65
VACUUM CONSOLIDATION IN PEAT Eric R Farrell AGL Consulting and Trinity College Brendan O’Kelly, Trinity College, Dublin and Juan Pablo OsorioSalas Universidad de Antioquia, Colombia, formerly Trinity College

Vacuum Consolidation in Peat

Embed Size (px)

DESCRIPTION

Vacuum Consolidation in Peat

Citation preview

Page 1: Vacuum Consolidation in Peat

VACUUM CONSOLIDATION IN PEAT

Eric R Farrell AGL Consulting and Trinity College

Brendan O’Kelly, Trinity College, Dublin and 

Juan Pablo Osorio‐Salas Universidad de Antioquia, Colombia, formerly Trinity College

Page 2: Vacuum Consolidation in Peat

Structure of Presentation Background

Principles of vacuum consolidation

Current practical applications

TCD/NRA test site Geotechnical/geology/hydroge

ology of test site Construction of test area Instrumentation Benchmark ground movement 

readings.

Results from vacuum consolidation test Vacuum achieved Settlement versus time Practical difficulties

Numerical modelling Laboratory parameters Soil model Comparison of field 

performance with predictions

Conclusions

Page 3: Vacuum Consolidation in Peat

PRINCIPLES OF VACUUM CONSOLIDATION• Vacuum consolidation is a construction method used to accelerate 

ground settlement by reducing the air pressure at the ground surface.

• Normally atmospheric pressure (patm ) is taken as the base line when computing σ′ = σ –u.  

• patm is about 100kPa, and the pwp can reduced below atmospheric using vacuum pumps. This increases the effective stress without generally increasing the shear stresses.

• Vacuum consolidation was originally proposed by Kjellman in Sweden in the 1950s.

Page 4: Vacuum Consolidation in Peat

Background ‐ Rampart roads• The construction of roads over peat 

bogs in the 18th & 19th centuries opened up the bog for harvesting

• The easiest place to harvest was adjacent to the roads, roads ended up elevated above the adjacent ground. These are called Rampart Roads. Heights of 9m have been recorded.

• Many of these roads are used today, resulting in narrow and very dangerous roads.

• Vacuum consolidation may assist in overcoming some of the challenges in widening these roads.

Page 5: Vacuum Consolidation in Peat

VACUUM CONSOLIDATION

Pump

PVDs

Granular bed

Airtight membrane

Atmospheric pressure = 100kPa

Pump creates a suction of up to 80kPa

u up to -80kPa (20kPa absolute)

Page 6: Vacuum Consolidation in Peat

VACUUM CONSOLIDATION

Pa

Pore pressure reduction

Vacuum pressure

Dep

th, z PVD

zo

Initial pressure

Reducedpressure

Page 7: Vacuum Consolidation in Peat

Current applicationsMenard vacuum

Baudrain – Cofra bv

Page 8: Vacuum Consolidation in Peat

Hayashi et al, 2003

Shang, Tang & Miao (1998)

Page 9: Vacuum Consolidation in Peat

TCD/NRA TEST SITE

Page 10: Vacuum Consolidation in Peat

Raised Bog under milled peat production, therefore some peat has been removed. Site is at edge of bog.

Top 1m ‐ very clayey sandy Gravel f% ≈ 30%Below – slightly clayey sandy Gravel f% ≈ 4‐11%K = 1.96x10‐6 to 1.15x10‐5 m/s

Page 11: Vacuum Consolidation in Peat
Page 12: Vacuum Consolidation in Peat

In‐situ vanes (55mm x 110mm)

Page 13: Vacuum Consolidation in Peat

Hydrogeology

Page 14: Vacuum Consolidation in Peat

Drain within 8m of edge of test area.

Page 15: Vacuum Consolidation in Peat

Layer Depth (m) Description Observations and properties

1 0 – 0.7 Man‐made fill Black peat; occasional plastic bags, gravel, pieces of geotextile, machine parts.

2 0.7 – 4.0 Pseudo‐fibrous peat

w = 561 – 1340%    GS = 1.38 – 1.59                 LOI = 87 – 99%         h = 9.57 – 10.56kN/m3

eo = 6.78 – 14.83     Sr = 94 – 100%                     pH = 4.5 – 6.2          Von Post = H4 – H7Cc = 2.2 – 6.4

3 4.0 – 7.0 Glacial till The clay fraction reduces with depth , about30% fines in top metre, reducing to 4‐11%.

