John Day Lock Instrumentation & Anchor Testing
As part of a comprehensive study into structural cracking experienced at the John Day Dam Navigation Lock, Cornforth Consultants (CCI) was retained by the Portland District Corps of Engineers to investigate, measure, and test water levels and navigation lock structural anchors. The first task advanced several borings into the structure. The objective of the borings was to locate and characterize several structural cracks identified earlier with geophysical methods.
A second objective was to recover large (6-inch diameter) core samples of concrete for lab testing. Six borings were advanced vertically through the Navigation Lock and into the bedrock foundation. Boring depths ranged from 180 to 220 feet. In addition to the large diameter borings, six HQ-sized core holes were advanced horizontally from inside the dewatered Navigation Lock. Horizontal borings were advanced 40 feet from a crane-supported drill platform. Hydrogeological testing with double packer systems and geophysical testing (including full waveform sonic, compensated density, single arm caliper, and natural gamma) were completed in each boring. Laboratory testing was completed on large diameter concrete core samples. Tests included unconfined compression, split tension, modulus of elasticity, Poisson's ratio, direct tension, and wet density.
Adjacent to the lock and downstream of the dam, five additional borings were advanced to investigate the lock foundation and hydrological characteristics. Nested piezometers consisting of vibrating wire piezometers (VWPs) and standpipe piezometers were installed in all five borings. Three angled borings were advanced into the toe of the Navigation Lock structure. Two VWPs were installed in each boring to monitor piezometric levels near the north filling/emptying culvert and near the concrete/bedrock contact during lock cycles. Two vertical core holes were advanced through the Navigation Lock and into the bedrock. The holes were located approximately four feet from the face of the Navigation Lock. A VWP was attached to slope indicator casing and grouted into the borings. Tiltmeter arrays were installed inside the slope indicator casings to monitor deflections of the monolith during lock cycles. Two additional borings were advanced to install 6-anchor Multi Position Borehole Extensometers (MPBX) to monitor deflections of the different anchors during lock cycles.
Cornforth Consultants also performed lift off tests of post-tensioned rock anchors located on the south side of the Navigation Lock. In 1981, 73 anchors were drilled and installed at angles between 55 and 65 degrees from horizontal. Thirty-seven strand anchors have 30-foot bond zones and total lengths between 137 and 174 feet. All anchors were originally tested to 1,700 kips and locked off at 1,500 kips. CCI was retained to perform lift off tests on random anchors to determine the current load locked into the anchors.
When the anchor head covers were opened to perform the lift off tests, several anchors were found to be damaged. Damage generally consisted of corrosion, broken strands, broken wedges, and seepage exiting the anchor head. The Corps modified the task order to include a detailed inspection and inventory of every anchor head. CCI completed lift off tests on five individual anchors. The lift off tests showed that undamaged anchors were carrying 90 percent of the lock off load. Follow-up lift-off tests were completed on fifteen anchors to compare known performance benchmarks and establish a functional life span for the anchors.