Difficult Site Access 2014Marvin Davis Difficult Site Access

MDA has made a specialty of working on hard-to-access Lake Tahoe project sites. Many sites are inaccessible to site exploration in winter, both due to snow cover and Tahoe Regional Planning Agency restrictions on grading between October 15 and May 1 of each year. For typical residential sites under these conditions, we may do preliminary evaluations based on ReMi shear-wave velocity surveys and dynamic cone penetrometer; some sites we have worked on defy conventional exploration even in the summer and have required the use of similar methods.

 Above is a project on the north shore – a guest house at lake level where the nearest road is 250 feet elevation above the building site. As you can see, the site surface is steep (1.5H:1V), uneven, and inaccessible to conventional exploration equipment. Temporary road construction was not an option. Some of our access was by barge. Geotechnical exploration consisted of dynamic cone penetrometer probes (DCP) and shear-wave velocity (SWV) surveys. The entire slope is plated with large boulders (4 to 12 feet diameter), however as seen in the following profile using both DCP and Remi SWV, despite boulders there is about 10 feet of loose sandy material indicated by SWV less than 1000 ft/sec, then about 30 feet of moderate SWV soil transition to actual bedrock. When the project was excavated, the depth to bedrock was 20 to 30 feet below grade. DCP penetration varied from about 3 feet (refusal on boulders) to 15 feet below grade.Marvin Davis Difficult Site Access2

Borings and probes have limitations in cobble and boulder soils due to refusal on shallow cobbles or boulders, as seen for the subsurface profile DCP on the shear wave velocity above. From borings alone, one may estimate the top of rock too high. Contrary to this, SWV surveys measure the slowest shear wave velocity (e.g. the inverted-weight average) or weakest materials in the profile, so that soil-filled voids between rocks or an abundance of weathered joints will likely show a deeper depth to “bedrock consistency.” This is important for anchoring structures in rock, but may overestimate the depth for which normal equipment can be used for excavation (variable depending on joint spacing). SWV will likely not represent the occurrence of fractured bedrock or boulders where they are present in a looser or weaker matrix. Occasional boulders, such as observed at the site surface, are likely to be present even in deposits with lower shear wave velocity.Marvin Davis Difficult Site Access0

The following is a small exploration we did in the north shore where a designer wanted to know if the existing footings would support an additional floor, and he needed bearing capacity in the range of 5,000 psf. The Remi survey in the back yard under the deck (again, no other way to get conventional drilling equipment on the site) allowed a measurement of approximate density and confirmed that fill was possibly only 2 to 3 feet depth (depth to top of footings) and dense soil was present underneath, and gave us confidence to recommend this high bearing for the existing conventional narrow strip footings. Marvin Davis Difficult Site Access3

Lastly, here is one of staff engineers, Federico, taking a DCP on a landslide/bluff failure in 2015. We had drilled borings in 2006 at the top of this slope, but for final design we needed new samples and data to determine depth of loose debris for the landslide slope, requiring us taking both tube samples and DCP manually.

Marvin Davis Difficult Site Access1

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