A micropile is a slim foundation component that transfers axial loads (both compressive and tensile) from a structure into deeper and better bearing layers of foundation soil. In principle, they are bored piles of up to 300 mm in diameter with a root fixed into the surrounding soil by grouting. The use of micropiles is especially suitable in cases of foundation of works in constructionally confined spaces, reconstruction of works and their foundations, underpinning of existing foundations or securing drivings of underground works. A micropile consists of three components: a head, which takes over the load of the building construction, a shaft that normally penetrates soft layers and a root fixed by grouting into the surrounding rock and transferring the building construction load on it.
An assembly of steel pipe pieces commonly forms micropile reinforcement, and then we talk about so-called pipe micropiles (70/12 mm; 108/16 mm; or other dimensions) connected with screwed splices. For grouting eventualities, parts in the root section are furnished with perforation overlaid with rubber sleeves. Micropile reinforcement can also be formed by a reinforcement cage made of building steel bars, e.g. 3-5 í 32 (so called reinforcement cage micropiles). Grouting of the root part is, in case of steel pipes, carried out directly over these pipes; in the second case the reinforcement cage is fitted with a standard cuff pipe made of PVC profile 32/3,6 mm. The root part is usually grouted in an upward way with the help of a double packer enabling multiple controlled grouting. The micropile head is usually formed by a lifting slab, which will subsequently be permanently built into the connected foundation construction. Micropiles transferring compressive loads have a lifting slab of a head welded to a reinforcement pipe, while micropiles transferring tensile loads have a lifting slab with a reinforcement pipe connected by a screwed splice with an inner safety screw.
A standard micropile profile
Technological process of carrying out micropiles
2a) realization of a borehole with the rotary technology
2b) pulling out drilling tools and filling the hole with grout
2c) setting a reinforcement thick-walled steel pipe
2d) grouting of the micropile root part
2e) finished micropile
Underpining of an object by braced micropile space piers
3a) grouted root micropile execution over original foundations;
3b) creation of braced space piers and its activation to existing ceiling construction;
3c) pulling down of original bearing construction within space piers;
3d) building up of new bearing construction and subsequent pulling down of auxiliary micropile
space piers; lower grouted part of micropile is used as a permanent part of foundation.
Depending on the type of load transfer to the foundation, micropiles can be divided into floating, leaning and clamped ones. According to how a micropile head is fastened to the building construction we distinguish between:
- Free micropiles (non-preloaded) concreted into a construction without their previous loading; the construction undergoes a complete deformation to its full extent.
- Micropiles preloaded, which conjoin with the construction after they are loaded to the value of force to be taken up from the construction. The micropile thus exhausts the permanent part of deformations and after the definitive load is installed only a flexible deformation occurs.
- Precast micropiles - a micropile is loaded to full load taken up from the construction before it conjoins with it. Under such pressure it will be firmly connected to the construction. In this case no further deformation will occur.
Examples of underpinnings of existing
| Securing overburdens of underground works (tunnels, galleries) with the use of a micropile umbrella |
 |  |