Project Background

The study used to compare FLAC2D and Paratie Plus involves the retaining structures planned as part of the detailed design for upgrading the Dalmine Interchange on the Milan–Bergamo section of the Turin-Venice Motorway. The retaining structures are required for the excavations necessary to construct abutment B of the interchange overpass. Tecne is the engineering company of the group who manages the Italian motorways.

The retaining wall section consists of micropiles reinforced with tubular steel elements with a diameter of 168.3 mm and a thickness of 12 mm, with a drilling diameter of 240 mm placed at a spacing of 40 cm. Tiebacks are designed with 4 strands of 0.6 inches tensioned in multiple rows. The micropiles are connected at the top by a reinforced concrete capping beam with dimensions of b=50 cm and h=70 cm. The stratigraphy consists of a layer of road embankment material, properly placed, resting on in-situ sedimentary deposits of fluvioglacial and fluvial origin (gravels in a sandy and sandy-silty matrix). The piezometric level does not interfere with the excavation and is assumed to be at an elevation of -13.0 m from the top of the capping beam.

Fig. 1. Model description.

Fig 2. Structural attributes.

Featured Software

FLAC2D

Paratie Plus

Paratie Plus vs. FLAC2D: Displacements

A comparison between the in-situ monitored values and the results from Paratie Plus follows. On the left, the displacement at the top of the retaining wall is displayed, and the different markers correspond to the various phases. On the right, the final load in the four rows of tiebacks is represented.

Fig 3. Displacement results from Paratie Plus.

A comparison of the results using Paratie Plus and those using FLAC2D is shown in the following figure. The dashed line represents the results of the classic Paratie Plus model. The solid orange line refers to the new algorithm that simulates the movement of the tieback’s foundation, and the thicker green solid line includes the arching effect.

The lines with markers represent the results of the FLAC2D analyses using the Mohr-Coulomb model, the Plastic-Hardening model with a virgin compression elastic modulus, and the Plastic-Hardening model with an unloading / reloading elastic modulus. With the new algorithm in the spring method, slightly higher displacements are obtained compared to the classic scheme, as suspected, especially at the excavation bottom. The arching effect, on the other hand, significantly reduces these displacements at the excavation bottom due to the redistribution of active earth pressure. The horizontal displacements calculated with FLAC2D are similar to those from the spring analyses near the wall’s embedded portion in the soil. However, above ground, the spring-based software tends to underestimate displacements compared to FLAC2D. Specifically, the displacements calculated with FLAC2D show a pattern likely linked to a generalized movement of the upper soil layer, even involving the region where the anchors are founded, as clearly illustrated in the FLAC2D displacement map.

Fig 2. Left: Comparison of Paratie Plus and FLAC2D results using the Mohr-Coulomb model (dark blue), Plastic-Hardening model with virgin compression elastic modulus (purple), and Plastic-Hardening model with unloading/reloading elastic modulus (light green).
Right: FLAC2D displacement map.

Fig 3. Bending moments and soil pressures.