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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. Underground Construction in Soft Ground, Fujila & Kusakabe (eds) © 1995 Balkema, Rotterdam. ISBN 90 5410 536 4 Italian national report on braced walls C. Mascardi U.Caste11otti & M. Mauro StudioG. Geotecnico Gatti Italiano Srl,A. Milano, Paviani Italy Ing. G Rodio & C Spa, Melegnano, Italy PolitecnicoG. Pugliese di Milano, Italy ETambara Trevi Spa, Cesena, Italy Metropolitana Milane§e Spa, Milano, Italy Geosud Spa, Roma, Italy / SYNOPSIS: The Italian practice for design, construction and monitoring of braced walls in soft and loose situation.soils is illustrated, with reference to the occurrence of such formations in the italian geological 1 OCCURRENCE OF SOFT/LOOSE SOILS IN ITALY, THEIR 2 THE USE OF BRACED WALLS IN ITALY GEOLOGY AND ENGINEERING PROPERTIES Braced walls have been widely in use in Italy for Soft and loose soils in Italy have been origi­ underground construction in many types of works, nated_ mainly within three geological environ­ and typically forz' ments, with different modes: - subway lines and stations; a) Continental and marine deposits. - road underpasses; b) Eluvial and colluvial deposits from soft - underground car parks; - water intakes; sedimentaryc) Pyroglastic deposits associated rocks. to volcanic p - harbour facilities. activity. The vertical retaining structures are generally The geographic distribution of these environ­ obtained by diaphragm walls or cast in situ ments is shown in figure 1, adapted from A.G.I. piles. Micropiles, heavily reinforced by steel (1985). pipes, are adopted where diaphragm walls are not The largest continental deposit is the Po valley feasible. Jet-grouting columns have been recently and the adjacent venetian plain; minor deposits introduced in association with micropiles in gra­ can be found along other rivers in the penin­ nular soils (Ground Engineering 1992); walls sula. Marine deposits are frequent along the obtained by deep mixing of cement in granular coasts and in tectonic depressions aside the Ap­ soils and placing vertical steel H beams in the pennine range, While the former are frequently fresh mix have also been experimented with recent and normally consolidated,g the latter success and in urban sites can be more economical consist mainly of ~overconsolidated clays and ‘and environmentally safe than diaphragm walls. silty clays of the Pliocene and Miocene ages. Steel sheet piles are relatively rare, their use ~ The grain size decreases from west to east in being confined_toUtemp9rary_works and to some the Po valley and from north to south in the cases of harbour works; soldier piles with timber venetian plain; the loosest and softest soils are lagging are practically unknown. p found in the zones adjacent to the coasts. The For excavation widths up to 10 m steel or r.c. mineralogical composition of these deposits struts are adopted, as in the so called "Milan reflects the varying nature of the parent rocks method" largely used in the construction of the distributed along the Alps granites, diorites, first metro line in Milan in the late fifties: gneiss, limestones, dolomites and others). the diaphragm walls, at each side of the tunnel In the Appennines widespread soft sedimentary were mutually braced by the upper r.c. slab, rocks (mudstones, clayey marls, very heavily 0.C. before the excavation was continued down to the clays, flysch) 'can frequently originate rela­ bottom. tively thin eluvial and colluvial fine grained Grouted anchors are generally preferred to the deposits superimposed to the parent rock, struts for wider excavations. scattered along major and minor valleys, mainly The above mentioned Milan method can be consi­ east of the main longitudinal divide. dered as a top-down construction and has also In a similar' way, restricted zones of soft been applied to some underground multi-level car redeposited clays have originated within larger parks. overconsolidated marine clay deposits. In wider excavations a bottom-up construction is The volcanic' zones around Rome and Naples generally preferred, unless the need of restoring exhibit a large presence of pyroclastic deposits, the former use of the surface as soon as possible both loose (pozzolanic sands and silts) or is of primary importance. cemented (tuff): they can originate respectively remoulded or redeposited pozzolanic soils and clayey residual soils, the density of which is 3 DESIGN PROCEDURES furtherly decreased by the presence of voids within the qrains of volcanic ashes. The Terzaghi and Peck approach-is seldom used, and only for strutted walls. _ 4 INSTRUMENTATION AND PERFORMANCE STUDY For simple cases, with one level only of struts or anchors, the design is based on the limit Major braced excavations in urban areas are state pressures; "passive" pressures are affected generally subject to some