Session report dams.pdf

Session report: dams,embankments and slope stabilityAdam Bezuijen PhDProfessor, Civil Engineering, Laboratorium of Geotechnics, Ghent University,Zwijnaarde (Ghent), Belgium; Deltares, Geo-engineering, Delft,the Netherlands

This session report discusses 13 papers that were presented at the 8th International Conference on Physical

Modelling in Geotechnics 2014 in the session ‘Dams, Embankments and Slope Protection’. It concentrates on whether

scaling is possible and what the consequences of scaling are in the various papers. Some tests do not really appear

to be scaled model tests, because it is difficult to define a prototype that behaves as the model. However, such

models can still be useful to study mechanisms that are relevant in prototype.

1. IntroductionThe 8th International Conference on Physical Modelling inGeotechnics 2014 (ICPMG 2014) attracted papers from over awide range of modelling applications. The 13 papers of thissession (references in italic) described tests in beam centrifuges,a drum centrifuge, 1g model tests and large-scale model tests.Some of these large-scale tests resemble field tests rather thanmodel tests.

An overview such as this has the problem of how to group thepapers: by topic, country of the author or kind of model test.All these groupings are possible. In this overview, however,another point of view is taken. This is inspired by one of thepapers (Koelewijn et al., 2014). This paper presented six testsin three different set-ups, all on a (nearly) 1:1 scale and it isargued that these tests cannot be performed on a smaller scale.Since most of the other tests described in the remaining 12papers were performed on a smaller scale, it is interesting toinvestigate as to what are the consequences of the scaling. Firstthe arguments of Koelewijn et al. (2014) will be discussed, toinvestigate later whether or not these arguments are valid forother tests.

2. Applications in which scale-model testsare difficult

The tests in the paper by Koelewijn et al. (2014) are wave over-topping tests on a slope with vegetation. This may be difficultin a small-scale test, although Takahashi et al. (2014) showedthat it is not impossible to make scale models of vegetationand test its influence. A second model described by Koelewijnet al. is of macro-stability. Craig (2014) and Zaytsev (2014)showed that this can be tested in a model even with reinforce-ment measures. The ‘classical’ centrifuge model tests describedby Craig (2014) are important tests in the development of

physical modelling. Zaytsev (2014) described tests on thereinforcement of railway embankments and showed thatreinforcement of the soil next to the embankment with fourrows of piles can be very efficient at reducing horizontalsettlements.

For layers of peat, as present in the model described byKoelewijn et al. (2014), which include reed and wood, it maybecome more difficult to make a centrifuge model. Other argu-ments played a role in the tests described by Koelewijn et al.(2014). One is that for non-specialists a 1:1 scale model ismore convincing than a centimetres-high centrifuge model.The question can be raised if that justifies very expensive1:1 scale tests.

The last four tests described by Koelewijn et al. were all onpiping. It is known that scaling is an issue in piping exper-iments (Bezuijen and Steedman, 2010) and thus experimentson a larger scale have to be performed to construct a modelwith similar piping properties as can be expected in the field.However, another approach is also possible, as shown by VanBeek et al. (2014). They performed small-scale model tests,knowing that these would not result in the same critical gradi-ents as reality, but by simulating the flow in these tests theyhave tried to find the mechanisms that govern piping.

3. Combination of modelsTamate et al. (2014) described a 50g centrifuge model to inves-tigate the stability and failure of slopes due to earthquakes. Toknow in detail what triggers the failure and if failure can beforeseen by certain preliminary events that normally precededfailure, a 1:1 model was made in which the damage caused bythe earthquake was included in the model. In this way, therewas no need to give an earthquake loading on the large model.

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International Journal of PhysicalModelling in GeotechnicsVolume 15 Issue 2

Session report: dams, embankments andslope stabilityBezuijen

International Journal of Physical Modelling in Geotechnics,2015, 15(2), 80–84http://dx.doi.org/10.1680/ijpmg.14.00042Paper 1400042Received 26/11/2014 Accepted 12/03/2015Published online 01/06/2015Keywords: dams, barrages & reservoirs/embankments/models (physical)

ICE Publishing: All rights reserved

Downloaded by [ CURTIN UNIVERSITY OF TECHNOLOGY] on [14/10/15]. Copyright © ICE Publishing, all rights reserved.

