-Book by Breitenbach. The e-Book contains considerably more information on the history of rock fill placement, technical methods for placing rock fill, and the testing and control of rock fill to minimize settlement.

The seismic stability of rock fill embankments is discussed by Barbosa, Morris, and Sarma in a paper Factor of safety and probability of rockfill embankments. They provide design charts to establish the factor of safety for rock fill embankments for critical acceleration coefficients.

Chrzanowski and Massiera discuss the monitoring of large earth and rock fill dams in a 2006 paper.

The U.S. Corps of Engineers discusses earth and rock fill placement and compaction at this link.

For general guidance on compaction and compaction equipment, go to the Caterpillar site.

WHAT IS ROCK FILL

In 1968, the USBR published the Earth Manual, which further defined cobble and boulder rock particles as follows: "Rounded particles are called cobbles, if they are between 3 and 12 inches (0.07 to 0.30 m) in size, and boulders, if they are greater than 12 inches (0.30 m) in size. Angular particles above 3 inches (0.07 m) in size are classified as rock fragments."

No further distinction was made about the rock fragment quality or gradation required for defining rock fill structures. From this author's experience, the "cobble" definition is commonly used by engineers to the present day for defining both rounded and angular rock fragments falling within the 3 to 12 inch (0.07 to 0.30 m) square mesh size range.

The term "rock fragment" is defined for this review as any rock (rounded to angular in shape) retained on the inch (19 mm) square mesh screen size.

The definition of rock fill materials includes an upper and lower rock size. The upper rock size is set by the allowable loose lift thickness for adequate compaction using conventional vibratory rollers or large loaded haul truck rollers for rubber-tired compaction. This should be about 2/3 of the maximum loose lift thickness with some allowance for larger rock fragments.

The lower rock size is defined as the point at which standard ASTM field and laboratory soils testing methods are no longer applicable to earth fill materials that contain excessive rock fragments. This earth fill soil limit is defined by ASTM test methods as no more than 30 percent of the sample retained by dry weight on a inch (19 mm) square mesh sieve size.

The definition of the rock fill lower limit is also suggested to limit the fine particle size to contain less than 15 percent silt and clay materials passing the No. 200 (0.074 mm) ASTM sieve size.

Rock used for rock fill embankments should preferably not be acid generating. Nor should it be subjected to other undesirable geochemical factors that may preclude it use in an embankment as described at this link.

GUIDELINES FOR ROCK FILL PLACEMENT AND COMPACTION

LIFT THICKNESS
Maximum loose lift thickness is governed by maximum rock size and type of compaction equipment. Optimum rock fill loose lift thicknesses are generally about 18 to 30 inches (0.5 to 0.8 m) with maximum rock sizes limited to two thirds of the lift thickness. Larger rock sizes can be incorporated into the fill provided the rock does not protrude above the fill surface to hinder compaction.

ROLLER TYPE
Experience indicates that the most efficient rock fill compactors are vibratory steel drum rollers with vibrations in the range of 1200 to 1500 vpm, roller speed of about 2 mph (3.2 km/h), a minimum static drum weight of 8 tons on level ground, and a minimum operating dynamic force of 15 tons.

Heavy loaded rubber-tired haul trucks (up to 240 tons) can provide some dynamic deep lift compaction, however the large haul trucks are limited to keeping a safe distance away from exterior fill slopes to prevent concentrated tire load bearing capacity failure near the edge of the fill slope or potential accidental truck roll over.

ROLLER PASSES
Optimum roller passes are determined from surveyed settlement versus roller pass curves developed in large-scale test fills. The general limit is between four to six passes. More than six passes tends to crush and pulverize the rock fill surface without adding significant compaction to the lower part of the lift. Each roller pass should overlap the edge of preceding passes for 100 percent roller pass coverage on the surface.

GRADATION
Rock fills for compacted embankment structures are generally placed in transitional zones with the coarser and more competent rock placed in the outer shell and finer, more weathered rock placed in the interior or adjacent to filter drain and core materials. A similar transition zone is developed for leach pad site grading fills with the finer rock materials placed beneath the pad subgrade soil and geomembrane liner system.

Oversized rocks are generally placed on the downstream or exterior rock fill slopes and in downstream outlet and spillway plunge pools for erosion and energy dissipation. Occasional extremely large oversized rock can be incorporated into rock fills provided no overhangs occur and the surrounding rock fill is compacted against the large rock pieces similar to compaction techniques against the rock abutments. Phased downstream raises to existing rock fill dams can incorporate the new rock fill into the oversized rock on the downstream slope of an existing dam, provided the large rock fragments are not clustered.

MOISTURE CONDITIONING
Moisture conditioning is desirable in the rock borrow areas for better mixing of moisture and materials during excavation, loading, dumping, and spreading for compaction. However, development of rock borrow areas involves blasting or ripping operations that sometimes make the borrow surface too rugged for conventional water trucks with spray bars.

Ideally the rock borrow should be sufficiently wetted so that no dust occurs when the haul truck or scraper dumps a load on the fill surface for spreading and compacting. Wetting of the rock fill in the fill area should be accomplished prior to spreading the new lift or following compaction of the lift. Wetting immediately prior to compaction by vibratory rollers significantly dampens the dynamic force of the compactor for inefficiency in compaction. The exception to this rule is a clean rock fill, which can be flooded with water and rapidly drained before compaction begins.

