Geotechnical Characterization for the Magneti Marelli Factory in Crevalcore (Bologna): DMT, CPTU and Laboratory Tests Comparison
Federico Fiorelli TELEIOS srl, Castel Maggiore (BO), Italy. E-mail: [email protected] Marco Franceschini TELEIOS srl, Castel Maggiore (BO), Italy. E-mail: [email protected] Rocco Carbonella RdR Geologia & Geotecnica, Bologna, Italy. E-mail: [email protected]
Keywords: ground response analysis, DMT vs CPTu, Plaxis Dynamic, HS Small model, G-γ decay curves
ABSTRACT: The present paper concerns the geotechnical aspects regarding the reconstruction interventions of the Magneti Marelli factory, located in the Municipality of Crevalcore (BO). Great relevance will be reserved to the interpretation of investigations and the definition of the vertical profile of the most important geotechnical parameters, focusing on the comparison between DMT, CPTu and laboratory tests results in the definition of geotechnical model. The seismic ground response analyses, that were conducted employing 1D EERA and 2D Plaxis Dynamic calculations, will be summarized in the second part of the paper.
1 INTRODUCTION From the geotechnical point of view the work provided: design and execution of the soundings, The 2012 Emilia-Romagna earthquakes caused geological characterization, assessment of several damages to the structures of the Magneti liquefaction susceptibility of soils, geotechnical Marelli factory, located in the Municipality of characterization for seismic ground response Crevalcore (Bologna). Indeed it was necessary to analysis and design of deep foundations, execution design strengthening interventions and seismic of the analysis and project of the new foundations. improvements of the structures. The present paper will describe the surveys and The whole geotechnical and structural design of their interpretation, the geotechnical model and the the interventions was committed to Teleios Srl results from site effect analysis. engineering company. Particular attention will be provided to the use of Those earthquakes showed relevant site effects, the flat dilatometer (DMT) for the geotechnical so geotechnical aspects had a primary importance characterization proposing a comparison between for the design. results obtained from DMT, CPTu and laboratory As part of the assignment before mentioned, a tests. wide campaign of surveys was planned for the geological and geotechnical characterization of the intervention area. 2 GEOTECHNICAL SURVEYS During a preliminary evaluation it was found the need to employ deep foundations for the new The surveys were chosen, in typology, number and structures because of the characteristics of soil and location, by the geotechnical designer considering actions that those new earthquake resistant structures the parameters necessary for the analysis. discharge at the base. For the seismic ground response analysis an In addition, a seismic ground response analysis accurate geological characterization was conducted was conducted to obtain site specific design tools, in addition to geophysical soundings for the such as response spectra and accelerograms, that estimation of shear waves velocity Vs and cyclic replace the use of the simplified methods proposed laboratory tests for the measurement of G-γ and D-γ in the Italian technical code NTC (2008). curves (where G is the shear modulus, D is the damping ratio and γ the shear strain).
