INTERNATIONAL COMMISSION ON LARGE DAMS ----- BARCELONA CONGRESS ON LARGE DAMS Spain, 2006 -----

FOUNDATION TREATMENT FOR SALUDA DAM

Paul C. Rizzo, P.E. Civil Engineer

Luis Gaekel, P.E.

Civil Engineer

Keith Kessler, P.E. Civil Engineer

Howard Gault, P.G.

Paul C. Rizzo Associates, Inc. USA

1. INTRODUCTION

Saluda Dam is a new (to be completed in 2005) RCC Dam, about 200

feet high and 2500 feet long near Columbia, South Carolina, USA. The foundation rock is comprised of high-grade metamorphic and igneous rocks – dominantly mica quartz feldspar schist, microcline gneiss and a granitic intrusive. These rocks have undergone at least 4 periods of deformation and the process of unloading many kilometers of overburden by erosion has resulted in the intense fracturing. They have also been cut by numerous pegmatites, quartz veins, amphibolite layers and faults.

Extensive foundation preparation, including hand excavation, cleaning with air and water jets, dental concrete and mass concrete was necessary to achieve a flat surface to receive the RCC. Narrow zones of weathered rock as deep as 60 ft (40 ft below water table) had to be removed by hand labor and micro excavators in a dewatered excavation. High pressure water was used to clean out the clay filled fractures.

This paper discusses the techniques adopted for around-the-clock work under difficult time constraints and water conditions. An attached photo of the work in progress illustrates some of the conditions encountered.

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2. BASIC REMEDIATION CONCEPT

The remedial design for Saluda Dam consists of the construction of a combination rockfill and roller compacted concrete (RCC) berm along the toe of the existing Saluda Dam. For about 5,900 feet of the 8,200 feet long berm, the remedial design calls for the construction of a rockfill berm. Where space constraints do not allow for the construction of a rockfill berm (i.e., at the Saluda Hydro Powerhouse, at McMeekin Station, and south of the Saluda Hydro Powerhouse), an approximate 2300-foot long RCC berm will be constructed. The crest of the new berm will be El. 372 NAVD (typical), which is the current minimum crest elevation of the existing dam.

The excavation for the new berm was generally divided into 250-feet sections, or cells, which were named north cells, south cells, and center cells relative to their location on the dam.

There were ten south cells, eight center cells, and five north cells. Their locations are illustrated on Figure 1. Center cells were excavated to competent bedrock. In this excavated area behind and extending north and south of the Saluda Hydro Powerhouse, 1.3 million cubic yards of roller compacted concrete (RCC) were placed to create the new RCC berm. During excavation, some cells were combined and boundaries of cells were adjusted for engineering and construction purposes. The principal stipulation during excavation of the cells was that removal of existing dam material was not allowed.

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North Cells Lake Murray

Center

S-1 S-2 S-3A/B

S-4 S-5/6 S-7

S-8

S-9 S-

10 C-9 C-7/8

C-6

C-5 C-4

C-3

C-2

C-1 N-5

N-4 N-3

N-2 N-1

Center

South Cells

Saluda River

Figure 1

Drawing Displaying Center Cells (RCC Berm) in Green with North and South Cells (Rockfill Berm) in Red

The rockfill and RCC berms have been designed to survive the Design

Seismic Event (DSE). The primary function of the remediation is to prevent an uncontrolled release of water from the reservoir (Lake Murray) during and immediately after the DSE. Following the DSE, remediation (and a temporary decrease in the pool elevation) will be required to verify that the berm can function as a permanent dam should the existing dam be determined by inspection to be unsafe. Both the RCC and rockfill berm have been designed to maintain stability during postulated post-seismic conditions.

The RCC berm at Saluda Dam is founded on competent rock, which was defined by the commencement of coring operation during the geologic investigations and verified during construction. Foundation rock was shaped to remove overhangs and steep surfaces. High rock surfaces were cut back to remain stable during construction and to maintain a smooth continuous profile to minimize differential settlement and stress concentrations within the RCC.

Treatment of the exposed rock surface after removal of unsuitable overlying materials depended on the type of rock and the irregularities present. The configuration of exposed hard rock surfaces was controlled largely by foliation, joints, fractures, faults, shear zones, and excavation methods. Depending on discontinuity orientations, these features sometimes resulted in vertical surfaces, benches, deep gouges or depressions, and overhangs. For example, a near-vertical bedrock surface (quartz plagioclase biotite schist) was uncovered between

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Penstocks 4 and 5, as seen on Photo 1. Features such as potholes, buried river channels, intrusive bodies, or shear zones created additional irregularities while excavating. Unsuitable materials were removed from the irregularities, and the foundation surface was shaped somewhat to provide a sufficiently regular surface on which RCC / dental concrete could be placed without differential settlement. Generally, the foundation surface was shaped adequately by conventional excavation.

