2019 | 72/2 | 129–135 | 10 Figs. | 2 Tabs. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society

Liquefaction potential of sands at the Krško-Brežice field, Jasna Smolar*, Matej Maček and Ana Petkovšek

University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova cesta 2, 1000 Ljubljana, Slovenia; (*corresponding author: [email protected]*; [email protected]; [email protected]) doi: 10.4154/gc.2019.12

Abstract Article history: The Krško-Brežice field is one of the most seismically active areas in Slovenia. The most dama­ th Manuscript received December 02, 2018 ging recorded earthquake with an intensity of VIII (EMS) occurred on 29 January 1917. It caused Revised manuscript accepted March 07, 2019 damage and claimed two lives. In the last 100 years, 9 earthquakes with intensity higher than VI Available online May 31, 2019 (EMS) have been recorded. At the investigated area, a top layer up to 5 m thick, consisting of recent deposit of very loose silts and sands (ML, SM, SP), covers the medium dense to dense Quaternary gravel, beneath which there are over-consolidated, uncemented Miocene silts and marls. The top layer could be prone to liquefaction, as reported for the close surroundings of Brežice, where the liquefaction phenomenon was observed during the earthquake in 1880 and during the Kupa Valley earthquake in 1909. The paper presents the results of laboratory index tests, cyclic simple shear tests and field in-

vestigations (SPT, CPT, (S)DMT, vs measurements), which were carried out to assess the lique- faction potential of the top layer at the location of the Brežice Hydroelectric Power Plant (HPP).

All results show that the top layer is prone to liquefaction for an earthquake with a 475 year re- turn period. Cyclic simple shear test results show that the liquefaction potential of horizontal Keywords: earthquake, CSR, liquefaction potential, ground for an earthquake with a 475 year return period can be reduced by the densification of silty sand, laboratory investigations, field tests the top layer to at least 95% of maximum Proctor density.

1. INTRODUCTION ground acceleration, amax, magnitude, M) and soil properties Earthquake induced liquefaction is the phenomenon in which (e.g. liquid limit, wL, plasticity index, IP, density, r, grain size loose saturated granular soil loses shear strength due to pore pres­ distribution, number of cycles to liquefaction, N, cyclic resistance sure increase, to the point where it is unable to support structures ratio, CRR). The liquefaction potential of soils can be assessed and to remain stable. Its devastating effects came to the attention from different laboratory and field investigations by several liq­ of geotechnical engineers in 1964 when the Good Friday earth­ uefaction susceptibility criteria, mostly developed for sands (PO­ quake (Magnitude, M = 9.2) in Alaska was followed by the Ni­ LITO & MARTIN, 2001, YOUD et al., 2001, ANDRUS et al., igata earthquake (M = 7.5) in Japan (KRAMER, 1996). The phe­ 2004, BOULANGER & IDRISS, 2016, 2006, IDRISS & BOU­ nomenon of liquefaction (including sand volcanoes) was observed LANGER, 2006, MARCHETTI & MARCHETTI, 2016, also in close surroundings of the Krško-Brežice field, in compa­ among others). rable geological conditions, during the Zagreb earthquake of This paper presents the assessment of liquefaction potential 1880, in the villages located in the valley of the River, and of the top layer at the location of the Brežice HPP, based on labo­ during the Kupa Valley earthquake of 1909 (VEINOVIĆ et al., ratory index and cyclic simple shear tests, as well as field cone 2007, HERAK et al., 2009; HERAK & HERAK, 2010). Re­ penetration tests (CPT/CPTu), standard penetration tests (SPT), cently, the liquefaction in Christchurch, New Zealand (2011) and flat (seismic) dilatometer tests ((S)DMT) and shear wave velocity in Indonesia (2018) reminded other professionals and scientists measurements (vs). of its devastating effects. Until the HAICHENG (1975) and TANGSHAN (1976) earth­ 2. STUDY AREA quakes it was believed that only clean sands were prone to lique­ faction (SHENGCONG & TATSUOKA, 1984). Recent investi­ 2.1. Geological and hydrological setting gations have highlighted that earthquakes can trigger shear The top layer at the location (Fig. 1) is up to 5 m thick and con­ strength loss in a broad range of types of saturated soils, from sists of loose alluvial silty sands and contains fine coal particles sand to low plasticity clays (BRAY et al., 2004, CHU et al., 2004, that originate from the flotation processes from the area around BOULANGER & IDRISS, 2006), although earthquake-induced 60 km upstream, where the coal mines have operated for the last ground failure is observed less frequently in clays than in sands. 300 years. Poplar tree roots appear up to 4 m depth and prove the CUBRINOVSKI et al. (2018) reported liquefaction in well- development of the “palaeo” ground during large flood events graded gravel with at least 30% of sands and silts during the (PETKOVŠEK et al., 2017). Kaikoura (New Zealand) earthquake (2016). Beneath the top layer, the medium dense to dense Quater­ Factors governing liquefaction may be grouped into a geo­ nary gravels classified as silty sandy gravel (GM) to poorly logical profile, consisting of seismicity at the location (e.g. peak graded gravel (GP) are deposited in thicknesses of between Geologia Croatica soft limestone. soft of inclusions some weak, with marl, silt and Miocene mented fects were theZagreb during (bluedots)andKupa observed Valley ( (yellow dots)earthquakes Figure fieldandinitsclosesurroundings at from 2.Epicentres theKrško-Brežice 567 to 2007 AD, ofearthquakes andthelocations where liquefaction-related ef 8 (NPP). Different colours represent design ground accelerations in soilrock orfirm (ARSO, 2001). Figure 1.Location oftheBrežice HPP–theinvestigated area inthechainoflower Sava river Power Hydroelectric NuclearPower plants andtheKrško Plant 130