In order to design deep foundations for the new have to be performed, the constitutive laws can earthquake-resistant structures, the homogeneous change and, consequently, the values of the soil layers were investigated in terms of strength and parameters. deformability parameters. Thus a 31.0 m deep 0 1 2 3 4 0 10 20 30 40 50 60 0 2 4 6 8 10 borehole with soil sampling and stratigraphy, two 0 0 0 40.0 m deep CPTu and a 20.0 m deep DMT were ID MDMT (MPa) KD performed for soil characterization from lithological, strength and stiffness point of view (see Fig. 1 and 5 5 5 Fig. 2). Considering geophysical tests, three surface wave tests (MASW), three passive seismic tests (HVSR) 10 10 10 and one Down-Hole test were conducted while one direct shear test, two oedometric tests, two consolidated drained (TX CD) triaxial tests, two 15 15 15 NC soil consolidated undrained (TX CIU) triaxial tests and KD = 2 two resonant column (RC) tests were performed in z (m) z (m) z (m) laboratory. 20 20 20
Fig. 2. DMT data: from left, material index ID, 0 5 10 15 20 0.00 0.10 0.20 0.0 0.5 1.0 1.5 2.0 constrained modulus MDMT, horizontal stress index KD. 0 0 0 qt (MPa) fs (MPa) u (MPa) 5 5 5 In this paragraph the principal geotechnical parameters will be treated while in the following one 10 10 10 the geotechnical model employed in the site effect
15 15 15 analysis will be shown. The model concerning the design of deep foundations will not be described. 20 20 20 The soil, differently from the building materials,
25 25 25 is characterized by a high heterogeneity and variability of its characteristics. For this reason the 30 30 30 principal parameters, obtained from geotechnical and geophysical soundings, will be compared to 35 35 35 evaluate the reliability of each test to investigate a z (m) z (m) z (m) 40 40 40 specific parameter. In particular, the comparisons will be focused on Fig. 1. Data from CPTu 2: from left, cone resistance qt, DMT and CPTu tests. sleeve friction fs and pore pressure u2.
In Fig. 2 the intermediate parameters from DMT are shown, as defined by Marchetti (1980), Eq. (1): 3.1 Litho-stratigraphic profile It is very important to define an accurate pp1−− 0 pu 00 ID=; K DD =; E = 34.7 ⋅−( pp10) (1) stratigraphic profile in order to identify soil layers pu− σ ' 00 v0 that can be considered homogeneous for the In Eq. (1) p0 and p1 are the corrected readings mechanical response. from flat dilatometer, u0 is the in situ equilibrium This profile was directly obtained from the pore pressure and σ'v0 is the effective overburden borehole and it was integrated applying correlations stress prior to blade insertion. with CPTu and DMT data, that are a very useful support. For the CPTu tests, three correlations for 3 GEOTECHNICAL CHARACTERIZATION lithological interpretation were applied: the ones by Robertson & Cabal (2010), Schneider et al. (2008) The geotechnical parameters were defined and Fellenius (2009). The results are shown in Fig. 3 interpreting the results from soundings; the whole and Fig. 4. Instead, for the DMT sounding the interpretation was curated by geotechnical designers. correlation proposed by Marchetti (1980) was used A geotechnical model is a set of parameters that, (Fig. 5). together with a constitutive model, allows to In Fig. 5 Ic is Soil Behavior Type index describe mathematically the mechanical response of (Robertson & Cabal 2010, Eq. (2)), while ID is the the soil. Indeed considering the calculations that material index (Marchetti 1980, Eq. (1)).
interesting to examine this parameter because of the important presence of fine grained soils on site. The undrained shear strength su was measured in the TX CIU laboratory tests and also obtained via correlations from DMT and CPTu. The comparison of those results, shown in Fig. 6, is useful to evaluate how the different surveys are able to investigate this parameter.
