Note: Descriptions are shown in the official language in which they were submitted.
CA 02171897 1996-04-03
PREW~.T c~.,~~',~~'.~~''N~~'~i~,T~~~~
The present invention relates to systems and methods
for the construction of underground structural concrete
walls.
It is often necessary to provide concrete walls
below grade, for example in the const.~ruction of railroad or
highway tunnels, underground parking garages, foundation
walls and retaining walls. These underground walls are
commonly formed by excavating a trench and pouring concrete
into the trench, i.e., casting the wall in place. The
physical properties of cast-rn-place slurry walls tend to
vary depending upon the composition of tk~e concrete, the
weather, and other variables present when the concrete is
poured.
~~~g~~arv of ~~,~ Invention
The present invention provides an improved method of
constructing underground structural concrete walls, using
pre-cast concrete panels. Preferably, the panels are also
pre-stressed to provide the walls with high strength and
resistance to bending moments, e.g., exerted by soil
pressure against the wall. Advantageously, the concrete
walls have a smooth, finished surface that is substantially
free of defects such as inclusions. Moreover, the method of
the invention may require less manpower than the
construction of conventional cast-in-place slurry walls.
Finally, the pre-cast concrete panels can be manufactured
under carefully controlled conditions and subjected to
quality control, and, if desired, a predetermined preload
may be applied to the panels, allowing excellent control
over the physical properties of the finished concrete wall.
CA 02171897 1996-04-03
In one aspect, the method includes: (a) providing,
e.g., by casting in place, a pair of parallel, opposing,
underground guide walls spaced a predetermined distance
apart; (b) excavating a trench, using the guide walls to
guide the excavation tool, the trench having a predetermined
width substantially equal to the space between the guide
walls; (c) pouring a footing at the base of the trench; and
(d) lowering a pre-cast panel into the trench in a desired
orientation relative to the trench walls.
Preferred embodiments of the invention include one
or more of the following features. The footing formed in
step (c) is formed of concrete, and is poured into the
trench using the tremie method. Trre method further includes
(e) pumping an excavating slurry containing a thickening
agent, e.g., bentonite or polymer, into the trench during
step (b) to prevent the walls of the trench from collapsing.
The method further includes (f) pumping a soil replacement
material, e.g., a cement/thickening agent slurry, into the
space between the panel and trench walls. This soil
replacement material provides support and resists movement
of the panels relative to the trench walls when the panel is
loaded, and also provides a waterproof barrier. Preferably,
each slurry contains from about 3 to ~~ bentonite as the
thickening agent, and the cement/thickening agent slurry
contains cement and bentonite in a ratio of from about 3:1
to 5:1, more preferably 4:1.
In a preferred embodiment of the invention, th~
guide walls are constructed to support the weight of the
panel, and the method further includes the step of
suspending the panel from a pair of beams placed
transversely across the guide walls and trench opening. The
panel is suspended by providing the panel with threaded
inserts extending verticall~r from the upper edge of the
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panel into the panel, inserting a threaded rod into each of
the threaded inserts, passing the threaded rod through a
through hole in the beam, and securing the threaded rod,
e.g., with a threaded nut. The elevation of the panel in
the trench can then be readily adjusted by raising the panel
to unweight the threaded nut and changing the position of
the threaded nut on the threaded rod.
In preferred embodiments, the wall is formed of a
plurality of pre-cast panels placed side by side in the
trench, and the method further includes joining the panels
along their adjoining edges. Preferably, the panels are
joined by engagement of corresponding interlocking members.
Preferred interlocking members include, on one of the
adjoining edges, a channel, and, on the opposite edge, a
member dimensioned to be received by the channel. It. is
particularly preferred that the channel and member be keyed
for interlocking engagement to provide a secure connection
between adjoining panels.
In a particularly preferred embodiment, each of the
adjoining edges includes a slot, the slots being positioned
so that when the interlocking members are engaged the slots
align to define a common slot, and the method further
includes introducing into the common slot a liquid, e.g., a
polymer, that is curable to form an elastomeric rubber
material. Preferably, the elastomeric material is water-
swellable. The presence of the elastomer-filled common slot
prevents leakage of water through the interface between the
panels.
