SUMMARY: This paper describes a novel underpinning system incorporating
Ankerbond and high capacity vertical piles bored through existing concrete
foundations. A special cutting head, with three diamond tipped cutters, is used to
cut three grooves in the cored holes through the existing concrete foundations.
These grooves provide a strong pile head connection to transfer additional load
from the existing foundations into the new piles, i.e. the existing foundations are
upgraded. The advantages of this system are its ease of installation, its reduced
construction time, and its efficiency due to the new vertical piles located closely
around the existing columns. The result is that no foundation beams are required
and that overall costs are significantly lower than other underpinning solutions.
Two case histories are presented which involve the use of the Ankerbond system
in conjunction with high capacity retrofitted minipiles. In these case histories, the
capacities of the existing foundations were increased to accommodate an
additional four storeys on top of existing buildings. Pile compression and tension
tests are presented which validate this novel underpinning system and demonstrate
its advantages for supporting increased foundation loads.
INTRODUCTION
When planning urban developments, the cost of demolition and the environmental and
sustainability regulations affecting the disposal of construction waste are encouraging
the re-use and upgrading of existing properties. When such developments involve adding
floors or additional loadings to a building, then the existing foundations may need
strengthening. A number of underpinning systems are available, but many involve
inclined piles with reinforcement incorporated into new beams connected to the existing
foundations to ensure an adequate transfer of load from the existing foundation to the
new underpinning system. The Ankerbond system described in this paper avoids the
need for foundation beams and hence the costs involved are significantly lower than in
the case of other underpinning solutions
ANKERBOND SYSTEM
The Ankerbond system was designed by Holemaster Limited as a means of enhancing
the pull out capacity of anchor bolts in concrete. This is necessary because high-speed
rotary-cored holes through reinforced concrete can often have very smooth bores, which
are difficult to ‘roughen’. Therefore, unless there are long bond lengths, there is a risk of
low grout to concrete bond values and subsequently low pull out forces for the anchor
bolts. The system had previously been used on core hole diameters of up to 100 mm,
however, Holemaster were willing to modify the system for use in 250 mm core holes
through existing ground bearing bases and piles caps. This was initially undertaken at
Colmstock House, Dublin, and then again at Leigh Mills Car Park in Coventry. The
original scheme at Colmstock House required many retrofitted minipiles, inclined in
several directions, in an attempt to overcome the issue of the minipiles punching through
the smooth cored holes. With the Ankerbond system, it was possible to halve the number
of minipiles by incorporating efficient vertical high-capacity minipiles within the
existing pile caps. The additional loadings could then pass down the columns, into the
pile cap and safely into the minipiles via the Ankerbond system, which would then
transfer the new loads deep into the underlying bedrock.
The first stage of the process is to form a 250 mm diameter vertical core through
the existing pile cap or ground bearing base, a minimum 500 mm thickness of high
strength concrete is required (>30 MPa). The modified Ankerbond tool incorporating
three diamond tipped cutters, is then lowered mid-way through the base to
form the three Ankerbond grooves and to try to roughen the remainder of the bore. The
same drill rig does the initial coring and the Ankerbond grooves .
Following coring, forming of the Ankerbond grooves and roughening, the bore is
cleaned with water. The minipiles are then constructed using 220 mm
diameter temporary drill casing installed through the 250 mm diameter core holes and
bored using 190 mm diameter augers or a 190 mm diameter down the hole hammer bit.
The temporary drill casing is extended through unstable strata until open hole boring is
possible. Upon reaching the required depth, the central reinforcement is placed and the
minipile is fully grouted using a 1:1 sand cement (OPC Grade 42.5 N) grout mix with a
0.45 water cement ratio. Finally, the temporary casing is removed, whilst keeping the
bore fully topped up with clean grout. After removal of the last section, a check is made
that the Ankerbond grooves are clean and full of fluid grout, prior to final topping up.Two pull out tests were undertaken to test the effectiveness of the Ankerbond
grooves. These comprised of 1.0 m lengths of 63 mm diameter GEWI reinforcing bar
cast into the existing ground bearing pads. The two tests were taken up to 1.5 x design
load or SWL (safe working load) of 1,075 kN = 1,612 kN, which is comfortably below
the reinforcing bar yield strength of 1,758kN. Care was taken to ensure that the reaction
beams did not exert pressure within 0.5 m of the reinforcing bar, to prevent any strut
effect. Both tests performed well and safely held the maximum test load, showing elastic
movement of ~5 mm, which is explained by the elastic extension of the free length of
reinforcing bar through the test jack
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