REINFORCEMENT OF A PILED FOUNDATION WITH SELF-

DRILLING MICROPILES

Racquel Nottingham (nottingham@ischebeck.de)

Friedr. Ischebeck GmbH, Ennepetal, GERMANY

Freddy Lopez (lopez@ischebeck.de)

Friedr. Ischebeck GmbH, Ennepetal, GERMANY

ABSTRACT

In the 20th century, deep foundations were widely used in building design and construction and has

greatly influenced and enhanced the industry as such. Using piles and micropiles have changed civil

design of structures allowing more robust designs in areas that were deemed unfit for construction. Over

time however, it is not uncommon that these deep foundations require further reinforcement, i.e. when

structural expansion is desired and as such the foundation is required to sustain greater loads beyond

that of its original design.

This paper presents a carpark structure in Cologne that required further reinforcement for expansion

purposes. The foundation system designed in 1999, was still fulfilling its original design intent however

to facilitate additional loads micropiles were designed. This paper will describe and summarise the

solution developed through numerical methods using 3D finite element software, PLAXIS 3D (Lopez,

et al., 2018) and the implementation and appropriateness of the micropiles selected.

INTRODUCTION

Several structures (buildings, bridges, industrial facilities, etc.) are built on piled foundations. In many

cases, the existing structures and their foundations need to be reinforced or retrofitted, because the

original use of these structures has been subjected to changes that require increased load bearing

capacities (i.e. former industrial facilities are reused for housing), or because the serviceability needs to

be enhanced, even though the original use remains the same. In very historic structures extra

reinforcement is more often required as the purpose of these buildings have changed over time.

Buildings that were originally used as simple houses have been since converted to office buildings

requiring higher loads and demanding more bearing capacity from the foundation. The reinforcement is

also required when national building codes are updated or when the structures behave differently than

originally planned (i.e. unacceptable absolute or differential settlements, etc.).

For the reinforcement of foundations, it is certainly common to undertake the retrofitting works from

the existing foundation levels, corresponding with significant space restrictions (limited working

heights). The most common constructive measures consider ground improvement (i.e. with injections)

(Dietz & Schürmann, 2006). Micropiles are often used to improve the bearing capacity of the ground in

cases where the existing foundation is founded on very weak material. The case study presented below,

summarises foundation underpinning as the most effective way to increase the existing foundation’s

bearing capacity. This system effectively engages skin friction due to low settlement values and

behaviour unlike that of traditional bored of drilled piles.

The present article explains the proposed reinforcement of a deep foundation (cast-in-place bored piles)

with self-drilling micropiles. The definition of the project and the interaction of the existing piles with

the reinforcement (micropiles) will be presented on the following pages.

PROJECT DESCRIPTION

This project discussed here is the extension of the Aggripabad building located in Cologne. The existing

two-storey parking structure was reinforced to facilitate a five-storey renovation and extension as given

X Incontro Annuale dei Giovani Ingegneri Geotecnici. Atti del Convegno ‒ F. Ceccato, M. Rosone e S. Stacul © 2021 Associazione Geotecnica Italiana, Roma, Italia, ISBN 978-88-97517-16-0

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in Figure 1 below. The current structure was developed on 80cm diameter, 9.0m long cast-in-place bored

piles. This foundation was designed according to the German Standards (DIN 1054 and DIN 4014) in

1999 for a compressive service load of Nserv, 0 = 1500kN. For the extension, the piles will be loaded with

a new characteristic service load of Nserv, 1 = 3000kN, i.e. a load increment of ΔNserv = 1500kN. As such,

the existing, bored piles required some additional reinforcement which was made possible by micropiles.

Figure 1: Imagery of existing structure and the planned extension (left) and a cross section of the existing

structure and planned extension (right) (Lopez, et al., 2018).

The numerical analysis was performed (Lopez, et al., 2018) previously within which, the bearing

capacity of the foundation with reinforcement from micropiles was effectively determined. The system

was directly analysed in a 3D numerical modelling software package to determine the load bearing

behaviour of the reinforced foundation. Essentially the important parameter here is settlement and the

reinforced system was incrementally loaded until a settlement of 10% of the pile diameter was achieved

(Pfähle, E. A., 2014). Figure 2 summarises the settlements achieved during the analysis. The graph

shows that the micropile reinforced foundation has a load capacity more than twice that of the single

bored pile. The bend in the settlement lines highlights the load in which the bearing capacity was

achieved.

Figure 2: Load bearing capacity of the bored pile alone (purple) and the foundation with additional four

micropiles with varying inclinations (Lopez, et al., 2018).

