Geotechnical site investigation of municipal solid waste landfills

D.A.F. Oliveira Caixa Econômica Federal and University of Brasília, DF, Brazil

P. Murrieta Environmental and Civil Engineering Department, University of Brasília, DF, Brazil

Keywords: municipal solid waste fills, mechanical behaviour, CPTU, SPT, PLT

ABSTRACT: In the actual stage of “waste mechanics”, conventional methods of soil mechanics are used.This paper discusses lessons learned in investigation of municipal solid waste landfills using standardpenetration tests (SPT), cone penetration tests with pore pressure measurements (CPTU) and plate loadingtests (PLT). The results are analyzed and compared to existing documents on properties of refuse. As mainconclusion it was possible to use common in situ tests of classical geotechnics like SPT, CPTU and PLT toevaluate, in qualitative terms, some behaviours, even with all difficulties that the material imposes

1 INTRODUCTION

Questions about municipal refuse safety disposal and the need for increased landfill capacity due to the declining of available new sites, have been leading many researchers to study the mechanical behaviour of municipal solid waste landfills (MSWL).

The quantification of mechanical properties of this material faces two major problems: the first is that it is an unusual kind of material with a complex structure with large heterogeneity that geotechnical engineering may not have appropriate equipment to deal with and the second one is that it is a degradable material with properties that may change with time. It is also an unusual topic in classical geotechnics.

In classical geotechnics three approaches have been used to evaluate refuse properties: laboratory testing, back calculation of field test and operational records and in situ testing. Trying to avoid problems of sample representatively, some authors have been making efforts to obtain data using the two last approaches.

Due the inexistence of appropriate methods, in the actual stage of “waste mechanics”, conventional methods of soil mechanics are still acceptable.

This paper discusses lessons learned in investigation of different municipal solid waste landfills using standard penetration tests (SPT), cone penetration tests with pore pressure measuring (CPTU) and plate loading tests (PLT).

2 STANDARD PENATRATION TEST (SPT)

Two landfills were investigated using standard penetration tests. One was Brasilia’s (DF-Distrito Federal-Brazil) sanitary landfill and the other was Salvador’s (Bahia-Brazil) sanitary landfill called AMC.

The tests performed in Brasilia’s landfill were executed in four experimental cells with different accelerated degradation systems. Besides evaluating mechanical resistance of the deposited material, these tests were also done at different times (date format dd/mm/yy) trying to verify the influence of degradation on penetration resistance.

Due to the limited depth of these cells, approximately 3,50m, the tests were stopped at maximums depth of 3,00m to avoid reaching the foundation drainage system. For the same reason the number of blows was counted continuously at every 0,15m. The SPT value (N) was adopted as the sum of the last 0,30m at every 0,45m tested. Figures 1 and 2 show the results of these tests.

The test performed in AMC was done to identify soil layers and also evaluate the mechanical resistance of the waste fill. In this case the blow number was counted at every 0,15m for the last 0,45m at every 1,00m depth. The SPT value adopted was also the sum of the last 0,30m at every 0,45m tested. Figure 3 shows the SPT results done in AMC.

Only for the AMC test a soil layer could be identified in a 15m depth.

Due the erratic behaviour refuse no energy correction was performed. In both cases a tendency

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Proceedings ISC-2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)© 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

of increase of blow number with depth can be verified, probably influenced by vertical stresses.

For the tests performed in the experimental cells the average value of blow number varies between 3 and 8. For the AMC test the average value of blow number is 11, excluding numbers greater than 20 that is probably the result of a rigid object.

Cell I

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 5 10 15

Blow number

De

pth

(m

) -

Nf (06/02/01)

Nf (26/04/00) F02

Nf (19/09/01)

Cell II

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 5 10 15

Blow numberD

epth

(m

) -

Nf (06/02/01)

Nf (19/09/01)

Figure 1. SPT results variations for cells I and II.

