Volume 13 (2017): Edizione 1 (March 2017)

The aim of the paper is to provide some aspects regarding the behaviour of laterally loaded piles in loessial soils, by presenting and analysing the results of several in situ tests on large diameter bored piles in this type of soil. The major feature of loess is that it exhibits a massive decline of its strength and stiffness parameters when it comes into contact with water, leading to the collapse of its structure even under self-weight and creating difficult conditions for foundations. The load tests were performed both in natural moisture content loess and also in saturated loess. The results obtained by means of instrumentation are back-analysed using current analytical methods and also by finite element method using a numerical model in the geotechnical computation software Plaxis 3D.

The Bistrita city bypass crosses obliquely at km 14+162 the Bistrita river and a local road. In the area where the bridge is situated the river has a width of about 50.00m and the local road has 5.00m, being located at 12.00m from the bank of Bistrita. The bridge should provide a roadway that is 7.80m wide and two sidewalks of 1.50m.

The challenge is to design a bridge that allows the crossing of the two barriers (the river and the local road) in the most efficient way possible from an economical point of view, but in such a way that both the geometrical constraints and the design requirements contained in the family of the European standards Eurocodes are respected.

In order to achieve this goal, the author has investigated the design situation by comparing different possible technical solutions, by conducting a series of parametric studies and by utilizing mathematical optimization techniques.

Following these investigations a 100.00m long bridge resulted. The superstructure is a continuous beam with three spans: 20.00m + 60.00m + 20.00m and consists of a composite steel - concrete deck. The deck cross section is composed of two steel beams with variable height and a reinforced concrete slab disposed on top. This configuration of the superstructure leads to the development of negative reaction forces in the bearings located at the end points of the deck.

The study has covered 8 key steps as follows:

- Establishing the technical solution.

- Establishing the number and the length of the spans.

- Setting the static scheme.

- Determining the optimal cross section of the steel beams.

- Setting longitudinal beam geometry.

- Establishing the number of beams in the cross section.

- Determining the optimal mounting order of the concrete slabs.

- Establishing the optimal type and distribution of the bearing devices.

The sectional stresses (M, N) in a gallery lining are computed comparatively numerical by FEM taking into account the interaction underground structure - surrounding ground and analytical by Protodiakonov method. The last method is based on coincidence arch born in ground over gallery.

Comprehensive analyses allow to settle the domain in which Protodiakonov method can be applied as well as the conditions in which discharge arches appear in ground above underground or embedded structures.

The study is available for galleries (tunnels) built so that their lining takes loads from undeformed ground (rock) mass surrounding gallery, existent before excavation.