Pump-out Tests in Alluvial Sands – A Case Study

1. PUMP-OUT TESTS IN
ALLUVIAL SANDS
– A CASE STUDY
Sorabh Gupta, Sanjay Gupta & Ravi Sundaram
Cengrs Geotechnica Pvt. Ltd.
1

2. PUMP-OUT TESTS IN
ALLUVIAL SANDS
– A CASE STUDY
Sorabh Gupta, Sanjay Gupta & Ravi Sundaram
Cengrs Geotechnica Pvt. Ltd.
1

3. The Need
2

4. Dewatering
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5. Dewatering
3

6. The Challenge
Effective Design of Dewatering Systems for
construction sites requires precise estimates of
hydrogeological parameters of the aquifer,
such as hydraulic conductivity / permeability,
etc.
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7. The Challenge
Effective Design of Dewatering Systems for
construction sites requires precise estimates of
hydrogeological parameters of the aquifer,
such as hydraulic conductivity / permeability,
etc.
..particularly on sites where alluvial sands are
encountered
4

8. Definitions
1. Hydraulic Conductivity (K), m/sec
Volume of water (m3) through unit section of soil (m2) in unit
time (s) under unit hydraulic gradient.
2. Transmissivity (T), m2/day (Productivity of an aquifer)
Water capacity which flows per unit length through an
aquifer under hydraulic gradient
3. Storage coefficient (%)
Volume of water it takes / releases per unit surface area of
aquifer under unit change of head.
4. Specific capacity
Discharge for unit time for unit draw down
c = Q / s
5

9. Laboratory Permeability Tests (Falling Head,
Constant Head)
Tracer Tests
Borehole Tests
Falling Head in Soils
Packer Tests in Rock
Full Scale Pump Out / Aquifer Tests
Geotechnical Investigations
6

10. Laboratory Permeability Tests (Falling Head,
Constant Head)
Tracer Tests
Borehole Tests
Falling Head in Soils
Packer Tests in Rock
Full Scale Pump Out / Aquifer Tests
Geotechnical Investigations
6

11. Pump Out / Aquifer Test
Advantages
Determine hydraulic parameters on global scale
Results directly used for design of dewatering
systems
Design the test as per site/project requirements
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12. Case Study 1 : Mall at Noida
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13. Project Location
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14. Project Location
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15. Project Overview
Located in a densely populated area
As per Initial Planning:
G+5, 3 basements
Foundation Level = 18 m below OGL
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16. Site Conditions
Site Stratigraphy:
✓ Medium dense sand (SP) to
35~40 m depth
✓ Static GWT at 8 m
Purpose:
✓ Lower the GW level to 21 m
depth below OGL?? (i.e. 13 m
drawdown!)
✓ Test performed from excavated
level (~7 m below OGL)
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17. 10.0m10.0m
PW
OW1 OW2
OW3
OW410.0m10.0m
Well Setup
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18. Pump Well Setup
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19. Pump Well Setup
25.0m
PUMP WELL DIA 300 mm
GRAVEL 250 mm
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20. Pump Well Setup
6.0m
3.0m
15.0m
BLANK CASING
200mm DIA
BLANK CASING
200mm DIA
SLOTTED CASING
200 mm DIA
BOTTOM CAP
25.0m
PUMP WELL DIA 300 mm
GRAVEL 250 mm
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21. Pump Well Setup
6.0m
3.0m
15.0m
BLANK CASING
200mm DIA
BLANK CASING
200mm DIA
SLOTTED CASING
200 mm DIA
BOTTOM CAP
Pea Gravels
25.0m
PUMP WELL DIA 300 mm
GRAVEL 250 mm
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22. Pump Well Setup
6.0m
3.0m
15.0m
BLANK CASING
200mm DIA
BLANK CASING
200mm DIA
SLOTTED CASING
200 mm DIA
BOTTOM CAP
Pea Gravels
PUMP
25.0m
PUMP WELL DIA 300 mm
GRAVEL 250 mm
13