It was found that enlargement of cracks in the upper part ofthe slope preceded failure.

Askarinejad et al. (2014) described another combination ofmodels. A centrifuge model was built to explain the results ofa field test. The effect of the bedrock shape and the drainageproperties could be studied in detail in a centrifuge model withknown boundary conditions. The influence of a buttress in thebedrock was studied in the centrifuge by performing exper-iments with and without a buttress. This is, of course, difficultin a field test. How the buttress influences the slope failurecould also be simulated by a numerical model. This is thus anexample where a small-scale test is a valuable addition to aprototype test, and it allows better analysis of the results of theprototype test.

4. Is there a prototype?The idea of a model test is that there is also a prototype.Nowadays that is not strictly necessary, because a physicalmodel is also in itself a realisation and it can be used tocompare results – for example, with the results of numericalcalculations. However, often attempts are made to make linksbetween a physical model and an existing or a possible fieldsituation. For the paper by Detert et al. (2014) this is question-able. They present an interesting new solution to building adam on soft soil, using high-strength geotextile. However,looking at their results it is not clear what geotextile strength isnecessary to make a prototype of their model.

Moreover, in model tests on static liquefaction it can be ques-tioned whether there is a prototype. As in the piping tests, inthese tests there are severe scaling issues. The onset of staticliquefaction will occur when, during shearing and consequentlycontracting of a loose-packed sand body, the pore pressuregeneration is larger than the dissipation. In a centrifuge, thedissipation is N times higher than in the prototype. Claimsthat static liquefaction is achieved in a model test (1g or Ng)must always raise some suspicion. Creating static liquefactionis very difficult in each kind of model. Increasing the watertable in a slope to create instability will lead to an increase inpore pressures, but this does not mean that there is really astatic liquefaction. For example, Take (2014) reported on cen-trifuge tests by Zhang and Ng (2003), who claim to havecreated static liquefaction. However, looking at the density ofthe sand (a ‘relative compaction density’ of 80% after somecompaction in the centrifuge) and at the pore pressures duringthe experiment, this is probably not the case. The characteristiccourse of the pore pressure during liquefaction is a very fastincrease and a slow decrease. During the fast increase of porepressure, the grains of the sand lose contact and there is somesettlement to a lower porosity. This is, for example, reported byBezuijen and Mastbergen (1988) in 1g experiments where

porosity and pore pressure were measured simultaneously(Figure 1). In these experiments, the relative density was closeto zero. The actual course of the pore pressures in time is diffi-cult to determine in the experiment by Zhang and Ng, due tothe relative low sampling rate, but they found a different distri-bution of pore pressures than expected during liquefaction.The experiment of Zhang and Ng as shown in the paper byTake (2014) indicates once more that the sampling rate shouldbe adapted to the time scale of the expected phenomena.

5. Modelling for a projectMost of the studies described so far are for research. Zepinget al. (2014) described centrifuge model tests to be useddirectly in a project. A rock-fill dam had to be built in a steepvalley and there were questions on arching effects and differen-tial settlements. It appeared that filling of the reservoir behindthe dam led to significant differential settlements and quitehigh compressive stresses (15–20MPa) in the diaphragm wall.In this case, the physical modelling tests certainly helped tooptimise the details of the dam project.

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Figure 1. Static liquefaction in a 1g test (modified afterBezuijen and Mastbergen (1988))

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International Journal of Physical Modelling in GeotechnicsVolume 15 Issue 2



6. Seismic testsSawamura et al. (2014) and Dafni and Wartman (2014)reported on seismic model tests.

Sawamura et al. (2014) tested the influence of an earthquakeon a culvert in the longitudinal direction. The results of thisstudy can be summarised as ‘let it go’. With connectionsbetween culverts, significant compressive and tensile forceswere measured during dynamic loading. When each culvertwas separated, the culverts behaved independently and hardlyany tensile force was generated. The same result was obtainedfor the connection between the wall and the culvert. For thesituations tested, the permanent deformation after the test wasmore or less comparable with or without connections.