form of instrumentation by a reduction factor of O.5+0.67 and "active" with the aim of directing worksite operations and pressures can be incremented at an intermediate of preventing or limiting litigation with value between "active" and "at rest" if adjacent properties. horizontal displacements of the soil have to be Generally, vertical and horizontal movements at limited. The "fixed earth support" condition is grade level are measured by repeated topographic generally preferred to the "free earth suppport". surveys. For multi-level anchored walls the most common Few major projects have been followed by more design procedure is based on a Winkler 2-D soil complete instrumentation, comprising: inclino­ model, with bi-linear elasto-plastic "springs". meter pipes, deep settlement sensors, load cells The stiffness of the wall and of the anchors at some of the anchors, strain gauges on the (struts) is taken into account and the sequence reinforcing bars of r.c. diaphragm walls, load of the construction phases is considered. cells at the soil/wall contact (Gatti and Cividi­ Calculations are carried out by appropriate ni 1978; Ceccoli et al. 1981). computer codes on p.c. Monitoring programmes normally include: The depth and flexural resistance of the wall, - Accurate preliminary inspection of the adjacent the size and prestressing of the anchors (struts) constructions, with mapping and survey of any have to he defined prior to the calculation, so apparent evidence of structural disease: the final dimensions are determined by trial and fissures, out of plumbs, misalignments. error. - Levelling of benchmarks on the adjacent In special applications, where a "precise structures. evaluation of the soil movements 'around the - Readings of extensometers installed on old - if excavation is required, a F.E.M analysis is run. any - and possibly new fissures detected on The factor of safety 'versus 'soil strength is adjacent buildings. evaluated_ by the ratio of the limit passive A zero reading has to be taken before any thrust to the mobilized soil force, as given by activity on the site; measures have to be the areas of the diagrams of the passive soil repeated at every relevant step of the works: pressure and respectively of the computed soil - Execution of diaphragm walls or vertical pressure against the buried depth of the wall._ micropiles along the perimeter. For a single anchor or strut level, the depth of - Each phase of excavation. the wall is such that a fixed end condition is - Anchors drilling, grouting and tensioning. reached; for multi-level anchored or strutted -- Water table lowering. walls the minimum acceptable depth is that 'providing the balance of active and passive pressure at wall bottom. If a water table is 5 CODES AND MANUALS present, the depth of the wall has also to be checked versus piping. The italian law (Presidential decree 7/1/1956 Vertical spacing of anchor lines is often n°164) compels to brace the excavations deeper dictated by the position of the floors in the than 1.50 m if persons have to work in, and soil building inside the excavation, as they will consistency does not provide "satisfactory eventually act as permanent struts. Horizontal guarantee of stability". spacing depends on the flexural strength of the The rules do not describe how the stability of horizontal connecting beam. 31.50 m can be the excavation walls has to be checked. regarded as a minimum-for both spacings and 4.00 Accidents and even casualties are relatively m/6.00 IH as a. maximum for horizontal/vertical frequent on minor excavation for buried services, spacing respectively, for most‘common applica­ due to inexistent or poor bracing, even in tions. ditches shallower than 1.5 m where~ personnel often works in a bent posture. assumedEarth pressure as in thedistribution active state above if cutthe depthwall is Braced walls must comply with the only italian allowed to move‘ horizontally; in case the law dealing specifically with geotechnical horizontal displacements are limited, earth aspects of civil engineering: the P.W. Ministry pressure will result accordingly .comprised decree 11/3/1988. The decree rules soil investi­ between the active and the at-rest value. 'gations, design of foundations, of retaining Earth pressure distribution below cut depth will walls (diaphragm walls included) and soil vary from the passive pressure just below the anchors; it gives general principles and guide­ excavation bottom to lower values at greater ~lines. A stability analysis is required for depths, as defined by the analysis. excavations exceeding 2 m in depth; the stability Anchor load is defined by the calculation, of the bottom versus heave has to be checked for allowable loads are calculated on the basis of braced excavations and if diaphragm walls are used, also taking into account piping actions.

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