Microstructure analysis of laboratory and in-situ compacted silts

Transcription

1 ES We of Conferences 9, 0600 (06) DOI: 0.0/ esconf/ E-UNSAT 06 Microstructure nlysis of lortory nd in-situ compcted silts Gicomo Russo Simon Wheeler,, Andre Rezz, Cludio Mncuso, Vincenzo Oliviero, Frncesc D Onz, Domenico Gllipoli nd Deprtment of Civil nd Mechnicl Engineering, University of Cssino nd Southern Lzio, Itly Deprtment of Deprtment of Civil, Architecturl nd Environmentl Engineering, University of Nples Federico II, Itly Itlin Ntionl Agency for the New Technologies, Energy nd Sustinle Economic Development, ENEA, Itly Lortoire SIAME, Université de Pu et des Pys de l Adour, Frnce School of Engineering, University of Glsgow, UK Astrct. The pper presents nd discusses some results of n experimentl reserch imed t nlysing the influence of compction vriles (w nd energy) nd method on the resulting microstructure of compcted silty soil. In prticulr, the experimentl dt here discussed llow to compre the microstructure induced y different dynmic compction techniques, compring tht chrcterising specimens otined y two lortory methods (Proctor stndrd nd Hrvrd) nd tht of smples compcted in-situ during the construction of n emnkment uilt for river regimenttion purposes. Both undistured nd distured smples hve een retrieved from the emnkment, the ltter one with the purpose of collecting the soil susequently used for lortory compction. Microstructurl nlyses (SEM, MIP) performed on lortory nd in-situ compcted smples evidenced sustntil similrity of the texture induced y the vrious compction techniques, highlighting tht lortory compction is suitle to provide soil smples representtive of erth in-situ compcted soil. Introduction At the phenomenologicl scle of lortory tests the experimentl evidence highlights tht the microstructure of compcted soils depends on wter content nd dry density dopted for the specimen preprtion. In fct, it is well known tht soil compcted wet of optimum, tends to hve homogenous texture, wheres those compcted on the dry side tends to exhiit imodl pore size distriution, referred to s doule structure [,,,, ], with two or more distinct clsses of pore dimensions chrcterising the soil texture. Typiclly, there re two distinct clsses of pores, the lrger one chrcterising the inter-ggregte pore spce nd the smller ones typicl of the ggregtes, thus nmed intr-ggregte pores [6, 7]. The distriution of the pore clsses is likely to influence the idro-mechnicl ehviour of the compcted soils. In the pper some results on microstructurl nlyses on in-situ nd lortory compcted smples re reported. Some of the tested smples hve een retrieved from n experimentl emnkment, identicl to the rel emnkments used for river regimenttion purposes, uilt in order to evlute the evolution of suction cused y soil-tmosphere nd simulted flooding in rel erth structure [8]. The emnkment ws constructed using the sme compction techniques dopted for service emnkments using fine-grined soil of medium plsticity tken from the floodplin. Other smples (from the sme soil) hve een compcted in the lortory using the Proctor stndrd nd the Hrvrd compction techniques. All the ove-mentioned group of smples hve een sujected to n extensive testing progrm in order to chrcterise their hydro-mechnicl ehviour nd to lortory tests to nlyse their microstructure. These ltter tests form the focus of the present pper nd re sed on mercury intrusion porosimetry (MIP) performed in order to otin quntittive description of the microstructure of soils nd the evolution of the pore size distriution (PSD) fter mechnicl nd hydrulic pths. MIP tests llowed the comprison of pore size distriutions induced y different lortory compction procedures nd initil wter contents. Scnning electron microscopy (SEM) oservtions hve een lso performed to further investigte the compction induced fric of the mentioned clsses of smples. Mteril nd experimentl procedures. Alluvil silt The soil is n lluvil sndy silt with reltively low cly frction, s showed y the grin size distriution reported in Figure. The min physicl properties of the soil re reported in Tle. Overll the soil is clssified s n inorgnic silt of medium/high plsticity. Corresponding uthor: girusso@unics.it The Authors, pulished y EDP Sciences. This is n open ccess rticle distriuted under the terms of the Cretive Commons Attriution License.0 (