Dafini and Wartman (2014) tested the dynamic responseof slopes and found, by modelling of models, that theground motion at the crest is frequency specific anddepends on the slope height and ground motion wavelength.In general, they concluded that ‘physical modelling in thecentrifuge proves to be a useful tool for fundamentallyunderstanding dynamic slope response and specifically topo-graphic effects provide that dense instrumentation can beemployed’.

In both papers, it is an advantage that the results could beobtained from small-scale tests, since this gave the possibilityof reproducible boundary conditions.

7. Piled embankmentsGirout et al. (2014) showed centrifuge experiments on piledembankments. An impressive set-up was created in which thevertical load on an embankment with up to 60 piles couldbe tested. The results were compared with the German Earthstructures with geosynthetic reinforcement (EBGEO) recom-mendations and with Plaxis numerical calculations. Theauthors followed a well-known procedure: one parameter, inthese tests, the maximum deflection of the geotextile betweenthe piles, is taken and the result of that parameter in the test iscompared with the results of calculations. It is implicitlyassumed that as the maximum deflection is the same then theloading on the geotextile is also the same. However, this is notthe case for piled embankments. Van Eekelen et al. (2012) haveshown that the vertical load distribution on the geotextilereinforcement can be determined from the shape of the geotex-tile deformation between the piles. A different distribution ofthe deflections means a different loading, although themaximum deflection is the same. The distribution of thedeflection is not measured in the model tests of Girout et al.(2014), but it is a result for the two calculation models used(see Figure 2).

In Figure 2, it is assumed that the maximum deflection is thesame in the two calculation models and the shape of the geo-textile is calculated for EBGEO and determined for Plaxisfrom figure 9 of Girout et al. (2014). It is clear that the shapeof the deformation is quite different. With the same maximumdeformation, the total loading on the geotextile will be morethan twice as large for the Plaxis calculation compared withthe EBGEO calculation. The example shows that comparing asingle parameter from a model test with a calculation result(or results from two different calculations) is not sufficient toknow whether there is agreement; it is also necessary to knowwhat mechanism occurs. Here, interpretation of a small-scalemodel centrifuge test may be difficult, because it is difficult tomeasure the deformation of the geotextile on differentlocations in such a test.

8. ConclusionsMost papers in this session on embankments showed thatinteresting results can still be obtained by doing small-scalemodel tests and centrifuge tests. Sometimes small-scale testsare even preferable to large-scale tests, because a better controlof the boundary conditions is possible. The scaling rules indi-cate where problems with scaled model tests can be expected.For the papers described here, this is the case with the pipingexperiments and liquefaction experiments. Moreover, it is stillpossible to use small-scale model tests – not as a representationof the field situation, but as a test on its own to study therelevant mechanism.

It is probably more difficult than in the old days, reported byCraig (2014), when some clay and a centrifuge where enoughto reach interesting results. Nowadays, more complicated set-ups and more instrumentation are often necessary.

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Figure 2. Results of Plaxis calculations compared with EBGEOassuming the same maximum deflection

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REFERENCES

Askarinejad A, Laue J and Springman SM (2014) Effectof bedrock shape and drainage properties on thestability of slopes. In ICPMG2014 – Physical Modellingin Geotechnics: Proceedings of the 8th InternationalConference on Physical Modelling in Geotechnics (GaudinC and White D (eds)). CRC Press, Boca Raton, FL, USA,pp. 1211–1217.

Bezuijen A and Mastbergen DM (1988) On the constructionof sand fill dams – part II: soil mechanical aspects.In Modelling Soil-Water-Structure Interaction SOWAS88: Proceedings of the International Symposiumon Modelling Soil-Water-Structure Interactions(Kolkman PA, Lindenberg J and Pilarczyk KW(eds)). Balkema, Rotterdam, the Netherlands,pp. 1211–1217.

Bezuijen A and Steedman RS (2010) Scaling of hydraulicprocesses. In ICPMG2010 – Physical Modelling inGeotechnics: Proceedings of the 7th InternationalConference on Physical Modelling in Geotechnics(Springman S, Laue J and Seward L (eds)). CRC Press,Boca Raton, FL, USA, pp. 93–98.

Craig WH (2014) Modelling slope failures by ‘gravity turn-on’.In ICPMG2014 – Physical Modelling in Geotechnics:Proceedings of the 8th International Conference onPhysical Modelling in Geotechnics (Gaudin C andWhite D (eds)). CRC Press, Boca Raton, FL, USA,pp. 1203–1209.

