PVD Case Study by Soham Foundations

Case Study for Ground Improvement Using PVD with Preloading for Coal & Iron Ore Stackyard

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CASE STUDY FOR GROUND IMPROVEMENT USING PVD WITH

PRELOADING FOR COAL & IRON ORE STACKYARD

Pallavi Bhosle Sohams Foundation Engg. Pvt. Ltd., Navi Mumbai400614, India. E-mail: [email protected] V.V. Vaishampayan MD, Sohams Foundation Engg. Pvt. Ltd., Navi Mumbai400614, India. E-mail: [email protected]

ABSTRACT: PVD with Preloading was used as the technique for the ground improvement below the Coal and Iron Ore Stackyards at port near Visakhapatnam, AP. The top layer of subsoil consisted very soft clay upto a depth of 12 m to 18 m. PVD was effectively used to accelerate the settlements. The settlements and pore pressure dissipation was monitored using instruments as Piezometers and Settlement Recorders. The paper describes the effectiveness of the technique and monitoring of the instrumentation. The settlements recorded were analysed by Asaoka Method and Hyperbolic Method. The analysis indicated close proximity between the percentage settlements observed and pore pressure dissipation. 1. INTRODUCTION

1.1 Project The development of the new port at Gangavaram located at about 15 kms south of Visakhapatnam Port, Andhra Pradesh commenced in December 2005. The site for Gangavaram Port is the best location for development of a modern all weather, deepwater, multipurpose and truly next generation port (Fig. 1). The development of port facilities included development of back up facilities for coal and iron ore storage and stacking and handling facilities. The proposed height of coal stack was 12.00 m and that of iron ore was 10.00 m.

Fig 1: Plan of the Project Site

The preliminary soil investigation revealed the presence of soft clay upto a depth of 10.00 to 18.00 m with very low safe bearing capacity and high consolidation parameters. It was hence decided to enhance the soil properties using a ground treatment method of Use of Band Drains/PVD with Preloading. The turnkey contract was awarded to M/s Sohams Foundation Engg. Pvt. Ltd., Navi Mumbai.

1.2 Geotechnical Investigation

As a part of the project, 8 Nos boreholes were taken to ascertain the design parameters. In general the area was fairly levelled and a thin layer of dredged sand of thickness 0.20 to 0.30 m was present at most of the locations. Immediately below the dredged sand was the marine clay with shells with thickness varying from 1.00 to 3.00 m. A layer of soft marine clay of thickness varying from 7 m to 15 m was observed following this layer of marine clay with shells. The standard penetration test was conducted in this stratum at various depths, indicated the penetration of 45 to 60 cms in one blows thus N resistance as 0 to 1. At a depth of 12.00 to 18.00 m below the existing ground level the penetration resistance N was observed to be increasing with depth. The laboratory tests conducted on selected disturbed soil samples and undisturbed soil samples indicated the following range of values (Table 1).

The safe bearing capacity of the existing soil was worked out as 3 T/m which was very low to take the loads. The consolidation settlements were worked out about 10001600 mm.

IGC 2009, Guntur, INDIA


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Table 1: Typical Soil Properties

Natural Moisture content 1281 (%) Specific Gravity 2.522.65 Bulk Density 1.241.52 (g/cc) Gravel 00 (%) Sand 231 (%) Silt + Clay 763 (%) Liquid Limit 21102 (%) Plastic Limit 1547 (%) Initial Void Ratio, e0 0.6272.249 Compression Index, Cc 0.380.92 Coefficient of Consolidation, Cv 0.721.95 (m/yr)

Cohesion, Ccu 0.191.05 (kg/cm) Angle of Friction, cu 1829 (deg) Shear Strength from VST 0.0950.991 (kg/cm)

2. GROUND IMPROVEMENT SCHEME

2.1 Salient Features of Scheme

Machinery Used: Hydraulic Stitchers Depth of PVD: 10.00 m to 18.00 m below

OGL Spacing of PVD: 1.00 m c/c in Triangular Grid

below the stacker reclaimers 1.50 m c/c in Triangular Grid in other area

Consolidation Period: For 1.00 m spacing: 65 days For 1.50 m spacing: 174 days

Thickness of sand mat: 300 mm

Horizontal Drainage System:

Combined system of geotextile pipe formed by boulders/gravels encased in geotextile and band drains laid horizontally connecting these pipes.

2.2 Post Treatment Assessment

The instruments as Piezometers; Settlement Recorders; Inclinometers were installed to assess the post treatment performance of the adopted method of treatment.