Cell III

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 5 10 15 20

Blow number

Depth

(m

) -

Nf (07/02/01)

Nf (19/09/01)

Cell IV

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0 5 10 15

Blow number

Depth

(m

) -

Nf (07/02/01)

Nf (19/09/01)

Figure 2. SPT results variations for cells III and IV.

According to Jucá et al. (1997) SPT values up to 10 are generally obtained. The average value obtained for the AMC test, is greater than this value

proposed by these authors, probably is result of the good field compaction that imposes unit weight greater than 10kN/m

3 and vertical stresses at depth.

Using the average SPT values and classical cohesionless soils correlations, considering that currently no one better was proposed, values of friction angle between 31º and 36º are obtained.

0,0

2,0

4,0

6,0

8,0

10,0

12,0

14,0

16,0

18,0

0 10 20 30 40 50

Blow number

Depth

(m

) -

Nf (14/11/01)

Figure 3. SPT performed on AMC landfill.

These friction angle values are in accordance to the recommended range of shear strength parameters proposed by Singh and Murphy (1990).

Due the large heterogeneity of the material it was not possible to verify any behaviour tendency of variation with time of penetration resistance. For Cell I the medium SPT value reduced from 8 in April 2000 to 6 in September 2001. For Cells II and IV the values increased from 6 to 8 and 3 to 5, respectively, from February 2001 to September 2001. And for Cell III the medium SPT value was the same for this period.

3 CONE PENETRATION TEST WITH PORE PRESSURE MEASUREMENT (CPTU)

Three CPTU tests were done in AMC landfill. The results are shown in figures 4, 5 and 6.

Due the possibility of negative measurements, all tests begin with a 100kPa pore pressure value (backsaturation). The first 0,50m measurements refer to a soil cover.

As could be seen in figures 4, 5 and 6, rigid objects were frequently found, resulting in tip resistance peaks.

1326 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Depth

(m

)

0.0 4.0 8.0 12.0

Tip resistance qc (MPa)

70.0 140.0 210.0

Friction Sleeve fs (kPa)

70.0 140.0 210.0

Pore Pressure u (kPa)

7.0 14.0

Friction Ratio (%)

Figure 4. CPTU01 results

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

Depth

(m

)

0 9 18 27 36 45

Tip resistance qc (MPa)

100 200 300 400 500

Friction Sleeve fs (kPa)

70 140 210

Pore Pressure u (kPa)

20 40 60 80

Friction Ratio (%)

Figure 5. CPTU02 results

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

Depth

(m

)

0 2 4 6 8 10 12 14 16

Tip resistance qc (MPa)

50 100 150 200 250

Friction Sleeve fs (kPa)

70 140 210

Pore Pressure u (kPa)

5 10 15 20 25 30

Friction Ratio (%)

Figure 6. CPTU03 results

1327Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

In almost all cases it could be verified a slight tendency of increase with depth of tip resistance, friction sleeve and pore pressure. It wasn’t verified only for CPTU02 for depths greater than 10,25m, because of a very high tip resistance peak (43Mpa), that probably had interfered in the last measure-ments.

Due the high permeability of municipal refuse drained analysis was carried out.

The mediums values of tip resistance for CPTU01, CPTU02 and CPTU03, excluding peaks, are 2920kPa, 3560kPa and 3200kPa respectively. The mediums values of friction ratio are 2,40%, 2,10% and 2,63% respectively.

Oliveira (2002) plotted these values in Schmertmann chart and observed that municipal refuse behaviour indicates classifications as clayey sands, sands and silts.

In the same way as done in SPT tests, using medium values of tip resistance and cohesionless soils correlations, friction angles between 27º and 38º are obtained.

These values of friction angle are also in accordance with the range proposed by Singh and Murphy (1990) and very close to the values obtained by Carvalho (1999) in CPT tests in Bandeirantes Sanitary Landfill (São Paulo, Brazil).

No correlation between the SPT (AMC) values and CPTU03 (CPTU nearest SPT-AMC) tendency line values could be established (figure 7).

y = 0,276x

R2 = -79,141

0,0

1,0

2,0

3,0

4,0

5,0

6,0

0 10 20 30

Blow number - N (SPT)

Tip

res

ista

nce

qc (

Mpa)

Regression

Figure 7. Correlation between SPT-AMC and CPT03.

