Boring Log - Water Table
Some boring logs only show one inverted triangle indicating the depth of the water table, and some other logs show two triangles: one indicating the 'initial' depth and another one indicating the 'after 24 hours (or sometime after 15 minutes)' depth.
If a problem asks for the water level depth, and the boring logs shows the two 'triangles', which one should we pick? I'd assume we would pick the one 'after 24 hours', but I'd like to confirm. Thanks.
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If there's a white triangle and a black triangle, use the black one. Otherwise, work with the shallower triangle value.
Why have two triangles on the same graphic?
Water table depths vary over time naturally, even within the same day. . . so two triangles may represent a typical range
The initial depth of the water table may be misleading--an artifact of the drilling process itself. . . sometimes technicians wait ten minutes after drilling a boring hole and take another reading, or drain the hole altogether, then wait a day for it to refill, then put both readings on the same boring log. The white triangle is the initial (not-as-accurate depth) and the black triangle is the "real" depth.
Water table depths vary by season as precipitation rates do, so . . . one triangle for each of two seasons
The site may have multiple aquifers at different depths, with an impermeable clay unsaturated zone between them. . . one triangle for each aquifer
We may be using borings to monitor groundwater levels in one area as we pump out ground water in another nearby site. "You can pump out the water from your construction site excavation pit, but monitor the groundwater near my existing adjacent buildings so you don't accidentally render their foundations unstable by pumping too much water out."
For building design purposes, if it's unclear which is the "real" reading, you'll probably use the shallower reading. If you want your basement floor to sit above the water table, take a conservative approach because leaking sometimes is still leaking.
For an example, visit https://i.sstatic.net/69cLL.png . . . Hot links don't work for that URL, so you'll need to copy-paste it into your browser.
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