STUDENTS´ VISIONS ON MICROPILE WALL APPLICATIONS: HOW TO CONSTRUCT A LARGE DIAMETER PILE OR AN UNDERPASS

SUBJECT: This paper introduces students´ visions on potential applications of micropile walls used in gravity foundations or large diameter piles and - when installed horizontally - in underpass construction.

The C pile (Fig. 1) is a patented new way of making a micropile. Unlike former micropiles, the top-hammered C pile is utilized in an open profile and installed into the ground with a new kind of eccentric drilling process.

Fig. 1 - The C pile is an open steel section, installed using a new kind of an eccentric drill

INSTALLATION OF MICROPILE WALLS

Drilled piles have been used for retaining walls constructed in Finland, Sweden and Norway since 2008. Drilled pile walls can be used in demanding soil conditions where installation of conventional sheet piles can face penetration problems or vibration risks. In Northern Scandinavia and Finland, hard and large boulders are common obstacles in the overburden limiting use of conventional sheet pile and retaining wall methods.

Two variations of drilled pile walls have been introduced recently in the Nordic countries based on either drilled steel pipe piles (RD piles) or an application of open section drilling utilizing C and CT profiles (Fig. 2). Using open section drilling, a drilled pile wall can be implemented starting with embedding of an open C section (Fig. 2A) and the wall can be extended using CT profiles (Fig. 2B). The first C profile contributes an open access to the next element. The T part of the CT profile penetrates to the previous C section creating a locking structure and the C part of the CT profile is an access to the next element, and so on.

Use of grout flushing improves structural capacity and water tightness of the wall. Grout can be used as insulation material, too. 

Drilled pile walls can be used as temporary or permanent structures. Typically, the wall has capacity to take both high vertical and lateral loads when needed.

Fig. 2 - Installation of a CT micropile wall: the first pile is type of C and

the following piles are drilled using CT profiles

CT MICROPILE WALL IN LARGE DIAMETER PILES

The CT micropile wall can be used to construct specific shapes suitable for use as the shaft of a large diameter pile (Fig. 3). The shaft can be implemented in form of an entire circle or using a butt joint at starting point of drilling (Fig. 3d). The center of the drilled shaft can be treated e.g. using deep mixing or jet grouting. The large diameter piles, made from CT micropiles, have applicability energy towers (monopole foundations) and bridges (multiple structures founded on large diameter piles).

Potential advantages favoring the use of CT micropile walls include:

• The diameter of a drilled shaft or a gravity foundation can be chosen between 0,5 m and several meters – there is no upper limit using CT micropiles.

• The CT micropiles can be lengthened and mechanical couplers have been developed for embedding of e.g. 1 to 6 m long elements.

• The center of the large diameter pile can utilized as part of the gravity foundation when the center is treated e.g. using deep mixing, jet grouting or pressure grouted micropiles.

• Drilled shafts or gravity foundations can be easily installed e.g. in demanding environments (mountain areas, isolated islands, pathless wildernesses) or even under basements using light machinery for CT micropile embedding. 

Fig. 3 -  A proposal to construct a large diameter pile using a C pile (a) and CT elements (b), with angled versions (c) when needed. The large diameter pile can be an entire circle e.g. in sandy soils or equipped with a butt joint (d) when drilling meets boulders or respective obstacles.

TUNNEL EXCAVATION

Until now, construction of underpasses beneath existing railway embankments has been challenged by limitations in breaks of train service. Traditionally, the underpass has be excavated extremely quickly and the bridge deck can be built simultaneously nearby the underpass.

It is proposed that if the CT micropile wall would be installed horizontally, the underpass could be constructed without any stop in train traffic. 

The CT micropile wall would be in the form of an arch and the compressed structure could be e.g. shotcreted. The tunnel for the underpass is possible to excavate in stages while maintaining uninterrupted rail service above: (i) drilling using e.g. cement grouting a horizontal CT micropile wall in an arch shape, (ii) excavation of the first 2 m in the tunnel, (iii) shotcreting of the first excavation, (iv) the second 2 m excavation, (v) the next concreting and so on.

Other types of tunnels could be constructed using CT micropile wall technology, too. The method could be applicable especially when the tunnel is constructed in demanding soil conditions, including e.g. boulders and respective obstacles.