and 12 m. The bedrock consists of over consists bedrock 12and The m. - consolidated, unce consolidated, ­ which frequently floods the area. During floods, the water level water the floods, During area. the floods frequently which and follows surface inthethe levelwater SavaRiver, fluctuation 6 m below the ground 4-6mbelow ground the from level water ranges ground The VEINOVIĆ etal., 2007, HERAK etal., 2009, HERAK &HERAK Geologia Croatica 72/2 , 2010). - Geologia Croatica ­ (5) (4) 131 g g g TS CEN moment - - - (3) -

0.370 0.350 0.290 (6) loose silt, sand (2)

, 2014). could be calculated terms are in radians. d could be calculated using g g g g g g

design ground acceleration ground design kPa

0.320 0.295 0.280 0.230 0.250 0.225 rock or firm soil rock 101 (1999), parameter r (1999), 1 atm = is between and 0.35. 0.30 =

′ =1 v ′ - - - v 5.8 5.6 6.25 s BOULANGER IDRISS & IDRISS magnitude varies with depth. In this study is evaluated it for =7.5, M =1 v s ≈ 1. s (m) is the depth the ground below (m) surface, M CSR = 7.5 and s = 7.5 z K =7.5, M M The case history values adjusted CSR to a reference magni Based on magnitude and the arguments inside the sin tude using eq. 2.

CSR where METHODS EXPERIMENTAL 3. Preliminary assessment the top of liquefaction layer potential was carried based out on its grain size distribution (SIST where depths between an m, m and for earthquake 0.5 4.0 with a 475 year return period, 1, design ground g (Table acceleration 0.350 of m. parameters Using 0.1 loose of silt, sand) listed and level GW above,

eq. 5 and eq. 6 ( 475 475 200 475 1000 1000 , ­ ­ ­ ­ return period return et et the ef ′ v s , the accele­ z et 2007 al., (1) ALEKSOVSKI et al., 2008) et al., VEINOVIĆ shear stress shear fac reduction d ALEKSOVSKI BOULANGER & IDRISS and r , 2010). et al., 2017). et 2017). al., the totalvertical stressdepthat v LAI s et al., 2017). Inside the influential area of the Bre et al., 2017) et al., LAI LAI is the peak ground surface acceleration, g max a Thedesign was ground acceleration 1) theat location (Table HERAK HERAK & PETKOVŠEK SHARE model ( detailed probabilistic seismic hazard analysis ( analysis seismic hazard detailed probabilistic method Slovenian national probabilistic seismic hazard analysis (ARSO, 2001) (ARSO, analysis hazard seismic probabilistic national Slovenian rationgravity, of tor that accounts the for dynamic response the soil profile. of 2014): plifiedLiquefaction Procedure ( where