Preferably, when the wall is formed of a plurality
of panels, the trench into which the panels are lowered is
excavated incrementally, i.e., the entire length of the
trench is not excavated prior to placing the panels, but
instead a first length of trench is excavated, one or more
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panels are positioned in this trench, then a further trench
is excavated adjacent the first trench. Preferably the
entire length of the guidewalls is formed initially, prior
to beginning excavation of the trench.
In another aspect, the invention features a method
of constructing an underground tunnel. The method includes
(i) forming a pair of spaced, opposed vertical walls by the
method described above, (iii excavating to a predetermined
roof level, (iii) casting the tunnel roof in place between
the vertical walls, (iv) excavating the tunnel to an
elevation suitable for the base slab, and (v) casting the
base slab in place between the vertical walls.
Preferably, the method further includes (vi)
scraping the cement~'bentonite slurry (from step (f), above)
off of the exposed side of the vertical walls to expose the
surface of the precast pane:. This step provides the
exposed side of each vertical wall with a smooth, finished
surf ace .
The invention also features methods of forming
retaining and foundation walls by forming a wall according
to a method of the invention and excavation a raglan
adjacent the wall to expose a surface of the wall.
Other features and advantages of the invention will
be apparent from the description of preferred embodiments
thereof, taken together with the drawings, and from the
claims.
Fig. 1 is a tog schematic view of the vertical walls
of a tunnel, formed according to ane embodiment of the
invention. Fig. la is a side W ew in elevation of one of
the vertical walls.
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Fig. 2 i$ a cross-sectional w~.ew of a pre-cast
panel, taken along line 2-~2 in Fig. ~. Fig. 2a is a detail
view showing the engagement of the interlocking members
joining two pre-cast panels.
Fig. 3 is a cross-sectional end view of a tunnel
formed according to one embodiment of the invention.
Figs. 4~4g are schematic diagrams illustrating the
steps of a method according to one embodiment of the
invention.
Figs. 5 and 5a are top plan and side cross sectional
detail views, respectively, showing the manner in which the
panel is suspended in Fig. ~g*
Fig. 6 is a top plan view of adjoining panels
according to the invention, showing a common slot containing
a water-swellable polymer.
Fig. 7 is a front view of a panel according to an
alternate embodiment of the invention.
As shown in Fig. 1, a tunnel 10 includes two
opposing vertical walls 12, 1~. Each of the opposing walls
includes a plurality of pre-cast panels 16 arranged side by
side, as shown i.n Fig. la.
Referring to Fig. 2, each pre-cast panel 16 includes
a plurality of prestressing strands 18 imbedded in concrete
20. The material used for prestressing strands 18, and the
preload applied, may be selected to suit s particular
application, as would be understood by a person skilled in
the art. In a preferred embodiment, prestressing strands 18
are 7-wire (0.6" diameter) low relaxation strands conforming
to ASTM A416 and having an ultimate tensile strength of
270,000 psi. The preload is agpiied by tensioning the
strands to a desired tension prior to pouring the concrete,
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and releasing the tension from the strands when the strength
of the concrete reaches ~OOt~ psi during curing. In the
tunnel application described herein a preload of about 2
million pounds was desired due to design considerations, By
way of example, to provide this preload, ~8 strands were
each tensioned to about 44,000 pounds. The strands can
typically be tensioned to about 75% of their total yield
strength, The total preload can be varied by changing the
number of strands used, or the material ~xsed to form the
strands.
Each pre-cast panel 1~5 also includes a first
interlocking member 22 on one of its longitudinal edges and
a second interlocking member 24 on its opposite longitudinal
edge, In a preferred embodiment, the first interlocking
member includes a channel. 25 having a substantially diamond-
shaped cross-section, and the second interlocking member
includes a rod 28 having a substantially round cross-section
arid being dimensioned to be received a.n the channel. by
sliding the rod downwardly through the upper opening of the
channel (see Fig. 2ay. Rod 28 is mounted on the panel by an
elongate member 25. The rod 28 preferably includes two
coaxially arranged rod portions that each have a length that
is much smaller than the length of the panel. The diamond-
shaped cross-section of the channel 2~ ~.s preferred over
other shapes because it has been found to have good
resistance to breakage during and after engagement of the
interlocking members, 'The channel may be formed of angle
iron or any other suitable high-strength material.
