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Chapter 3
Hydraulically-Driven Push Piers
CHAPTER 3
HYDRAULICALLY-DRIVEN PUSH PIERS
depth or length of sleeve and pier over which
the bending force dissipates is a function of the
soil stiffness. The depth is greater in soft clay
and loose sand, and less in stiff clay and dense
sand. Finite element analysis software was used
to analyze how the external sleeve and the pier
interact with soils of various properties. Bracket
rotation is resisted not only by the rigidity of the
pier system, but also by the passive pressure
of the soil surrounding the external sleeve and
pier. Therefore, the capacity and performance
of the pier system is in part governed by the
stiffness of the confining soils.
Benefits/advantages of an external sleeve include:
• Sleeved system separates the bending forces
from axial compression forces.
• Sleeve resists most of the bending forces and
behaves purely as a beam.
• Pier tube is protected from the bending forces
preserving its axial capacity so it can behave
more like a column.
• Sleeve is easy to install. It’s driven at the same
time as the starter tube.
• No cumbersome reinforcement to install after
driving pier tubes.
• The extra steel is where it needs to be. Much
more efficient than using thicker pier tube
sections for the entire length. It’s a local
solution to a local issue.
• Sleeve is in place during the system’s
maximum load (driving tubes).
• Relieves friction between pier bracket and pier
tube. Drive and lift pressures more accurately
reflect the load on the pier system.
3.3.2 Starter Tube
The starter tube is the first pier section pushed into
the ground and is installed at the same time as the
external sleeve, where applicable. Under-footing
and flush-mount bracket systems utilize a friction
reduction collar at the bearing end of the starter
tube. The friction reduction collar will be discussed
in more detail in the following section. Models
PP237 and PP288 starter tubes come in standard
lengths of 32 and 50 inches. The 32-inch starter
tubes are used in limited access and low headroom
applications, such as within a crawl space, along
with a modified (shorter) drive stand, shorter drive
cylinder, a 30-inch long external sleeve and 18-inch
long pier tubes. The PP350 system uses a 50-inch
long starter tube only, to coincide with the standard
48-inch long external sleeve. The PP350 is a higher
capacity system and generally not ideal for crawl
space applications. The PP400 system uses a
starter tube length consistent with the standard
36-inch pier tube length. The PP400 system is
unique in the FSI push pier product line in that the
under-footing bracket assembly does not include
the 48-inch long external sleeve and, therefore, no
special design or installation considerations have
to be made for starter tube length. The PP288 slab
pier system does not utilize a starter tube with a
friction reduction collar. The first section advanced
consists of a field-modified standard pier section
(see Section 3.12).
Figure 3.3.1.2.a
External sleeve and pier bear
against confining soil when system is under load