23. LVL (-7.0 m)
3..0m
PRESENT EXCAVATION
WORKING, LVL
OBSERVATION WELL DIA 150 mm
GRAVEL 250 mm
SLOTTED CASING
100 mm DIA
BLANK CASING
100mm DIA
18.0m
LVL (-25.0 m)
15.0m
OW2
BOTTOM CAP
3..0m
LVL (-7.0 m)
GRAVEL 250 mm
LVL (-31.0 m)
OW1
OBSERVATION WELL DIA 150 mm
21.0m
24.0m
Pea Gravels
Observation Well Setup
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24. Well Installation
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25. Pump Well Setup
12.5
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26. Pump Well Setup
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27. Pump Well Setup
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28. Pump Well Setup
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29. Pump Well Setup
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30. Flow Meter
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31. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step drawdown test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
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32. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step drawdown test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
19

33. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step drawdown test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
20

34. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step drawdown test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
20

35. Step Drawdown Test
Purpose
To estimate...
Short-term yield-drawdown relationship
Estimate ‘Q’ for CD test
Determine Well Efficiency, etc.
21

36. Step Drawdown Test
Overview
Steps estimated on maximum yield of the well
based upon trial runs
Discharge is increased in 4~6 steps of 60~100 min
each
Discharge controlled by Control Valve
Average discharge and drawdown recorded with
time for each step
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37. Step Drawdown Test
Step
Discharge
(lt/min)
Discharge
(lt/hour)
I
II
III
IV
V
VI
619,200 10,320
1,022,400 17,040
1,260,000 21,000
2,016,000 33,600
2,484,000 41,400
2,793,600 46,560
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38. Location : Mall at Noida
Time vs Drawdown in all wells
Step Time Flowmeter Discharge PW OW-1 OW-2 OW-3 OW-4
No Interval Reading (Litr/hr) Draw Draw Draw Draw Draw
(min) (m) down(m) down(m) down(m) down(m) down(m)
I 0 1.72 0.000 0.300 0.300 0.300 0.300
5 1.91 0.190 0.600 0.600 0.600 0.600
10 1.925 0.205 0.900 0.900 0.900 0.900
15 1.925 0.205 1.200 1.200 1.200 1.200
20 1.925 0.205 1.500 1.500 1.500 1.500
25 1.93 0.210 1.800 1.800 1.800 1.800
30 1.93 0.210 2.100 2.100 2.100 2.100
35 1.93 0.210 2.400 2.400 2.400 2.400
40 1.93 0.210 2.700 2.700 2.700 2.700
45 1.93 0.210 3.000 3.000 3.000 3.000
50 1.93 0.210 3.300 3.300 3.300 3.300
55 1.93 0.210 3.600 3.600 3.600 3.600
60 1.93 10320 0.210 3.900 3.900 3.900 3.900
II 65 2.23 0.510 4.200 4.200 4.200 4.200
70 2.24 0.520 4.500 4.500 4.500 4.500
75 2.245 0.525 4.800 4.800 4.800 4.800
80 2.25 0.530 5.100 5.100 5.100 5.100
85 2.25 0.530 5.400 5.400 5.400 5.400
90 2.255 0.535 5.700 5.700 5.700 5.700
95 2.255 0.535 6.000 6.000 6.000 6.000
100 2.255 0.535 6.300 6.300 6.300 6.300
105 2.255 0.535 6.600 6.600 6.600 6.600
110 2.255 0.535 6.900 6.900 6.900 6.900
115 2.255 0.535 7.200 7.200 7.200 7.200
120 2.255 17040 0.535 7.500 7.450 7.500 7.500
III 125 2.3 0.580 7.800 7.750 7.800 7.800
130 2.35 0.630 8.100 8.080 8.080 8.100
STEP DRAWDOWN TEST DATA
Step Drawdown Test Data
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39. Drawdown Measurement
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40. Drawdown Measurement
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41. Step Drawdown Test Results
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42. Step Drawdown Test Results
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43. Step Drawdown Test Results
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44. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step drawdown test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
27

45. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step drawdown test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
27