For depths greater than 3,50m the increase of pore pressure values is almost linear. This increase is probably influenced by both leachate and gas pore pressure. The level of leachate in the waste fill could be verified in a test pit in a 4,50m depth. The use of this measurement in CPT tests can give in qualitative terms some idea of pore pressure level in the waste fill.

4 PLATE LOADING TEST (PLT)

Two tests were done in AMC Sanitary Landfill (Salvador, Bahia, Brazil) and two other in the West

Residential Sector of “São Sebastião” city (Distrito Federal, Brazil). This waste fill will be called in this paper as DF landfill.

These two tests were performed trying to evaluate the bearing capacity and verify the stress vs. deformation behaviour.

Comparison of results of the two tests was done only in qualitative terms because of the differences between the two refuses, like the composition, age, degradation stage, etc and the different tests methodology adopted.

The plate used in the tests executed in AMC was 0,80m diameter, directly founded on refuse (without any soil cover) and because of the long time that settlements takes to stabilize and considering that this is a sanitary landfill still in operation, these tests were performed in accordance to ASTM D1196 method (Standard method for non repetitive static plate load test of soil in flexible pavements).

For the tests executed in the DF landfill, also directly on the refuse (without any soil cover), a 0,60m diameter plate was used. As this landfill is no more in operation, the consideration of load stabilisation for the DF test called PLTCL was the smaller value between the necessary time to obtain a settlement ratio (difference between consecutive settlement reads divided by total settlement in current load stage) less than 5% or 30 minutes. For the DF test called PLTCR the consideration of load stabilisation was 8min for loading stage and 5min for unloading stage. The PLTCR was performed immediately after the PLTCL and in the same place.

Figures 8 and 9 show the results plotted in natural scale of these tests.

In both cases the load vs. settlement curves were almost linear which turned impossible evaluation of the ultimate capacity load with this scale, except for the test called PLTCR which is considered a quick test. The quick test makes possible to reach greater loads because the settlements don’t have sufficient time to stabilize.

Trying to establish more clearly the ultimate loads for all tests, the results were plotted in semi-logarithm graphics as shown in figure 10 and 11.

For the tests performed in AMC landfill, even using semi-logarithm scale, it is very difficult to identify clearly its ultimate load. This is a result of the high compressibility of young municipal refuses (less than 2 years) that imposes a linear load vs. settlement behaviour. In spite of it, a slight increase of curve inclination can be verified at 39kN load.

For the DF tests it could be observed that for loads greater than 78kN the rupture process of the material is increased.

These ultimate loads (39kN and 78kN) result, for each plate diameter and a factor of safety equal 3, in 26kPa and 92kPa bearing capacity stresses.

1328 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9

0

40

80

120

160

200

240

Settle

ment (m

m)

PLT02

PLT03

0 40 80 120

Load (kN)

Figure 8. PLT02 and PLT03 (AMC tests) results in natural scale.

PLTCL

PLTCR

0 40 80 120 160 200

Load (kN)

0

10

20

30

40

50

60

Settle

ment (m

m)

Figure 9. PLTCL and PLTCR (DF tests) results in natural scale.

0

40

80

120

160

200

240

Settle

ment (m

m)

10.00 100.00

Load (kN)

PLT02

PLT03

Figure 10. PLT02 and PLT03 (AMC tests) results in logarithm scale.

0

10

20

30

40

50

60

Settle

ment (m

m)

PLTCL

PLTCR

1 10 100

Load (kN)

Figure 11. PLTCL and PLTCR (DF tests) results in logarithm scale.

These values, allied to the settlement ratios verified in the tests (showed in figures 12, 13 and 14) disallow the use of shallow foundations directly founded on this refuses.