IC from CPTu ID from DMT 0 1 2 3 4 5 10 1 0 0 0 Sand Silt Clay Sand Silt Clay
5 5
Fig. 3. Robertson (2010) profiling chart. 10 10
15 15
20 20
25 25
30 30
35 35
z (m) z (m) 40 40
Fig. 5. Comparison between stratigraphy from CPTu and Fig. 4. Schneider et al (2008) and Fellenius (2009) DMT. profiling chart. Table 1. Stratigraphy resume. 0.5 =−22 ++ Layer 1 From ground level to 3.0 m from g.l. IC((3.47 log QF tr) ( log 1.22) ) (2) SUPERFICIAL ALTERED LAYER Where Qt is the normalized cone resistance Eq. (3) Layer 2 From 3.0 m from g.l. to 24.0 m from g.l. LAY ILTY CLAY and Fr is the normalized friction ratio Eq. (4): C – S Layer 3 From 24.0 m from g.l. to 30.0 m from g.l. Qqt=( tv −σσ00) ' v (3) SILTY SAND Layer 4 From 30.0 m from g.l. to 40.0 m from g.l. Fr=( fq st( −⋅σ v0 )) 100% (4) CLAY – SILTY CLAY As shown in Fig. 6, the agreement between the As shown in Fig. 5, the feedback from different test is reasonably good. su from DMT, that lithological characterization via DMT and CPTu is was derived applying Marchetti (1980) correlation good. (Eq. (5)), fit quite well the data obtained from TX Please note that a sandy layer is situated between CIU tests. 24.0 m and 30.0 m from the surface. The 1.25 liquefaction assessment, that is not included in this SKu=0.22 ⋅⋅( 0.5Dv) ⋅σ ' 0 (5) paper, has shown that this is a non liquefiable layer. Table 1 summarizes the stratigraphic profile. For CPTu, it was looked for the correlations that best fit the su from DMT and TX CIU. Best results were obtained employing the formulation reported in Eq. (6). 3.2 Undrained shear strength su −σ The undrained shear strength su describes fine qcv Su = (6) grained soil from the resistance point of view in Nk short term condition and it is very important for the evaluation of axial capacity of piles. Hence it is The su profile, shown in green in Fig. 6, was obtained using Nk = 16. This is a site specific value
of the parameter Nk, calibrated according to the derived. Then a comparison with the constrained DMT and TX CIU data. Instead the su profile, modulus MDMT, obtained from DMT, was proposed. shown in blue in Fig. 6, was calculated applying the (Please note that MDMT is equal to Eoed). factor Nk according to Robertson (2012, see Eq. In the PLAXIS HS Small constitutive model it is (7)): very important the E50 modulus, i.e. the stiffness that soil shows at 50% of the yielding stress. This NF=10.5 +⋅ 7 log (7) kr( ) modulus can be directly derived from TX CD, and it was also proposed in the comparison in order to 0 25 50 75 100 125 150 compare with the oedometric ones. 0 S (kPa) Su CPTu Fig. 7 illustrates the correlations of Mitchell & u Nk = 16 Gardner (1975, Eq. (8)), Kulhawy & Mayne (1990, 5 Su CPTu Nk Eq. (9)) and Tonni & Gottardi (2012, Eq. (10)). Robertson Su TX CIU = ⋅ 10 Eqoed 2.5 c (8) Su DMT Eqoed =8.25 ⋅−( cσ v0 ) (9) 15
Eoed =1.35 ⋅⋅ Ic( qc −σ v0 ) (10) 20 Considering the MDMT profile, it can be observed (Fig. 7) that CPTu correlations highly underestimate 25 the moduli. The results from oedometric tests are aligned with CPTu correlations and this is 30 compatible with what mentioned about disturbance caused to the samples. 35 In order to establish a better correlation for CPTu data, a site specific coefficient α = 12 was defined z (m) 40 for Eq. (11), using DMT data. Eq=α ⋅ (11) Fig. 6. Undrained shear strength su of cohesive soils from oed c laboratory tests, DMT and CPTu. 