2 ES We of Conferences 9, 0600 (06) DOI: 0.0/ esconf/ E-UNSAT 06 g/cm of the Proctor stndrd mximum dry density t wopt = 0.7%, s requested y the ASTM procedure. In Figure the Hrvrd compction curve is lso reported nd compred with the Proctor one. The initil stte of the in-situ smple B is lso shown together with some points (crosses) otined y the Hrvrd compction procedure. Tle. Physicl properties of lluvil silt. Specific grvity Gs.7 Plstic limit wp 0 Liquid limit wl 7 Plsticity Index PI d (g/cm ) PP (%) Hrvrd compction curve..0 0 Hrvrd compcted smples dimeter (mm) B A in-situ undistured smples Proctor compction curve w (%) Figure. Stndrd Proctor compction curve nd undistured in-situ compcted smples. 0 Figure. Grin size distriution of lluvil silt.. Microstructure nlyses. Compction MIP tests were performed y doule chmer Micromeritics Autopore III pprtus. In the filling pprtus (diltometer) the smples were outgssed under vcuum nd then filled y mercury llowing the increse of solute pressure up to the mient one. Using the sme unit the intrusion pressure ws thn rised up to pproximtely 00 kp y mens of compressed ir. The detected entrnce pore dimeters rnges etween m nd 7. m (pproximtely 0.0 MP - 0. MP for mercury contct ngle of 9 ). After depressuristion to the mient pressure, the smples were trnsferred to the high-pressure unit, where the mercury pressure ws incresed up to 0 MP following previously set intrusion progrm. At ny intrusion step time sufficient to oserve qusi-sttic penetrtion of mercury [0] ws llowed. Corrections to the pore-size distriution due to the compressiility of the intrusion system were pplied performing lnk test. The inner centrl region of ech compcted smple ws crefully cut in smll pieces of pproximtely g to otin the MIP specimens. Smple dehydrtion requested efore performing MIP nlyses ws otined ccording to the freeze-drying method []. SEM nlyses were performed on dehydrted smples in order to highlight their fric. The Stndrd Proctor compction curve otined for this soil y the ASTM D stndrd is reported in Figure, showing n optimum wter content (wopt) of 0.7% nd (mximum) dry density ( dmx) of.6 g/cm. The nlysis of the in situ undistured smples retrieved during the construction (here not reported) confirmed tht soil construction of the emnkment ws performed in wet conditions [8], with wter contents higher thn the reference Proctor optimum. It is worth noting, however, tht the dt shown refer to smples retrieved from the emnkment some yers fter its construction, thus the ctul wter content vlues re likely to e lso influenced y wetting-drying cycles tht the emnkment itself underwent under service conditions. In order to reproduce initil conditions similr to the in-situ ones, it ws decided to dopt the Hrvrd procedure [9] s it llows to otin smples smller of the Proctor ones with comprle physicl properties. As well known, the Hrvrd compction pprtus consists in smll volume mould (6. cm) hold y n extension collr, se nd clmps. The tmper consists of rss rod crrying spring, with wooden hndle on one end. Springs of different stiffness re ville to perform compction. The soil is compcted in five lyers under the lod trnsferred y pushing down selected numer of times the hndle, with the possiility of selecting the spring stiffness. The Hrvrd compction procedure hs een clirted with the stndrd Proctor procedure following the ASTM D A numer of pressures of the spring of stiffness 0 l resulted in dry density of the soil within the intervl of ± 0.06 Results The cumultive intruded volume hs een represented s function of the entrnce size of pores in terms of intruded void rtio emip, defined s the rtio etween the mercury intruded volume nd the volume of solids. At the end of the intrusion stge, emip cn e directly compred to the reference void rtio of the smple e0 determined in

