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มหาวิทยาลัยอุบลราชธานี 12-14 พฤษภาคม 2553

MODIFICATION OF PRELOADING BUND FOR THE CONSTRUCTION OF OUTLET STRUCTURE OF SLUDGE LAGOON, MAHASAWAT WATER TREATMENT PLANT Khomkrit Wetchasat1 Nopparat Thuampradit2 1

Civil Work and Building Design Section Chief, Metropolitan Waterworks Authority, Thailand E-mail: [email protected] 2 Engineer, Ratchaburi Electricity Generating Holding Plc., Thailand E-mail: [email protected]

ABSTRACT : Metropolitan Waterworks Authority (MWA) had engaged into the construction of Mahasawat Water Treatment Plant and related works (Third phase). Three outlet structures are constructed on sludge lagoons for draining water from sludge disposal. In the original design, preloading bunds 9 m (EL+10.00 m MSL) high have been constructed by one step with waiting time 730 days. This paper is purposed to present the modification of preloading bund for reducing the preloading time. The alternative state loading method has been purposed, the first step loading was carried out to the high of bund up to 6 m (EL+7.00 m MSL) with the waiting time 300 days and then the second state was constructed up to the high of bund 10 m (EL+11.00 m MSL) with the waiting time 108 days. Parameters for the analysis are obtained form surface settlement plates which were install under the bund by the Asaoka theory and also the calculation of settlement is carried out by Terzaghi theory. The analysis results shown that settlements from the calculation by those above methods can be well fitted with the actual settlements obtained form the field and the preloading time was reduce 223 days form the original design. KEYWORDS : Preloading bund, Sludge lagoon, Settlement, Terzaghi theory, Asaoka theory

1. INTRODUCTION According to the supplemental specification section S2BB-4 and contract drawing no. 3SL-C-06 [3][4], the original design preloading bunds shall be constructed at the construction area of outlet structures by compacting earth bunds raised in one step up to EL+10.00 m MSL and shall be left in place until Month 24. After the construction of those original design bunds have been constructed up to EL+7.00 m MSL. There is the request to modified preloading bund by the state loading method. These modifications are to accelerate the loading period and to prevent the stability failure of bunds. The modified preloading bunds were constructed in two steps, the 1st step raised the bund up to EL+7.00 m MSL and left until Month 9 then the final step raised the embankment up to EL+11.00 m MSL and left until month 16. This construction method, it can be accelerated the consolidation settlement from 24 months to 16 months. The objective of this research is assessed experimentally and numerically the settlement of preloading bunds for the reducing settlement time. This research emphasizes to determine settlement behavior of preloading bund from the actual data obtained from the instruments in the field, to back analysis for achieving soil parameters from the actual field data (the modified preloading bund) and the related theory [1][2], to predict soil parameters of original design preloading bund by

estimating form those above back analysis parameters, to determine settlement vs. time both of original design bund and modified bund, and to find the time which can remove the preloading bund. The work is a part of construction of sludge lagoon in an attempt at minimizing the construction time. The research effort primarily involves compilation of available data on the soil boring log, the field instrumentation, and computation for the modified preloading bund design.

2. STUDY PROCEDURE Figure 1 shows study procedure. These procedure is consist of 1) collect settlement data from the field instruments under preloading bund, 2) evaluate settlement parameters of Cv, ρcf from those data for each state loading by Asaoka, 3) determine settlement vs. time from Terzaghi, 4) compare that determined settlement with actual settlements data, if that comparisons can be accepted, it will proceeded, 5) predict settlement parameters (Cv, ρcf) of the original design bunds from the parameters which are obtained from step two, 6) calculate settlement vs. time of original design bunds, 7) estimate settlement (ρcf1) at preloading time of original design bund which is specified in the specification (24 months, t1), 8) estimate preloading time from the modified preloading bund which has equal settlement with the original design bund (t2), and 9) estimate reducing time (t1-t2).

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Collect settlement Data from Instrumentation. Evaluate data from instruments. Evaluate settlement parameters ( Cv, ρcf) from field settlement data by Asaoka (1978) form each loading elevation (+ 3.90 m MSL, +7.00 m MSL and +11.00 m MSL). Calculate settlement vs. time from Terzaghi (1925) of modified preloading bund. Figure 2 Instruments which are installed at preloading bund.

