Dynamic compaction to mitigate liquefaction potential

1. Sanjay Gupta, Ravi Sundaram, Sorabh Gupta
CENGRS GEOTECHNICA PVT LTD.

2. Project Details
Gautam Budh University at Greater Noida,
Uttar Pradesh
Covers an area of about 500 acres
84,000 m2 of constructed area, 30% green
cover
Site in the flood plains of the River Yamuna,
about 2 km from river

3. Vicinity Map

4. The Project Today..

5. .. And what it will be.

6. Site Conditions
Site is in Earthquake Zone IV - IS 1893: 2002
Loose alluvium - fine sand (Yamuna Sand)
Groundwater met at shallow depth
Sand to 8-12 m depth is prone to liquefaction
Ground Improvement by Dynamic Compaction
carried out on site
Each structure was individually assessed

7. Investigations Before CompactionInvestigations Before Compaction

8. Investigations After CompactionInvestigations After Compaction

9. Plethora of Data..
Over 600 boreholes and
150 static cone penetration
tests completed in the
University area over the last
2 years

10. This Paper Presents..
Ground Improvement by
DYNAMIC COMPACTIONDYNAMIC COMPACTION was done for
FACULTY BLOCKFACULTY BLOCK

11. Faculty Block Location

12. Scope of Work- Faculty Block
At Faculty Block:
Before Compaction:
4 boreholes – 15 m
1 SCPT
After Compaction:
4 boreholes – 15 m
1 SCPT

13. Before Compaction

14. After Compaction

15. Borehole Data – Before Compaction
SPT values
typically 10-15 to
8 m depth
Fines content:
5-10 %
Groundwater at 4-
5 m depth, may
rise to GL

16. Liquefaction Assessment
Seed & Idriss (1971) method – NCEER
Summary Report
Cyclic Stress Ratio (CSR)
Maximum Credible Earthquake (MCE)
Design Earthquake Magnitude: 6.7
Peak Ground Acceleration: 0.24g

17. Liquefaction Assessment
Cyclic Resistance Ratio (CRR) determined
from SPT & SCPT
As per the analysis, the fine sands to 8 m8 m
depthdepth at the Faculty Block are susceptible
to liquefaction in the event of the design
earthquake

18. Dynamic Compaction
Dropping a heavy
weight can compact
loose sands to
substantial depth
Effective for sands
only
Done on a grid pattern

19. Conceptual Illustration
The maximum depth of improvement
(Df) at the project site was estimated
using Mitchell & Katti, 1981:
where
n = modification factor (taken as 0.7)
W = weight of pounder, and
H = height of drop
WHnD = WHnD =

20. Compaction Details
Conventional Crane –
TLC 955A
Compaction in 3 Phases:
2 Compaction Phases and
Ironing Phase
1 week time lag in
between – to allow pore
pressures to dissipate

21. Compaction Phase
Area divided into 4 x 4 m grids
11.65 T pounder falling from
height of 14 m
10 drops at each grid point
Energy Imparted: 1600 kN-m
Corresponding depth of
improvement: 9 m
2nd Phase staggered 2 m

22. Ironing Phase
Craters filled with GSB Grade II
material
Hammer weight: 11.65 T
Height of fall: 6 m
No. of drops: 5
Energy: 2114 kN-m
Area graded with 10 passes
of 10 T vibratory roller

23. SPT before compaction

24. SPT before & after compaction
75 % Improvement to
about 4 m depth
25~30 % Improvement to
about 10 m depth

25. SPT & SCPT before & after
compaction
25~30 % Improvement to
10 m depth
75 % Improvement to
4 m depth

26. SPT & SCPT before & after
compaction
50-170 % Improvement
to 4 m depth
25~50 % Improvement
to 10 m depth

27. Extent of Improvement Achieved
After Compaction, N>20, qc > 50 MPa
Peak Improvement: Between 1 and 5 m
depth
Improvement below 10 m depth is
marginal

28. CRR Evaluation (Before Compaction)

29. CRR Evaluation (After Compaction)
After Compaction, CRR > CSRAfter Compaction, CRR > CSR

30. FOS against Liquefaction
(based on SPT & SCPT)
LiquefiableZone
No LiquefactionLiquefaction to
8m depth
Before Compaction After Compaction

31. Liquefaction Mitigation
Untreated ground (before compaction) is
susceptible to liquefaction to 8 m depth
After compaction, Factor of Safety against
liquefaction > 1
Susceptibility to liquefactionSusceptibility to liquefaction
successfully mitigatedsuccessfully mitigated

32. Foundation Selection
For unimproved ground, pile foundations transferring
the loads below the liquefiable zone would be
necessary. Pile lengths would be on the order of 15
to 20 m
Open foundations on improved ground: Isolated
footings with connecting beam
Design Net Bearing Pressure: 175 kPa175 kPa
RESULTED IN SUBSTANTIAL SAVINGSRESULTED IN SUBSTANTIAL SAVINGS
IN COST AND CONSTRUCTION TIMEIN COST AND CONSTRUCTION TIME

33. Concluding Remarks
Case Study demonstrates successful
mitigation of liquefaction potential by use of
dynamic compaction
Successful in areas of loose clean sands
Field trials are necessary to confirm feasibility
Sufficient in-situ testing before and after
improvement should be done to confirm
efficacy of the improvement

34. Thank You!Thank You!