Similarly, the elongate member 25 and rod 28 may be formed
of any suitable high strength, rigid material.
Preferably, the interlocking members also include a
tongue 2a and groove 29 which interlock in tongue and groove
fashion. The combination of the keyed channel and rod and
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1"~~.~
the tongue and groove engagement provides a particularly
stable, secure connection between panels, providing
dimensional stability to the wall when the wall is placed,
positioned and loaded.
Referring to Fig. 6, in preferred embodiments the
pre-cast panels further include a slot 31 extending along
each of the vertical edges of the panel. These slots align
when the panels are in place, defining a common slot 33
extending vertically between each pair of adjoining panels.
A liquid that is curable to form an elastomeric material is
poured into the common slot and allowed to cure to form a
gasket-like seal 35 between the panels. A suitable liquid
is a liquid rubber commercially available from Asahi Denka
under the tradename ADEKA tnLTRA SEAL A-~50. This preferred
rubber, after curing, has a tensile strength of more than 47
kg/m and an elasticity of greater than 1000. Importantly,
this seal provides a second:~ry waterproofing barrier against
water leakage between the ad~oix~ing panels in addition to
the barrier formed by the soil replacement material.
In addition, each panel includes a plurality of
vertical through holes 30 extending through the interior of
the panel. These through holes act as a relief, allowing
the concrete at the base of the trench and the bentonite
slurry in the trench, displaced by the volume of the panel,
to flow into the through holes rather than cause the panel
to be buoyed up in the trench. The panels may also include
transverse through holes (not shown) through which ti.e-backs
(soil anchors) may be inserted to further stabilize the
finished vertical wall, as is well known in the art.
ane side of a finished tunnel 10 is shown in F'ig. 3.
In addition to vertical walls 14, the tunnel includes a roof
slab 32 and a base slab 34. Threaded steel bar inserts 36,
3~8 are provided at the intersection of the roof slab and
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z~~~~~~
vertical wall and base slab and vertical wall, respectively.
These inserts assure the structural continuity of the tunnel
struCturea
A preferred method of installation of the pre-cast
panels is illustrated in Figs. 4-4g. Figs. 4-4b illustrate
the initial steps used to form the guide walls 40. First,
as shown in Fig. 4, a relatively wide ~e.g., 7-8 ft.),
shallow (e. g., 4-6 ft.j trench 42 is excavated and formwork
44 and rebar 46 are placed on each side of the trench. Then
concrete 4'7 is poured into each side of the formwork 44 and
allowed to cure (Fig. 4a), and the formwork is remo ed (Fig.
4b), leaving a pair of opposed guide walls 40 spaced a
predetermined distance (the width of the formwork 44) apart.
Once the guide walls have been completed, a narrow (e.g., 1
to 3 ft.), deep trench 48 is excavated using the guide walls
to guide the excavation bucket, as shown ire Fig. 4c. During
this excavation step, a bentonite slurry 50 is continuously
pumped into the trench 4~ to prevent the walls of the trench
from collapsing. Preferably, the bentonite is a high
swelling, Wyoming type, sodium based bentonite consisting
mainly of montmorillonite, and the slurry contains a
concentration of from about 3 to 6~ bentonite in water. The
concentration of bentonite will vary depending upon the type
of ground to be excavated, as is understood in the art. The
slurry is formed by mixing bentonite and water at high
shear, as is well known. When excavation has been
completed, i.e., when the depth of the trench is
substantially equal to the desired height of the vertical
wall, th~ trench bottom is cleaned. The trench preferably
has a depth substantially equal to the height of the wall to
be formed. Then, concrete footing 51. is placed in the
bottom of the trench, preferably using the tremie method as
shown in Fig. 4d, to a depth at which a small portion (e. g.,
CA 02171897 1996-04-03
1~.~'~
1 to 3 ft.) of the bottom edge of the pre-cast panel will be
embedded in the concrete when the panel is in place in the
trench. Before the concrete has set, pre-cast panels 16 are
then inserted into the trench, a.g., by a crane (not shown),
one at a time, as shown in Fig» 4e, the position of each
panel being adjusted until the panel is plumb and level.