Compare settlement between

Reject

Accept

Predict settlement parameters ( Cv, ρcf)of original design bund, EL+ 10.00 m MSL, by data which are obtained from embankment EL+ 3.90 m MSL, +7.00 m MSL and +11.00 m MSL. Calculate settlement vs. time from Terzaghi (1925) of original design bund. Estimate settlement, ρcf1, at the preloading time of original Estimate time from the modified preloading bund curve which has the equal settlement with the original design bund, t2.

2.2 Settlement parameters The soil settlement parameters for the modified preloading bund will be obtained by Asaoka [1]. This method is proposed for estimate the consolidation settlement form the field settlement under constant stress. This method can be done by plot settlement data from field (Figure 3). The settlement (ρc) at each interval of time Δt has been estimated since the consolidation process has been occurred. Asaoka [1] proposed the relationship between ρcj and ρcj+1 (Figure 4) by equation (1),

ρ cj +1 = β 0 + β1 ρ cj .

When: β1 is the slope of regression line, β0 is the value which intercept ρcj+1 axis. So, the total of consolidation settlement can be obtained by equation (2),

Estimate reducing time = t1-t2. End of Study Figure 1 Study procedure for this research.

2.1 Settlement Data According to the Figure 2, there are 21 surface settlement points and 6 subsurface settlement points have been installed to observe settlement behavior of preloading bund. Majority of those instruments were damaged during the compaction of preloading bund. However, there are 3 settlement points (S-23, S-24 and S25) which have enough continuation and reliable data. In this research, those settlement points have been used for studying.

(1)

ρ cf =

β0 . (1 − β1 )

(2)

Asaoka [1] proposed the equation for finding Cv which was occurred in the field,

Cv =

−5 2 ln β1 . Hd Δt 12

(3)

Soil parameters will be evaluated at each state loading including at preloading elevation +3.90 m, +7.00 m and +11.00 m MSL. Then the soil parameters of preloading elevation +10.00 m MSL (Original Design) can be estimate by relationship between log(σ) and soil settlement parameter (Cv, ρcf).

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2553 plotted together for checking reliable of those method. If plotted data can be fitted with the equation y = x and R2 value is more than 0.800, it will mean that equation of Asaoka [1] and Terzaghi [2] can be accepted and reliable.

Figure 3 Relationship between settlement and time from field data [1].

2.5 Estimate Preloading Time According to the supplemental specification section S2BB-4 and contract drawings no. 3SL-C-06 [3][4], the original design preloading bunds shall be constructed at the construction area of outlet structures by compacting earth bunds raised in one step up to EL+10.00 m MSL and shall be left in place until Month 24. These criteria, settlement of original design preloading bund can be measured at the preloading time specified in that specification (t1). Then, preloading time of modified preloading bund can be measured by the equal settlement of that original preloading bund (t2). Therefore, reducing time will be measured by, Reducing time = t1 - t2.

(6)

3. STUDY RESULTS

0.00

2

.

(4)

-0.25

U z = 1− ∑( m=0

M = (2m+1)π/2.

(5)

(6)

Cv is coefficient of consolidation. Hd is drainage path, for this research it can be measure from the thickness of soft and medium stiff clay layer (13.00 m). 2.4 Reliable of Theory and Prediction Method Predicted settlements which obtained by Asaoka [1] and Terzaghi [2] and field actual settlement will be

432 4

-0.40

2

-0.45

S-25

2 Mz ( − M 2Tv ) ) sin e , M Hd

405

6

-0.30

-0.50

m =∞

378

8

-0.20

-0.35

When: Tv is time Factor which can be found by equation (5),

351

10

-0.10

Embankment Elevation (m.MSL.)

Hd

324

-0.05

Settlement(m.)

Tv = C v

297

12

-0.15

t

270

243

216

189

162

135

108

81

54

Time(days)

27

2.3 Settlement vs. Time After all of necessary soil parameters both of modified preloading bund and original design bund are achieved. The relationship between settlement and time shall be calculated and plotted by equation which proposed by Terzaghi [2],

0

Figure 4 Method for obtaining ρcf [1].

3.1 Field Settlement Data Field settlement data form surface settlement nos. S23, S24 and S25 can be plotted vs. time as shown in the Figure 5. The average settlement from S23, S24 and S25 will be used for back analysis the soil settlement parameters.

0 S-24

S-23

Average

Embankment Fill(m. MSL.)

Figure 5 Field settlement data obtained from S23, S24 and S25.