Table 1 – Simplified soil profile

TCD/NRA VACUUM CONSOLIDATION FIELD TRIAL

Page 16: Vacuum Consolidation in Peat

TCD/NRA VACUUM CONSOLIDATION FIELD TRIAL

0.85m sq. spacing

1.2m sq. spacing

Page 17: Vacuum Consolidation in Peat
Page 18: Vacuum Consolidation in Peat
Page 19: Vacuum Consolidation in Peat
Page 20: Vacuum Consolidation in Peat
Page 21: Vacuum Consolidation in Peat

Instrumentation

• 10 No. Vibrating wire (VW) piezomters (also calibrated for suction)

• 6 No. push‐in VW settlement cells 0.9m, 1.5m & 2.65m

• Settlement plates

• Standpipes

• Barometric pressure/temp.

• Rain gauge

• Water meter

• Air pressure gauges

Acknowledge assistance of NVM Ireland Ltd.

Page 22: Vacuum Consolidation in Peat

TCD/NRA VACUUM CONSOLIDATION FIELD TRIAL

Page 23: Vacuum Consolidation in Peat

26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66

26

30

34

38

42

46

50

54

58

62

66

26

30

34

38

42

46

50

54

58

62

66

26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66

Test area

StandpipesBoreholes

Shearvanes tests

21

16

10

1415

26

12 11

24

25

13

17

BH-1

BH-2

Vane 3

Vane 2

Vane 1

N

12

37

8 945

6

18

19

2023

22

A

A'

B

B

Page 24: Vacuum Consolidation in Peat

Pumping system: 30th Nov 2009 – 23rd Jun 2010• 1.5kW Centrifugal pump• 38mm diameter jet pump

Page 25: Vacuum Consolidation in Peat

Pumping system: 29th Jul 2010 – 29th Oct 2010• 2.2kW Liquid ring pump• 1.5kW Centrifugal pump• 38mm diameter jet pump

Page 26: Vacuum Consolidation in Peat

MONITORING BEFORE PUMPING

• Prior to starting the TCD/NRA vacuum consolidation field trial, four months of baseline monitoring were conducted.

• The vacuum consolidation trial was run from 30thNovember 2009 and was terminated on 29thOctober 2010

Page 27: Vacuum Consolidation in Peat
Page 28: Vacuum Consolidation in Peat

Btm of ditch

Page 29: Vacuum Consolidation in Peat
Page 30: Vacuum Consolidation in Peat
Page 31: Vacuum Consolidation in Peat
Page 32: Vacuum Consolidation in Peat
Page 33: Vacuum Consolidation in Peat

Flooding in August 2009

Page 34: Vacuum Consolidation in Peat
Page 35: Vacuum Consolidation in Peat

MONITORING DURING PUMPING• The TCD/NRA vacuum preloading field trial commenced on the 30th November 2009.

• The results for the eleven months of pumping presented here.

• Rain, water table, vacuum, settlement and pore water pressure are presented.

Page 36: Vacuum Consolidation in Peat
Page 37: Vacuum Consolidation in Peat

Sett=1.11m for 0.85m spacingSett=1.09m for 1.20m spacing

Page 38: Vacuum Consolidation in Peat

Volume of water extracted

Page 39: Vacuum Consolidation in Peat
Page 40: Vacuum Consolidation in Peat
Page 41: Vacuum Consolidation in Peat
Page 42: Vacuum Consolidation in Peat
Page 43: Vacuum Consolidation in Peat

umax = 38.9 ‐ 53.2kPa for depths 0.90 to 2.65m.umax = 13.9kPa for 4.05m depth.

Page 44: Vacuum Consolidation in Peat
Page 45: Vacuum Consolidation in Peat
Page 46: Vacuum Consolidation in Peat
Page 47: Vacuum Consolidation in Peat
Page 48: Vacuum Consolidation in Peat
Page 49: Vacuum Consolidation in Peat

MAXIMUM SURFACE SETTLEMENT  1.23m (Strain of 33%)

Page 50: Vacuum Consolidation in Peat

Visual 1st June 2010

Page 51: Vacuum Consolidation in Peat

CRACKS AT EDGE

Chai et al. (2005)

