soil improvement for a road using the vacuum preloading method

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Soil improvement for a road using the vacuumpreloading method

S. W. YAN and J. CHUyGeotechnical Research Institute, Tianjin University, China;ySchool of Civil and Environmental Engineering, Nanyang Technological University,Singapore 639798

A case of using the vacuum preloading method to improve

the foundation soil for a road in Tianjin, China, ispresented. A vacuum load of 80 kPa was applied for 90days to consolidate a 20 m thick soft clay layer. The groundsettled for about 15 m. The average degree of consolida-tion estimated using settlement data was 90%. The un-drained shear strength of the soil increased and the watercontent decreased after vacuum preloading. The proce-

dures used for soil improvement, the instrumentation andthe field monitoring data are described. Several issuesconcerning the practical aspects of the vacuum preloading

method are discussed.

Keywords: case history; consolidation; ground

improvement

Nous presentons un cas utilisant une methode de pre-

charge sous vide pour ameliorer le sol de fondation duneroute a Tianjin en Chine. Une charge sous vide de 80 kPa aete appliquee pendant 90 jours pour consolider une couchedargile tendre epaisse de 20 m. Le sol sest tasse surenviron 1,5 m. Le degre moyen de consolidation estimedapres les donnees de tassement etait de 90%. La resis-tance au cisaillement non draine du sol a augmente et le

contenu en eau a diminue apres la precharge sous vide.Nous decrivons les procedures utilisees pour ameliorer lesol, linstrumentation et les donnees de contro le sur le

terrain. Nous examinons plusieurs problemes concernantles aspects pratiques de la methode de precharge sousvide.

Introduction

A section of a road leading to a container terminal at TianjinPort, China, had to be constructed on a 20 m thick soft claylayer. The top 56 m of the clay layer was reclaimed recentlyusing clay slurry dredged from the seabed. The remaining1415 m was original seabed clay. The soil in both layerswas soft, and was still undergoing consolidation. This softclay layer needed to be improved before any constructionwork could be carried out.

Preloading using a fill surcharge was not feasible as it wasdifficult to build a fill embankment several metres high onsoft clay. The vacuum preloading method was adopted as itwas considered the most cost-effective method for thisproject.

The vacuum preloading method has been widely used inTianjin for land reclamation and soil improvement worksince 1980. The technique has been well developed over theyears as a result of intensive research and field trials (Chenand Bao, 1983; Ye et al., 1983; Yan and Chen, 1986; Choa,1990; TPEI, 1995; Chu et al., 2000). Prefabricated verticaldrains (PVDs) have often been used to distribute vacuumload and discharge pore water. A vacuum load of 80 kPa orabove can be maintained as long as it is required. Compared

with the fill surcharge method for the equivalent load, thevacuum preloading method is cheaper and faster. Accordingto a comparison made by TPEI (1995), the cost of soilimprovement using vacuum preloading is only two thirds of

that by fill surcharge, based on the local prices of electricityand materials.

The principles and mechanism of vacuum preloadinghave been discussed in the literature (e.g. Kjellman, 1952;Holtz, 1975; Chen and Bao, 1983; Qian et al., 1992. A casestudy on the use of the vacuum preloading method for thesoil improvement for a road project is presented in thispaper. The site conditions, the soil improvement procedure,and the field instrumentation are described. The fieldmonitoring data are presented. The achieved degree of

consolidation and the effect of soil improvement are eval-uated. Several issues concerning the practical aspects of thevacuum preloading method are also discussed.

Soil conditions

The section of road to be improved is shown schematicallyin Fig. 1. It was 3645 m long and 51 m wide. For theconvenience of construction, the site was divided into twosections. The idealised soil profile is shown in Fig. 2. Theliquid limit (LL), plastic limit (PL), water content (Wo), voidratio (e), and undrained shear strength (cu) profiles in bothsections are shown in Fig. 3. The shear strength, cu, was

measured by unconsolidated undrained (UU) tests. It can beseen that the soil properties vary erratically with depth. Thewater content of the soil was as high as or even higher thanthe liquid limit at most locations. The undrained shearstrength (cu) of the soil as shown in Fig. 3 was generallysmaller than 20 kPa.

