A specific protocol to characterize Intermediate ...

Geotechnical and Geophysical Site Characterization 4 – Coutinho & Mayne (eds) © 2013 Taylor & Francis Group, London, ISBN 978-0-415-62136-6

A specific protocol to characterize Intermediate Geomaterials (IGM). The case of Porto granites N. Cruz Rodoviária-Mota-Engil, Porto, Portugal Universidade de Aveiro, Aveiro, Portugal

A. Viana da Fonseca Faculdade de Engenharia, University of Porto, Porto, Portugal

C. Rodrigues Mota-Engil & Polytechnic Institute of Guarda, Portugal

ABSTRACT: The suitability of a specific geotechnical survey is dependent on several issues such as installation needs, time of performance, cost-effectiveness and adequacy of results to geotechnical projects. Residual soil profiles are usually erratic, frequently showing hard horizons and/or boulders included and dispersed in a weathered to decomposed rock mass, being also commonly in unsaturated conditions. The usual practice, in Portugal as in many other regions of the world, is to use dynamic probing (SPT or DPSH) as the main source of geotechnical information, from which the limitation of derived parameters is rather inadequate to take advantage of modern numerical tools available for design. However, by combining other more comprehensive and powerful testing techniques, such as PMT, DMT and CPTu tests and also, when it is possible, geophysical surveys specifically for the evaluation of shear wave velocities (SDMT, SCPTu and cross-hole tests are excellent means for that), it is possible to access good quality and proficuous information for the whole range of intermediate granitic geomaterials (W4 to loose soil) with no special extra-cost. Herein, departing from the available scientific references and in a specific calibration framework performed within a PhD work (Cruz 2010) a practical characterization test protocol that can be easily applied to engineering practice in residual soils is outlined, in order to contribute to a better geotechnical parameterization and, as a consequence, to increase efficiency level in practical engineering design. Keywords:

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DMT tests, cemented soils, suction

INTRODUCTION

The north and central regions of Portugal are largely dominated by upper layers of residual soils from different nature, namely originated in granite and schist rock masses. The field work for the research work present herein is located in Porto Metropolitan area where Porto Granite Formation dominates. The geomorphology associated to the area of Porto is based in a set of hills that are going smoothly down in height towards the Atlantic Ocean, while the Douro valley is confined by abrupt side walls. Up north, after the Ave Valley, the platform is covered by marine erosion. The fundamental geological unit of this area (Porto Granitic Formation) can be described as a leucocratic alkaline rock, comprising a mixture of glassy quartz, white alkali-feldspar often in mega-crystals, biotite and muscovite with the latter prevailing, white

sodic plagioclase and minor amounts of dark minerals. The alkali feldspar usually presents the higher grain size and is mostly orthoclase, sometimes microcline. As for plagioclases, oligoclase-albite and albite are commonly present (COBA 2003; Viana da Fonseca et al. 2006). The residual soils arising from these formations are the result of mechanical and chemical weathering, respectively by means of grain fracturing disaggregation and hydrolysis of K-feldspar and Na-feldspar, which lead to the formation of kaolinitic clay, while quartz and muscovite remain stable due to their high weathering resistance. Biotite undergoes oxidation to form iron oxides. From mechanical point of view, these granitic masses are very complex and mostly characterized by its gradation from upper levels to lower sound rock, generally improving its behaviour with depth. Typical weathering profiles in the area show a

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global decrease of its levels to deeper sound rock, and so, inherent improvement of its geomechanical properties, from upper residual soils to the correspondent slightly weathered (W2) rock. In general, the typical profile is composed by a medium dense residual soil, referenced by NSPT ranging from 10 to 30 blows, often followed by a transition layer (30 < NSPT < 60) to the decomposed to weathered rock massif (W5 to W4). Local experience reveals that the medium dense layer can reach depths of 15–20 m, while the transition layer, when existing, is generally thinner than 4–5 m, with their behaviours basically commanded by micro-fabric. Decomposed to highly weathered rock massif, underlay these residual soils, and are usually characterized by NSPT values higher than 60, showing a balanced influence of macro and microfabrics in the global mechanical response. Finally, in depth the weathering degree gradually decreases to slightly (W2) or medium weathered (W3) rock massif, represented by rock mechanics. Although this may suggests an homogeneous evolution with depth, these formations show erratic profiles, either horizontally or with depth as a consequence of diverse weathering factors, such as composition of the parent rock, intensity and continuity of joint systems, as well as climate conditions. From theoretical background, these residual masses are widely known as showing mechanical behaviour different from those established for sedimentary transported soils, mainly due to the following characteristics (Vaughan et al. 1988; Viana da Fonseca 1996; Schnaid et al. 2004; Viana da Fonseca & Coutinho 2008; Cruz 2010): a. Variable grain strength within the same soil mass, with void ratios or densities depending on weathering level, instead of stress history. b. Presence of a cemented matrix that plays an important role on strength and stiffness behaviour, especially at shallow depths (low confining stresses); c. Interparticle bonding that generates a cohesivefrictional material expressed in Mohr-Coulomb strength criterion; the cohesion intercept and the angle of shearing resistance cannot be derived by the common sedimentary correlations; d. High stiffness, at small strain levels, due to the presence of cementation structure; significant drop in this stiffness property for moderate to high stress-strain levels is expected, close to the high non-linearity of mechanical constitutive laws, typical of meta-stable geomaterials; e. Macro-fabric can have an important influence in general behaviour of decomposed to highly weathered rock masses (W4 to W5);