Dafni J and Wartman J (2014) Centrifuge modelingof dynamic response in slopes. In ICPMG2014 –

Physical Modelling in Geotechnics: Proceedings ofthe 8th International Conference on PhysicalModelling in Geotechnics (Gaudin C and White D(eds)). CRC Press, Boca Raton, FL, USA,pp. 1227–1232.

Detert O, König D and Schanz T (2014) Centrifugemodelling on a self-regulating foundation systemfor embankments on soft soils. In ICPMG2014 –


Girout R, Blanc M, Thorel L and Dias D (2014) Piledembankments on soft soil reinforcements withgeosynthetic. In ICPMG2014 – Physical Modelling inGeotechnics: Proceedings of the 8th InternationalConference on Physical Modelling in Geotechnics(Gaudin C and White D (eds)). CRC Press, Boca Raton,FL, USA, pp. 863–869.

Koelewijn AR, De Vries G, Van Lottum H et al. (2014)Full scale testing of piping prevention measures: threetests at the IJkdijk. In ICPMG2014 – Physical

Modelling in Geotechnics: Proceedings of the 8thInternational Conference on Physical Modelling inGeotechnics (Gaudin C and White D (eds)). CRC Press,Boca Raton, FL, USA, pp. 891–897.

Sawamura Y, Kishida K and Kimura M (2014) Dynamiccentrifuge model tests on culvert embankment withperpendicular wall in culvert longitudinal direction.In ICPMG2014 – Physical Modelling in Geotechnics:Proceedings of the 8th International Conference onPhysical Modelling in Geotechnics (Gaudin C andWhite D (eds)). CRC Press, Boca Raton, FL, USA,pp. 883–889.

Takahashi A, Nakamura K and Liktiersuang S (2014)On the seepage-induced failure of vegetation-stabilisedslopes. In ICPMG2014 – Physical Modelling inGeotechnics: Proceedings of the 8th InternationalConference on Physical Modelling in Geotechnics(Gaudin C and White D (eds)). CRC Press, Boca Raton,FL, USA, pp. 1233–1239.

Take WA (2014) Physical modelling of instabilityand flow in loose granular slopes. In ICPMG2014 –


Tamate S, Hori T, Mikunu C and Suesmasa N (2014)A combined study of centrifuge and full scale modelson failure of seismically damaged slopes. InICPMG2014 – Physical Modelling in Geotechnics:Proceedings of the 8th International Conference onPhysical Modelling in Geotechnics (Gaudin C andWhite D (eds)). CRC Press, Boca Raton, FL, USA,pp. 1219–1225.

Van Beek VM, Vandenboer K, Van Essen HM and

Bezuijen A (2014) Investigation of the backwarderosion mechanism in small scale experiments. InICPMG2014 – Physical Modelling in Geotechnics:Proceedings of the 8th International Conference onPhysical Modelling in Geotechnics (Gaudin C andWhite D (eds)). CRC Press, Boca Raton, FL, USA,pp. 855–861.

Van Eekelen SJM, Bezuijen A, Lodder HJ and Van Tol

AF (2012) Model experiments on piledembankments – part II. Geotextiles and Geomembranes32: 82–94.

Zaytsev AA (2014) Physical modeling embankment onpeat foundation with reinforcing of wooden piles. InICPMG2014 – Physical Modelling in Geotechnics:Proceedings of the 8th International Conference onPhysical Modelling in Geotechnics (Gaudin C andWhite D (eds)). CRC Press, Boca Raton, FL, USA,pp. 877–881.

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Zeping X, Yujing H and Jianhui L (2014) Three dimensionalcentrifuge modeling test of high CFRD in narrowValley. In ICPMG2014 – Physical Modelling inGeotechnics: Proceedings of the 8th InternationalConference on Physical Modelling in Geotechnics

(Gaudin C and White D (eds)). CRC Press, Boca Raton,FL, USA, pp. 899–902.

Zhang M and Ng CWW (2003) Interim Factual TestingReport I – SG30 & SR30. Hong Kong University ofScience & Technology, Hong Kong, China.

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Documents

Session report dams.pdf