2.3. Earthquake induced Cyclic Stress Ratio (CSR) canCSR be estimated as developed part using eq. 1, the Sim of al., 2008) and (3) recent Seismic hazard and 2008) (3) al., harmonization in Europe - SHARE ( model predicted based on three different currently(1) methodologies: nationalvalid Slovenian probabilistic seismic hazard analysis detailed probabilistic (2) seismic hazard 2001), (ARSO, 1) (Fig. analysis at the location of the HPP Brežice ( 2.2. Seismicity Figure the epicentresshows 2 andmagnitudes earthquakes of at surroundings to close 567 its from in and Krško-Brežicethe field symbols and represent2007 yellow Blue AD. locations in where liquefaction was observed and during the Zagreb (1880) earthquakes (1909) (after Kupa Valley rises the ground above surface and may remain days for ( žicethe HPP, groundwater regime has been changed due to the operation the HPP of Brežice. and Cyclic simple shear apparatus (left) and schematic illustration of the investigation (right). of the investigation (left) illustration simple shear apparatus and schematic 3. Cyclic Figure Smolar et al.: Liquefaction potential of sands at the Krško-Brežice field, Slovenia HPP. the Brežice of the location at accelerations ground 1. Design Table fective vertical stress at depth z Geologia Croatica protective latex membrane without reinforcement. without membrane latex protective a in sheathed were layer. top specimens The from specimens tact (void density ratio), desired the in on to of height and and 30 mm of 70 mm a diameter with a mould into content water optimal their at compacted statically layer specimens top on performed were by described that to similar procedure A mitigation. potential liquefaction the on impact fication densi the evaluate to void and ratio layer atnatural of top the tial poten liquefaction the study to used Inc., was veloped by Seiken 17892- *SPCT –Standard Proctor test*SPCT compaction duced from Figure 4.Grain size ofthetop distribution layer from thetest site for (grey Grain size curves distribution curves). Toyoura sand, Nova loza dvor andMoj are repro 17892ISO/TS Table ofthree samplesfrom properties thetop 2.Index layer (adapted PETKOVŠEK etal., after 2017). 132 based on the results of in results on the based before cyclic loading. stress of effective vertical fined whentheincrease of to was pore equal waterpressure 95% de was of liquefaction (Fig. 3,conditions right). beginning The undrained in ratio cyclic stress desired at 0.5 at load Hz sinusoidal horizontal with loaded were specimens The stress. effective of 75 at kPa consolidated isotropically were specimens the ration, 0.95. satu­ than After higher Bwas parameter Skempton the when completed successfully was saturation The technique. translation ISHIHARA Particle density, r Natural water content, w Parameter USCS Classification Maximum dry density, r dry SPCT*, Maximum Plasticity index,I Plasticity Optimum water w content, SPCT*, Void ratio, natural state, e Dry density,Dry r Fines content, <0.063mm(%) Natural density, r Liquid limit,w Saturation was achieved by using the CO the by using achieved was Saturation The laboratory cyclic simple shear apparatus (Fig. 3, left), de apparatus cyclic simple shear laboratory The Liquefaction potential of the top layer was evaluated also also evaluated layer of was top the potential Liquefaction 12). etal. (1980). HOSONO & YOSHIMINEHOSONO d L (t/m (%) P (%) (t/m s (t/m - 3 4) and Atterberg limits (SIST limits 4) Atterberg and ) 3 3 ) ) 0 0 (%) (-) dmax - opt situ Standard penetration tests (SPT) tests penetration Standard situ (t/m (%) (2004)and 3 ) DAS VEINOVIĆ (1992) Tests used. was - 2 TS CEN ISO/TS ISO/TS CEN TS , water, and axis axis , water, and etal. for (2007), whileboundaries potentially liquefiable andmostliquefiablesoils are reproduced from SIST-TS CENISO/TS 17892-12 SIST-TS CENISO/TS 17892-12 SIST-TS CENISO/TS 17892-2 SIST-TS CENISO/TS 17892-2 SIST-TS CENISO/TS 17892-3 SIST-TS CENISO/TS 17892-1 SIST-TS CENISO/TS 17892-4 ASTM D2487-10 Test Method DIN 18127 DIN 18127 ­ ­ ­ ­ ­ – ( 22476 ISO/TS CEN (SIST ISO 22476 EN (SIST ISO 22476 EN ments are appropriate for all soil types ( types soil for all appropriate are ments of most liquefiable soils ( soils liquefiable most of 1880, in group belong the dvor, to appeared liquefaction where Nova Moj loza and from sands graded poorly samples, tigated inves the with comparison In soils. liquefiable most to uefiable (1980), al. et liq of potentially group layer the belongs top to the After sand. graded poorly silt to sandy from vary may and layer not of is uniform top the distribution size grain The specimens. investigated of the all curves distribution size Fig. and 4 layertive specimens, top of representa properties geotechnical Table index the 2presents liquefaction potential 4.1. Indexproperties–preliminaryassessmentof RESULTS4. DISCUSSION AND CPT are recommended only for only non recommended are CPT ( experience and database extensive of more the because potential liquefaction v s ). SPT and CPT are generally preferred for the evaluation of evaluation for the preferred generally are ). CPT and SPT YOUD YOUD sample 1 - ML/CL 11) wave shear velocity measurements and et al., 2001). While the preferred SPT and and SPT al.,et 2001). preferred the While 1.15 17.4 1.23 1.65 1.30 2.64 5.06 - 30 54 1), Flat dilatometer tests (DMT) (SIST (DMT) 1), tests dilatometer Flat 9 - 3), Cone penetration tests (CPT/CPTu) tests 3), penetration Cone VEINOVIĆ (grey curves) shows the grain shows grain curves) the (grey sample 2 24–26 1.53 15.0 1.10 1.77 1.28 2.78 16.3 - et al., 2007). The Toyoura The al., 2007). et 6–7 SC 38 gravel soils, v soils, gravel ANDRUS Geologia Croatica 72/2 et al., 2004). et ISHIHARA ISHIHARA sample 3 s measure 1.61 27.5 1.01 1.34 1.23 2.64 21.5 SM 39 19 – - TS TS ­ ­ ­ ­ - Geologia Croatica - - - - - , ­ ­ ­ ­ ­ 133 IDRISS & (2014). (2006). BOULANGER IDRISS & values calculated from SPT 60 ) 1 N values calculated from CPT measure c1N BOULANGER IDRISS & IDRISS BOULANGER & , 2010). Recommended deterministic, 2010). SPT based – corrected SPT N value for 60% energy efficiencyoverbur 60% energy and for SPT N value – corrected 60 – normalized cone tip resistance. cone – normalized ) 1 c1N These findings arevalid for thehorizontal ground without Results of CPT also confirm that the top layer is prone to liqu to prone is layer confirmtop alsothe CPTthat of Results Results of SPT confirm that the top layer is prone to lique ­BOULANGER surfaceloading and at the with foundation the level GW ground groundm below surface). 