0%

10%

20%

30%

40%

50%

0 20 40 60 80 100

Load Time (min)

Se

ttle

men

t ra

tio

19kN

39kN

59kN

78kN

98kN

Figure 12. PLT02 settlements ratio

0%

10%

20%

30%

40%

0 20 40 60 80 100

Load time (min)

Set

tlem

ent ra

tio

19kN

39kN

59kN

78kN

98kN

Figure 13. PLT03 settlements ratio

0%

10%

20%

30%

40%

50%

60%

70%

0 100 200 300Load time (min)

Set

tlem

ent

rati

o

137kN

54kN

41kN

27kN

Figure 14. PLCR and PLTCL settlements ratios

The engineering practices indicate that with settlements ratios smaller than 5% the buildings presents no foundation problems. For all tests this ratios were greater than 5%. In AMC case these ratios vary between 15% and 20% and in the DF case are near 10%. More analysis of these PLT tests can be found on Oliveira et al. (2003)

In this regard the use of these bearing capacities obtained imposes elevated risks.

5 CONCLUSIONS

The use of conventional methods (SPT, CPTU, PLT) of site investigation in municipal solid waste landfills could be verified, even all difficult that the material imposes, like rigid object, etc. These tests

1329Proceedings ISCʼ2 on Geotechnical and Geophysical Site Characterization, Viana da Fonseca & Mayne (eds.)

can give qualitative data about the mechanical behaviour of the waste fill.

Both in SPT tests as in CPTU tests increases of mechanical resistance in depth (blow number, tip resistance, sleeve friction) could be verified. It can indicate the influence of vertical tension stresses in densification of the material.

All values of friction angles obtained by cohesionless correlations were in accordance to range of shear parameters recommended in literature.

The values of friction angle obtained and the Schmertmann chart classification indicates that refuses has also frictional behaviour.

Because of the complexity structure of the material and large heterogeneity it was not possible to verify any variation with time of penetration resistance using SPT test.

The values of pore pressure obtained indicate that the material has not a completely drained behaviour.

The large deformations observed in PLT tests, the large settlement rates and the impossibility to verify clearly the failure load induces the adoption of low bearing capacities that disallow the use of shallow foundations directly founded on refuses.

ACKNOWLEDGEMENT

Special thanks to LIMPURB (Salvador/Bahia/ Brazil), VEGA Environmental Engineering S.A., LCL Consulting and Engineering Ltda., JSE Foundations Ltda. and Environmental Geotechnics Laboratory and Geotechnics Laboratory of Federal University of Bahia.

REFERENCES

Carvalho, M.F. Mechanical Behaviour of Municipal Solid Waste. Doctorate thesis - Escola de Engenharia de São Carlos, Universidade de São Paulo, São Paulo, Brazil, 1999: 300p.

Jucá, J.F.; Cabral, J.J.P.S; Monteiro, V.E.D; Santos, S.M. & Perrier Jr., G.S. Geotechnics of a Municipal Solid Waste Landfill in Recife, Brazil. Proceedings of Recent Developments in Soil and Pavement Mechanics, Almeida (ed), Balkema, Rotterdam, ISBN9054108851,1997: p.429-436

Oliveira, D.A.F. Slope stability of Municipal Solid Waste Landfills. Master Dissertation, Environmental and Civil Department, University of Brasília, Distrito Federal, Brazil, 2002: 155p.

Oliveira, D.A.F; Passos, P.G.O; Camapum de Carvalho, J.; Cunha, R.P. 2003. Bearing capacity of solid waste fills. Proceedings of V Brazilian Conference on Environmental Geotechnics, REGEO’2003, March 23-26, Porto Alegre, Rio Grande do Sul, Brazil, 2003: 8p.

Singh, S.; Murphy, B. J. Evaluation of Slope Stability of Sanitary Landfills. Geotechnics of Waste Fills - Theory and Practice, ASTM STP 1070, Arvid Landva, G. David Knowles, (ed), American Society for Testing and Materials, Philadelphia, 1990, p.240 - 258.

1330 © 2004 Millpress, Rotterdam, ISBN 90 5966 009 9