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 CPTu Mitchell Eoed (MPa) 3.3 Oedometric modulus Eoed fine grained soils & Gardner 5 CPTu Tonni & Among the geotechnical parameters, stiffness Gottardi parameters are of great importance but are also 10 CPTu Kulhawy delicate and complex to be investigated. & Mayne 15 It is known that in a borehole the soil sampling DMT causes a disturbance to the specimen that influences the laboratory results in terms of deformation 20 oedometric moduli. Indeed it is preferred to rely as much as tests possible on in situ tests, that are able to investigate 25 the natural condition of the soil. However, not all in situ tests are characterized by 30 the same reliability. For example, in a CPTu the cone causes the rupture of the soil and so there is 35 less accuracy about stiffness parameters. Instead the z (m) DMT was designed to investigate soil stiffness and 40 its reliability was widely demonstrated (Baligh & Fig. 7. Constrained modulus Eoed of cohesive soils from Scott 1975). DMT, CPTu and lab tests. Laboratory data are available from oedometric and TX CD tests that were carried out on two In Fig. 8 the lab data are referred to E50 moduli samples taken from the silty-clay layer (from 3.0 m obtained from TX CD. At least in the present case to 24.0 m from surface). In addition there are MDMT measured with flat dilatometer is similar to correlations with CPTu and DMT. E50 values. However, considering that TX CD tests From CPTu and from oedometric lab tests are also influenced by disturbance caused to the oedometric moduli (Eoed) for cohesive soils were samples, this is not sufficient to state that MDMT
correspond to E50 in HS Small model instead of For DMT Marchetti formulation (Marchetti et al. Eoed. 2008) was used to estimate G0, from which Vs can be easily derived (see Eq. (14)). 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 The comparisons between Vs measured and 0 DMT Eoed (MPa) estimated from CPTu and DMT are shown in Fig. 9. 5 fine grained soils TX CD (E50) The CPTu-Vs correlations provide good results in tests the first 15.0 m but below that depth they are not 10 CPTu alpha able to predict the stiffness change shown by user defined geophysical investigations. 15 −1.066 ID <⇒0.6 GM0 DMT =26.177 ⋅ KD 20 << ⇒ = ⋅−0.921 z (m) 0.6ID 1.8 GM0 DMT 15.686 KD (14) 25 −0.7967 ID >⇒1.8 GM0 DMT =4.5613 ⋅ KD Fig. 8. E from CPTu: site specific correlation. oed The DMT-Vs correlation is able to predict correctly the geophysical results, and in particular 3.4 Shear waves velocity Vs the stiffness increase at the depth of 15.0 m from ground level. The good agreement between MASW, HVSR and a Down Hole have been geophysical tests, CPTu and DMT gives to Vs performed to evaluate the shear waves velocity profile, that was very important for the aim of the vertical profile. work, a great reliability. Those investigations were useful not only for seismic ground response analysis but also for the 0 100 200 300 400 0 design of deep foundations because, via the small MASW 1 VS (m/s) strain stiffness G0, they were used for example to 5 estimate the load-settlement curves and in numerical MASW 2 BEM codes. 10 MASW 3 The Vs profile was very important for the ground response analysis, as for one-dimentional 15 Down-Hole approaches Vs is a direct input, while in Plaxis bi- 20 Colombi-Fioravante dimensional finite element analysis Vs are used to Rix&Stokoe obtain G values, one of the input parameters in HS 25 Colombi-Fioravante 0 Hegazy&Mayne Small model. DMT HS Small differs from Hardening Soil model 30 because it is able to account for the soil stiffness at 35 small strain, thanks to two additional input z (m) parameters, the tangent modulus G0 and the value of 40 shear strain γ at which G is decreased to 70% of 0,7 Fig. 9. Vs profiles from geophysical tests, CPTu and its initial value, used to define the stiffness and DMT. damping vs. shear strain curves (Benz 2007). It is possible to obtain a Vs profile also via correlation with DMT and CPTu. The correlations 3.5 G-γ decay curves that are best able to predict Vs from CPTu, in the present case study, are those of Rix & Stokoe (1991, The stiffness decay curves G-γ play a primary role in Eq. (12)) and Hegazy & Mayne (1995, Eq. (13)). seismic ground response analyses. In 1D EERA Colombi et al. (2007) have defined A, α and β analyses they