Pile Design and Construction Practice, Fifth edition

in Section 2.2.3 and Figure 2.15. Typical jacked piles require a pit excavation beneath
the foundation, and a hole in the floor of the pit to receive the bottom-pointed unit of the
pile. A hydraulic jack is placed on top of this unit and is surmounted by short lengths of steel
plate or beam sections to spread the load onto the underside of the existing foundation. The
bottom unit is then jacked down until it is flush with the bottom of the pit. The jacks and
packers are removed and another precast concrete section added, which is in turn jacked
down. Additional units are now added and jacked down until the bearing stratum is reached
or until the resistance of the pile as measured by the pressure gauge on the jack indicates that
the desired ‘preload’has been attained. The elements are next bonded together by inserting
short steel bars into the longitudinal central hole and grouting them with cement. On the
completion of the jacking operation short lengths of steel beam are driven hard into the
space between the pile head and the foundation, or between the pile head and the spreader
beams. The jack is then removed and the head of the pile, packers, and spreaders encased
solidly in concrete. Grout bags which are inflated and pressurized with cement grout are a
convenient alternative to the steel packing.
It is essential to maintain the load on the jack until the packing is completed. This is
to avoid any rebound of the pile head, and subsequent settlement when the load from the
structure is transferred to the piles.
An alternative method which ‘pre-tests’the jacked-in pile once it reaches the bearing
stratum, or the desired value of preload has been attained, requires a pair of hydraulic jacks
to be inserted between the head of the pile and a bearing plate packed up to the underside
of the existing foundation. The thrust on the rams of these jacks is adjusted to apply a load
of 1.5 times the working load onto the pile. When downward movement of the pile has
ceased a short length of steel H-section with end-bearing plates is wedged tightly into the
space between the jacks (Figure 9.6). The latter can then be removed and used for the same

442 Miscellaneous piling problems

Figure 9.6Underpinning with pre-test load (a) Jacking down underpinning pile (b) Insertion of steel
strut (c) Steel strut wedged into place before encasement in concrete.

Pile jacked

into place

Bearing

plate

Steel strut Dense

concrete

packing

(a) (b) (c)