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A Case History with Combined Physical and Vacuum Preloading in Colombia

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A Case History with Combined Physical and Vacuum Preloading in Colombia A Case History with Combined Physical and Vacuum Preloading in Colombia D.G. Yanez, F. Massad Abstract. The scope of this paper is the back analysis of soft soil treatment case studies in which prefabricated vertical drains together with physical and vacuum surcharge were applied. Initially, the vacuum surcharge systems in vertical drains used worldwide are reviewed. The radial consolidation theory specifically developed for vacuum and physical surcharges is presented, highlighting the influence of the vacuum efficiency parameter. This parameter represents whether or not there is a loss of vacuum pressure in depth. In the second moment, the local geological-geotechnical parameters of the construction are presented, from which data were used as case studies. This characterization was based on the study of regional geology, field geotechnical tests, the collection of undisturbed samples and further laboratory tests. Based on the specific consolidation theory and the geological-geotechnical characterization carried out, detailed analyzes are presented for 2 specific sites of the case study. The detailed analysis is composed of two steps of numerical modeling. The first step analyzes the vacuum loss in depth, as presented in the literature review and the second step is used to check the geotechnical parameters interpreted by the investigation campaign. At the end, comments and considerations are made about the vacuum consolidation theory and the soil characteristics in the case studies. Keywords: consolidation theories, sample quality, settlement back analysis, soft soil, vacuum surcharge. 1. Introduction 100 kPa next to sea level - a value that is lower in practice. Differently from a traditional embankment, suction does There are several constructive methodologies for im- not lead to an increase in total stress, risking stability. Vac- proving soft soil, such as partially or completely removing uum suction can also be used to accelerate consolidation and substituting the soft soil layer; building light embank- time compared to conventional surcharge, even with PVD. ments; using stability berms; building piled embankments The vacuum preloading technique was presented by Kjell- and embankments reinforced with geosynthetics; building man (1952) after initial tests in the 1940s. Kjellman’s sys- in stages; preloading with temporary surcharge; installing tem consisted in applying vacuum in a sand layer on the Prefabricated Vertical Drains (PVDs) along with the use of surface with an impermeable membrane over it. This im- preloading; and using PVDs together with both embank- permeable membrane needs to completely isolate the area ment and vacuum surcharge combined. The most adequate where vacuum is to be applied, therefore, it can be neces- solution depends on: the geological-geotechnical charac- sary to build a peripheral trench in the area, maintained with teristics of the deposits; the use of the constructed area and a water or slurry seal. Despite the knowledge available re- its vicinity; the time available for construction; and the cost garding the vacuum application theory (Holtz & Wager, of construction. (Almeida & Marques, 2010). Partially or 1975), it was only after the 1980s that this method became completely removing and substituting the soft soil layer has popular, especially in the Asian (Choa, 1989; Qian et al., been less and less applicable because of its high cost and the 1992) and European geotechnical communities (Massé et environmental impact given the need for both borrow and al., 2001), due to advances in vacuum pump technology. spoil banks. Staged construction is often an economical ap- The main advantages of vacuum surcharge (sometimes also proach, but the time required for consolidation is often in- called “virtual” surcharge) in relation to physical surcharge compatible with the available schedule. PVDs can be are the following, as presented by Pilot (1981): (i) there are installed to improve drainage conditions and geosynthetic no problems with embankment instability to be considered; reinforcement, i.e. geogrids or geotextiles, used to increase (ii) it eliminates the need for material quarries for tempo- the embankment height in every stage, thus reducing the rary surcharge, which are usually expensive and/or unavail- consolidation time, but sometimes it is not enough to meet able in modern projects; and (iii) the installation, applica- modern construction restraints. tion and removal of vacuum surcharge are quickly carried A current alternative to preloading in embankments out. In opposition, vacuum surcharge adds more variables on soft soil is vacuum surcharge, which results in negative to the already complex problem of soft soil treatment with pore pressure (suction) on the soil that is approximately PVDs and preloading. So much is that Indraratna et al. Diego Gazolli Yanez, M.Sc. Student, Escola Politécnica, Universidade de São Paulo, Av. Prof. Almeida Prado 271, trav. 2, São Paulo, SP, Brazil. e-mail: [email protected]. Faiçal Massad, Ph.D., Full Professor, Escola Politécnica, Universidade de São Paulo, Av. Prof. Almeida Prado 271, trav. 2, São Paulo, SP, Brazil. e-mail: [email protected]. Submitted on July 25, 2016; Final Acceptance on May 16, 2018; Discussion open until December 31, 2018. DOI: 10.28927/SR.412217 Soils and Rocks, São Paulo, 41(2): 217-234, May-August, 2018. 