are a direct input while in 2D analyses, coefficients for Ferrara area. that were performed using Plaxis Dynamic, the HS α Small constitutive model includes laws that, starting qc from input parameters, allows to generate the Vs= A ⋅ (12) corresponding G-γ curves (in terms of stiffness) and pa D-γ (in terms of damping ratio). To directly αβ qf investigate the decay curves two cyclic laboratory Vs=⋅⋅ A cs (13) tests in resonant column were performed. Fig. 10 ppaa shows one of the measured G-γ curves. Recently researchers worked on the relationship between the G-γ decay curves and the flat
dilatometer (Amoroso et al. 2014, Amoroso et al. stiffness decreases with respect to its initial tangent 2012, Marchetti et al. 2008). The basic concept is value. In Fig. 12 the yellow points represent the that, performing a SDMT, or a standard DMT and G0/MDMT ratio at various depth: it can be seen how knowing the Vs profile via other geophysical they are aligned with the bibliographic data (white soundings, two points of the curve are known: the points from Marchetti et al. 2008). initial one G0, with the initial tangent value of the 1.2 stiffness linked to Vs, and a second point GDMT in G/Gmax RC1 -11.0m which the initial value of the stiffness is decreased to 0 1 after Maugeri, 1988 the one obtained by flat dilatometer. Hence a shape after Amoroso et al, of the curve that is coherent with those two points 0.8 2014 could be defined to obtain the complete G-γ curve. Gdmt/Go from DMT
Using this approach, the strain value corresponding 0.6 gammaDMT after to GDMT, i.e. γDMT, is defined through a range. Amoroso et al. 2014 0.4 gammaDMTafter 1.2 Maugeri, 1988
1 normalized shear G/G modulus normalized 0.2 0.8 0
0.6 0 G/G 0.0001 0.001 0.01 0.1 1 10 100 0.4 shear strain γ (%) γDMT = 0.75% ÷ 1.7% 0.2 0 Fig. 11. Comparison of G-γ curves from RC and DMT. 0.0001 0.001 0.01 0.1 1 10 100 shear strain γ (%) –1.0066 –0.921 clay - G0/MDMT = 26.177KD silt - G0/MDMT = 15.686 KD Fig. 10. G-γ curve measured in one of the RC tests. 30 30 Clayey sample taken at a depth of 11.0 m. 20 20 DMT DMT /M
In particular, two shapes of the curves were /M 0 0 G considered. The first was taken from Maugeri & G 10 10 Carrubba (1988, Eq. (15)) while the second from Amoroso et al. (2014, Eq. (16)). 0 0 0 10 20 30 0 10 20 30 K K G 1 D D = (15) β Fig. 12. G /M ratio for clayey and silty soils. G0 1+⋅αγ (%) 0 DMT G 1 = (16) G G γ 0 11+0 −⋅ 4 SEISMIC GROUND RESPONSE ANALYSIS GDMT γ DMT Comparing data from DMT at the same depths at The aim of the seismic ground response analysis is which the RC tests were performed and decay to evaluate how a seismic signal changes while curves from RC tests it was possible to calibrate the crosses soil layers. parameters of Eq. (15), i.e. α and β, and the The analyses were performed using EERA parameter γDMT of Eq. (16) to obtain the curves that (Equivalent-linear Earthquake site Response best fit those from RC. Then the obtained parameters Analyses of layered soil deposits, Bardet et al. can be applied to evaluate the curves also at other 2000), a one-dimensional 1D equivalent-linear depth where lab tests have not been performed but in approach, and Plaxis Dynamic (Plaxis 2014), a 2D which DMT data are known. γDMT was found in a non linear FEM software. range between 0.75% and 1.7%, in agreement with All the calculations are based on 10 bibliographic data, reported in the background of accelerograms that are compatible with the Fig. 11. parameters defining the seismic hazard of the site The great advantage of this approach is that, with respect to Italian building code (NTC 2008). while lab testing are rarely performed and are The 1D calculations need the definition of the however punctual, G-γ curves from DMT provide litho-stratigraphic profile, the Vs vertical profile and continuous information with depth. the G-γ and D-γ decay curves. The decay of stiffness was also studied The 2D Plaxis analyses were performed employing, once again, the flat dilatometer and data employing the HS Small model. Table 2 summarizes from Down-Hole to evaluate, in function of the the principal parameters of the geotechnical model. different lithology and with depth, how much the Only those concerning the first 40.0 m of soil are