Photo 1

Near Vertical Outcrop of Schist in Cell C-5

Geology played an integral role in determining depth of excavation. Over-excavation in limited areas was required due to presence of weathered, broken, fractured, or faulted unstable materials. RIZZO geologists mapped all exposed sections of bedrock, observing discontinuities, lithologies, fracture patterns, foliation orientations, and overall competency of the foundation on which the RCC berm was to be built.

3. OVERALL EXCAVATION PROGRAM FOR THE RCC BERM

A primary consideration in the development of the excavation design was

to exclude excavation into or below the toe of the original existing dam. Excavation into and /or below the riprap was acceptable, but penetration into embankment soils was not. Space constraints in the area of the Saluda Hydro Powerhouse required vertical or near-vertical cuts and temporary excavation support schemes. Anchoring of an existing un-reinforced concrete retaining wall,

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behind the powerhouse; as well as construction of new tieback walls north and south of this wall, provided excavation support.

In general, the excavation behind and adjacent to the powerhouse was unsupported from the top (approx. El. 240) down to about El. 200. This excavation was primarily through the previously placed riprap and was sloped at 1.5H to 1V. Excavation through the riprap exposed the top of the existing concrete retaining wall, where an 8-foot horizontal bench was established. As the excavation proceeded below this level, soldier pile and lagging tieback walls north and south of this existing wall and inclined anchors installed through the wall were required to assure stability. All anchors and tiebacks were drilled into un-weathered, competent rock.

For the existing concrete retaining wall, anchors were installed between the existing penstocks. Though the existing concrete retaining wall was stable during the 1930’s construction project without soil in front of it and without anchoring support, stability concerns were driven by the additional weight of the riprap that was placed above the wall at a later date.

In addition to exposing competent rock and the penstocks, excavation in the vicinity of the powerhouse also undermined the foundations of two existing anchor blocks for two above-grade steel water conduits branching from two of the penstocks, which provided circulating water for the McMeekin Station. Photo 2 shows the original retaining wall behind the powerhouse and the circulating water pipes. These circulating water pipes required temporary support to remain operational during the initial stages of the excavation. The original pipes were later demolished and replaced with new conduits during a limited McMeekin Station outage closely coordinated with the RCC construction.

Original concrete retaining wall

Lagging Wall

Photo 2

Aerial View Upstream of the Powerhouse

Note: Circulating Water Pipes, Unearthed Penstocks, Original Retaining Wall,

and Lagging Wall Construction

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While the intent of the powerhouse area excavation was to expose a suitable rock foundation for the RCC berm, tight working conditions immediately behind the powerhouse due to the penstocks for Saluda Units No. 1 to No. 4, an arch conduit for Saluda Hydro Unit 5, and the relocated circulating water lines precluded the use of RCC material at the rock interface in all areas. In these areas, un-reinforced mass concrete was placed from the prepared rock foundation (approx. El. 165) up to El. 193, an elevation above most of these structures. Thereafter, RCC was placed to complete the berm.

Typical bedrock excavation activities and equipment used can be seen on Photo 3.

Photo 3

Bedrock Excavation in Cell C-6

Note: Upstream Lagging Wall

Excavation for the bedrock cells north and south of the area behind the powerhouse proceeded in like fashion; however without the space restrictions and structure concerns, excavation occurred at a faster rate. Excavation of the first center cell (C-5) began in June of 2003. Bedrock conditions proved unpredictable as excavation progressed to the north and south of cell C-5. Several deep bedrock trenches precluded excavation from progressing normally without using vertical or near-vertical slopes. Upstream retaining walls that were not originally included in the excavation design were required. Tied-back soldier-pile and lagging walls, similar to those on either side of the powerhouse, were built along the upstream berm-axis alignment of cells C-7 to C-8 and C-3 to C-1. Total length was approximately 950 feet with height as much as 30 feet.

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The final center cell (C-1) was excavated to competent bedrock in May

2004, thus ending the excavation program for the RCC berm. Photo 4 is a general photo of bedrock cells being excavated north of Saluda Hydro Powerhouse.

Photo 4

Representative Photo of cell excavation at Toe of Saluda Dam

Rock Cleaning and Dental Work

The bedrock foundation for the RCC berm was cleaned to provide acceptable conditions of contact between the body of the dam and its foundation and to provide for observation and documentation of details of foundation conditions at that interface. Exposure of potentially adverse conditions during cleanup provided the chance to undertake remedial activity.