3 ES We of Conferences 9, 0600 (06) DOI: 0.0/ esconf/ E-UNSAT 06 trditionl wy y mesuring the volume nd wter content of the smple used for the MIP tests. The initil properties of the MIP smples considered in this pper re reported in Tle. intruded void rtio emip Tle. Physicl properties of MIP smples. Smple w (%) e0 emip Hrvrd w_opt e0(prct)=0.67 e0(hrvd)=0.670 Hrvrd w_opt pore entrnce dimeter (μm) 0.89 demip/d(logp) Hrvrd w_opt Hrvrd in situ 00 6 In Figure comprison etween MIP tests on stndrd Proctor nd Hrvrd compcted smples t optimum wter content is reported, evidencing the similr texture of the specimen despite the different compction techniques. In prticulr, Figure shows difference in the cumulted intrusion void rtios fter the intrusion stge, with higher intruded void rtio emip for the Hrvrd smple, closer to the initil void rtio e0. Both the pore size distriutions show (Figure ) monomodl curve, with the most frequent vlue of pore entrnce size etween m. In Figure some SEM imges of the two smples re reported, confirming the similrity of their microstructurl orgniztion, independently on the compction method. The occurrence of slightly lrger pores cn e oserved for the Hrvrd smple (Figure ). As result, the similr fetures induced y different compction techniques t optimum wter content confirmed the suitility of the Hrvrd technique for prepring MIP smples comprle with the Proctor stndrd one. Figure reports the results of the MIP tests performed on the in-situ compcted smples A nd B (see Figure ) nd the Hrvrd one prepred t the sme initil wter content of B. Compring the cumultive intruded void rtio of the in-situ smples (Figure ), it cn e oserved tht for the smple A the intruded void rtio is higher thn the smple B, s expected from its higher initil wter content nd lower dry density. The cumultive intruded volume of the Hrvrd smple compcted t insitu wter content is closer to the B in situ smples, s expected for the initil stte of the smples. The intruded void rtio t the end of the intrusion stge emip is lower thn the initil void rtio e0 of the smples, highlighting similr degree of not intruded porosity of the three smples pore entrnce dimeter (μm) Figure. MIP results on Stndrd Proctor nd Hrvrd compcted smples t optimum wter content. ) intruded void rtio, ) pore size density function. Figure. SEM nlyses on ) Stndrd Proctor nd ) Hrvrd compcted smples.

4 ES We of Conferences 9, 0600 (06) DOI: 0.0/ esconf/ E-UNSAT 06 intruded void rtio emip e0(a)=0.909 e0(b)=0.870 e0(hrvd.)=0.89 The frequency of pore entrnce dimeters is very similr for oth the in-situ nd lortory compcted smples. The mono-modl curves of the pore size density functions showed in Figure highlight close correspondence, with the most frequent entrnce pore dimeter in the rnge m. A slighter difference cn e found in the frequency of smller pores (pore entrnce dimeter < 0. m), which is systemticlly higher for the in situ undistured smples A nd B with respect to the Hrvrd smple. Finlly, Figure 6 shows the SEM imges of the in-situ B smple nd Hrvrd one. The ggregted fric of the two smples re quite similr prt for the detection of pores of reltively lrge entrnce size for the smple B in the rnge etween m nd 0 m (consistent with the slightly higher frequency evidenced y the MIP results in the sme rnge), nd n pprent less frequency of smll pores inside the ggregtes chrcterizing the fric of the Hrvrd smple. Hrvrd_in situ pore entrnce dimeter (μm).0 Hrvrd_in situ demip/d(logp).00 Conclusions The pper reports some results of joined reserch progrm etween severl institutions (see the heding of the pper) on the hydro-mechnicl ehviour of compcted lluvil silt. The results of MIP tests nd SEM imges from undistured smples tken from n experimentl emnkment nd specimens otined in the lortory y different compction techniques re presented nd discussed to nlyse possile effects of the compction vriles (w, energy) nd method on the microstructure of the resulting soils. To this end, the microstructure of Proctor stndrd, Hrvrd nd in-situ undistured smples hs een nlysed nd compred. The comprison of Proctor stndrd nd Hrvrd specimens t the optimum wter content nd density highlighted very similr soil frics, suggesting tht the compction technique does not ply such n importnt role on the finl texture tken on y compcted soils. The comprison etween the microstructure of in situ undistured smples nd Hrvrd ones compcted on the wet side t very similr wter content nd dry density lso highlighted significnt common fetures of the microstructure. In oth the ove mentioned cses, the results seem to confirm the vlidity of using lortory compcted smple s reference for the nlysis of the hydro-mechnicl ehviour of soils used s construction mterils pore entrnce dimeter (μm) Figure. MIP results on in-situ smples A nd B nd Hrvrd compcted smple t in-situ wter content. ) intruded void rtio, ) pore size density function. Acknowledgments The contriution of the Europen Commission vi the Creer Integrtion Grnt SAFES (Service Assessment nd Filure of Erth Structures), contrct n. PCIG09-GA0-977, is grtefully cknowledged. Figure 6. SEM nlyses ) on in-situ smple B nd ) Hrvrd compcted smple t in-situ wter content.

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