3.2 Settlement Parameters measuring By Asaoka [1], field settlement data is plotted between ρcfi and ρcfi+1 at the time interval 10 days of each state of preloading elevation (EL+3.90 m, +7.00 m and + 11.00 m MSL) as shown in the Figures 6-8, respectively. Soil parameters which are measured by Asaoka [1] can be concluded in the Table 1.

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ρcfi+1(m)

2553 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.00

y = 0.719x + 0.033 R² = 0.982

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

ρcfi(m) Monitoring Data

Y=X Line

Linear (Monitoring Data)

Figure 6 Relationship between ρcfi and ρcfi+1 of modified preloading bund at loading state EL+3.90 m MSL. 0.60

After the soil parameters of modified preloading bund are measured, then the soil parameters of original design bunds will be estimated by plot data between stress in soil mass (σ) in term of log scale and soil settlement parameters as shown in the Figures 9 and 10. According to those relationships, regression equation is proposed. Therefore, soil settlement parameters of original design preloading bund, at embankment elevation +10.00 m MSL can be calculated by those equations. It can be concluded that soil parameters of original design preloading bund are: 1) coefficient of consolidation Cv = 1756.48 cm2/day, and 2) total consolidation settlement ρcf = 0.5374 m.

0.50

ρcfi+1(m)

0.40

45,000 y = 0.926x + 0.035 R² = 0.999

0.30

40,000 35,000

0.20

30,000

0.00 0.00

0.10

0.20

Monitoring Data

0.30

ρcfi(m) Y=X Line

0.40

0.50

0.60

Cv,cm2/day.

0.10

Linear (Monitoring Data)

y = -41031x + 58685 R² = 0.850

25,000 20,000 15,000 10,000 5,000

Figure 7 Relationship between ρcfi and ρcfi+1 of modified preloading bund at loading state EL+7.00 m MSL.

0 0.50

0.70

0.47

1.10

1.30

1.50

Log(σ), t/sq.m

0.46

y = 0.975x + 0.013 R² = 1

0.45

ρcfi+1(m)

0.90

Figure 9 Relationship between stress in soil mass and coefficient of consolidation.

0.44 0.43 0.42

Log(σ), t/sq.m

0.41

0.50 0.00

0.40 0.39 0.39

0.40

0.41

0.42

0.43

0.44

0.45

0.46

1.10

1.30

1.50

0.10 Linear (Monitoring Data)

Figure 8 Relationship between ρcfi and ρcfi+1 of modified preloading bund at loading state EL+11.00 m MSL.

ρcf,m.

Y=X Line

0.90

0.47

ρcfi(m) Monitoring Data

0.70

0.20

y = 0.747x - 0.5 R² = 0.825

0.30 0.40

Table 1 Summary of soil parameters of modified preloading bund which is measure by Asaoka theory. Embankment Elevation +3.90 +7.00 +11.00 (m MSL) Equation y = 0.7190x y = 0.9268x y = 0.9758x + 0.0336 + 0.0351 + 0.0133 0.0133 β0 0.0336 0.0351 0.9758 0.7190 0.9268 β1 0.54959 0.11957 0.47951 ρcf (m) 10 10 10 Δt (days) 13 13 13 Hd (m)

0.50 0.60

Figure 10 Relationship between stress in soil mass and total consolidation settlement (ρcf).

3.3 Estimation of Settlement vs. Time Relationship between Settlement and time both of original design preloading bund and modified preloading bund can be calculated by Terzaghi [2] as shown in Figure 11.

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600

800

0.00

12.0

-0.10

10.0

-0.20

8.0

-0.30

6.0

-0.40

4.0

-0.50

2.0

-0.60

0.0 Modified Preloading bund

Original Design

Embankment Elevation

Figure 11 Relationship between settlement and time both of original design bund and modified preloading bund.

3.4 Reliability of Prediction Method Figure 12 is shown the relationship between settlement which is predicted by Asaoka [1] and actual settlement data from field. Reliability of prediction method can be checked by plot data between actual field settlements data and prediction data as show in Figure 13.