217 Yanez & Massad (2005) developed a specific radial consolidation theory for second system connects vacuum straight to the PVDs and vacuum surcharge which can (or not) describe a linear suc- was developed in the Netherlands in the early 1990s (San- tion pressure decrease with depth. droni et al., 2012). In both systems, it is common to tempo- A vacuum suction system was used as preloading in a rarily lower the water table during the operation of vacuum, highway duplication in Colombia where alluvial soft soil is which leads to superficial dehydration and occasionally to present. This deposit formation gives the soil a heteroge- tension cracks. neous profile in which the stratigraphy includes erratic peat One of the advantages of the “drain-to-drain” system deposits with very low shear strength, eventual fine sands in comparison with the “membrane” one is that the former and the upper layers are affected by seasonal desiccation. can be used in terrains where there are intermediate sand Vacuum surcharge was especially attractive in this case be- layers in the soil. In this case, any sand layers below as well cause the existing highway lane’s operation could be com- as above the water level must be isolated from the vacuum promised if the new embankment were to cause instability. by blind stretches of the drains in order not to lose effi- Two sites 900 m apart from each other were chosen to be ciency of the vacuum, avoiding the need for the impervious presented in this paper, namely A and B. SPT and piezo- slurry wall around the area to be treated in the “membrane” cone testing were executed in the former and a complete in- system. If the lower layer is a draining one, the PVD must vestigation, including laboratory consolidation tests, was remain approximately 1.0 m above this stratum, forming a conducted at the latter. Better settlement predictions were lower impermeable zone. Contrarily, one possible disad- expected to arise from the parameters measured at the latter vantage of the “drain-to-drain” method regards its suction site B but, given the heterogeneous nature of the soft soil, reach. In the “membrane” system, vacuum pressure is ap- they did not. Thus, in order to provide reliable results and plied in the sand blanket and to the soil through the vertical interpretations, back analysis was carried out to infer the drains, while in the “drain-to-drain” system vacuum is only vacuum efficiency and representative soil parameters. applied directly inside the vertical drains. Hence, as Li et al. (2011) verified in laboratory with samples whose diameter 2. Practical Systems for Applying Vacuum was 30 cm and 1 m high, the “membrane” system was Currently in Use slightly more efficient than the “drain-to-drain” one, possi- bly due to the application of vacuum not only in the vertical There are currently two well-known systems for ap- drain, but also on the surface, resulting in both radial and plying vacuum on soft soil: the “membrane” and the vertical suction combined. However, when the soft soil is “drain-to-drain” systems, as shown in Fig. 1. The former, too thick, this effect is negligible. an older system, uses an impermeable membrane mounted on a sand blanket over a peripheral trench in the whole area The highest vacuum pressure that can be applied is to be treated, just like the system created by Kjellman limited by atmospheric pressure, which is higher than (1952), as is commercially employed in Europe and several 90 kPa up to an altitude of 1,000 m. In practice, however, Asian countries, especially China and Japan. In some cases, the highest feasible pressures are between 60 and 80 kPa, it is necessary to dig an impervious slurry wall below the due to vacuum losses in the system (in the vacuum pumps, peripheral trench in order to isolate sand lenses and perme- connections, hoses, vertical drains, etc.) and on the soil. A able layers, whereas the more recent system allows inter- nominal suction (pv) of 80 kPa is usually associated with mediate sealing sheets to be used (Chai et al., 2008). The vacuum consolidation and, when the necessary surcharge Figure 1 - Systems for applying vacuum on the field (a) with membrane, and (b) drain to drain (or “membraneless”). Source: Indraratna (2010). 218 Soils and Rocks, São Paulo, 41(2): 217-234, May-August, 2018. A Case History with Combined Physical and Vacuum Preloading in Colombia for the construction is higher than 80 kPa, physical pre- loading is also employed. When the vacuum pump is lo- cated above the level where suction is applied, part of the pressure dissipates throughout the suction height (Hs).
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