OVERBUILD
Modern day compacted rock fills that are relatively well graded experience post-construction settlements of the order of 0.2 ft per 100 ft of height (0.2 m per 100 m). For compacted large earth-rock fill dam structures with a relatively thin central core or upstream earth fill impervious core liner facing, about 0.5 ft (0.15 m) crest overbuild per 100 ft (30 m) of dam height appears conservative.

For large compacted earth-rock fill dams with relatively thick central earth fill cores, about 1 ft (0.3 m) minimum crest overbuild per 100 ft (30 m) of dam height is reasonable to counteract the long-term consolidation of the low permeability core materials (post-construction dissipation of excess pore water pressures in the fine-grained core materials).

Additional overbuild may be required in high seismicity dam locations to accommodate potential dynamic deformations and related settlement in the dam crest. The engineering analyses of potential earthquake related dam crest movements typically includes the maximum design earthquake (MDE) during operations and the long-term maximum credible earthquake (MCE) at closure.

TESTING ROCK FILL

GENERAL
Test fills are generally conducted in rock fills during construction to suit available rock borrow and site conditions. The test fills are conducted to determine specific acceptable procedures for placement and compaction including moisture conditioning, loose lift thickness, rock type and gradation, compaction equipment, and number of passes by the specified compactor. Some limitations are initially set during design concerning the specified rock types, maximum rock sizes, lift thickness, and compaction equipment requirements.

The test fill limits are determined by the size of the construction equipment and the number of lifts to be used for testing the rock fill placement and compaction.

TEST PAD SIZE
The minimum width of the test fill subgrade area is generally set at three times the width of the compaction roller and three times the height of the final test fill surface above the base level, as shown in Equation 1. The same test fill width is suggested for loaded haul trucks, due to the potential for lateral spreading of the rock fill along the exterior slopes of the test fill pad from the more concentrated and dynamic haul truck tire loads.

Test Fill Minimum Base Width = (W x 3) + (N x T x 3) (Eq. 1)

Where:
W = Roller drum width,
N = Number of lifts to be placed, and
T = Planned loose lift thickness.

A typical 10-ton vibratory roller with a drum width of 7 ft (2.1 m) and say two test fill lifts of 1.5 ft (0.46 m) each should have a 30 ft (9.1m) minimum test fill subgrade base width, as shown in Equation 2. Assuming approximate 1 ft (0.3 m) side overlaps in the steel drum roller passes for ideal 100 percent pass coverage, this base width spacing allows the steel drum roller compactor to stay about 1 ft (0.3 m) away from the edges of the final lift fill level for support purposes. Loaded rubber-tired haul trucks would have approximately half the truck width of lateral spacing from the edge of the final test pad lift level, as the tire tracks are staggered across the center of the test pad area for 100 percent tire pass coverage.

The compactor length dictates how much level fill surface length is required between the ramp and test area for level compaction across the test section. The test fill length is generally at least two times the width to allow the vibratory compaction operator to set and adjust his speed and vibration controls before crossing the planned control area on a level test fill surface. Ramps are used at both ends of the test fill as needed to place, spread, and compact each lift horizontally; similar to planned operations.

In the example above for the steel drum roller, the minimum length at the base of the test fill pad would be about 60 ft (18 m), depending on the time required to set compaction controls. Shorter test fill lengths are possible when the operator does the machine adjustments outside of the test fill limits before reaching the ramp to the level test fill surface.

LIFT THICKNESS
An 8 to 15 ton (static drum weight) smooth steel drum vibratory compaction rollers generally has an effective rock fill compaction lift thickness of between 1.5 to 3 ft (0.5 to 0.9 m) in about 4 roller passes on moistened rock fill. The 20 ton (static drum weight) smooth steel drum vibratory compaction rollers generally have a deeper effective rock fill compaction lift thickness of between 3 to 5 ft (1 to 1.5 m) in about 4 passes on moistened rock fill. The vibratory compaction roller weight versus general rock fill lift thickness estimates on moistened rock fill are approximate, based on visual observations and recorded field densities by this author in large hand excavated test pits at several dam sites. The definition of moistened rock fill for this discussion is minus inch (19 mm) earth fill materials within a range of 2 percent dry to 2 percent wet of optimum moisture content.

ROCK FILL DENSITY
Large scale rock fill density and gradation tests can generally be limited to about one day of testing time with a single field engineer or senior technician assisted by a technician for measuring excavated material and water replacement buckets, and to complete the laboratory gradation testing. The typical diameter size for the test hole ring should be 4 times the excavated maximum square mesh rock size for acceptable test accuracy. The typical test hole depth should extend through the entire rock fill lift, or through multiple lifts if practical, to account for the overall change in effective roller compaction with depth.

Water replacement techniques are used to determine the volume of the lined test ring and excavated and lined test hole. The excavated test hole material is weighed and test hole volume calculated to determine the moist unit weight rock fill density. Moisture content is generally determined on the finer minus 0.75 inch (19 mm) square mesh rock fraction for calculating the rock fill dry density. Some rock fills may require moisture content measurements in the larger weathered or moisture absorbing rock fragments, as needed.

Bulk gradations include measuring the larger plus 0.75 inch (19 mm) rock fraction in 2 inch (50.8 mm) square mesh increments for discarding. The minus 0.75 inch (19 mm) rock fraction can be accurately quartered and split by ASTM procedures to reduce the amount of finer rock materials for ease in completing the gradation testing.

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