Monotonic and dilatory excess pore water dissipations in silt following CPTU at variable penetration rate

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1 Monotonic and dilatory excess pore water dissipations in silt following CPTU at variable penetration rate P. Paniagua, R. Carroll, J. S. L'Heureux Norwegian Geotechnical Institute, Trondheim & Oslo, Norway S. Nordal Norwegian University of Science and Technology, Trondheim, Norway 5 th International Conference on Geotechnical and Geophysical Site Characterization ISC

2 Background Contractive Dilative behaviour following dissipation tests in silty soils Samples in study intact and reconstituted silts Characteristics of depth interval Dissipation test data collected Methods to analyse dissipation tests Outcome: Effect of rate on t 50 Outcome: Evaluation of methods used for t 50 analysis

3 Scope of work for study Laboratory conditions Vassfjellet silt, Norway reconstituted Test using miniature cone mm diameter (University of Colorado) Overburden stress 80 kpa during testing In situ conditions Halden, Norway intact Test using standard cone 35.7 mm diameter Assess the influence of the penetration rate on t 50 for further interpretation of c h.

4 Analysis of CPTU dissipation tests Monotonic application From shoulder pore water decay Teh and Houlsby (1991) Dilatory application From shoulder pore water decay Square root method with Teh and Houlsby (1991) Chai et al. (2012) Burns & Mayne (1998) Mantaras et al. (2010)

5 Characteristics of depth interval Vassfjellet and Halden Parameter Vassfjellet Halden silt at silt 6.5 m or 6-7 m Water content, w (%) Total unit weight, (kn/m³) Density of solids, s (kn/m³) Organic content, < 2% < 0.5% Plasticity Index, I p Non plastic 10.8% Friction angle, (o) c v * cm²/s k* at 0% strain m/s x10-8 CPTU 6 mm/s 20 mm/s q t (MPa) B q *Measured in CRS tests, k at 0% axial strain, c v at in situ effective vertical stress. Halden - SBT: Transitional or (3-4) Description Vassfjellet (%) Halden (%) Sand 7 20 Silt Clay

6 Characteristics of depth interval Vassfjellet and Halden Vassfjellet Variable rates 0.06 mm/s 6 mm/s 50 mm/s At slow, medium and fast rates q t MPa B q 0.004, 0.04, 0.01 F r 3, 1, 4 % Halden Variable rates 2 mm/s 20 mm/s (Std) 320 mm/s At 20 mm/s q t 1 MPa B q F r 1.3 %

7 Dissipation test data collected Vassfjellet silt (lab) Slow Medium Fast Velocity mm/s V (-) (vd/c h ) Response Monotonic Monotonic Dilation Final u on penetration kpa u max kpa *c v at effective vertical stress cm 2 /s

8 Dissipation test data collected Halden (in situ) Slow Standard Fast Velocity mm/s V (-) (vd/c h ) Response Monotonic Dilation Dilation Final u on penetration kpa u max kpa *c v at in situ effective vertical stress cm 2 /s

9 CPTU dissipation test results: Vassfjellet silt Burns & Mayne (1998) Burns & Mayne (1998) All rates Medium rate Fast rate Teh & Houlsby (1991) Mantaras et al. (2014) Mantaras et al. (2014) All rates Medium rate Fast rate

10 CPTU dissipation test results: Halden silt All rates Burns & Mayne (1998) Medium rate Burns & Mayne (1998) Fast rate Teh & Houlsby (1991) Mantaras et al. (2014) Mantaras et al. (2014) All rates Medium rate Fast rate

11 Challenges Uncertainties associated with dissipation test Identification of u 0 Determination of u i in tests with dilation Partial drainage Assumed fully undrained in analysis Disturbance effect Anisotropy of soil Soil rigidity index (I r )

12 Outcome: Rate effect on t 50 for Vassfjellet Fastest penetration resulted in longest t 50 time for most methods and overall mean value widest band of t 50 time using all methods associated with dilation Slowest rate resulted narrowest band of t 50 time using all methods associated with monotonic decay Velocity (mm/s) V ( ) (vd/ch) Mean t 50 (sec) Standard deviation of t 50 (sec) Values based on t 50 results from all methods

13 Outcome: Rate effect on t 50 for Halden Fastest penetration resulted in shortest t 50 time for all methods and overall mean value narrow band of t 50 time using all methods associated with dilation Slowest rate resulted longest t 50 time for most methods and mean value associated with monotonic decay Velocity (mm/s) V ( ) (vd/ch) Mean t 50 (sec) Standard deviation of t 50 (sec) Vaules based on t 50 results from all methods

14 Outcome: Evaluation of methods used for Vassfjellet 1. From shoulder pore water decay unrealistic basis of analysis for dilatory dissipation 2. Burns and Mayne (1998) parameter input not representative of soil and some unknows 3. Chai et al. (2012) simple but shortest t 50 values 4. Mantaras et al. (2010) simple but longest t 50 values 5. Teh and Houlsby et al. (1991) with sq root time correction reasonable results

15 Outcome: Evaluation of methods used for Halden 1. From shoulder pore water decay unrealistic basis of analysis 2. Burns and Mayne (1998) parameter input not representative of soil and some unknows, gave longest t 50 values. 3. Chai et al. (2012) simple to use, widest range of t 50 contrasting evaluation 4. Mantaras et al. (2010) simple to use, reasonable t 50 values 5. Teh and Houlsby et al. (1991) with sq root time correction reasonable results overall

16 Conclusions Vassfjellet: c v cm²/s and c h ~ cm 2 /s Halden: c v cm 2 /s and c h ~ cm 2 /s Dilation response: Variation in t 50 and more difficult to interpret with high dilatory response Identification of soil properties and site characterisitcs are critical Partial drainage associated for some tests can apply correction Future work: Further testing in uniform silty material ongoing Evaluation of additional data with existing methods Comparison with laboratory results

17 NORWEGIAN GEOTECHNICAL INSTITUTE