f. Water levels at significant depth are frequent in residual profiles, generating suction phenomena with significant influence in strength and stiffness properties; in Porto region, as in many other residual environments, it is rather common to observe vertical excavations in these materials, as a consequence of both interparticle bonding and suction, natural or induced by pumping in excavations (Topa Gomes 2009). As a consequence of these features, geotechnical behaviour has to be described in a different manner and using specific correlations to derive geotechnical parameters. The work presented in the following sections is a result of several years of research related with the use of in-situ testing as fundamental frames to characterize residual soils and decomposed rock massifs (intermediate geomaterials, IGM), in this case arising from Porto and Guarda Granitic Formations. Available data from Porto and Guarda granitic formations was gathered creating a continuous chain that combines general trends and high quality data, based in wide sort of testing equipments. The final goal of the research aimed at the establishment of a practical characterization set of procedures that could be easily and efficiently applied to engineering practice in these materials

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THE FUNDAMENTS OF THE METHOD

The geology and geotechnical properties of the ground underlain the city of Porto is well characterized in Porto Geotechnical Map, PGM (Coba 2003), a document published by Porto City Hall site, and congregates a wide sort of qualitative and quantitative information collected during decades of projects and investments in civil engineering infra-structures around the city area, from which registers of the conventional geological and geotechnical in-situ testing were indexed and statistically treated to generate local definition of geotechnical units for engineering purposes. This information was organized by different materials and its evolution with the weathering degrees (ISRM 1981), allowing to define trends and ranges of magnitude of practical geotechnical parameters throughout weathering. This data was enlarged outside the city limits (in the same granitic formation) by the data collected in geotechnical campaigns performed since 1995 by the first author of this paper, within his professional activity, using mainly more recent in-situ tests techniques, such as flat dilatometer (DMT), seismic piezocone (SCPTu), pressuremeter tests (PMT) (Cruz et al. 1997; Cruz et al. 2004a; Cruz & Viana da Fonseca 2006a), as well as a wide range of geophysical techniques. This information

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Table 1.

CPTu and DMT test parameters from residual soils.

NSPT

Unified class. qc (MPa

fs (kPa)

P0 (MPa)

P1 (MPa)

ID

ED (MPa)

KD

30–60 10–30 5–10

SM SM-SP SC-ML

>300 250–400 100–250

>0.5 0.1–0.5 0.05–0.3

>2 0.5–3 0.2–1.5

1.5–4.5 1.5–4.5 1.0–1.75

>50 5–60 3–20

>15 5–20 3–7

10–20 1–10 60 >60 30–60 10–30

SM (W5-4) SM (W5) SM SM-SP

1.5–2.0 1.4–3.6 0.8–1.2 0.3–0.5

3.0–4.5 3.5–6.5 1.4–3.0 0.8–2.0

60–400 50–200 30–60 10–30

PY—Yield pressure; Pl—Limit Pressure; EPMT—Presseremeter modulus.

helped to calibrate ranges of geotechnical parameters arising from PGM, while it was also used as a link to high quality experimental sites of Porto and Guarda, referenced in international literature (CICCOPN, ISC2/CEFEUP, Av. França, Hospital de Matosinhos, IPG Guarda). It is important to mention that generally all these experimental sites were related to PhD research programs, which increases the confidence on recoiled data and on the performed analysis (Viana da Fonseca 1996; Rodrigues 2003; Topa Gomes 2009; Ferreira 2009; Cruz 2010; Rios da Silva 2011, among others). As a consequence, the whole set of information allowed to establish major characteristics and its evolution with weathering, supported by a statistically significant data base, indexed to high quality information arising from the analysis of other well controlled experimental sites. Summarizing the main physical trends, the studied materials (considering the range within W4 to loose soils) are usually well graded, revealing an increase of fine content, plasticity, porosity and void ratios with weathering, while in-situ permeability seems to decrease with depth, although some exception can be detected due to of some rock masses, in the opposite direction of weathering. Unified (ASTM 1988) and AASHTO classifications show convergent information, revealing silty sands to clayey sands or low plasticity silts, while in terms of Wesley Classification (Wesley 1988; Wesley & Irfan 1997) these soils don’t show strong mineralogical influence (Group A) ranging from A(a) to A(b) groups, respectively soils in which macrofabric plays an important role in mechanical behaviour (in this case, W4 and W5 rock massifs, generally represented by NSPT > 60) and soils where macro-fabric no longer exists (generally with

NSPT