Field and Laboratory Testing of Soil for Surcharge Design

Transcription

1 Field and Laboratory Testing of Soil for Surcharge Design Steven Bartlett. University of Utah Required from Field and Pre-Construction Investigations 1. Design profile and X-sections with natural ground of current embankment (as applicable) and proposed embankment geometries and right-of-way constraints. 2. Understanding of general ground conditions (Is this a soft soil site?). 3. Details of construction schedule (Is there enough time for surcharging). With PV drains? Without PV drains? 4. Detailed geotechnical borehole logs and CPT soundings 5. Undisturbed sampling of all cohesive layers (Piston sampling preferred). 1

2 2D Embankment X-section Existing Final (Design) Embankment / Wall Height (m) Final w/ Surcharge Grnd El Horizontal Distance (m) Field Investigation Locations UDOT Research Drilling # Site Latitude Longitude BH Depth (ft.) Drilling Date South 40 45'40.02"N '45.61"W 92 12/5/ South Layton 41 3'23.25"N '47.63"W 142 2/5/ Springville 40 9'39.78"N '18.43"W 129 4/1/ Provo 40 12'26.41"N '39.43"W 127 4/5/2013 2

3 400 South and 800 West Salt Lake City Layton Parkway and Main Street 3

4 400 South and 700 East - Springville University Ave I-15 On-ramp, Provo 4

5 CPT Sounding 400 South Borehole Log 5

6 Preconsolidation Stress from CPT Correlations P ' p a vo Pa qt Pa vo R 2 = 91% Ozer, 2005 S. Temple Site Comparison of CPTU with CRS Consolidation Tests p' vs. Elevation CPTU 9 CRS Tests IL Tests N. Temple Site Comparison of CPTU with CRS Consolidation Tests p' vs. Elevation CPTU 1 CRS Tests CPTU 10 IL Tests S. Temp Embankment Site Comparison of CPTU with CRS Consolidation Tests p' vs. Elevation CPTU 12 CRS Tests CPTU 11 svo' Preconsolidation Stress from CPT Correlations Elevation (meters) 1276 Elevation (meters) 1276 Elevation (meters) Ozer, p' (kpa) 1268 p' (kpa) 1268 p' (kpa) 6

7 Time Rate of Consolidation, Field and Laboratory Performance Time Rate of Consolidation (CRS Cell Testing) 7

8 Useful correlations for preliminary design of Bonneville deposits (UT UDOT Research Report) Consolidometer 8

9 Sample Extrusion Sample Preparation 9

10 Sample Preparation (cont.) Screening of Samples Recommend Screening Criteria for Undistrubed Samples (UT UDOT Research Report) 10

11 Determination of C and C 1. Determine pre-consolidation stress, p, from incremental load tests using Casagrande s method and Work Method. 2. Using another specimen from same depth determine C at an OCR = 1 to obtain C normally consolidated. Do this using the preconsolidation stress determined from the incremental load test performed in step 1. Load the sample to 1.5 to 2 times that of the preconsolidation stress. This ensures that the samples have reached a new normally consolidated state and any effects of aging or past preconsolidation have been removed. When in virgin compression, perform a time-rate-of-settlement plot and calculation. This test may require 1 to 2 weeks laoratory time. Plot C versus CR. 3. Increase the state of stress to OCR = 1.25 on the same specimen and perform time-rate-of settlement evaluations (square root of time) until 90% EOP has occurred. (Do not go beyond this point to avoid going into secondary compression; hence potentially aging the soil!) Determination of C and C 4. Unload the specimen back to the state of stress corresponding to OCR = 1.0 and perform time-rate-of settlement evaluations to determine C as a function of OCR. 5. Repeat steps 2 thru 5 using OCR values of 1.25, 1.5, Plot C / C versus OCR and AAOS. 7. Determine the time for surcharge to resume, ts and plot this against OCR or AAOS. 11

12 Useful Plots You Should Make 1. Plots of sp (Casagrande and Work Strain Methods) 2. Pre-consolidation Stress versus Depth 3. Compression ratio (CR) versus in-situ moisture content 4. C versus CR 5. C and C versus OCR p Determinations Using Casagrande s Method Step A F B G D E Strain (ΔH/H) H C 0.25 Preconsolidation Pressure 0.30 Stress (tsf) A - Stress 0.1strain curve from data points B - Point of maximum curvature C - Tangent line to curve at point B D - Horizontal line through point B E - Line bisecting angle between lines C and D F - Tangent to linear portion of curve in vergin compression range G - Intersection of lines E and F (vertical effetive stress at point G equals the preconsolidation pressure) H - Tangent to linear portion of the curve in the recompression range. 12

13 p Determinations Using Work Method Step Work (ton ft/ft 3 ) σ' p Stress (tsf) Preconsolidation Stress vs. Depth Step σ'p (psf) Depth (ft) South Springville Provo S. Layton 400 S. σ'v (estimated) Springville σ'v (estimated) Provo σ'v (estimated) S. Layton σ'v (estimated)

14 C Determinations Log of Time Method Step 2 Log of time vs. Strain for 2.5 tsf OCR Strain (ΔH/H) Cα = Log of Time (min) C versus CR (Research Sites) Step Rate of Secondary Compression, C a C α = CR SD= R² = N=4 C α = CR SD= R² = N=4 C α = CR SD= R² = N=4 C α = CR SD= R² = N= South Springville C α = CR SD= R² = N=5 Provo S. Layton Virgin Compression Ratio, CR 14

15 C versus CR (Research Sites and I-15 Data) Step 2 Time Rate of Consolidation Square Root of Time Method Step 3 B 0 1 Strain (%) A C 5 D 6 E Root of Time ( min) A - Time strain curve from data points. B - Extention of initial linear portion of cure A to time = 0 minutes. C - Constructon line with slope = 1.15 times inital linear portion of curve A. D -ε 90 strain at point where curve A crosses line C. E -t 90 time at point where curve A crosses line C. 15

16 C Determinations Log of Time Method Step 4 Log of time vs. Strain for 2.5 to 2 tsf OCR Strain (ΔH/H) Cα' = Log of Time (min) C Determinations Log of Time Method Step 5 Log of time vs. Strain for 3 to 2 tsf OCR Strain (ΔH/H) Cα' = Log of Time (min) 16

17 C Determinations Log of Time Method Step 5 Log of time vs. Strain for 4 to 2 tsf OCR Strain (ΔH/H) Cα' = Log of Time (min) C α and C α vs OCR Step 6 Normalized Rate of Secondary Compression Cα'/Cα C' α /C α = e (OCR) R² = C' α /C α 0.2 C' α /C α = e (OCR) R² = = e (OCR) R² = South Springville Provo Expon. (400 South) Expon. (Springville) Expon. (Provo) Expon. (ALL) 0.1 C' α /C α = e (OCR) R² = OCR 17

18 C α and C α vs OCR Step 6 UDOT Study Ng, I-15 MIT Time of Secondary Compression after Unloading, t s - Step 7 18

19 Time of Secondary Compression after Unloading, t s - Step 7 Strain (ΔH/H) Log of time vs. Strain for 3 to 2 tsf OCR Log of Time (min) t s = max. value Time of Secondary Compression after Unloading, t s - Step 7 19