Table 2 shows the details of the instruments installed in project area (Fig. 2).

Table 2: Details of Instruments

Section

Casa

gran

de

piez

omet

er

Vibr

atin

g w

ire

piez

omet

er

Plat

e se

ttlem

ent

mar

kers

Mag

netic

se

ttlem

ent

reco

rder

Section 1 CP1 VP1, VP2, VP3 PS1, PS2 MS1, MS2

Section 2 CP4 VP9, VP10,VP11 PS8, PS9, PS10 MS6

Section 3 CP3 VP6, VP7,VP8 PS5, PS6, PS7 MS5

Section 4 CP2 VP4, VP5 PS3, PS4 MS3, MS4

Section 5 CP5 VP12, VP13, VP14

PS11, PS12, PS13

MS7

The readings were taken every 4 days when loading started, continued for a month after final preload height is achieved. The frequency was later extended to 7 days.

AREA - B

AREA - A

Section 3 Section 2 Section 1

CP2(9m)

VWP4(6m)

CP3(6m) CP1(9m)

CP4(9m)

VWP5(6m)

VWP6(9m)

Vwp8(9m)

VWP7(6m)

VWP9(6m)

VWP10(6m)

VWP11(9m)

VWP3(6m)

VWP2(6m)

VWP1(9m)

MS3

MS4

MS5

MS6 MS1

MS2

PS3

PS4

PS5

PS6

PS7 PS10

PS9

PS8 PS1

PS2

IN 3

IN 6

IN 7

IN 8

IN 1

IN 10

IN 9

VWP14 (9m)VWP13(6m) VWP12(9m)

PS13PS12 PS11

CP5 (6m)MS7

C'

C

IN 5

IN 2IN 4

SECTION - C-C'

Hard / Stiff Strata

Pre

load

As

Per D

esig

n

GL

Soft Clay 6m9m

3m6m9m

CP PS MS

VWP

Section 4

Section 5

Fig. 2: Location Details for the Instruments


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2.3 Methods of Data Analysis

From the observed data of excess pore pressure variation and settlements the degree of consolidation can be assessed as,

2.3.1 Excess Pore Pressures The piezometers indicate the increase in excess pore pressure on application of the load. As the load is transferred to the virgin soil i.e. with consolidation is in progress, the increased excess pore pressure dissipates. The degree of consolidation is then determined as,

100*% maxit

t

UUUU

U =

Umax: Maximum Excess Pore Pressure after load application

Ut: Excess Pore Pressure at time t Ui: Initial Excess Pore Pressure

2.3.2 Settlements

From settlements, the degree of consolidation can be assessed by Asaoka Method and Hyperbolic Method. By these methods the ultimate settlement S100 is evaluated based on the observed data. Then the degree of consolidation is determined as,

100*%100SS

U t=

St: Settlement at time t S100: Ultimate Settlement Asaoka Method: This method is based on the Barrons solution for radial drainage. From the timesettlement curves the settlements at different time (with equal interval t) is determined. Then the points (Si, Si-1) are plotted. The interception of this line and the line drawn with slope = 1 gives the ultimate settlement S100.

Hyperbolic Method: This is based on the Terzaghis theory for pore water pressure distribution. The average degree of consolidation when plotted in the form of U Vs T/U is a hyperbola. Hence, the graph is plotted for time/settlement Vs settlement. The settlement is determined as the inverse of the slope of this hyperbola.

However, for the case of Terzaghis theory, the slope of the hyperbolic line is not constant. In fact, it is valid for Uav between 60% and 90%, and hence it must be evaluated between these two points. Theoretically, the average slope of the first straight line portion is equal to 0.824. The inverse of this slope gives a corresponding settlement of 1.21%, which overestimates the ultimate primary consolidation settlement by 21%. Consequently, Tan (1994) proposed that if the slope of the first linear portion, , of the hyperbolic plot is used, the

predicted ultimate primary settlement, p, should be multiplied by the theoretical slope such that p = /.

2.4 Analysis of Data

Piezometers were installed at two different depths 6 m and 9 m.

Figures 3 & 4 indicate the excess pore pressure variations with time. For the piezometers installed below the stacker reclamation the degree of consolidation was observed as 5565%. For the piezometers installed in other area (PVD spaced at 1.50 m c/c) the degree of consolidation was observed as 3040 %.

