Field Testing

Field Operation of TDR/DMT

In this study the TDR penetrometer waveguide was fitted immediately behind the DMT blade as shown in Fig. 6-4. The DMT electric/pneumatic tubing passed through the inside of the hollow TDR penetrometer waveguide. The TDR/DMT probe was attached to 90 m long A rods. The A rods had a total weight of approximately 900 kg, enough to offset the buoyancy and provide reaction force to penetrate the TDR/DMT probe 10 m into the sediment. A portable drill rig mounted on a barge was used to hold the drill rods from the water surface as shown in Fig. 6-5. The DMT tubing along with the TDR co-axial cable were threaded to the outside of the A rods through an adaptor and then connected to their respective control unit on the barge. The function of the drill rig was to hang the drill rods and passively let them be lowered instead of pushing the drill rods. Thus, the arrangement should avoid the potential

reference point on the dam crest was determined with a total station. The barge was fixed to a rather massive dredging boat which was in turn fixed to the shore with cables. All drainage tunnels of the reservoir were shut down during TDR/DMT tests to prevent fluctuation of the water surface elevation. With these arrangements, the barge vertical movement during a single DMT is expected to be less than 30 mm.

The water surface was at an elevation of 244 m at the time of field testing. A total of 10 profiles were conducted, five of them used the TDR/DMT probe (numbered TDR/DMT-1 to TDR/DMT-5), and the other five profiles used DMT only (numbered DMT-1 to DMT-5). Fig.

6-6 presents a location diagram of all the DMT and TDR/DMT operations. In plan view and at water surface level, the test locations were at 50 m to as much as 130 m from the shore line. The power plant inlet was located on the surface of a natural rock formation with a slope of approximately 2 (vertical):1 (horizontal). The DMT readings started at elevation 185 m, TDR tests began at elevation 235 m, all tests ended at elevation 160 m. Thus, the bottom of the penetration could be as close as 10 m from the rock surface. The test interval varied from 5 m in clean water to 20 cm in dense sediment. This arrangement prevented any possibility of water leakage and provided an opportunity to calibrate the DMT po readings against the hydrostatic pressure (uo) in clean water while lowering the DMT.

DMT

TDR

Derlin Stainless

DMT

TDR

Derlin Stainless

Fig. 6-4 (a) The photo of TDR/DMT probe and (b) the schematic illustration of TDR/DMT probe

(a)

(b)

Fig. 6-5 Operation of TDR/DMT from a barge

TDR/DMT-3

TDR/DMT-5 TDR/DMT-4

TDR/DMT-2

North DMT-5

DMT-4 DMT-3

DMT-2

scale 50m

DMT-1 Dam crest

TDR/DMT-1 inlet

Power plant PRO

Dam crest

Fig. 6-6 The test locations

Interpretation of Test Results

Fig. 6-7 shows a series of waveforms recorded in TDR/DMT-3, of reflection coefficient versus the sequential number of data points. At elevation 212.5 m, TDR was in clean water, the waveform at elevation 182.5 m indicated that the TDR had entered bottom mud. The depth or elevation of all the TDR and DMT was referred to the center of the DMT blade. The reflection coefficient towards the end of the record where the reading had reached a stable value was referred to as the terminal value, Vr, ∞. A laboratory calibration between Vr, ∞ and (σ-σw) at various sediment concentrations was conducted using the sediment and water collected from the test location. With the Vr, ∞ - (σ-σw) correlation and relationship between (σ-σw) and sediment concentration as shown in Fig. 6-3, the sediment concentration in terms of ppm is inferred from Vr, ∞. The solid concentration by volume (θs) and thus the density ratio of bottom mud over water (γt / γw) can then be calculated based on the specific gravity of the solid.

Fig. 6-8 shows the results from the interpretation of all the TDR readings. Except for TDR/DMT-1, the tests indicated a water/mud inter-face at elevation 183 m where solid concentration had a significant increase to 4 x 10⁵ ppm. At elevation 171 m, the γt / γw reached approximately 1.4. Below elevation 171 m, the TDR readings became unstable. This is likely due to the fact that the bottom mud had become solid below that elevation, and the inevitable waving of the barge caused disturbance or cavitations within the solid mud around the TDR waveguides.

The original plan of using the chart Marchetti and Crapps (1981) to determine the bottom mud density could not materialize as in most cases, po was very close to uo, and that resulted in unreason-able material index, ID. Thus, the interpretation of DMT results was mostly based on po and p1. In diluted bottom mud, where the strength was close to zero, po should represent the ambient total stress. Thus a comparison between the increase of po and that of hydrostatic

increase and the mud turned into solid, there should be significant differences between po and p1

and thus the ED values can be inferred. The results of DMT-1 to DMT-5, following the above concept are shown in Fig. 6-9. Significant differences between po and uo could not be identified until elevation 176 m, which was 7 m lower than the TDR prediction.

500 1000 1500 2000 2500 3000 3500 1400

1600 1800 2000 2200 2400 2600 2800 3000

No. of data points

Reflection coefficient

212.5m 182.5m 172.5m 163.5m

Fig. 6-7 TDR waveforms from TDR/DMT-3.

1E+004 1E+006 1E+008

Fig. 6-8 The interpreted TDR test results

500 600 700 800 900 P_o, kPa

500 600 700 800 900 P₁, kPa