A firm foundation11 May 1998
Eastside, in California, will be the largest earth and rock filled dam in US history. The foundations had to be laid with care
If there are problems with the base of a dam, water seepage can cause great structural damage and people could lose their lives. That is why such care is being taken with the foundations of the three dams that will contain the new Eastside reservoir. The reservoir will set records: it will become Southern California’s largest surface water storage reservoir and will be the nation’s largest earth and rock filled dam.
To stabilise the rock strata under the west dam, the highest of the three dams that make up the Eastside project, grouting holes are being drilled to an exceptional 200ft (61m) deep and at inclinations from vertical to 70° off vertical. Drill accuracy is essential when drilling to such depths, to reduce deviation to a minimum and ensure there is an intact grouting curtain.
The grouting subcontractor, Nicholson Rodio, is using Tamrock drill rigs to carry out the work, in the shape of six Tigers and two 660s. They are equipped with Sandvik T38 guide tubes, T38 rods and 2.5in (64mm) button bits. This equipment is said to have been able to achieve the very low levels of hole deviation required. The guide tube is fitted, instead of the the first steel of the string, and prevents deviation by creating a stiff joint between the drill bits and the steel.
The tubes are 56mm in diameter, compared with conventional 38mm drill steels, and it is this which is largely responsible for the increased stiffness. The increased diameter also speeds up the flow of the medium, which flushes the cuttings from the hole. The resulting accuracy, when using this tool arrangement, is a deviation of no more than 3% at depths of over 100ft (30.5M).
Holes of the depth being drilled at Eastside require a technique of drilling in steps of 25-30ft (7.5-9m). After drilling the first step, an expandable rubber packer tube is fed manually down the hole with a smaller breather tube. Grout is pumped down the packer and backs up the side of the hole, filling rock fissures as the level climbs. Air is expelled through the breather. Grout pressure registers on a gauge and shows when the hole is full.
Following this step the packer is withdrawn and the rig moves on to the next drilling site while the grout, which is cement based, is left for a week or so to cure. When the rig returns, it repeats the procedure and carries the hole down another step. The sequence may be repeated up to eight times to attain a hole of the required depth.
The plant manager for contractor Nicholson Rodio, which is working on the west dam, is Oscar Smaniotti. He is enthusiastic about both the equipment and the technique. ‘Having had this experience,’ he said, ‘I would not now drill a grouting hole without a Sandvik guide tube’. This is high praise, considering that Smaniotti is in the process of drilling 9000 holes, each with an average length of 130ft (40m) for the curtain grouting work. He is also drilling consolidation grouting holes 30-50ft (9-15m) deep, making a total of about one million feet (305,000m) of drilling.
The axial length of the west dam is over 8000ft (2700m) with additional grout work running1400ft (430m) in lateral adits associated with the dam structure. The prime contractor for the east dam appointed the Suhakar company as grouting contractor and Tamrock equipment is also being used to fulfil this contract.
The size of the job
The east dam is slightly longer than the west — 2.2miles, compared to the west dam’s 1.7miles — but it is only 185ft (56m) high. The third dam, the smaller saddle dam, will be 130ft (40m) above the lowest point on the ridgeline. The three dams will together enclose the Domenigoni and Diamond valleys, some four miles southwest of the city of Hemet, forming a reservoir of some 4500acres (1800ha) in area, varying in depth from 160ft to 260ft (49m to 79m).
The reservoir will be supplied with water from the Colorado river from the east, via the San Diego canal and a new inland feeder canal from the north. It is slated to be completed in 1999 and filled by 2003, doubling Southern California’s surface water storage capacity and guaranteeing a six-month supply of emergency water in times of drought.
The reservoir will be surrounded by a 9000acre (3645ha) reserve. Some US$10M will be spent on habitat for the endangered Stephens kangaroo rat; a special reserve will be home to the threatened gnat-catcher songbird; and 16 other bird, plant and animal species will be protected. Included in the plan is US$100M to develop outdoor recreation facilities including several marinas, fishing, a golf course, campsites and recreational trails.