Photos 5 and 6 display methods used during bedrock foundation cleaning after excavation.

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Photo 5

Pressure Washing Near Thrust Block in Cell C-5

Photo 6

Vacuuming Bedrock with Suction Hose in Cell C-6

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After excavation, all loose or objectionable (weathered) material was removed by handwork, barring, picking, brooming, water jetting, and/or air jetting. Accumulated water from washing operations was typically removed by strategically placed sump pumps. Loose or unsuitable material in cavities, shear zones, cracks, or seams were removed by hand as shown in Photo 7.

Photo 7

Material Removed by Hand

The rock surface and all pockets or depressions were carefully cleaned of soil and rock fragments before RCC or concrete could be placed, which required compressed-air/water cleaning and handwork.

In some cases, excavation uncovered faults, seams, deep shear zones, and shattered, fractured, weathered, or inferior rock extending to depths that were impractical to remove. Geologic discontinuities affected both the stability and the deformation modulus of the foundation. In these cases, dental concrete was used instead of RCC to fill deep faults, shears and fractures.

Dental concrete was also used to fill or shape holes, grooves, and extensive areas of vertical surfaces created by fractures, shear zones, buried channels, joints, and other irregularities. An example where dental concrete was placed in a

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deep weathered zone where a mafic intrusive body extended through the cell is seen on Photo 8.

Thin areas of dental concrete over rock projections on a jagged rock surface are likely places for concrete cracking and were avoided by using a sufficient thickness of dental concrete. The rock surface was thoroughly cleaned as described above and moistened prior to concrete placement to obtain a good bond between the concrete and the rock surface. When overhangs were filled with dental concrete, the concrete was well bonded to the upper surface of the overhang. Concrete was formed and placed so that the head of the concrete was higher than the upper surface of the overhang.

Photo 8

Dental Concrete Filling Extensive “Valley” in Cell C-6

Dental concrete was typically cured with water and operations were not permitted over the dental concrete until strength and temperature were tested and fell into the acceptable range as defined in the technical specifications.

4. ROCK BURSTS AND REACTION TO STRESS RELEASE

The metamorphic rocks in the dam foundation were crystallized at great pressures and temperatures. RIZZO geologists were able to document long 60 million year period of ductile deformation that stopped some 230 million years ago. From that time to approximately 100 million years ago, 10 to 15 kilometers of rock was eroded away, at which time Coastal Plain sediments were deposited on top of the erosion surface. These rocks still retain at least part of the stress associated with deep burial. When overburden was removed these rocks relaxed,

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resulting in fracturing. Most of these fractures were sub-horizontal. Most were one to two centimeters wide with some much wider. Commonly they were filled or partially filled with clay. This clay is likely the result of deposition of clay from turbid groundwater. And is evidence of intercommunication of surface water and groundwater in the old river bed.

These fractures are more common in the non-foliated pegmatites and intrusive rocks that the foliated schists and gneisses. Photo 9 shows an open fracture within a large pegmatite in cell C-7.

Photo 9

Pegmatite with Clay-Filled Fracture

These fractures were washed with high-pressure water and backfilled with dental concrete to the extent possible.

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5.0 SUMMARY

The design of an RCC berm in the central portion of the Saluda Remediation structure was dictated be the limited space between the existing Saluda Dam and the Hydro and Coal Fueled Power Stations. To provide an adequate foundation for the RCC berm required excavation to rock at the downstream toe of Saluda Dam. To accomplish that task the foundtion area was divided into Cells. The first cell to be excavated and the most challenging was Cell C-5 immediately upstream on the Saluda Hydro Powerhouse. The task involved anchoring an existing retaining wall, constructing new retain walls and placing mass concrete over the foundation rock to an elevation above the penstocks.

As the excavation moved to the north and south of Cell C-5 deep trenches and irregularities in the rock were encountered the construction of additional retaining walls. The walls allowed a vertical cut on the upstream edge of the excavation and provided an area where fill could be placed if need to stabilize the slopes of the existing dam.

The need to provide a sound foundation for the RCC berm required particular attention to the cleaning and preparation of the foundation rock. During the excavation every effort was made to remove all the week, weathered or otherwise unsuitable rock and to shape the rock for foundation preparation. Cleaning was accomplished with high-pressure air and water spray. Ponded water, sand and rock fragments were removed by hand and with large vacuum trucks. Trenches, cracks, joints and other irregularities were cleaned and filled with dental concrete. The large irregularities such as the trenches and the old riverbed required significant amounts of mass concrete and pumped concrete to level the foundation areas for RCC placement.