The aim of this paper is to evaluate soil shear strength from the Dobkovicky landslide. The landslide was activated on June 6, 2013 after heavy rains and the D8 motorway, which was under construction at the time, was damaged. The laboratory tests were carried out on two types of soils, clay and tuff, both from the surface of the rupture. Critical and residual friction angles were evaluated on both types of soils

Boháč, Jan

Kycl, Petr

Mašín, David

Roháč, Jakub

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/APP.2017.10.0048Acta Polytechnica CTU Proceedings 10:48–51, 2017 © Czech Technical University in Prague, 2017available online at http://ojs.cvut.cz/ojs/index.php/appSHEAR STRENGTH OF SOILS FROM THE DOBKOVIČKYLANDSLIDE IN THE CENTRAL BOHEMIAN UPLANDSDETERMINATED BY LABORATORY TESTSJakub Roháča,b,∗, Petr Kyclb, Jan Boháča, David Mašínaa Charles University, Faculty of Science, Albertov 6, Pragueb Czech Geological Survey, Klárov 3, Prague∗ corresponding author: jakub.rohac@geology.czAbstract. The aim of this paper is to evaluate soil shear strength from the Dobkovičky landslide.The landslide was activated on June 6, 2013 after heavy rains and the D8 motorway, which was underconstruction at the time, was damaged. The laboratory tests were carried out on two types of soils,clay and tuff, both from the surface of the rupture. Critical and residual friction angles were evaluatedon both types of soils.Keywords: Laboratory tests, the Dobkovičky landslide, shear strength.1. IntroductionThe plans for a highway connecting Prague with Dres-den have been debated since the 1930s. Constructionof the first 4.2 km of the 92 km long D8 motorwaybegan with section 0806 Řehlovice - Ústí nad Labemnearly 50 years later in 1984. The construction ofthe last remaining 16.2 km in section 0805 Lovosice -Řehlovice was launched in 2007. This section crossedthe Central Bohemian Uplands, which represent themost geologically complex and problematic part of themotorway, in particular due to the presence of a largearea of dormant and abandoned landslides known as“Prackovice landslides” intersected by the motorway.On Friday, June 7, 2013 the works were interruptedand the motorway under construction was damagedwhen heavy rains triggered a landslide near the villageof Dobkovičky (km 56.3-56.5). The damage was notrestricted only to the motorway but also affected thesingle-track railway connecting Lovosice with Teplicebetween the stations Dobkovičky and Radejčín. Therailway track was completely destroyed in the lengthof 200 m, including a 6 m high embankment.2. Dobkovičky landslideDobkovičky landslide (Fig. 1) was activated in thewell-mapped landslide area of Prackovice, which ischaracterized by landslides in its geological history.The whole area from the river Labe up to the railwayembankment is covered by morphologically visiblelandslides. On June 3, 2013, the first cracks wereobserved in the area of Dobkovičky quarry after 4days of heavy rains. The Dobkovičky landslide wasactivated in the evening of June 6, 2013.2.1. Landslide descriptionThe planar surface of the rupture was detected at adepth of 4-6.5 m below ground level by an engineering-geological survey provided by the company AZ ConsultFigure 1. Picture of the landslide after collapse(summer 2013).[1]. The length was 470 meters and width was 200 me-ters. The maximum length of the movement in thecenter of the landslide was 49 m and the maximumspeed of the movement was 1 meter per hour on June8 [2].2.2. Geology and hydrogeologyThe surface of the rupture of Dobkovičky landslideconsists of siltstone, altered basalt, tuff and anthro-pogenic material. The anthropogenic material is lo-cated in the center of the landslide in the form of arailway embankment and on the top of the landslidein the form of a dump of material from Dobkovičkyquarry. Weathered chalky siltstone (clayey character,R6) represented the slip surface from the toe to therailway line as documented in the minor scarp and inthe place of the railway embankment. Basalt (R4) was48vol. 10/2017 Shear strenght of Soils from the Dobkovičky LandslideFigure 2. Map of the landslide with sampling points.Type Sample Depth [m] WL WP IPKts HV-P220 8.3m 59.3 24.8 34.5Kts1 HV-P212 2.3m 62.5 25.5 37Tf IG 208 6.5m 87.1 52.2 34.9Table 1. Index propertiesdocumented above the railway embankment up to themain scarp. Neogene tuff had a sandy clay characterwith pieces of coal. It had an irregular representationin the surface of the rupture.The main source of groundwater is quaternary de-bris. The stratum of fresh and weathered siltstoneis considered as a hydrogeological insulator [3]. Thegeneral groundwater flow direction is from north-westto south-east. Hydrogeology of Dobkovičky quarry ischaracterized by fissured permeability, it can be as-sumed that most of the rainwater is drained throughthe fissures to the top of the impermeable siltstonesand drained to the slope under