reported because are those that were directly Among the results, response spectra have a investigated by the surveys. In the analysis the data particular interest because not only they are more were extrapolated with depth, referring also to frequently applied in the design but they also allow geological characterization and to other deep to evaluate changes induced in the input signal by soundings performed in the Po valley to reach the the soil in function of the vibration period. Results bedrock depth considered in the calculations. from 10 accelerograms have been statistically treated to obtain an average spectrum and a Table 2. HS Small model parameters. confidence interval, as can be observed in Fig. 13. Layer: C1A 1.0 From: 0 m To: -16.0 m NTC2008 soil class A NTC2008 soil class C Parameter U.M. Value NTC2008 soil class D Unit weight γ kN/m3 18 SeismHome1 0.8 SeismHome2 Cohesion c' kPa 7 SeismHome3 Friction angle Φ’ ° 16 SeismHome4 ref SeismHome5 Secant stiffness E50 MPa 24 0.6 SeismHome6 ref SeismHome7 Oedometric modulus Eoed MPa 16 ref ER L112/2007 01 Unloading-reloading modulus Eur MPa 80 ER L112/2007 02 Power function m - 0.4 0.4 ER L112/2007 03 ref Average spectrum MEDIO Small strain shear modulus G0 MPa 65 Average spectrum + SIGMA Average spectrum - SIGMA Shear strain γ0.7 - 4E-4 0.2 Layer: C1B
From: -16.0 m To: -24.0 m (g) Se(T) acceleration Spectral Parameter U.M. Value 0.0 3 Unit weight γ kN/m 18 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Cohesion c' kPa 7 Period T (s) Friction angle Φ’ ° 16 ref Secant stiffness E50 MPa 30 Fig. 13. Response spectra from Plaxis 2D FEM analysis. ref Oedometric modulus Eoed MPa 20 ref Unloading-reloading modulus Eur MPa 100 With the simplified method proposed by technical Power function m - 0.5 code NTC 2008, the effects induced by stratigraphic ref Small strain shear modulus G0 MPa 90 amplification can be taken into account by defining Shear strain γ - 4E-4 0.7 the soil type in terms of Vs,30, that represents a Layer: S1 weighted value of Vs in the first 30.0 m of depth. From: -24.0 m To: -30.0 m Geophysical tests show values of V close to Parameter U.M. Value s,30 Unit weight γ kN/m3 19 the boundary between class C and D, since Cohesion c' kPa 1 Vs,30 ≅ 180 m/s. As illustrated in Fig. 14 there are Friction angle Φ’ ° 29 significant differences between the two ref Secant stiffness E50 MPa 20 corresponding spectra. The choice depends on the ref Oedometric modulus Eoed MPa 20 designer’s choice. ref Unloading-reloading modulus Eur MPa 60 Power function m - 0.5 1.0 ref NTC2008 soil class A Small strain shear modulus G0 MPa 80 NTC2008 soil class C Shear strain γ0.7 - 2E-4 Layer: C2 0.8 NTC2008 soil class D From: -30.0 m To: -40.0 m EERA 1D average spectrum Parameter U.M. Value PLAXIS 2D average spectrum Unit weight γ kN/m3 18 0.6 Cohesion c' kPa 7 Friction angle Φ’ ° 21 ref 0.4 Secant stiffness E50 MPa 36 ref Oedometric modulus Eoed MPa 24 ref Unloading-reloading modulus Eur MPa 120 0.2 Power function m - 0.5 ref Small strain shear modulus G0 MPa 70 (g) Se(T) acceleration Spectral Shear strain γ0.7 - 4E-4 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 From 1D and 2D analysis response spectra and Period T (s) accelerograms were calculated to be used in the design of reinforcement interventions and seismic Fig. 14. Average response spectra from 1D EERA improvements of the structures required after the analyses and Plaxis 2D Dynamic FEM analyses. earthquakes of 2012.
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