Ground Improvement (2003) 7, No. 4, 165172 165

1365-781X # 2003 Thomas Telford Ltd

(GI 2165) Paper received 11 October 2002; accepted 20 May 2003


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Soil improvement procedure

The specification for the soil improvement was set toachieve an average degree of consolidation of 85% based onsettlement data under a minimum vacuum loading of80 kPa, and a minimum bearing capacity of 80 kPa.

The soil improvement work was carried out as follows. A03 m sand blanket was first placed on the ground surface.PVDs were then installed on a square grid at a spacing of10 m to a depth of 20 m. Corrugated flexible pipes (100 mmin diameter) were laid horizontally in the sand blanket tolink the PVDs to the main vacuum pressure line. The pipeswere perforated and wrapped with a permeable fabric textileto act as a filter layer. Three layers of thin PVC membrane

were laid to seal each section. Vacuum pressure was thenapplied using jet pumps.The schematic arrangement of the vacuum preloading

method used is shown in Fig. 4. The vacuum pressure wasapplied continuously for 90 days until the required degreeof consolidation was achieved. Curves of applied vacuumload against time and ground settlement against time areshown in Fig. 5. It can be seen that a vacuum pressure of80 kPa or above was maintained for the whole duration ofthe vacuum preloading.

Instrumentation and field

measurementsInstrumentsincluding pore water pressure transducers,

surface settlement plates, multi-level settlement gauges,standpipes and inclinometerswere installed in both sec-tions to monitor the consolidation performance. The loca-

tions of those instruments are shown schematically in Fig. 1(plan view) and Fig. 6 (elevation view). Undisturbed soilsamples were taken. Laboratory and field vane shear testswere conducted both before and after the soil improvement.

Before vacuum preloading

As excess pore water pressures existed in the soil, someconsolidation took place as soon as the vertical drains wereinstalled. An average 058 m of settlement had occurred

before the application of vacuum preloading. Other factorsthat contributed to the settlement included the disturbanceto the soil caused by vertical drain installation, and theconsolidation that took place under the sand blanket and theself-weight of soil.

During vacuum preloading

Pore water pressuresThe pore water pressures in the soil reduced with the

application of vacuum preloading. The reductions in thepore water pressure at different depths are plotted againstduration in Fig. 7(a) and (b) for sections I and II respectively.Fig. 7(a) indicates that, for section I, there was a vacuumdistribution period of about 10 days for the effect of vacuumload to be felt. The vacuum distribution period for section IIwas smaller owing to the adjacent effect of section I onsection II, as shown in Fig. 7(b). After this initial vacuumdistribution stage, the pore water pressures reduced quicklywith time. The pore water pressure reduction becamesmaller after about 40 days at most locations.

SettlementsThe settlements monitored at different depths during

vacuum preloading are plotted against duration in Fig. 8.The maximum surface settlements are 0959 m and 1147 mfor sections I and II respectively. Although the pore waterpressure took some time to reduce (see Fig. 7), the settlementtook place as soon as the vacuum preloading was applied.After the vacuum preloading stopped at 90 days, thesettlement still increased for a few days before it becameconstant, as shown in Fig. 8.

Analysis of results

Reduction in pore water pressures

Based on the pore water pressure measurements shown inFig. 7, the pore water pressure distributions with depth atdurations of 30, 60 and 90 days are shown in Fig. 9. Theinitial pore water pressure profile, uo(h), and the suctionline, us, are also plotted in Fig. 9. The initial pore waterpressures were obviously greater than the hydrostatic porewater pressure in both sections, indicating that the subsoilwas still under consolidation. These were mainly theremaining pore water pressures that were generated by theplacement and consolidation of the top 6 m of slurry fill.