the quarry.3. Sample descriptionThe laboratory tests were carried out on two typesof soils, clay and tuff, both from the slip surface.Samples were taken during the engineering-geologicalresearch in 2014 and 2015. There are 3 main IG typesof material: Kts1-cretaceous silt stone, olive-colored(borehole HV-P 212), Kts-cretaceous siltstone, blue-gray colored (borehole HV-P 220), and Tf -tertiarytuff, yellow colored (borehole IG 208) [1]. Samplingpoints are labeled on Fig. 2. The grading curves ofthe three main IG types are shown in Fig. 3. Indexproperties are described in Tab. 1, where WL is theliquid limit, WP is the plastic limit and IP is theplasticity index.4. Sample preparation and testproceduresAll of the laboratory tests were carried out in thelaboratory of soil mechanic at the Faculty of Science.Critical parameters were evaluated in a conventionaltriaxial apparatus and in a shearbox. Residual pa-rameters were evaluated in a ring shear apparatus.All of the laboratory tests were carried out on recon-stituted soils - the material was thoroughly mixedwith distilled water until a homogeneous slurry wasobtained. The final water content of the slurry wasapproximately 1.5 times higher than the liquid limitof the material. The material was then sieved througha 2 mm mesh sieve.4.1. Triaxial testCIUP triaxial test (Consolidated Isotropic Undrainedwith Pore pressure measurement). The slurry waspoured carefully into a consolidometer and consoli-dated to a vertical stress of approximately 80 kPa.The load was applied with increments of 10 kPa everyday to prevent squeezing up the sides of the piston.After two weeks, the sample was extruded and itsends were trimmed to the correct length of 78 mm,the diameter was 38 mm. The sample was measuredand the mass of the sample was determined. Thenthe sample was placed into the triaxial cell on a basepedestal. Radial drains were used for all of the sam-ples to reduce the duration of the tests. Top drainagewas not used. The samples were isotropically consol-idated and sheared in an undrained condition withpore pressure measurements.4.2. Shear box testTests were carried out on reconstituted slurry, whichwas consolidated to a vertical stress of 60 kPa, 100 kPaand 150 kPa. The speed of shear was lower than0.0013 mm/min to ensure drained conditions.4.3. Ring shear testA Bromhead ring shear apparatus was used for eval-uation of the residual shear strength. The test wascarried out, similarly to the previous apparatus, onreconstituted soils. Slurry was consolidated to a verti-cal stress of 30 kPa, 50 kPa and 70 kPa. The speedof shear was lower than 0.023 mm/min.5. ResultsTable 2 shows the results, the types of tests and theused stresses. Eighteen laboratory test were carriedout for evaluation of the shear strength of the soilsfrom the Dobkovičky landslide. Nine of them wereCIUP (Consolidated Isotropic Undrained with Porepressure measurement) triaxial tests, three were shearbox tests and six were ring shear tests. The resultsof the three selected tests are shown in Fig. 4. Theshear parameters of the tuff are slightly higher thanthe clays. The residual parameters are approximately49J. Roháč, P. Kycl, J. Boháč, D. Mašín Acta Polytechnica CTU ProceedingsFigure 3. Grading curves of the three main IG types of soils, after [4]Table 2. Results of shear strength tests12 of the critical parameters of the same sample, whichconfirms the known theory of soil mechanics [5].6. ConclusionsThe presented laboratory tests confirm that shearstrength depends on the movement of the slip surface.Residual shear strength reached a value of between13 and 15◦, which is approximately one half of thecritical values [5]). It is planned to extend the numberof laboratory tests within the study to create a detailednumerical model of the Dobkovičky landslide.References[1] J. Suchý. D8 - odstranění sesuvu v km 56,300 - 56,500+ stabilizace oblasti kolem stavby D8 - 0805. Tech. rep.,AZ Consult, 2014.[2] P. Kycl. Situační zpráva o sesuvu Dobkovičky D8 km56,300 ke dni 11. června 2013. Tech. rep., Českágeologická služba, 2013.[3] J. Záruba. Inženýrskogeologický doprůzkum, stabilitnívýpočty v km 56,160 - 56,660. Tech. rep., Arcadisgeotechnika, 2011.[4] Z. J. S. J. Plšková, M. Předběžná zpráva d8-0805lovosice-Řehlovice, II. Etapa inženýrsko-geologickéhodopruzkumu v oblasti sesuvu Dobkovičky a v jehopředpolí v km 56.300-56.500. Tech. rep., 2016.[5] J. Atkinson. The mechanics of soils and foundations.CRC Press, 2007.50vol. 10/2017 Shear strenght of Soils from the Dobkovičky LandslideFigure 4. The results of performed tests51

SHEAR STRENGTH OF SOILS FROM THE DOBKOVICKY LANDSLIDE IN THE CENTRAL BOHEMIAN UPLANDS DETERMINATED BY LABORATORY TESTS

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SHEAR STRENGTH OF SOILS FROM THE DOBKOVICKY LANDSLIDE IN THE CENTRAL BOHEMIAN UPLANDS DETERMINATED BY LABORATORY TESTS

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