46. Constant Discharge Test
Overview
To determine hydraulic characteristics of the
aquifer within the radius of influence of PW
PW continuously pumped to ensure desired
drawdown at steady state.
Water level readings are recorded in PW and
OW’s at regular intervals.
Test continues till steady state / equilibrium
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47. CONSTANT DISCHARGE TEST DATA
Location: Mall at Noida
Time vs Draw Down in all wells
Discharge = 48,000 lph = 48 m3
/hr
PW ROBW-1 ROBW-2 TOBW-1 TOBW-2
r = 0 m r = 10 m r = 20 m r = 10 m r = 20 m
0 0.000 0.000 0.000 0.000 0.000
1 1.330 0.160 0.000 0.200 0.000
2 1.340 0.190 0.060 0.210 0.010
3 1.350 0.195 0.100 0.210 0.100
4 1.360 0.200 0.130 0.210 0.100
5 1.370 0.200 0.130 0.210 0.110
6 1.370 0.200 0.130 0.220 0.160
7 1.380 0.210 0.130 0.220 0.160
8 1.390 0.210 0.130 0.220 0.160
9 1.400 0.215 0.130 0.230 0.160
10 1.410 0.220 0.130 0.230 0.160
15 1.410 0.225 0.135 0.240 0.160
20 1.410 0.227 0.140 0.250 0.160
25 1.410 0.230 0.140 0.250 0.160
30 1.410 0.230 0.140 0.260 0.160
35 1.410 0.235 0.140 0.260 0.160
40 1.410 0.240 0.140 0.260 0.160
45 1.410 0.245 0.140 0.270 0.160
50 1.410 0.250 0.140 0.270 0.160
55 1.410 0.260 0.140 0.280 0.160
60 1.410 0.260 0.140 0.280 0.160
75 1.410 0.270 0.140 0.280 0.160
90 1.415 0.290 0.140 0.290 0.160
105 1.420 0.320 0.150 0.290 0.165
120 1.430 0.320 0.160 0.290 0.170
135 1.440 0.320 0.170 0.300 0.175
150 1.450 0.325 0.175 0.305 0.175
165 1.460 0.330 0.180 0.320 0.180
Time
(min)
Drawdown, metres
CD Test Data
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48. CD Test Results
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49. CD Test Results
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50. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step draw down test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
31

51. Testing Procedure
Installation of PW & OW
Well Development
Trial Runs
Step draw down test – SDD test
Constant discharge test – CD test
Recuperation / Recovery test
31

52. Recuperation / Recovery Test
After CD test is complete, Pump is stopped
Take water level readings in same sequence
as for CD
Test continues till water level rises up to
static level
Provides independent check of CD Results
32

53. Formation Method
Aquifer Parameters
Coefficient of
Permeability
from Borehole
Data (cm/sec)
Tav
(m2/day)
Kav
(cm/sec)
Sav
Alluvium
Strata
(Yamuna
Sands)
CD Test 1782 1.9 × 10−1 4.1 × 10−2
2.0 × 10−4
~ 5.0 × 10−4
Recovery Test 4046 4.3 × 10−1 -
Summary of Results
33

54. Formation Method
Aquifer Parameters
Coefficient of
Permeability
from Borehole
Data (cm/sec)
Tav
(m2/day)
Kav
(cm/sec)
Sav
Alluvium
Strata
(Yamuna
Sands)
CD Test 1782 1.9 × 10−1 4.1 × 10−2
2.0 × 10−4
~ 5.0 × 10−4
Recovery Test 4046 4.3 × 10−1 -
Summary of Results
33

55. Test Summary
Large unconfined aquifer with very high
permeability and storage capacity
Actual K three orders of magnitude higher
than original estimate (based on borehole
tests)
Very low drawdown (<2.0 m) even at high
discharge (48,000 lph)
For 13 m drawdown:
• > 3,00,000 lph discharge required
• 7.2 million litres of water would need to be
discharged into the adjoining drain every day!
• Affect on adjoining structures?
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56. Test Consequences
Design altered - raised final excavation level
Deep diaphragm wall constructed, which acts as
cut-off-barrier to GW flow, restricts the aquifer
extent, and protects nearby structures from
subsistence due to migration of fines etc.
Installed dewatering wells spaced at 25~30 m
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57. Geotechnical Engineering is a Science,
But its Practice, an Art...
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