200

Time, days 400

600

12.0

-0.10

10.0

-0.20

8.0

-0.30

6.0

-0.40

4.0

-0.50

2.0

-0.60

0.0

Modified Preloading bund

Actual Settlement

3.5 Reducing Time According to Figure 13, the predicted settlement will be estimated at the time 24 months (730 days) specified in specification is 0.4702 m. This value is equal to the settlement of modified preloading bund at time 497 days. Modified preloading bund are designed to raise the embankments in 2 step of construction, the first step preloading bund shall be raised up to EL+ 7.00 m MSL and shall be maintained for 9 month (270 days) and the final step it shall be raised up to EL+11.00 m MSL and shall be maintained for 7 month (210 days). Due to the delayed for the construction of the final step, the first step loading was maintained more than 10 months (July 4, 2004 to May 7, 2005). However, analysis results in Figure 14 are shown that the final step shall be maintained more than 108 days (3.6 months or May 14, 2005 to August 31, 2005), it can be fulfilled requirements in the specifications. Therefore, the modified preloading bund can be removed after the date August 31, 2005.

800

0.00

0

Embankment Elevation(m. MSL.)

Settlement,m.

0

According to the Figure 12, it can be fitted data with the curve of equation y = x with the R2 value 0.9733. It indicate that settlements which is predicted by Asaoka [1] and Terzaghi [2] are reliable.

Embankment Elevation

Figure 12 Relationship between predicted settlement and actual settlement.

100

200

300

Time, days 400 500

600

700

800

0

12.00

-0.1

10.00

-0.2

8.00

-0.3

6.00

-0.4

4.00

-0.5

2.00 0.00

-0.6 Actual Settlement Original Design

Embankment Elevation,m.MSL

Time, days 400

Settlement,m.

200

Embankment Elevation(m. MSL.)

Settlement,m.

0

Modified Preloading bund Embankment Elevation

Figure 14 Relationship between actual settlement time and embankment elevation.

4. CONCLUSIONS 0.50 0.45

Predicted Settlement(m.)

0.40 0.35 0.30 0.25 0.20 0.15 0.10

Trend Line Equation y=x R2 = 0.9733

0.05 0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Actual Settelment(m.)

Figure 13 Relationship between actual settlement and predicted settlement.

The study results indicate that the settlement which are estimated by back analysis form Asaoka [1] and Terzaghi [2] can be fitted with the actual settlement in the field. Therefore, the predicted settlement from that method is reliable. The prediction of this method, modified preloading bund, shall be maintained more than 497 days after commencement of the construction by 2 step of loading. The 1st step embankment shall be raised up to EL+7.00 m MSL and maintained that load for 10 month (July 4, 2004 to May 7, 2005) (conform to the actual situation in the field) and then the final step is taken by raising the embankment up to EL+11.00 m MSL and maintain that load at least 3.6 months (108 days or until August 30, 2005). Those above conditions can give the equal

การประชุมวิชาการวิศวกรรมโยธาแหงชาติครั้งที่ 15 2553 settlement to the original design preloading bund which can fulfilled the requirements specified in the specification.

ACKNOWLEDGEMENTS The authors are most grateful to Metropolitan Waterworks Authority (MWA) by Mr. Chareon Passara, Deputy Governor / Chairman of the Committee on Research and Development for Knowledge Management for providing financial support throughout this presentation. Permission to publish this paper is gratefully acknowledged. The authors also would like to thank Mr. Chutchawaln Wirawongnusorn, Director of Survey and Design Department, Mr. Pairote Darapongsataporn, Director of Water Treatment and Transmission System Construction Supervision Department, Mr. Prawit Julatitta, Director of Production System Construction Supervision Division and Mr. Suthirug Buchagul, Chief of West Bank Production System Construction Section for providing data, material supports.

REFERENCES [1] Asaoka, A., 1978. Observation Procedure of Settlement Prediction, Japanese Society of Soil Mechanics and Foundation Engineering, Vol 18, No 4. [2] Terzaghi, K., 1925. Erdbaumechanik auf. Bodernphysicalischen Grundlagen. Deuticke,Vienna. [3] MWA, 1999. Contract Drawings (3/3) for Construction of Maha Sawat Water Treatment Plant and Related Works (Third Phase), Volume IV, 3SL-C-06, p. 327, Bangkok: Metropolitan Waterworks Authority. [4] MWA, 1999. Special Condition of Contract and Supplemental Specifications for Contract of Maha Sawat Water Treatment Plant and Related Works (Third Phase), Volume I Part B, pp. S2BB-1 - S2BB-5, Bangkok: Metropolitan Waterworks Authority.

มหาวิทยาลัยอุบลราชธานี 12-14 พฤษภาคม