Liquefaction - The Shaking and Sinking Earthcache
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This earthcache is designed to explore the geological effect of soil liquefaction, a common and potentially disastrous type of soil subsidence. The reason for placing the earthcache upon this dam will be clear as you learn about liquefaction and the construction of the dam.
This earthcache is wheelchair accessible.
Soil liquefaction, in its simplest sense, is a property of loose, sandy soils that are fully saturated or partially saturated with water to begin to act like a liquid under the presence of shaking or rapid pressure changes. Essentially, soils that are loosely packed contain many spaces between the individual grains of sand, and in saturated environments these spaces are filled by water. Under normal conditions the sand mixture maintains cohesion (friction between the individual grains 'holds' the grains in place) and therefore a solid surface is maintained. However when intense pressure changes and/or rapid, repetitive shaking stresses are applied (as during an earthquake), the loose soil grains tend to compress and loose cohesion and the pressure of the water rises dramatically and pushes the soil grains further apart. The water attempts to 'escape' from the higher pressure area under the ground to the lower pressure area above the ground, and water and sand will travel through weaker areas or cracks in the soil and then 'boil' out of the ground. This results in the ground beneath you transforming from a solid to a liquid state, and any structures built in the area may suffer from sinkage into the ground. At angular soil locations (such as river and lake banks) the liquefied soil tends to slide into the water as lateral spreading, thus weakening anything built upon or held back by the banks. Highly compressed and compacted soils, unsaturated loose soils, and areas that are largely filled with rocks, are not subject to liquefaction.
You may have experienced a mild form of soil liquefaction in your past. Imagine that you are walking upon a sandy beach on the coast of South Carolina. As you step upon the saturated soil of the beach right at the water line, you may notice that water and sand beneath your feet seems to bubble up from around your foot and between your toes. You are applying pressure upon a saturated, sandy soil and creating soil liquefaction beneath your feet! If you stand there long enough you may even notice that you are sinking into the sand.
South Carolina, Earthquakes and the Saluda Dam
On August 31st, 1886, what is now estimated to have been a magnitude 7.3 earthquake occurred near Summerville, SC (north of Charleston). The earthquake did massive damage to the coastal area of the state, but also caused damage hundreds of miles away and the earthquake itself was felt as far away as Boston. Earthquakes are very common in South Carolina but generally average closer to 2.0-3.0 in magnitude.
This earthquake has since become a benchmark moment for South Carolina’s construction and engineering fields, and many of the structures built throughout the state have been designed with an eye on that event. This includes the dam upon which you now stand, which was designed and built around 1930.
The Saluda Dam was built primarily for production of electrical power, a role that has now been greatly reduced in favor of maintaining the Lake Murray recreational area. According to ‘Saluda Dam, Then and Now” (by Elena Sossenkina, Scott Newhouse and Samuel Moxley) the dam was “a semi-hydraulic fill structure with no internal seepage control (filter or drain protection) and essentially no seepage cut off.” The dam was built in three sections – an upstream section, a downstream section, and a void in between. The void was filled with water and then 5 scows floated across the surface of the water. These scows contained enormous pumps and nozzles and sprayed the water with wet soil that was designed to settle down and condense, filling the void. The Saluda Dam was completed without regard to the saturation level of the fill dirt used to construct it, as was the practice of the engineers of the time (and with their then current knowledge of seismology). “When completed in 1930, the Dam was 208 ft high and nearly a mile and a half long. The maximum width of the Dam at the bottom is 1,150 ft and crest width is 25 ft.” (Saluda Dam, Then and Now)
As you can see, the Saluda Dam was built of loose, sandy soil material that was hydraulically filled into the dam, and which has been consistently saturated with water due to dam’s creation and impounding of Lake Murray. For many years the dam was considered to be able to withstand an earthquake similar to the 1886 Charleston earthquake, but as knowledge about earthquakes and the liquefaction events that occurred grew, the dam’s ability to withstand these affects came into question. The dam is only about 10 miles from downtown Columbia, and the failure of the dam due to a seismic event could severely affect 120,000 people downstream.
So the challenge of this earthcache is to think like an engineer, and to determine what measures have been taken to mitigate (reduce) the threat of liquefaction causing a failure of the earthen dam upon which you stand.
Requirements
To get credit for this earthcache, you must walk the length of the dam and determine what features of the dam you can identify that are designed to reduce the threat of liquefaction and the potential danger to the downstream populace. Make visual observations only - no collection of items or special entries into restricted areas are required. I have been able to identify at least 8 features (there may be more) and I would like you to identify at least 4 and then email your answer to me through the Geocaching website.
While it is not required to attach photos for credit, this earthcache is at a very picturesque location and you would be a poor cacher indeed if you did not take the opportunity to attach a lovely picture to your log post.
There are also some other close by caches and a benchmark or two that you can discover nearby.
BittenApple also wishes to acknowledge and thank 2 SCANA employees, Matthew Bebber and Jeffrey Wider, who were conducting surveys of the earthern dam on October 3, 2013 when the CO visited. They enabled the CO to review their work and look at their equipment, as well as discussing some of the features to be identified. Thanks again guys!
Additional Hints
(Decrypt)
Uvagf sbe vqragvslvat srngherf:
1. Nal creznarag vzcebirzrag gung ervasbeprf be fhofgnagvnyyl vzcebirf gur fgehpgher bs gur rnegura qnz pbhyq uryc zvgvtngr yvdhrsnpgvba naq gur erfhygvat yngreny fcernqvat. Nyfb erzrzore jung vf arprffnel (trbybtvpnyyl) sbe yvdhrsnpgvba gb bpphe naq gura ubj pbhyq lbh jbex gb erqhpr be erzbir bar bs gur snpgbef.
2. Na vzcbegnag cneg bs zvgvtngvat yvdhrsnpgvba vf tngurevat xabjyrqtr nobhg gur cebprffrf npgvat ba gur qnz – ubj jbhyq bar qrgrezvar gur rssrpgf bs gurfr cebprffrf? (Gur pbbeqvangrf nobir fubhyq oevat lbh pybfr gb na nafjre, nf jvyy gur uvag cubgb va gur tnyyrel.)
3. Abg ivfvoyr qhevat gur jnyx: Guvf pbzcyrk nyybjf crbcyr gb qevir qbja vagb n ibvq oruvaq gur rnegura qnz (juvyr urnqvat gb Vezb). Nf lbh qevir nybat gur qbjafgernz fvqr bs gur qnz lbh jvyy frr yvggyr fgngvbaf jvgu znexref – nsgre erivrjvat gur bevtvany qnz qrfpevcgvba nobir jung qb lbh guvax gurfr znexref ner? QB ABG FGBC NALJURER NYBAT GUR UVTUJNL - WHFG NA 'RQHPNGRQ THRFF' JVYY QB.
4. Vs nyy ryfr snvyf, gnxr n cubgb bs gur ynxr, naq gura ghea nebhaq naq ybbx ng gur “onpxhc” guvat oruvaq lbh. Vg vf n $275 zvyyvba qbyyne vafhenapr cbyvpl.