Pore pressures during stone column installation

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1 Pore pressures during stone column installation Jorge Castro Group of Geotechnical Engineering University of Cantabria, Santander, Spain

2 Outline Introduction Site characterization Instrumentation Data analysis Peaks of pore pressures Influence of previous column installations Pore pressure dissipation Conclusions 2/16

3 Introduction 3/16

Site characterization Summary of index soil tests CPTU Borehole Between columns Outside columns Depth (m) γd (kn/m 3 ) W (%) w L (%) w P (%) 4-4.6 17. 21 35.

4 Site characterization Summary of index soil tests CPTU Borehole Between columns Outside columns Depth (m) γd (kn/m 3 ) W (%) w L (%) w P (%) /16

5 Site characterization Summary of oedometer, triaxial and unconfined compression tests Borehole Depth(m) c'(kpa) Φ(º) c u (kpa) c c c s Between columns Outside columns C v 1-3 /1-4 cm 2 /s 5/16

6 Site characterization Column properties Triangular pattern Dry bottom-feed method Diameter d c =.8 m Spacing d e = 2.8 m Area replacement ratio a r =.74 Length L = 9 m 6/16

7 Instrumentation ROAD SUECA 1 STONE COLUMN 6 PZ1_1-PZ1_2 5 7 PZ2_1-PZ2_2 2 PZ3_1-PZ3_ m 7 m 6,6 m PZ1_2 PZ2_2 PZ3_2 PZ1_1 PZ2_1 PZ3_1 7/16

8 Instrumentation PZ 1 PZ 2 PZ 3 Pore pressure measured (kpa) Depth (m) Current (ampere) 16:: 16:3: 17::. 17:3: 18:: 18:3: :: 16:3: 17::. 17:3: 18:: 18:3: :: 16:3: 17::. 17:3: 18:: 18:3: Time 8/16

9 Data analysis 3 First column installation Peak of pore pressure (kpa) 2 1 COLUMN k=22.2 kpa R=5.37m k=22.8 kpa R=5.63 m 4m 7m Δu = k ln k = 2c u R r Elastic-perfectly plastic behaviour Distance to SC 1 (m) 9/16

10 Data analysis All column installations Peak of pore pressure (kpa) COLUMN k=34.4 kpa R=4.89 kpa k=73.4 kpa R=4.51 m 4m 7m Δu = k ln k = 2c u R r Elastic-perfectly plastic behaviour Distance to the column (m) 1/16

11 Data analysis 1 Peak of pore pressure at 7m (kpa) PZ 1 PZ 2 PZ Peak of pore pressure at 4m (kpa) 11/16

12 Data analysis 5 4 PZ 1 PZ 2 PZ 3 u/u (7m) u/u (4m) 12/16

13 Data analysis 5 PZ1-1 5 PZ1-2 Peak of pore pressure (kpa) Distance to column (m) Peak of pore pressure (kpa) Distance to column (m) PZ2-1 PZ2-2 Peak of pore pressure (kpa) Peak of pore pressure (kpa) Distance to column (m) Distance to column (m) 13/16

14 Data analysis Excess pore pressures (kpa) PZ3-1 / SC 2 PZ3-1 / SC 3 PZ2-1 / SC 5 :: :3: :6: :9: :12: :15: Time (hh:mm:ss) 14/16

15 Data analysis Isochrones without fractured zone Consolidation curves at piezometer (r=1.6 m) 25 1 Excess pore pressures (kpa) COLUMN 1 days 1 day.1 day u Excess pore pressures (kpa) No fractured zone r f =2a r f =3a r (m) Time (days) C h =1-2 cm 2 /s 15/16

16 Conclusions Results of field instrumentation to study pore pressures during stone column installation Peaks of pore pressures fairly well matched by cavity expansion theory Depth influence Previous column installation influence Difficult to explain the fast dissipation 16/16

1.8 Unconfined Compression Test

1-49 1.8 Unconfined Compression Test - It gives a quick and simple measurement of the undrained strength of cohesive, undisturbed soil specimens. 1) Testing method i) Trimming a sample. Length-diameter