Each panel is suspended from the upper surface of the guide
walls 40, e.g., by a pair of structural steel box beams 52
(Fig. 4f). As shown in detail in Figs. 5 and 5a, each panel
includes two pairs of threaded inserts 60. A threaded rod
62 is inserted into each of the threaded inserts, and each
threaded rod is inserted through apertures in box beam 52
and retained in this position by threading a nut 5~4 canto
each rod above the beam 52. The elevation of the panel is
then adjusted by raising the panel sufficiently to unweight
the threaded nuts and then adjusting the position of each
threaded nut 64 on the threaded rod,
As subsequent panels are placed, the second
interlocking member 24 an the panel being placed is engaged
with the first interlocking member 22 an the adjoining,
previously placed panel (see Fig. 2a), to securely join the
panels together. Finally, a sail replacement material 54,
e.g., a cement/bentonite slurry, is pumped into the area
between the panels and the trench walls (Fig. 4g) to provide
support to the panels and resist movement of the panel
relative to the trench walls when the panel is loaded.
Preferred soil replacement materials, such as the
cement/bentonite slurry, also provide a barrier to
infiltration of water or moisture tram the soil through the
wall or between the panels.
To construct a tunnel as shown in Fig» 3, panels are
installed as shown in Figs. 4-fig, to farm a first vertical
wall, and installed in similar manner to form a second
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vertical wall spaced a predetermined distance from the first
vertical wall and substantially parallel thereto. Then, the
area between the vertical walls is excavated to the design
level of the lower surface of the tunnel roof, and the roof
slab is cast in place. Next, the tunnel is excavated, under
the cured roof slab, to the level of the lower surface of
the base slab, and the base slab is cast in place.
Other embodiments are within the claims. For
example, although the method has been described above in the
context of tunnel construction, the method is useful in many
other underground wall applicat:i.ons.
Moreover, while a preferred method of prestressing
the pre-cast panels has been described, other prestressing
techniques could be used, as would be understand bar a person
skilled in the art.
Further, while preferred interlocking members are
illustrated herein, other interlocking arrangements can be
used, e.g., the channel and/or °~he rod may haws a different
cross-sectional shape.
Additionally, while the precast panels have been
illustrated as being suspended from the guide walls so that
their upper edge is slightly below grade level, the panels
could be placed so that a portion oaf the panel extends above
grade level, or suspended in a manner that would allow the
upper edge of the panel to be further below grade revel, if
desired.
rf desired, the concrete that is placed in the
bottom of the trench prior to insertion of the panel may
instead be placed in the bottom of the trench after
insertion of the panel, e.g., by providing a vertical
through hole through the interior of the panel having a
sufficient diameter to allow the concrete to be tremied
through the panel.
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While bentonite has been described as the preferred
thickening agent for use in both slurries, either slurry can
include a different thickening agent. ether suitable
thickening agents, e.g., polymers, for excavating slurries
are well known in the oil dr~.lli.ng art.
Moreover, other soil. replacement materials could be
used instead of a cement/bentonite slurry. Suitable
materials are those that could be pumped into the space
between the panel and trench walls and that would provide
support equal to or greater than that of soil, e.g», lean
concrete, cement/atapulgite slurry and flowable fill.
Moreover, rather than using two slurries, an
excavating slurry and a soil replacement slurry, a single,
thickening agent/concrete slurry can tae used for both steps.
Alternatively, a curing additive can be added to the
thickening agent/water slurxy, after insertion of the panel,
to cause the slurry to solidify.
The pre-cast panels need not be rectangular, as
shown. For example, as Shawn in Fig. 7, in an alternate
embodiment the precast panel includes a pair of "a:rms" 102
that allow the upper edge of the panel to be placed below
grade level. These arms may be cut off after the ;panel is
in place, allowing the soil above the upper edge of the
panel to be excavated after the wall has already been
formed.
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