0.1 (e.g. (SPT) penetration tests 4.3. Standard ( shows Figure symbols) 7 (red measurements in the top layer at the location of the Brežice HPP, HPP, Brežicethe of location the at measurements layer top thein in comparison with SPT based liquefaction case history data, ­including soils with different contents fines of ( CPT – based soil liquefaction potential assessment with the case his 8. CPT – based soil liquefaction assessment potential Figure con fines boundary toryrecommended and data different curveswith soils for (FC).tent q tory data and recommended boundary curves for soils with different fines con fines boundary toryrecommended and data different curveswith soils for (FC).tents (N of 1 atm. den pressure SPT – based soil liquefaction potential assessment with the case his 7. SPT – based soil liquefaction assessment potential Figure ments in the top layer at the location of theinBrežiceHPPcom thelocationof at ments in thelayer top parison with CPTbased liquefaction case history data, including ( fines of contents different with soils lations (boundary clean curves) sands for and cohesionless for soils with various amounts of fines (FC) presented inFig. 8 are summarized after 2014). Recommended deterministic2014). CPT based triggering corre ­triggering correlations (boundary cohesionless curves) for soils having various amounts of fines (FC), presented 7,inFig. are summarized after efaction an for earthquake year return with a 475 period. 4.5. Flat dilatometer tests (DMT) faction an for earthquake year return with a 475 period. tests (CPT/CPTu) 4.4. Cone penetration Figure the 8 shows q - ­ ­ ­ ­ HO (2001), the ), determined after after determined ), POLITO N (2004). Key: S1, S2, S3 – sample 1, (2001). , 2004). Mechanically, 2004). improved speci POLITO at the investigated area 6, (Fig. is about 0.30 =1 ′ v HOSONO HOSONO YOSHIMINE & s (1996), for an for earthquake (1996), withmagnitude a is 7.5 of ), expressed), as the ratio between achieved dry density =7.5, pr M D Results of the laboratory cyclic simple shear tests confirm CSR tion criteria proposed by by tion criteria proposed Assessment of the liquefaction on plasticity based 5. Assessment liquefac potential Figure SONO & YOSHIMINE & SONO KRAMER mens compacted maximum of to at least 95% Proctor dry density will year return very 2) resist probably the 475 (Table period earthquake, although high excess pore pressure be gene­ would rated. horizontal grey while solid the line), corresponding equivalent number stress of to liquefaction cycles ( that the intacttop isprone layer to liquefaction an for earthquake year returnwith a 475 period. Results obtained on intact top layer specimens are comparable sand with ( those the for Toyoura about 15 (Fig. 6, vertical (Fig. grey solid about 15 line). sample 2 and sample 3, e_i – initial void ratio (at preparation of the specimen), preparation (at ratio sample 2 and sample 3, e_i – initial void cyclic D_pr;s prior to loading (after – the ratio consolidation), ratio e_s – void dry density cyclic prior to loading and maximum SPCT dry specimens’ between curve red – assessed boundaryliquefaction where conditions between density, liquefaction to (above is sufficient curve)is expected resistant (below and where curve). Results of cyclic simple shear tests. Data for the Toyoura sand were sand were Toyoura the for Data Results 6. of cyclic simple shear tests. Figure aftersummarized sand also tobelongs thesame group, andwas in thepast widely used as reference material the for study liquefaction. of Using plasticity liquefaction criteria proposed by 4.2. Laboratory cyclic simple shear tests 4.2. Laboratory cyclic simple shear Figure the shear results 6 shows simple tests cyclic of as a link between the induced and stress measured cyclic ratio (CSR) num top layer also belongs to the group of potentially liquefiable to 5). liquefiable soils (Fig. to liquefaction. cycles ber of The tests were carried on intact out specimens and specimens prepared at different degreesof com paction ( Smolar et al.: Liquefaction potential of sands at the Krško-Brežice field, Slovenia and maximum dry SPCT 2. density given in Table Geologia Croatica 4.6. Shearwavevelocitymeasurements(v period. a475 with return year earthquake for an uefaction (2000) with case history data andcalculated values for(2000) withcasehistory thetop layer at theloca (2012 after Fig. in summarized 9 is presented sand clean for curve) (boundary correlation triggering based DMT istic 2016). determin Recommended REMUND, & 2016,ROLLINS regardless of the fines content silt, sandy and sand for silty data history case liquefaction based Figure 10Figure shows v the ceptibility criteria proposed by different authors. by different proposed criteria ceptibility sus liquefaction using evaluated were and curves liquefaction boundary recommended and data history case with compared were Results tests. field and laboratory using assessed field was Krško-Brežice the at layer top the of potential liquefaction The 5. CONCLUSIONS 9shows K the Figure tion oftheHPPBrežice. v for data andcurve recommended clean sand.tory K boundary Figure potential 9.DMT assessment –basedsoilliquefaction withthecasehis 134 at the location of the HPP Brežice in comparison with DMT- with comparison in Brežice HPP ofthe location the at Figure resistance proposed by 10.Liquefaction curves stress index. by by proposed curves resistance liquefaction with comparison in HPP Brežice the of location the at layer top the in measurements ity return period. return a475 with year earthquake for an liquefaction to layer prone is DRUS ANDRUS & STOKOE & ANDRUS Results of DMT also show that the top layer is prone to liq to layer prone is top the of show also DMT that Results Shear Shear thatwavethe top also velocity confirm measurements ). et al., 2004). et D s1 measured values using DMT in the top layer layer top the in DMT using values measured s1 - stress-corrected shear-wave -stress-corrected velocity. values calculated from shear wave shear veloc from calculated values (2000) and case history data ( data history (2000) case and (MARCHETTI &MARCHETTI, ANDRUS &STOKOE ROBERTSON s ) D - horizontal AN K D ­ ­ ­ ­ ­ - -

