The geotechnical properties and behaviour of Carboniferous shale at the Balderhead Dam

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Abstract
Summary: The Balderhead Dam was constructed for the Tees Valley and Cleveland Water Board between 1961 and 1965, on the River Balder which is a tributary of the Tees. The dam is 157 ft high, 3 000 ft in length and is of rolled fill construction, comprising a central core of 222 000 cu yd of boulder clay and 2 508 000 cu yd of shale fill forming the shoulders. The fill materials were wholly obtained from within the reservoir basin. Shale is a notoriously unpredictable engineering material and, as shale had not been used previously in the United Kingdom for an embankment of this magnitude, considerable investigations and testing were carried out both prior to, and during, construction. The bedrock in the Balder valley comprises part of the Yoredale sequence of the Cotherstone syncline and includes dominant shales with thin sandstones, limestones and coal seams. The shale materials disintegrate on weathering and decayed, uncemented shales occur immediately below the thin cover of superficial clays. The preliminary investigations included a trial bank at the adjacent Selset reservoir, a trial pit in the Burnhope Dam, built 25 years previously, and the examination and testing of relevant shale samples from the area. From these various investigations, the design properties of the shale fill were taken as: c ′ = 0 φ ′ = 33° Coefficient of consolidation C υ = 800 sq ft/month Coefficient of permeability K = 2 × 10 −6 cm/s Density γ = 135 lb/cu ft It was assumed that, over a long period of time, the finegrained shales could disintegrate and eventually assume the properties of clays. For this reason, the design approach was programmed to develop a rational design, to be backed by continued investigation and observation during construction so that the basic assumptions could be checked and appropriate amendments carried out. Provision was made in the design for the more weathered or disintegrated shales, occurring directly below the superficial cover, to be placed in a zone immediately upstream of the clay core and the design allowed for construction pore pressures to be set up in this zone. A 5 ft wide filter drain on the downstream side of the clay core prevents seepage into the fill. During the first season of construction a poor zone of brecciated shale, resulting from near-surface, glacial disturbance of the shale forming the valley-sides, was encountered in the borrow pit. This shale was placed in a special downstream zone and subsequently pore pressures developed within this material. Further investigations, testing and analysis were carried out at the end of the first season. Apart from routine control tests, a special study was made of the in situ density and permeability of the shale fill from trial pits. It was apparent, from these observations, that the compaction which had been carried out with a grid roller could be improved and tests were undertaken with a 8½ ton vibrating roller, which gave better results and was used throughout the next two seasons. The instrumentation of the embankment included settlement gauges and piezometers in the shale fill. The embankment fill was finished in 1964 and the reservoir completed and filled in 1965.