PSI - Issue 84

Marco Barla et al. / Procedia Structural Integrity 84 (2026) 441–448

443

• The retaining system is structured as follows: • The lower level is formed by a 170 m long anchored pile wall with heights ranging from 7 to 12 m. The wall is composed of 1.0 m diameter reinforced concrete piles connected by a reinforced concrete capping beam. The structure is stabilized by two to three rows of active anchors, depending on wall height, and local drainage is provided by sub-horizontal drains. Immediately behind the pile wall, a rockfill retaining wall allows a morphological transition to the adjacent slope. • The upper level comprises a 130 m-long anchored reinforced concrete slab system, consisting of two superimposed panels designed to distribute anchor forces. The maximum exposed height of this system is about 7 m and decreases toward the ends to match the slope geometry. Additional drainage is ensured by sub-horizontal drains. • Between the two stabilization levels, a 30 m-long anchored pile section with smaller-diameter piles (0.8 m) provides structural continuity. This section is stabilized by two rows of active anchors with lower design stressing levels (200 kN). The stabilization system is completed by a gabion structure and a continuous surface drainage channel along the upslope area to promote controlled runoff. Along the slope, the soil stratigraphy comprises two main lithotechnical units: a superficial eluvio-colluvial layer consisting of silty sands with heterometric clasts, and the underlying Rocchetta-Monesiglio Formation, composed of alternating silty marls, sandy marls, calcarenites, sands, and conglomerates. Fig. 2 shows a schematic representation of the soil stratigraphy along a cross-section of the slope in the zone affected by larger movements. Despite the apparent lithostratigraphic simplicity, the presence of an antiform structure in the central portion of the slope, defined by the top of the marly units, is associated with a gradual thickening of poorly cemented sandy lithotypes toward Savona (Trincea Valera 2 in Fig. 1). Overall, the geotechnical properties of both stratigraphic units are moderate to poor. These geological and geotechnical conditions, combined with the saturation of the superficial eluvio-colluvial layer during prolonged rainfall events, have caused slope stability problems since the early stages of the A6 motorway construction. Moreover, the original retaining systems have undergone progressive deterioration over time due to inadequate or absent protective measures and improper installation of the tie rods, which exposed them to unexpected corrosion.

Fig. 2. Schematic representation of the soil stratigraphy. RMF = Rocchetta-Monesiglio Formation.

In 2020, Autostrada dei Fiori SpA, the motorway concessionaire, identified significant deterioration affecting a portion of this structure (Trincea Valera II), manifested through settlement, lateral displacement, cracking of the concrete elements, and degradation of several anchors. In response, they commissioned a geological investigation and the design of new stabilization and retaining works, intended to replace the existing structures. After visual annual