PROJECT OF THE FOUNDATIONS OF A NEW INDUSTRIAL COMPLEX LOCATED IN BOLOGNA IN VIA CERODOLO N°2/3
CLIENT: FIVE s.r.l. – BOLOGNA
LOCATION OF INTERVENTION: Bologna, via Cerodolo n°2/3
SERVICES RENDERED: design of the foundations of the various buildings of the new industrial complex
BEGINNING/END OF WORKS: 2014
The design of the foundations of several buildings that will be built as part of the construction of a new industrial complex located in Via Cerodolo n.2/3 in Bologna (BO).
1. BUILDING 1
In the existing building 1, the foundations of new works foreseen inside, a freight elevator and a staircase in particular, have to be realized. Two superficial plinths of the following dimensions have been provided:
- Freight elevator: plinth 3.55m x 3.55m x 0.4m
- Stairs: plinth 4.75m x 4.3m x0.55m.
The two foundations are set at an elevation of -3.97m from the finished floor of Building 1.
2. BUILDING 2
Building 2, a new construction, has a surface foundation consisting of inverted beams, with a slab 140 cm or 180 cm wide (depending on the magnitude of the transmitted loads) and 30 cm thick and with a rib 80 cm wide and 50 cm thick.
The foundation shall be set at an elevation of -1.35 m from the finished floor of Building 2.
3. BUILDING 3
On the basis of the geotechnical investigations carried out in situ, the project of the foundations of the building identified with the name “Building 3” that will be built in the new plant in Via Cerodolo n.2/3, Bologna (BO) has been carried out.
This building is an office building, with 5 floors above ground and has a seismic-resistant structure made of reinforced concrete frames.
The foundations were calculated according to non-traditional criteria, now contemplated also by the technical standards NTC2008 at paragraph 126.96.36.199; in particular reference was made to mixed foundations, or as it is more correct to say, piled raft foundations [PRF].
With a traditional project, instead, in case we have to/want to use piles for any reason (insufficient bearing capacity of the surface slab, excessive sagging of the same, particular geotechnical conditions, etc.) it is necessary to entrust 100% of the load to them, thus neglecting the contribution offered by the connection slab which, being placed directly on the ground, discharges a sensible part of the load on it.
The Teleios study has developed an automatic calculation process that follows the PDR method, which can be seen as an evolution of it as it considers the non-linearity of the behavior of the piles (Fiorelli & Franceschini, 2014).
Note in the following figure how, compared to the tri-linear constitutive bond of the bibliographic PDR, its refinement contemplates, in the range of interaction between piles and slab, a non-linear response of the piled raft.
45 continuous helix CFA piles with a diameter of 500 mm placed at 2500 mm spacing, equal to 5 times their diameter, have been foreseen. The net length of these piles is 20.0 m.
A comparison was made in terms of subsidence between the PDR (bibliographic version and Teleios modified version) and the geotechnical finite element calculation software PLAXIS 2D.
From the FEM calculation, using nonlinear constitutive models of the materials (soil, in particular) and also nonlinear numerical solvers, with successive load increments, the following deformed configuration was obtained:
Calculation of the foundation using the PDR method resulted in a constant settlement, equal to:
- Calculation in LINEAR regime of the pile-soil complex: δ = 43.7 mm
- Calculation in NON-LINEAR regime of the pile-soil complex: δ = 17.5 mm
This last approach is the one developed within Teleios and consists in a non-linear development of the analytical method PDR (from the name of the authors, Poulos, Davis & Randolph).
The order of magnitude of failure is optimally found by the PDR analytical method.
4. BUILDING 3 SHOP AND BUILDING 4
According to the geomechanical characteristics of the ground and the loads transmitted by the elevated structures, a foundation typology consisting of reinforced concrete plinths supported by 250 mm diameter valved micropiles, length L = 19.0 m and valved in the last 4.0 m, was chosen for both buildings.
5. Planimetry of the various buildings
Plan of the foundations designed for buildings 3, 3 shop and 4.
6. CFA Pile Technology
Continuous helix CFA piles were used in this project.
Continuous helix technology has the fundamental aspect of reduced soil removal compared to traditional drilled piles.
The soil, in fact, remains essentially in its initial condition as it does not undergo the detensioning linked to the excavation of the hole.
The classic execution procedure is as follows:
- positioning of the drilling equipment;
- start of excavation with the tool in rotation and continuous and automatic advancement;
- once the final depth has been reached, the opening of the tool tip is commanded and the pile is cast, again using a computerized automatic ascent system: during the casting phase, the concrete is introduced through the hollow shaft and fills the volume previously occupied by the ground. The introduction of the concrete takes place in a continuous and controlled way at the same time as the gradual lifting of the tool. In the CFA pole there is not therefore a phase of open hole to fill with concrete, but it is the same concrete that creates the space to fill, always pushing upwards with a positive pressure the base of the tool;
- once the casting is finished, the pile head is carefully cleaned to lay the metal reinforcement, which is done with the hole filled with concrete;
- the only volume of soil removed is that which remains trapped in the helix.
6. Micro-pile technology
Given the peculiarities of these piles, especially with regard to the injection and formation of the valved section, the main phases of realization are briefly described below.
- Drilling conducted by rotation. Where cemented material is encountered, roto-percussion is used.
- Self-supported drilling. If necessary, support the walls of the hole to be made by means of a metal casing of adequate diameter and thickness. The use of bentonite mud is forbidden.
- Drilling length at least 20 cm longer than the theoretical one shown in the drawings.
- Clean the hole once the drilling is completed.
- Micropile reinforcement consisting of metal tubing laid in pieces joined by external sleeves of length greater than or equal to 200 mm, threaded at full height, with a useful steel section equal to the nominal section of the tubing.
- In-situ welding of reinforcing tubes is not permitted.
- Reinforcing pipe closed at the bottom with a metal bottom.
- The part of the pipe equipped with sleeve valves for the injection of second phase under pressure is indicated in the drawing.
- Sleeve valves: rubber sleeves at least 3.5 mm thick, adhering to the pipe and held in place by steel wire rings (∅ = 4 mm) welded to the pipe at the sleeve edges (see photo below). The lowest valve will be placed immediately above the base of the pipe.
- The gap between the hole and the reinforcement pipe (sleeve) will be filled by pouring cement mortar through the reinforcement pipe from the lowest valve level.
- If the metal casing is used to support the hole, it will be removed after the completion of the sheath.
- Sheath injection performed at low pressure: P ~ 0.2 MPa = 2 bar.
- Once the injection of the sheath has been completed, the inside of the reinforcement pipe will be washed.
- Once the sheath has set, in any case within 24 hours, proceed with the injection valve by valve starting from the lowest valve level, using a packer.
- As an indication, the volume of grout injected will not exceed 50 lt for each valve under a residual injection pressure of less than 1 MPa (10 bar).
- The contractor must submit a report where, for each micropile, the volume of grout injected and the residual pressure values are indicated, valve by valve.