4. FIELD VANE SHEAR TEST (VST) One of the objectives of this research is to correlate the high quality CRS

Transcription

1 4. FIELD VANE SHEAR TEST (VST) One of the objectives of this research is to correlate the high quality CRS laboratory results discussed in Section 3 with in situ methods so that the latter can be used in geotechnical evaluations. The field vane shear test (VST) is the most widely used method for estimation of the in-situ undrained shear strength of soft clays. In this report, the undrained shear strength of the Lake Bonneville clays was determined by using the VST and by a new method developed herein that uses the constant rate strain (CRS) consolidation test results Vane Shear Test Apparatus The VST tests were performed using the University of Utah s VST device manufactured by Geotech Inc. of Sweden. The VST device comes with an electrical control box for applying and recording the torque (torquemeter), a series of vanes and rod and a slip coupling (Figure 4.1). The extension rods are fed through the recording box to lower the vane to the appropriate test depth. The tapered vanes are of high quality nickelchromium steel and are specially designed to penetrate the soil with minimal disturbance. Their maximum measuring range is 100 kpa for 65 x 130 mm vane and 200 kpa for 50 x 110 mm vane. When testing, the extension rod is coupled with the recording box by locking mechanism and thereafter the box rotates the vane. 4-1

2 4.2. Procedure The vane testing was done in conjunction with hollow-stem auger drilling. The hollow stem was advanced to a depth approximately 0.3 m above the test interval. The vane was further advanced to the test depth by pushing the vane rods with the drill rig. For the testing, a rotation speed 0.1%/s was chosen, which is in the range suggested by ASTM standard D2573. The applied torque is measured with strain gauges, and the rotational angles are recorded every half of a degree. Once the yield point of the soil has been reached, the rotation of the vane is continued in order to characterize the soil s strength at very large strain (i.e., residual strength). In a second part of the test, the vane was released from the recording torquemeter and rotated clockwise ten times with a pipe wrench to completely remold the soil. As soon as thereafter, the torquemeter was once again locked onto the extension rods and the test was repeated to determine the soil s remolded undrained shear strength. To aid in the interpretation of the test, the shape of the curve can be seen from a lap top computer, which is connected to the torquemeter (Figure 4.2). 4-2

3 Figure 4.1 Geotech Vane Shear Test System (Torquemeter) Figure 4.2 Field Vane Shear Test 4-3

4 4.3. Field Vane Shear Test Results An appropriate sized vane was selected based on the anticipated peak undrained shear strength for the soils encountered at the site. A large vane (65x130 mm) was used for the upper Lake Bonneville Clay and small vane (50x110 mm) was used for lower Bonneville Clay, which are little stiffer than the upper zone. Typical undisturbed and remolded vane shear test results from North Temple research site can be seen in Figures 4.3 and 4.4, respectively. Note that the first 23 degrees of rotation shown in the Figure 4.3 is due to rod friction only, before the vane has started to shear the soil. (The couple made by Geotech is intentionally made with a certain amount of rotational play so that the rod friction can be measured independently of the torque required to shear the soil. This is done so that the rod friction can be subtracted from the total torque to determine the part contributed by the soil s shear strength). North Temple Site Vane Shear Test Result Undisturbed Tapered Vane Size: 13.0x6.5 cm Test Elevation: m Peak Torque (Nm) Rod friction prior to engaging vane Rotation Angle (Degrees) Figure 4.3 VST undisturbed shear strength results for N. Temple m 4-4

5 North Temple Site Vane Shear Test Result Remolded Tapered Vane Size: 13.0x6.5 cm Test Elevation: m 18 Peak Torque (Nm) Friction Rotation Angle (Degrees) Figure 4.4 VST remolded shear strength results for N. Temple m As shown by the Figures 4.3 and 4.4, the amount of torque required to turn the rods is first registered, then the total torque applied to both the rods and the vane until failure is registered. The difference between these values was used to calculate the shear strength of the soil. VST results were summarized in Table 4.1 and Figure 4.5. The complete set of VST test curves are presented in Appendix F. 4-5

6 Table 4.1 Summary of VST tests Test Vane Research Depth Elevation su sur S Comments # Size Site mmxmm m m kn/m2 kn/m2 su/sur 11&12 65x130 N. Temple Insensitive 13&14 65x130 N. Temple Insensitive 15&15a 65x130 N. Temple Insensitive 16&17 65x130 N. Temple Insensitive 18&19 65x130 N. Temple Insensitive 20&21 50x110 N. Temple Insensitive 22&23 50x110 N. Temple Insensitive 24&25 50x110 N. Temple Insensitive 26&27 50x110 N. Temple Insensitive 28&29 65x130 S. Temple Insensitive 30&31 65x130 S. Temple Insensitive 32&33 65x130 S. Temple Insensitive 34&35 65x130 S. Temple Insensitive 36&37 65x130 S. Temple Insensitive 38&39 50x110 S. Temple Insensitive 40&41 50x110 S. Temple Insensitive 42&43 50x110 S. Temple Insensitive 8&9 65x130 S. Temple Embankment * x130 S. Temp. Embankment * ** su = Peak undisturbed undrained shear strength, sur = Peak remolded undrained shear strength, S= Sensitivity * Still increasing, ** No remolded done 4-6

