11/19/09

Aspect on the job in the North Cascades

Three Aspect field staff members spent much of the late summer and early fall logging rock core in the logistically challenging but beautiful North Cascades. The goal of the project was to characterize the Skagit Gneiss rock throughout the length of a proposed unlined deep bore tunnel which will parallel the existing 11,000-foot-long Seattle City Light Gorge power tunnel. The second tunnel will help to optimize efficiency of water transport between the Gorge Dam in Diablo and the Gorge Powerhouse in Newhalem, and thereby increase power output.

The team worked with Jacobs Associates and Crux Subsurface to complete 6 borings between 360 and 800 in length for a total footage of nearly 2800 feet of hard rock drilling, in demanding terrain including steep mountain slopes and dark underground tunnels.

Three of the borings required helicopter mobilization, in which drill rigs, core boxes, and safety equipment (but unfortunately not personnel) were transported to and from the project sites.

Field members’ morning commutes ranged from a 30 minute hike using ropes to negotiate particularly steep sections, to a thousand-foot rubber boot-laden splash into a dark tunnel.

A few of the particular challenges of the project included heavy bit wear and at-times slow drilling through the quartz-rich rock; encountering heavy artesian flow from horizontal borings and circulation loss in heavily fractured rock; providing continual water supply to a drill site half a mile away and 700 vertical feet above its tank; mobilizing a wheel-less drill rig by hand and in the dark nearly a quarter of a mile over uneven, puddle-filled ground (and carting 800-feet of delicate rock core out); supplying power, lighting, and air circulation to the back of the tunnel; and working long hours for ten-day stretches at a time.

Fortunately, the field staff enjoyed hearty meals and comfortable accommodations thanks to Seattle City Light, and were able to carve out a few evening hours to enjoy the splendor of the North Cascades. Fishing poles, bikes, bathing suits, running shoes, and climbing harnesses were all put to good use.

The fieldwork was the culmination of months of effort and years of planning by many parties, and the first step towards realizing Seattle City Light’s decades-long dream of optimizing power output at the Gorge Dam and Powerhouse.