Home subsidence: the case study of consolidation with preloaded micropiles

Type of disaster

This is a house built partly with a basement and partly without. The part without a basement immediately showed more pronounced settlement movements that then worsened. The structure is built on rectangular foundations in good quality reinforced concrete.

The presence of highly compressible soils, especially in the first layers, associated with a probable incorrect water management, has caused over time a significant collapse of half of the structure. The deformation undergone by the structure has therefore generated a crack pattern with 45° cracks alarming.

Given the severity of the landslide and the presence of poor quality soils at great depths, we immediately opted for an underpinning intervention with small diameter piles.

The client’s technicians have identified the SYStab Preloaded Poles as the most suitable solution to resolve the failure and stabilize the structure.

Consolidation of the seabed with pre-loaded piles

The SYStab pile driving methodology consists of fix suitable steel plates to the existing foundation. This metalwork allows you to exploit the contrast offered by the structure for the insertion of the poles into the ground with hydraulic jacks, set on the collars of the plates themselves. The insertion occurs gradually, without producing harmful vibrations for the damaged superstructure and without the need for a pre-drill.

In this case, the intervention was carried out according to the following operational phases:

• excavation to make the existing foundation accessible;
• fixing of steel plates using threaded rods and chemical anchor;
• driving of piles with monitoring of installation pressures, until the design capacities/depths are reached;
• execution of the step load test according to the regulations;
• pre-loading of each micropile with pressures higher than the operating pressures (an extremely important phase and a peculiar characteristic of this type of pile, it allows to guarantee the effectiveness of the connection and avoid future settlements, making the pile immediately active!);
• final fixing of all the posts to the collar of the plates, using high-strength steel bolts;
• backfilling and restoration of external pavements.

Advantages of foundation consolidation

The consolidation of foundations with preloaded micropiles has allowed us to stabilize permanently the portion of the building without a basement, restoring safety and allowing the crack repair on the load-bearing walls.

The consolidation methodology illustrated has many advantages compared to traditional techniques:

• very rapid implementation times compared to traditional methodologies;
• does not produce any waste material;
• Normally neither mud nor water are used (dry construction site also suitable for narrow and internal environments);
• it does not produce any vibrations harmful to the superstructures;
• preload for the cancellation of primary settlements for all piles;
• the capacity of each micropile is tested during construction;
• possibility of lifting and recovery of flatness if the structure allows and requires it.

The overall work as regards the phase of Micropile drivinglasted only 5 days and allowed the insertion of 18 micropiles 114 mm in diameter and with an average depth of 13.5 metres, including the final fixing of the piles to the steel plates which guarantee the safety of the final load transfer result.

The technology used has allowed us to quickly run the poleswith minimal inconvenience And without producing harmful stresses for the inhabited structure.