analysed in in analysed tion and Soil(RIC) The efficiencyMixing. was of eachtechnique Roller Vibratory techniques: Rapid Compaction, Impact Compac different three HPP, Brežice using the of design the of scope the in investigated layer was of top the mitigation potential efaction B Engineering for Soils of Classification for Practice Standard (2010): D2487-10 ASTM ARSO (2001): Slovenian Environment Agency. http://www.arso.gov.si/potresi/podatki/ REFERENCES ground improvement (densification). improvement ground require sites the mitigation, potential for liquefaction Thus, period. a 475 with return year earthquake for an surface ground 0.1 table a water mbelow layer, the top the assuming in efaction of liqu­ threat aclear is there that indicate analyses The methods. test various the on based potential of liquefaction evaluation the and technicians for their vital field work and data collection. fielddata and vital work for their technicians and engineers many to and program investigation the help atrealising (HSE Invest)to Mr. UNETIČ B.Sc.for Andrej his thanks special express to wouldLjubljana like SLP authors Ljubljana. and The IRGO with close No.ment cooperation in 700748 realised and agree grant under programme innovation and 2020 research zon Union’s European and HESS by Hori the work funded was This ACKNOWLEDGEMENT DIN 18127(2012):Soil,investigationandtesting-Proctor-test. 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