Pressure/Tensiometer Sensors in an Engineered Dam Failure Experiment
Don Baker, former Postdoctoral Associate
USDA-ARS Hydraulics Engineering Research Lab Stillwater, OK



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Pressure/Tensiometer Sensors in an Engineered Dam Failure Experiment Don Baker, Postdoctoral Associate USDA-ARS Hydraulics Engineering Research Lab Stillwater, OK

In an overtopping experiment an earthen dam is built and then caused to fail, usually by the erosion caused by water passing over unprotected soil.

Since this kind of failure happens with working dams that have had reservoirs full for some time, the experimental reservoir is kept partially filled for some days before the experiment.

The phreatic surface

SEEP2D Phreatic Surface Solution

Measure Phreatic Surface with Peizometer/tensiometer

Peizometer-Tensiometer Tube

Design for Measuring Bulk Conductivity

Building an Embankment in a Flume

Mark the height of the next lift, 6 inches

Dump dirt in the flume

Level the dirt

Roto-till the clumps out

Fine level

Two passes with the vibratory compactor

Compact the edges

Recheck lift height Get ready for another cycle

Auger a 2 1/4 inch hole

Tamp in 1 inch dry soil to protect the porous cup from the concrete floor

Add about 4 to 6 inches of mud

Insert tube and twist into mud

Tamp in several inches of dry soil to avoid bentonite contamination near the porous cup

Fill and tamp the rest of the hole with a mixture of soil and 30% bentonite

Clean up bentonite mix, avoiding conductivity changes in inflow or outflow

Shave inflow and outflow surfaces off flat

Install soil retention screen

Install weir to set inflow head at 4.6 feet

Gravel pack between screen and weir to hold inflow pipe and soil

Outflow screen and 6-inch v-notch weir

Bows added against wind

First flow over the inflow weir

Initial tensiometer responses to inflow head

Outflow weir w/ level sensor

Outflow weir

Diurnal variation in sensor readings

Diurnal variation in sensor readings

Sensor 1&3 readings w/o sun shield

Sensor 1&3 readings w/ sensor 1 sun shield

Reduced diurnal variation in Sensor 1

Stove pipe sun shield on sensor 1

Post mortem shows that the probe had good hydraulic contact with the soil and the bentonite mix formed a good seal against vertical flow above it.

Manometer tube sensor calibrations in tower

Two 6 psi and four 15 psi sensors on manometer sight tube

Sensors and instrument shack installed at dam

South sensors, 27S, 19S, 15S and 9S, one day after 4.5 foot fill of reservoir

Sensor response to filling of reservoir

Sensor responses to filling of reservoir

SEEP2D solution for phreatic surface (blue) and (uncalibrated) estimates of phreatic surface from sensor readings just before the reservoir was raised to overtopping, about 12 days after the initial flood.

Sensor response to overtopping and failure

Sensor response to overtopping and failure

Camera 2 Time Lapse

0830 hrs: Overtopping begins, note wet dirt lower down where grass is stripped off

0836: Exposed dirt is eroding

0851: Head cut is wider

0918: Head cut is wider yet

0957: Head cut is about 1/3 overtopping width

1012: Mass failure on the right

1030: Head cut more than half width of overtopping flow

1103: Head cut is full width of overtopping flow

1257: More mass failure on the right

1351: Reservior drops as breach jet begins on right about 0.7 from left to right

1354: Mass failure on right undermines I-beam pad, breach jet stronger, tailwater higher

1357: Breach jet bigger, reservoir lower, tailwater higher

1453: Near the end, breach jet smaller, reservoir lower, tailwater lower

Dam safety officials cheering the mass failures

Ours are volunteers

Author: Donald L. Baker