During vacuum preloading, the pore water pressurereductions were different at different points. As there was alayer of silt or silty clay between 6 and 11 m that was

relatively more permeable, the pore water pressure in thislayer reduced faster. As shown in Fig. 9, the pore waterpressures had reduced to nearly the suction line within thefirst 30 days. The pore water pressure reductions wereslower in the slurry fill (between 0 and 6 m) and at the

bottom of the stiff silty clay layer (from 14 to 20 m), as the

Section IISection I

BoreholeWater standpipeMulti-level settlement gauge

Field vaneInclinometerPore water pressure transducer

364.5 m

51m

Fig. 1. Project site and plan view of instrumentation

0

6

8

11

16

20

Silty clay consolidated from slurry, yellow and grey in colour,

high compressibility

Silt, grey in colour

Soft silt to silty clay, grey and brown in colour, high compressibility

Silty clay, grey and brown in colour, high compressibility

Stiff silty clay, grey and brown in colour, medium compressibility

Fig. 2. Simplified soil profile

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S. W. Yan and J. Chu


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soils there were relatively less permeable. This is consistentwith the initial pore water pressure profile, where largerinitial pore water pressures presented at the same locations.

Degree of consolidation

The results presented above show that the ground hadsettled more than 1 m and substantial pore water pressurehad reduced at the end of vacuum preloading. Therefore theconsolidation of soil under vacuum preloading was effective

in both sections I and II. The degree of improvement can befurther quantified by the degree of consolidation.

The degree of consolidation can be estimated using eithersettlement or pore water pressure. When calculating the

degree of consolidation using settlement, the ultimate settle-ment has to be predicted. Several methods are available inestimating the ultimate settlement (Asaoka, 1978; Sridharanand Rao, 1981; Zeng and Xie, 1989). Based on previousexperiences with similar projects, Zeng and Xies methodwas specified as the method to be used for this project. In

0 10 20 30 40

cu: kPa

0

4

8

12

16

20

Depth:m

0.5 1.0 1.5 2.0

Void ratio, e

0

4

8

12

16

20

Depth:m

0 20 40 60 80

LL, PL, and Wo

0

4

8

12

16

20

Depth:m

(a)

0 10 20 30 40

cu: kPa

0

4

8

12

16

20

Depth:m

0.5 1.0 1.5 2.0

Void ratio, e

0

4

8

12

16

20

Depth:m

0 20 40 60 80

LL, PL, and Wo

0

4

8

12

16

20

Depth:m

Wo

PL

LL

Wo

PL

LL

(b)

Fig. 3. Basic soil properties: (a) at section I; (b) at section II

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Soil improvement for a road using the vacuum preloading method


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this method, the average degree of consolidation, Uavg, isassumed to be of the following form:

Uavg 1 et (1)

where t is time, and and are two parameters. In applyingthis method, three time intervalst1, t2, and t3need to bechosen from the curve of settlement (S) against time (t) inthe region where the full vacuum load has been applied. t1,t2, and t3 have to be chosen in such a way that t3 t2 t2 t1. If the settlements corresponding to t1, t2 and t3 are S1, S2and S3, then from equation (1) the ultimate settlement, S1,can be derived as

S1 S3(S2 S1) S2(S3 S2)

(S2 S1) (S3 S2)(2)

where S1 the ultimate settlement, and S1, S2 and S3 are

the settlements measured at time t1, t2 and t3 respectively.The average degree of consolidation can then be estimatedas the ratio ofSt and S1.

The ultimate settlements calculated with equation (2) were

1078 m for section I and 1278 m for section II. The surfacesettlements measured at t 90 days were 0959 m for sectionI and 1147 m for section II. Thus the average degree ofconsolidation calculated was 89% for section I and 90% forsection II, which values were higher than the 85% averagedegree of consolidation requested in the specification.