7 Field Vane Shear Test Results Undisturbed Undrained Shear Strength vs. Elevation S. Temple N. Temple Field Vane Shear Test Results Remolded Undrained Shear Strength vs. Elevation S. Temple N. Temple Field Vane Shear Test Results Sensitivity vs. Elevation S. Temple N. Temple Elevation (meters) Elevation (meters) Elevation (meters) Undisturbed Shear Strength (kpa) Remolded Undrained Shear Strength (kpa) Sensitivity Figure 4.5 VST Results 4-7

8 4.4 Sensitivity and undrained shear strength from CRS tests In determining of the consolidation properties of the Lake Bonneville Clays, a total of 42 CRS consolidation tests were conducted for the three different research sites as explained in Chapter 3. Typical void ratio, e, versus effective stress,, curves can be seen in Figure 3.14 and in Appendix D. As seen from Figure 3.14, the normally consolidated portion of the curve is relatively linear on a semi log plot. However, some of the curves shown in Figures 4.6 or 3.9 have departed from linear behavior. They show a more S-shaped behavior, with an inflection point. Out of 42 CRS e vs. curves, 20 of them behaved like that of Figure 4.6. This departure from linear behavior has been attributed to the sensitivity of the tested clay. (Clays that are more sensitive have a higher degree of curvature in the virgin part of the consolidation curve). ' v ' v 2.1 Void ratio vs. Effective Stress North Temple B3 Sample Elevation: m Strain Rate: inch / min = 0.54 %/h 2 p'= kpa vo'=92.87 kpa 1.9 vd'= kpa Void Ratio, e Effective Stress, 'v (kpa) ' Figure 4.6 e vs. v curve for N. Temple m 4-8

9 The following variables have been used in Figure 4.6: vd ' = Effective stress where the linear behavior departed p ' = Effective preconsolidation stress vo ' = Effective overburden stress This research proposes a new interpretation method for finding the sensitivity and undrained shear strength from the CRS e vs. curves. The sensitivity from CRS results is defined as: ' v S CRS vd ' 4.1 ' p and undrained shear strength is: s u CRS vo' 4.2 S CRS To use this method, first S CRS is calculated using Equation 4.1 and the CRS consolidation curve. Then the peak undrained shear strength is calculated using Equation 4.2 and the effective overburden stress. As an example calculation for the CRS test conducted in North Temple site at the elevation of m, vd ' equals kpa ' vo equals kpa and ' equals kpa. Using Equation. 4.1 produces a p sensitivity of S and an undrained shear strength of CRS s kpa. The calculation of the undrained shear strength from CRS u consolidation tests are summarized in Table

10 Table 4.2 Determination of Undrained Shear Strength Properties from CRS tests CRS Test Test vo ' Elevation Site ' ' vy vd S CRS s u CRS kpa kpa kpa kpa N. Temple N. Temple N. Temple N. Temple N. Temple N. Temple N. Temple N. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple S. Temple

11 The method described in Equations 4.1 and 4.2 was validated by comparing the CRS results with those obtained from VST. Comparison of the s u and S from VST and CRS methods for the North Temple and South Temple sites can be seen on Figure 4.6 and 4.7, respectively. As seen in these figures, the s u and S values from the proposed CRS method are reasonably similar to field measured VST values. Thus, the proposed method appears to produce reasonable estimates of the VST shear strength using CRS test results. It is recommended that this method be applied at other sites to evaluate if the method can be more generally applied to other Lake Bonneville sites and to other clay deposits. If the method appears to be valid, then it will be possible to estimate s u from CRS tests, which could be a considerable cost savings for some geotechnical evaluations. 4-11

12 North Temple Site Field Vane Shear Test Results Undisturbed Undrained Shear Strength vs. Elevation North Temple Site Field Vane Shear Test Results Sensitivity vs. Elevation VST CRS Method VST CRS Method Elevation (meters) Elevation (meters) Undisturbed Shear Strength (kpa) Sensitivity Figure 4.7 Comparison of the CRS Method and VST shear test results for N. Temple Site 4-12

13 South Temple Site Field Vane Shear Test Results Undisturbed Undrained Shear Strength vs. Elevation South Temple Site Field Vane Shear Test Results Sensitivity vs. Elevation VST CRS Method VST CRS Method Elevation (meters) Elevation (meters) Undisturbed Shear Strength (kpa) Sensitivity Figure 4.8 Comparison of the CRS Method and VST shear test results for S. Temple Site 4-13

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