Pore Pressure variation

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

0 25 50 75 100 125 150 175 200 225 250 275 300No of Days

Por

e P

ress

ure

(ksc

)

CP 1 CP 2 CP 3 CP 4 Fig. 3: Variation of Excess Pore Pressures with Time

Pore Pressure variation

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

0 25 50 75 100 125 150 175 200 225 250 275 300

No of Days

Por

e P

ress

ure

(ksc

)

VP 2 VP 11 VP 10 VP 9 VP 8 VP 5 Fig. 4: Variation of Excess Pore Pressures with Time

Settlement markers were installed on virgin soil with minimum 0.50 m embedment. Some of the plate settlement markers were damaged during the preloading activity whereas the magnetic settlement markers showed the erratic results.

Figure 5 indicates the settlement data with time. The observed settlements were in the tune of 300500 mm.


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Settlement

0

100

200

300

400

500

600

0 25 50 75 100 125 150 175 200 225 250 275 300

No of Days

Set

tlem

ent (

mm

)

PS1 PS2 PS4 PS6 PS9 PS10

Fig. 5: Settlements with Time

The graphs for ultimate settlement prediction by Asaoka Method and Hyperbolic Method are indicated in Figures 6 and 7.

Asaoka Method - PS1

0100200300400500600700800

0 100 200 300 400 500 600 700 800

Settlements (Si-1)

Settl

emen

ts (S

i)

Asaoka Method - PS2

0100200300400500600700800

0 100 200 300 400 500 600 700 800Settlements (Si-1)Se

ttlem

ents

(Si)

Asaoka Method - PS4

0100200300400500600700800


ttlem

ents

(Si)

Asaoka Method - PS6

0100200300400500600700800


ttlem

ents

(Si)

Asaoka Method - PS9

0100200300400500600700800


ttlem

ents

(Si)

Asaoka Method - PS10

0100200300400500600700800


ttlem

ents

(Si)

Fig. 6: Ultimate SettlementAsaoka Method

The degree of Consolidation by both the methods was observed as (Table 3).

Table 3: Degree of Consolidation from Settlement Data

Asaoka Method

Hyperbolic Method (Slope = 0.824)

Hyperbolic Method (Slope =

1.0) Spacing 1.00 78115 6384 5270 Spacing 1.50 8592 3953 3243

Hyperbolic Method - PS 1

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 50 100 150 200 250 300 350 400 450 500

Time (Days)Tim

e / S

ettle

men

t


0.000.100.200.300.400.500.600.700.800.901.00

0 50 100 150 200 250 300 350 400 450 500Time (Days)Ti

me

/Set

tlem

ents


0.000.100.200.300.400.500.600.700.800.901.00

0 50 100 150 200 250 300 350 400 450 500

Time (days)Tim

e / S

ettle

men

ts


0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

0 50 100 150 200 250 300 350 400 450 500

Time (Days)Tim

e / S

ettle

men

ts


0.200.300.400.500.600.700.800.901.001.101.20

0 50 100 150 200 250 300 350 400 450 500Time (days)Ti

me

/ Set

tlem

ents


0.000.100.200.300.400.500.600.700.800.901.001.101.20

0 50 100 150 200 250 300 350 400 450 500

Time (Days)Tim

e /S

ettle

men

ts

Fig. 7: Ultimate SettlementHyperbolic Method

3. CONCLUSIONS

Plate Settlement Recorders are more reliable than the Magnetic Settlement Recorders for marine clays.

With the application of the load the pore pressure increased and dropped down slowly with time. The pore pressure variation indicated about 5560% dissipation i.e. degree of consolidation.

The Degree of consolidation based on Asaoka Method was evaluated as 78115% whereas based on Hyperbolic method it was evaluated as 3270%.

Hyperbolic Method is more comparable with the Pore Pressure Dissipation Results. Further the results obtained with theoretical slope of hyperbola as 1.00 are more closer to the predicted by pore water pressure analysis.

The consolidation settlements worked out theoretically from laboratory test results were much higher than that predicated by Asaoka and Hyperbolic Method.

REFERENCES

Al-Shamrani M.A., Settlement Prediction of Sabkha Formations Using Rectangular Hyperbolic Method.

Holtz R.D., Jamiolkowski M.B., Lancellotta R. and Pedroni R. (1991). CIRIA Ground Engineering Report: Ground ImprovementPrefabricated Vertical drains: Design and Performance.

Bo M.W., Arulrajah A. and Choa V. (1997). Assessment of degree of consolidation in Soil Improvement Project, Proc. of International Conference on Ground Improvement Techniques, 7180.


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PVD Case Study by Soham Foundations