Liquefaction Risk Mitigation for a Power Plant in the Indo Gengatic Alluvium

1. SANJAY GUPTA RAVI SUNDARAM
SORABH GUPTA
Cengrs Geotechnica Pvt. Ltd.
New Delhi, INDIA

2. 108 MW Gas Based Power Plant
Site in northern part of Delhi city (India)
Alluvial Plains of River Yamuna
Earthquake Zone IV as per IS 1893-2002
Facilities planned include STG, GTG, Steam
Turbine, Boiler, Chimney, Cooling Water
System, Switchyard, etc.
Loose sands to 8 m depth prone to
liquefaction during major earthquakes

3. DelhiDelhi
Vicinity Map

4. Geological Setting
Quaternary Deposits
Recent Alluvium
Deposits of River
Yamuna and its
tributaries
Younger Alluvium –
Loose fine sand –
Yamuna Sand
Older Alluvium –
Pleistocene – medium
dense to dense silts
DelhiDelhi
IndoIndo--GangeticGangetic
AlluviumAlluvium

5. Detailed Geotechnical Investigation
15 boreholes – 30 m depth
6 static cone penetration tests
SASW tests along 8 lines
3 cross-hole seismic tests

6. Layout Plan
Exploratory Boreholes
Electrical Resistivity Test
Cone Penetration Test
Cross-hole Seismic Test

7. Typical Borehole Data
Loose surficial fill to 1-2 m
depth
Natural deposits primarily
fine sand / silty sand with
intermediate layers of
sandy silt
Groundwater at 5.2 –
6.4 m depth

8. SPT using automatic trip hammer

9. SPT Profile
N<15
11 m

10. SCPT Profile
Loose
11 m
0 - 10 m depth qc = 8-10 MPa
10 - 14 m depth qc = 8-16 MPa
14 - 16 m depth qc = 4-8 MPa
16 - 18 m depth Refusal

11. Schematic: Cross-Hole Seismic Test

12. CHST in progress

13. No Boreholes Required – Ground Surface Test
Schematic: SASW Test

14. SASW Method
1 Hz
Geo
phone
4.5 Hz
Geo
phone
NDT PC
Dispersion
Geo
phones
SASW
… courtesy IIT DelhiIIT Delhi

15. SASW test in progress
Sledgehammer as source Receiver Geophone

16. Shear Wave Velocities

17. Shear Wave Velocity: Design Profile
Vs = 135-180 m/s to 5 m depth
= 200-240 m/s to 10 m depth
= 250-315 m/s to 30 m depth
Vs from SASW were 10-20%
higher than from CHST
Both methods indicate
relatively lower velocities to
8~9 m depth

18. Seismic Zones of India
DelhiDelhi

19. IS: 1893 (Part 1) – 2002 states
Liquefaction likely in loose sands below water
table
0-5 m depth : N < 15
> 10 m depth : N < 25
linear interpolation for 5-10 m depth
If this criteria suggests liquefaction potential,
detailed analysis may be carried out

20. Detailed Liquefaction Analysis
Youd & Idriss (2001) – NCEER Summary
Report
Cyclic Stress Ratio (CSR)
Cyclic Resistance Ratio (CRR)
Factor of Safety = CRR / CSR
For this project, Factor of Safety against
liquefaction specified as 1.2

21. CSR & CRR – SPT

22. CSR & CRR – SCPT

23. CSR & CRR – Vs

24. Factor of Safety against Liquefaction

25. Engineering Solutions
For critical / heavily-loaded facilities such as
TG, Boiler, Chimney, etc:
Bored PilesBored Piles –– 600 mm diameter, extending
well below the liquefiable zone
For medium-loaded facilities such as Clari-
flocculator, Cooling Towers, etc:
Ground improvementGround improvement by vibro-
replacement (stone columns)

26. Safe Pile Capacities – 600 mm dia
Normal Condition
Compression : 810 KN
Uplift : 480 KN
Seismic Condition
Compression : 740 KN
Uplift : 420 KN
Factor of Safety : 2.5
For seismic condition, skin friction in
liquefiable zone was ignored

27. Vibro-Replacement
Dry Vibro Stone columns installed by
bottom-feed method
500 mm dia extending to 10 m depth
Centre-to-centre spacing: 1.5 m
Design Net Bearing Pressure: 160 kPa

28. Vibro-Replacement in progress

29. Vibro-Probe in Action

30. Dynamic Cone Penetration Tests
After CompactionAfter Compaction
Blow Counts exceed 15
below 2 m depth
Substantial improvement
in penetration resistance
Medium dense to
5 m depth
Dense below 5 m depth
Improved soils notImproved soils not
likely to liquefylikely to liquefy
during earthquakeduring earthquake

31. Load Test on Vibro-Column

32. Load Test on Improved Ground
300 mm thick sand pad
placed over the vibro
stone columns
Test plate size 1.5 m x
1.5 m with stiffeners
Load applied cyclically
1st cycle to 250 kPa
2nd cycle to 500 kPa
FOUNDATIONS SAFEFOUNDATIONS SAFE
FOR 160 kPaFOR 160 kPa

33. Conclusions
Case study demonstrates successful
mitigation of liquefaction susceptibility
Detailed geotechnical investigation including
shear wave velocity measurements needed
to identify and quantify depth of liquefaction
Piling and ground improvement carried out at
site to ensure that the power plant
foundations are safe during earthquake

34. Thank You!Thank You!