VI. UNDRAINED BEHAVIOR AND LIQUEFACTION
6.6 Colloidal Silica (LUDOX SM-30) Treated Specimens 112
6.6.1.1 Experiments
A total of 2 cyclic tests were conducted to evaluate the effect of electro-osmotic injection of LUDOX SM-30 colloidal silica grout mix on the soil resistance to liquefaction (Table 6.6). A summary of the test parameters is shown in Table 6.7.
The undrained cyclic strength of grouted soil was evaluated in terms of the number of cycles and energy required to reach +/- 5% axial strain.
Table 6.6 Undrained Cyclic Triaxial Tests on LUDOX SM-30 Colloidal Silica Treated Specimens Test # Electric
Current
Anode Fluid (1st shot)
Anode Fluid (2nd shot)
Cathode Fluid
Final e Silica-2 Yes Silica Grout Mix - Water 0.421 Silica-4 Yes Silica Grout Mix - Water 0.425 Note: Silica grout mix was prepared by mixing a certain amount of LUDOX SM-30 colloidal silica (Aldrich Chem. Co.), water, and CaCl2 solution. Concentration of silica in the grout mix was 15% by weight, and the concentration of CaCl2 was 0.02 M. Final e was void ratio based on water content after the undrained cyclic triaxial test.
Table 6.7 Test Parameters for Undrained Cyclic Triaxial Tests on LUDOX SM-30 Colloidal Silica Treated Specimens
Parameter Value Test Type Undrained Cyclic Test
Soil Mix Soil Mix (a) - (Refer 6.2.1) Initial void ratio 0.50
Specimen Size 162 mm by 70.7 mm (Length by diameter) Effective Confining
Pressure 100 kPa
Cyclic Stress Ratio 0.2 Loading Frequency 0.2 Hz
6.6.1.2 Results
Figure 6.10a shows the test results in term of void ratio versus number of cycles to reach +/- 5% axial strain. Figure 6.10b shows the test results in term of void ratio versus (energy to reach +/- 5% axial strain). For comparison purpose, the results of the 6 cyclic tests conducted on ungrouted soil specimens prepared with the same soil mix (Refer to section 6.5.1) are also shown in the figure. The test result indicates that for grouted specimens, more cycles of load and more energy were required to cause liquefaction.
EL
For Sample Silica-2, base on visual observation, there was apparent increase in soil strength after grouting. While from data point in the drawing, there was no significant strength increase. The possible reason for this discrepancy is that errors were introduced in the void ratio measurement and the void ratio measured was lower than actual value.
0.3 0.4 0.5 0.6 0.7
1 10 100 1000
Number of Cycles (5.0% Strain)
e
Ungrouted Soil Silica-2 (LUDOX Treated) Silica-4 (LUDOX Treated)
0.3 0.4 0.5 0.6 0.7
1000 10000 100000
EL (J/m3)
e
Ungrouted Soil Silica-2 (LUDOX Treated) Silica-4 (LUDOX Treated)
8000
(a) Number of Cycles (5.0% Strain) versus e (b) EL versus e Figure 6.10 Undrained Cyclic Test Result
(CSR = 0.2, σv0’ =100 kPa.)
6.6.2 Undrained Monotonic Strength 6.6.2.1 Experiments
Table 6.8 shows the undrained compression triaxial tests conducted on colloidal silica treated specimens. The results are shown in Figure 6.11. For comparison purpose, the results of Test Tri-02 (Control test, used water as grout material) and the 8 undrained compression tests conducted on ungrouted soil specimens prepared with the same soil mix (Refer to section 6.5.2) are also shown in the figure. A summary of the test parameters is shown in Table 6.9.
Table 6.8 Undrained Compression Triaxial Tests on LUDOX SM-30 Colloidal Silica Treated Specimens Test # Electric
Current
Anode Fluid (1st shot)
Anode Fluid (2nd shot)
Cathode Fluid
Final e
Tri-02 Yes Water N/A Water 0.469
LUDOX-002 Yes Silica Grout Mix - Water 0.527
LUDOX-004 Yes Silica Grout Mix - Water 0.515
LUDOX-006 Yes Silica Grout Mix - Water 0.558
Note: Silica grout mix was prepared by mixing a certain amount of LUDOX SM-30 colloidal silica (Aldrich Chem. Co.), water, and CaCl2 solution. Concentration of silica in the grout mix was 15% by weight, and the concentration of CaCl2 was 0.02 M. Final e was void ratio based on water content after the undrained cyclic triaxial test.
Table 6.9 Test Parameters for Undrained Compression Triaxial Tests on Colloidal Silica Treated Specimen
Parameter Value Test Type Undrained Compression Triaxial
Soil Mix Soil Mix (b) - (Refer 6.2.1) Initial Void Ratio 0.55
Specimen Size 81 mm by 37.3 mm
(Length by diameter) Effective Confining Pressure 100 kPa
Rate of Shearing 36mm/hr
0 300 600 900 1200
0 5 10 15 20 25
ε (%)
q (kPa)
Tri-02 LUDOX_002
LUDOX_004 LUDOX_006
Ludox_006 Ludox_004
Ludox_002 Tri_02
-300 -200 -100 0 100
0 5 10 15 20 25
ε (%)
U (kPa)
Tri-02 LUDOX_002
LUDOX_004 LUDOX_006
Ludox_006
Ludox_004 Ludox_002 Tri_02
(a) Deviatoric Stress versus Axial Strain (b) Pore Pressure versus Axial Strain
0.3 0.5 0.7 0.9 1.1
1 10 100 1000 10000
p' (kPa)
e
Ungrouted Soil LUDOX Treated Tri-02
No Grout Silica + CaCl2
(b) Effective Confining Stress versus e
Figure 6.11 Undrained Compression Triaxial Strength of Colloidal Silica Treated Specimens
6.6.2.2 Results
Figure 6.11a shows the deviatoric stress versus axial strain data. Figure 6.11b shows the pore pressure versus axial strain data. The result shows that both the peak and steady-state shear strength of the specimen increased after electro-osmotic injection of colloidal silica grout mix. Figure 6.11c shows the mean effective confining stress ' p
(=(σ1'+2*σ3')/3) at the end of shear tests versus void ratio. It indicates that specimens grouted with colloidal silica grout mix show high strength (Figure 6.11c).
The results from all these tests indicate that electro-osmotic injection of colloidal silica grout mix increases the peak strength and steady-state strength of the soil.
Therefore, for flow failure to occur in grouted soil, a higher load is needed to “push the soil over the peak”(NRC 1985) of the undrained shear strength. The potential for liquefaction is also reduced due to increase in the undrained steady-state strength.