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The final solution and micropile configuration consisted four micropiles to each pad foundation, i.e. for

every bored pile, four (4) micropiles are required. Figure 3 below shows the (axisymmetric) layout of

the reinforced deep foundation. The micropiles are inclined at 15 degrees from the vertical and have a

characteristic axial load of 310.6 kN each.

To incorporate the new micropiles into the existing foundation, the pile cap was extended as shown in

Figure 3 (right).

Figure 3: Plan view of the pad foundation (left) Cross-section through the foundation displaying the bored pile,

micropile and extension of the pile cap (right).

MICROPILES

The micropiles consists of continuously threaded hollow bars, made of seamless, fine-

grained steel (S460NH according to DIN EN 10210) pipes, installed via rotary

percussive drilling. During the drilling process, the micropiles are continuously

grouted (dynamic injection), building a rough interlocking at the interface grout-soil,

increasing the skin friction (Lopez & Fernandez, 2017). As a self-drilling system, the

micropiles can be easily installed with specific flexible and lightweight equipment

(Figure 5). Although these machines are much smaller than traditional rigs, they are

still able to obtain a high drilling performance with minimal vibration effects, making

the installation possible, especially in confined locations (i.e. low headroom).

Micropiles can be implemented in many situations in both existing structures and new

development. They have a wide range of sizes and can very often result in cost saving

due to a reduction in reinforced concrete. As opposed to bored or driven piles, the

installation of micropiles requires a minimum workforce but can still achieve a high-

quality result in minimum time.

The design utilized Ischebeck TITAN 52/26, 12 metres long with diameter 13cm drill

bits. The hollow bar has an external diameter of 52mm and internal diameter of 26

mm and is supplied in 3m lengths including couplers to connect the bars. The

micropile configuration is shown in Figure 4. Each micropile includes the sacrificial

drill bit that as the name suggested cannot be recovered, forming part of the final

micropile. The selection of the drill bit is of paramount importance in order to achieve

a successful penetration of the existing ground. The system also makes use of

centralizers to ensure that the hollow bar is constantly at the centre of the borehole,

especially when grouting. The grout here, provides corrosion protection through grout

cover which was calculated to be sufficient once correctly installed. In the case of

connections especially to the concrete slabs or foundations at the head of the

micropile, a plastic or steel tube is required. This tube is inserted after the hollow bar

is grouted. It provides additional protection for the grout and in the case of a steel

tube, facilitates the effective transfer of loads from the structure to the micropile. This

is a composite system in which the strength is achieved through the combination of

grout and hollow bar steel reinforcement.

Figure 4:

Micropile

configuration

(Ischebeck

TITAN, 2018)

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Figure 5: Installation of micropiles in confined spaces (Lopez & Severi, 2017).

CONCLUSION

The improvement of foundations in older buildings are not an uncommon but often daunting depending

on the method of reinforcement. The case study in Cologne presented an existing structure, where the

original deep foundation, still satisfies the requirements but an expansion was required thereby

increasing the loads exerted on the foundation. To facilitate the additional loads, self-drilling micropiles

were proposed to reinforce the existing foundation.

Self-drilling micropiles have many advantages and can be incorporated in varying applications reducing

time and efforts spent on site. These systems operate through skin friction and rely heavily on the friction

at the soil grout interface for the necessary resistance. As such, a system that allows maximum possible

friction by implementing dynamic injection which will finally guarantee a rough grout body was

selected.

These systems can also be installed in areas where the head room is limited and thus perfect for

foundation underpinning and other areas of possible height constraints. This paper discussed the

application of Ischebeck TITAN micropiles to a swimming pool compound in Cologne Germany.

REFERENCES

Dietz, K. & Schürmann, A., 2006. Foundation Improvement of historic buildings by micro piles,

Museum Island, Berlin and St. Kolumba Cologne. Schrobenhausen, In Proceedings of the 7th

International Workshop on Micropiles.

Ischebeck TITAN, 2018. Titan Micropiles. An Inovation Pevails. Design and Construction..

Ennepetal: s.n.

Lopez, F. & Fernandez, J., 2017. Case study of an uplift reinforcement project.. Vancouver, s.n.

Lopez, F. & Severi, G., 2017. Micropiling in Urban Infrastructure: adnvantages, experience and

challenges, Ennepetal: s.n.

Lopez, F., Terceros, M. & Achmus, M., 2018. Reinforcement of Existing Deep Foundations with

Micropiles, Germany: s.n.

Lopez, F., Terceros, M. & Achmus, M., 2018. Verstäarkung bestehender Tiefgründungen mittels

Mikropfählen: das Aggripabad in Köln, Deutschland: s.n.

Pfähle, E. A., 2014. Recommendations on Piling (EA-Pfähle). Berlin: Wilhelm, Ernst & Sohn.

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