Geotechnical aspects of the design of retention basins in difficult hydrogeological conditions: a case study in Stefanestii de Jos, Ilfov County

 In this article, a case study is presented about the technical solution adapted for two rainwater retention ponds in Stefanestii de Jos, Ilfov County. The design theme established the need to build two retention basins with a capacity of 4,200 m³, and 3,600 m³, for collecting rainwater from the roof of several warehouses (logistics), respectively their external platforms. The ground conditions revealed consistent plastic clay land, with the groundwater level at approx. 2.8 m depth from ground level and 0.8 m below the upper elevation of the basin.

To comply with the needed capacity, the solution of a temporary enclosure diaphragm wall, general raft and dedicated piles for the problem of hydraulic lifting in the definitive stage was found. The diaphragm wall has a depth of 7.5 m, the distance from the upper elevation to the upper face of the raft being 4.0 m. The foundation system, dimensioned for taking over up-lift, consists of a general raft of 45 cm, inclined in the perimeter area, and piles with a spacing of 3.0 .. 3.5 m in order to ensure the bearing capacity for the periods when the basin is empty.

INTRODUCTION

The scope of the works was the design and execution of two circular pit enclosures having the function of retention basins, respectively displacement piles acting as an anchorage meant to prevent hydraulic lifting.

The basins have diameters of 48 m, respectively 39.4 m, and the depth in both cases is 6.25 m from the natural ground level.

General description

The design of the project was based on a permanent retaining system, as the diaphragm walls act as structural walls for the basins. The retaining walls are connected to the 45 cm thick raft, and, in order to counter the up-lift effect, the raft is anchored using 330/500 mm Screwsol^® displacement piles.

Figure 1: General layout of the site

The two basins, with a cumulative volume of about 11,000 m³, serve the 2 warehouses on the site, built over an area of about 8 ha.

The ±0.00 m architectural level of the building is +83.00 msl.

Due to the high groundwater table (about -3.00 m from natural ground level), as well as the large diameter of the retention basins, it was decided for circular enclosures with permanent and structural functions and pile-anchoring both rafts of the rainwater retention ponds. To facilitate the execution phases, a dewatering system was installed and used to pump water outside the excavation pit.

Figure 2: Basin cross section

Ground conditions

The groundwater level was revealed as infiltrations and quantitatively significant accumulations in the mass of cohesive clay materials from depths of around 3.50 m, having a stabilization tendency at depths of 3.00 / 3.30 m.

DESIGNED SOLUTION

Retaining structure

In order to counter earth and water pressure, it was chosen the solution of a self-bearing circular enclosure of 60 cm thick diaphragm wall, executed from the working platform level of 2.0 m down to -9.50 m, with the internal diameter of 48.00 m, respectively 39.4 m, inside which the earth was excavated down to -6.75 m. A reinforced concrete raft of 45 cm thickness is connected to the diaphragm walls and anchored by Screwsol^® 330/500 mm, 10-meter-long piles.

Figure 3: Enclosure pit geometry

Screwsol^® anchor piles

Considering site features (ground conditions, high water table), the use of Screwsol^® displacement piles was decided, acting as anchoring piles for the reinforced concrete rafts.

The execution of the displacement piles is carried out by the combined action of a torsion and a vertical pressure, the amount of soil discharged being minimal. After reaching the designed pile toe level, the concrete is pumped with constant pressure through the top of the tube inside the auger, followed by its simultaneous rotation and withdrawal. In that way, the void beneath the tube is filled with fresh concrete.

The specially designed drilling head of the Screwsol^® system distinguishes it from the usual displacement piles. This tip – a tronconic shape with helical turns – has an applied propeller shaped as a blade and a spur. The drilling is carried out by rotating the drilling head in a clockwise direction. The position of the propeller is designed in such a way that it enters the ground through its minimum section, the „rejection” effect being negligible.

Figure 4: Displacement piles execution stages            

Figure 5: Screwsol® pile

The pile concreting – as shown above – is achieved by a simultaneous rotation and retraction of the drilling head, but the direction of rotation is identical to that applied during drilling – clockwise – precisely so that in this phase the propeller leaves its mark in the ground through its maximum section of trapezoidal shape.

By using it, turns are made in the already compacted ground material around the pile, which, after concreting, become concrete threads; in this way the outer diameter of the executed piles is increased (see Figure 5).

Concreting is carried out using a concrete pump that delivers the concrete through the inside of the auger to the base of the drilling hole. The flap at the base of the auger is opened by the pressure concrete is expelled.

Simultaneously with the concreting operation, the extraction of the augers is also carried out. The concreting is finished when the augers are completely extracted from the ground and the drilling hole is filled with concrete.

Steel rebar cages are lifted down the drilling hole as soon as the concreting has finished and the cleaning of the upper part of the pile has been done. It consists in the removal of both mechanized and manually contaminated concrete with ground material from the drilling process.

Pit enclosure execution stages

The execution of the pit enclosure and foundation system includes the following characteristic stages:

• Realization of the working platform at -2.00 m related to the natural ground level;
• Execution of guide walls;
• Execution of diaphragm walls from -2.0 m down to -9.5 m (7.5 meters deep);
• Trimming contaminated concrete and execution of the capping beam;
• Execution of the anchor piles;
• Trimming the pile caps until excavation level;
• General excavation down to -6.75 m;
• Execution of the reinforced concrete raft and its connection to diaphragm walls.

Figure 6. Displacement piles and d-wall execution

Figure 7. General excavation and piles trimming

Figure 8. Steel reinforcement for raft foundation

CONCLUSIONS

The objective of this solution was to aim for an effective structural system, in terms of bearing capacity, and watertightness, with increased productivity output and high quality standards, ensuring uplift resistance of the rafts for both rainwater retention ponds.

REFERENCES

[1] *** Eurocod 7: Proiectarea geotehnica. Partea 1: Reguli generale, SR EN 1997-1:2004;

[2] ***   Eurocod 7: Proiectarea geotehnica. Partea 1: Reguli generale. Anexa Nationala,  SR EN 1997-1:2004/NB:2007;

[3] *** Executia lucrarilor geotehnice speciale. Pereti mulati, SR EN 1538+A1:2015;

[4] *** Normativ privind proiectarea geotehnica a lucrarilor de sustinere, NP 124-2010;

[5] *** Normativ privind proiectarea geotehnica a fundatiilor pe piloti, NP 123-2010;

[6] *** Executia lucrarilor geotehnice speciale. Piloti forati, SR EN 1536+A1:2015;

[7] *** Executia lucrarilor geotehnice speciale. Piloti de indesare, SR EN 12699.

Authors:

Lorand SATA, Catalin BALACEANU − SBR Soletanche Bachy Fundatii SRL, Bucharest, Romania

[Proceedings of the 17th Danube European Conference on Geotechnical Engineering (17DECGE), June 7-9, 2023, Bucharest, Romania – https://17decge.ro/]

…citeste articolul integral in Revista Constructiilor nr. 212 – aprilie 2024, pag. 62-65