As the pore water pressures during the consolidation weremeasured, the average degree of consolidation can also becalculated based on pore water pressure. Referring to Fig.9(a) or 9(b), the average degree of consolidation can becalculated as

Uavg 1

[u t(h) us]d h[u

0(h) u

s]d h

(3)

and

us w h 80 (3a)

where u0(h) is the initial pore water pressure at depth h;u t(h) is the pore water pressure at depth h at time t; us is thesuction applied; h is depth; and w is the unit weight ofwater. The integrals in equation (3) can be calculated usingthe area between the curve u t(h) and the line us in Fig. 9(a)or 9(b). Applying equation (3) to Fig. 9(a) and 9(b), theaverage degree of consolidation calculated was approxi-mately 75% for both sections.

The average degree of consolidation calculated based onthe measured pore water pressures is smaller than thatbased on settlements. Similar problems have been pointed

A A

1 2 3 4 5 6 7 8

8 7 4 10 11

9

1

AA

Fig. 4. Schematic arrangement of vacuum preloading method: 1, drains; 2,filter piping; 3, revetment; 4, water outlet; 5, valve; 6, vacuum gauge; 7, jetpump; 8, centrifugal gauge; 9, trench; 10, horizontal piping; 11, sealingmembrane

100.0

80.0

60.0

40.0

20.0

0

0.2

0.40.6

0.8

1.0

Vacuuml

oad:kPa

Settlement:m

10 20 30 40 50 60 70 80 90 100Duration: days

Fig. 5. Applied vacuum pressure and ground settlement measured with duration

1.0

4.0

6.0

8.5

11.0

14.5

18.0

02.0

5.5

7.5

9.5

13.0

15.5

Slurry

Silt

Softclay

Siltyclay

Siltyclay

0

2

4

6

8

10

1214

16

18

22

2021.0

Pore water pressure transducer

Inclinometer

Multi-level settlement guage

Water standpipe

Fig. 6. Elevation view of instrumentation

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out by Hansbo (1997). The difference could be attributed tothe following factors:

(a) Both the settlements and the pore water pressures weremeasured at specific points only. Thus the data may not

be representative of the average values for the whole

layer.(b) There are uncertainties involved in the prediction of the

ultimate settlement.(c) As the load was applied over a strip, it was not a truly

one-dimensional problem.(d ) It was a large strain consolidation problem.

0 20 40 60 80 100

Duration: days

0

10

20

30

40

50

60

70

80

90Porewaterpressu

rereduction:kPa 1.0 m

4.0 m

6.0 m

11.0 m

18.0 m

8.5 m

14.5 m

(a)

0 20 40 60 80 100

Duration: days

0

10

20

30

40

50

60

70

80

90Porewaterpressure

reduction:kPa 1.0 m

4.0 m

6.0 m

11.0 m

18.0 m

8.5 m

14.5 m

(b)

Fig. 7. Pore water pressures measured at different depths against duration:(a) at section I; (b) at section II

0 20 40 60 80 100

Duration: days

0

0.2

0.4

0.6

0.8

1.0

1.2

Settlement:m

0.0 m

2.0 m

5.5 m

9.5 m

15.5 m

7.5 m

13.0 m

(b)

120

0 20 40 60 80 100

Duration: days

0

0.2

0.4

0.6

0.8

1.0

1.2

Settlement:m

0.0 m

2.0 m

5.5 m

9.5 m

15.5 m

7.5 m

13.0 m

(a)

120

Fig. 8. Settlement measured at different depths against duration: (a) atsection I; (b) at section II

Initial

30 days

60 days

90 days

uo(h)

us

0

2

4

6

8

10

12

14

16

18

20

Depth:m

100 50 0 50 100 150 200 250

Pore water pressure: kPa

(b)

Initial

30 days

60 days

90 days

uo(h)

us

0

2

4

6

8

10

12

14

16

18

20

Elevation:m

100 50 0 50 100 150 200 250

Pore water pressure: kPa

(a)

Fig. 9. Pore water pressure distribution: (a) at section I; (b) at section II

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For performance assessment, there are two advantages inusing the pore water pressure to evaluate the degree ofconsolidation: (a) the calculation is based purely on measur-ing data; and (b) the pore water pressure dissipation processin soil can be visualised clearly from the pore water pressuredistribution curve.