Page 52: Vacuum Consolidation in Peat

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

11.5

12

12.5

0.1 1 10 100 1000 10000

Cha

nge

in H

eigh

t mm

Log Time minTIME SETTLEMENT

12.5 kN/m2

100kPa

200kPa

400kPa

800kPa

Primary24 hr

creep

SETTLEMENT OF PEAT

Rate of settlement

cv;    cvh;   chh

k ;     kv;    kh

Page 53: Vacuum Consolidation in Peat

Δe=Ck Δ logtCk ≈  0.25eo

Page 54: Vacuum Consolidation in Peat

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

0 50 100 150 200 250 300

Strain %

Stress

2.4m to 3.2m Oed 4 Spreadsheet

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

1 10 100 1000

Void ra

tio e

Effective stress kPa

2.4m to 3.2m Oed 4 Cc Model

Page 55: Vacuum Consolidation in Peat

Soil models investigated• Simple EOP  Cs & Cc (CR = Cc/{(1+eo)} & RR) and vc’

• Soft Soil model (SS) Plaxis (*= Cc/{2.3(1+eo)}, к*)

• Soft Soil Creep model Plaxis (*= Cc/{2.3(1+eo)}, к*, *)

Page 56: Vacuum Consolidation in Peat

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

1 10 100 1000Stress

2.4m to 3.2m Oed 4 Spreadsheet 1.5m to 2.3m

1 10 100

Strain %

Stress

E'=100kPa

E'= 100kPa

Page 57: Vacuum Consolidation in Peat

Modelling vacuum consolidation

x

y

1

1

1

2

2

2

3

3

3

4

4

4

5

5

5

6

6

6

7

7

7

8

8

8

9

9

9

10

10

10

AA AA

A

A

A

A

A A

0 1

23

4 5

67

89 10

11

1213141516

17 18 19 20 21

22 23 24 25

26 27 28 29

30 313233 34 35 36 37 38 39 40 41

42 434445 46 47 48 49 50 51 52 53

5455

56

5758

59

60 61

Page 58: Vacuum Consolidation in Peat
Page 59: Vacuum Consolidation in Peat
Page 60: Vacuum Consolidation in Peat

Upper peat Middle peat Lower peatPOP 10 5 11l* 0.125 0.16 0.16k* 0.05 0.055 0.04m* 0.0065 0.0078 0.009g 10.45 10.1 10.06eo 7.42 13.81 12.05

kv=kH(m/day)

0.3 0.2 0.01

ck 1.8 3.44 3.44

VALUES USED ON BACK ANALYSIS WITH SSC MODEL

PARAMETERS INTERPRETED FROMLABORATORY OEDOMETER TESTS

Upper peat Middle peat Lower peatPOP 10 5 11l* 0.11 to 0.125 0.14 to 0.16 0.16 to 0.2k* 0.022 to 0.05 0.03 to 0.055 0.033 to 0.04m* 0.0065 to 

0.0080.0078 to 0.01 0.0065 to 0.009

g 10.45 10.1 10.06eo 6.78 – 8.69 13.43 – 14.5 11.5 – 12.5

kv=kH(m/day)

0.011 to 0.128 0.0053 to 0.011 0.0015 to 0.0029

ck 1.8 3.44 3.44

Page 61: Vacuum Consolidation in Peat

• Swell using simple soil model = 0.161m

• Swell measured = 0.13m

• Equivalent E’ ≈ 100kPa at this very low effective stress.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25 30 35 40

18th August

18th Sept

pwp kPa

Dep

th (m

)

Page 62: Vacuum Consolidation in Peat

• The TCD/NRA vacuum preloading field trial was implemented and showed that this technique can be successfully used in peat soils.

• The drainage system comprising PVDs, horizontal drains and a granular bed, was effective in distributing the applied vacuum pressure and collecting the drained water.

CONCLUSIONS

Page 63: Vacuum Consolidation in Peat

Practical difficulties/Observations• Summer conditions general water table was lower than side barrier.

• Higher suctions achieved with vacuum pump but more stable values with liquid ring pump

• Pore pressure reduction at edges was slightly lower (2kPa to 8.5kPa) than at centre.

• PWP roughly uniform with depth

• Vacuum pressure adequately transmitted in 0.85m and 1.2m spacings.

Page 64: Vacuum Consolidation in Peat

continued

• Calcification of pumps a significant issue

• Freezing, tears, bursts electrical cuts affected the performance.

• Airtight cover could be improved, use of water seal should be considered.

• Vacuum consolidation had little effect on the water table outside the test area.

• Behaviour can be simulated using standard soil models.

Page 65: Vacuum Consolidation in Peat

ACKNOWLEDGEMENTS

• National Road Authority.

• Trinity College Dublin.

• Geotechnical Trust Fund Award (Engineers Ireland).

• Universidad de Antioquia

• Bord na Móna