Safe and environmentally sound

Officine Maccaferri’s Claudio Ferracuti explains how the engineering principles that inspired the design of a motorway in Albania demonstrate that structures built using reinforced earth technologies can be very large and, at the same time, inserted into complex environmental situations which need to be protected.

“Of all the retaining walls so far built, Wall 6-7 is particularly interesting, both from a technical point of view - with its height of 40m - and from an aesthetical one, as it fits in perfectly in the mountainous landscape”
-Claudio Ferracuti

The motorway connecting Durazzo - the main Albanian port on the Adriatic Sea - with Kosovo is the largest project ever financed in Albania. The motorway will enable the border to be reached in just two hours instead of six hours along hazardous mountain roads, and it will be a major stimulus for the development of this area.

The 64km section from Rreshen to Kalimash was awarded to the international construction group Enka Bechtel and most of the project had been completed by June 2009, after about one year - including the 70 planned retaining walls, with a total length of 6.4km.

Officine Maccaferri SpA has been a global reference point over the last 130 years for the design and construction of state-of-the-art solutions for erosion control and support works. It was responsible for the design, supply and installation (the latter through its local sub-contract partner Albania Draht) of 30 reinforced earth walls with a total surface area of approximately 35,000m².

These walls were designed and constructed as hybrid reinforced earth structures with the combination of two Maccaferri products. The first, Terramesh System - as the secondary reinforcement - consists of horizontal flat reinforcement elements, made from double twist hexagonal wire mesh (8x10 cm) and 2.7 mm diameter steel wire protected with Galfan alloy and coated with a polymeric layer to guarantee the maximum service life of the project.

The second, Paralink 300 - as the primary reinforcement - is a uniaxial geogrid (produced by Linear Composites Ltd. of the Maccaferri Group) consisting of a core of densely packed, parallel and perfectly aligned, ultra strong polyester filaments, encased in a protective resin sheath incorporated in polyethylene strips with a width of between 80 mm and 90 mm.

Of all the retaining walls so far built, with various heights and lengths, Wall 6-7 in section 1 at km 19 is particularly interesting, both from a technical point of view - with its height of 40m - and from an aesthetical one, as it fits in perfectly in the mountainous landscape.

During the design - and once the assumption of a new bridge had been rejected - the contractor appointed Officine Maccaferri to design the support works that would enable the motorway to run at the design elevation along the mountain slopes. The result was a reinforced earth structure in which Maccaferri used its considerable know-how and demonstrated the feasibility of a structure, which, at first sight, might have appeared somewhat reckless.

During the design of such structures it is absolutely essential to correctly identify the potential failure mechanisms in the soil in order to assess the stability contribution offered by the presence of the reinforcements. A correct design of a reinforced earth structure therefore implies a correct choice of the length and vertical spacing of the reinforcements necessary to guarantee stability, with known geotechnical parameters for the structural embankment (friction angle, specific weight) and the mechanical characteristics of the reinforcements (failure load, soil adhesion factor).

The examination of the stability conditions of the embankments was carried out using the usual limit state methods. The evaluation of the stability safety factors was carried out with the MacStar2000 software package (developed by Officine Maccaferri for reinforced earth structures) in which the search for the critical surfaces is carried out by the automatic generation of a high number of potential slip surfaces.

Moreover on the slopes above the walls, rockfall protection, erosion control and re-vegetation measures were installed to protect the motorway below and to promote rapid establishment of stabilising, vegetative cover. Maccaferri Macmat R16822GN was used for these two last functions instead of SteelGrid MO 150, all in conjunction with anchors, and rock bolts of various types and 500 kJ high resistance rockfall barrier CTR 05-07-B were installed as rockfall protection. Specialist design assistance was provided by Maccaferri.

 

Claudio Ferracuti is a civil engineer who has specialised in geotechnical, basal reinforcement, paving, rockfall protection system and landfill throughout his career. Over the last 12 years he has branched into the commercial sector in Europe, Africa and the Middle East, now working as the Export Manager for Officine Maccaferri S.p.A, responsible for coordinating solutions related to strategic development across the Maccaferri export business.