Increase in undrained shear strength

Field vane shear tests were conducted before and aftervacuum preloading in both sections I and II, and the resultsare presented in Fig. 10. It can be seen that, overall, the vaneshear strength has increased by about 2030%. Aftervacuum preloading a minimum bearing capacity of 80 kPa,

which was estimated as 514cu according to the design codeJTJ 250-98 (1998), was achieved.

Change in water content

The changes in water content for soil before and aftervacuum preloading are plotted in Fig. 11(a) and 11(b) for

both sections I and II. Generally the higher the initial watercontent, the greater the reduction in water content. How-ever, the change in water content is not directly proportionalto the increase in the undrained shear strength. For example,although the vane shear strength at 6 m at section I

Before

After

0 10 20 30 40 50 60

Undrained shear strength: kPa

0

2

4

6

8

10

12

14

16

18

20

Depth:m

(a)

Before

After

0 10 20 30 40 50 60

Undrained shear strength: kPa

0

2

4

6

8

10

12

14

16

18

20

Depth:m

(b)

Fig. 10. Vane shear strength profile: (a) at section I; (b) at section II

Before

After

0 20 40 60 80

Water content: %

0

2

4

6

8

10

12

14

16

18

20

Depth:m

(a)

Before

After

0 20 40 60 80

Water content: %

0

2

4

6

8

10

12

14

16

18

20

Depth:m

(b)

Fig. 11. Water content profile: (a) at section I; (b) at section II

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increased from 20 kPa to 30 kPa (see Fig. 10(a)), there wasvery little change in the water content, as shown in Fig.11(a). Therefore the change in water content is not a goodindicator in qualifying the result of soil improvement.

Lateral displacement

One special feature of the vacuum loading method is thatit causes an inward lateral movement in soil. The lateral

displacements were monitored by inclinometer for bothsections, and the data are presented in Fig. 12. It can be seenthat the lateral displacement was the greatest at ground leveland reduced sharply with depth.

As shown in Fig. 12(b), the lateral ground movementinduced by vacuum preloading was as high as 350 mm. Thecurves of ground lateral displacement against duration are

shown in Fig. 13. It can be seen that the ground lateraldisplacements approached a plateau towards the end of thevacuum preloading in both sections. This is different fromthe case presented by Chu et al. (2000), where a convergencein the ground lateral displacement was not observed. Thedifference in the lateral displacement development was duemainly to the geometry of the site. In the case reported inChu et al. (2000), the area improved was more than 400 mwide. In this case, the area preloaded was a long strip with awidth of only 51 m, and thus the lateral displacementdeveloped faster. Cracks were seen to develop on theground surface at a few metres away from the edge of thepreloaded area. As there were no adjacent buildings or

facilities, the lateral displacement and cracks were tolerable.However, for sites where adjacent structures are present,lateral displacements can cause problems.

Conclusions

A case study on the application of the vacuum preloadingmethod to the improvement of a 20 m thick soft clay layerfor a road project was reported. Based on the study, thefollowing conclusions can be drawn:

(a) The vacuum preloading method can be used effectivelyfor the improvement of a 20 m thick soft clay layer,

which would be difficult to be treated using fillsurcharge.

(b) The vacuum distribution system comprising PVDs at asquare grid of 10 m together with horizontal 100 mmdiameter corrugated flexible collector pipes was effec-tive in distributing the vacuum pressure and collectingdrained water. A vacuum pressure of 80 kPa wasmaintained throughout the whole project.

(c) After the application of an 80 kPa vacuum pressure for90 days, the average degree of consolidation achievedwas 90% based on the settlement data. The total groundsettlement was more than 15 m. The undrained shearstrength as measured by field vane shear tests increased

by 20 30% as a result of vacuum preloading.

0 50 100 150 200 250 300 350

Lateral displacement: mm

0

2

4

6

8

10

12

14

16

18

20

Depth:m

0 day

4 days

7 days

11 days

17 days

24 days

42 days

80 days

92 days

(a)

0 50 100 150 200 250 300 350

Lateral displacement: mm

0

2

4

6

8

10

12

14

16

18

20

Depth:m

0 day

4 days

7 days

11 days

17 days

24 days

42 days

80 days92 days

(b)

Fig. 12. Lateral displacement: (a) at section I; (b) at section II

Section I

Section II

0 20 40 60 80 100

Duration: days

400

350

300

250

200

150

100

50

0

Groundlateraldisplacements:mm

Fig. 13. Ground lateral displacement plotted against duration of vacuumloading

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(d ) The application of the vacuum caused an inward lateralmovement. The maximum lateral displacement meas-ured was 350 mm. Cracks on the ground surface wereobserved.

References

Asaoka A. (1978) Observational procedure of settlement prediction.Soils and Foundations, 18, No. 4, 87101.

Chen H. (1987) Analysis of the mechanism of vacuum preloadingmethod. In The Soft Soil Foundations of Tianjin (eds Z. Hou et al.),Tianjin Science and Tech. Publishing House, Tianjin, pp. 7383.

Chen H. and Bao X. C. (1983) Analysis of soil consolidation stressunder the action of negative pressure. Proceedings of the 8thEuropean Conference on Soil Mechanics and Foundation Engineering,Helsinki, 2, 591596.

Choa V. (1990) Soil improvement works at Tianjin East Pier project.Proceedings of the 10th Southeast Asian Geotechnical Conference,Taipei, 1, 4752.

Chu J., Yan S. W. and Yang H. (2000) Soil improvement by thevacuum preloading method for an oil storage station. Geotechni-

que, 50, No. 6, 625632.Hansbo S. (1997) Practical aspects of vertical drain design. Proceed-ings of the 14th International Conference on Soil Mechanics andFoundation Engineering, Hamburg, 3, 17491752.

Holtz R. D. (1975) Preloading by vacuum: current prospects.Transportation Research Record, No. 548, 2679.

JTJ 250-98 (1998) Chinese Code for Soil Foundation of Port Engineering.Ministry of Construction, China.

Kjellman W. (1952) Consolidation of clayey soils by atmosphericpressure. Proceedings of a Conference on Soil Stabilization, Massa-chusetts Institute of Technology, Boston, pp. 258263.

Qian J. H., Zhao W. B., Cheung Y. K. and Lee P. K. K. (1992) Thetheory and practice of vacuum preloading. Computers andGeotechnics, 13, 103118.

Sridharan A. and Rao S. (1981) Rectangular hyperbola fittingmethod for one-dimensional consolidation. Geotechnical TestingJournal, 4, No. 4, 161168.

TPEI (1995) Vacuum Preloading Method to Improve Soft Soils and CaseStudies. Tianjin Port Engineering Institute.

Yan S. W. and Chen H. (1986) Mechanism of vacuum preloadingmethod and the numerical method. Chinese Journal of Geotechni-cal Engineering, 8, No. 2, 3544.

Ye B. Y. et al. (1983) Soft clay improvement by packed sand drain-vacuum-preloading method. In The Soft Soil Foundations ofTianjin (eds Z. Hou et al.), Tianjin Science and TechnologyPublishing House, Tianjin, pp. 126131.

Zeng G. X. and Xie K. H. (1989) New development of the verticaldrain theories. Proceedings of the 12th International Conferenceon Soil Mechanics and Foundation Engineering, Rio de Janeiro, 2,14351438.

Discussion contributions on this paper should reach theeditor by 1 April 2004

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