Note: Descriptions are shown in the official language in which they were submitted.
21902 1~ /
~ 1 --
~OEl~OD OF rKIsvL .~ lN6~ OEANGES IN ~Lll~i NATCIKE OF VISCOIJS
GROlJr~v FOl~ IN 1~; FO~NDATION CREATED FOR ROADS, BANRS OR
T~IE LIRE, AND OF ~usv~ lNG EAKl-~QuARE DISASTER
The present invention relates to a method of
preventing changes in the nature of viscous ground found in
the foundation created for roads, banks or the like, and a
method of prevention of earthquake disasters as well.
For Japan or a small country having limited open
fields, techniques of how soft ground is improved or
reconstructed are increasingly important for developing
industrial infrastructures such as water fronts, and slope
fronts, and putting them in good condition. Recently steady
increases in the number of traffic vehicles, combined with
their size increases, pose problems that must be urgently
solved; it is required to put existing road networks
immediately in good condition, and to repair, structurally
improve or reconstruct, and widen available roads ranging
from principal to local roads, with additional requirement
for creating new roads.
In recent years, it has been pointed out that the
Japanese Islands may be at the active stage of earthquakes
due to the fact that earthquakes, whether small or big in
magnitude, occur frequently at all parts of the country. In
view of social considerations, how seismic diasters are
effectively prevented is an urgent problem.
U.S. Patent No. 4,309,129 issued January 5, 1982 to Y.
Takahashi describes a method and apparatus for improving
the strength of soft viscous ground. The method involves
injecting a hardenable liquid (ie. a cement milk) into each
of a number of selected points in the ground. U.S. Patent
No. 4,540,316 issued September 10, 1985 to the same
inventor also discloses a method of improving soft ground
f or buil ding purposes.
Conventional techniques taking aim at improving
ground-constituting soil itself include a piling method
designed to consolidate and drain soft ground by a piling
structure placed thereon, so that the soft ground can be
improved and reinforced, and a combined piling and drainage
method designed to reduce consolidation time as much as
possible.
-2- Zl~Q212
,
However, the piling method or the combined piling and
drainage method takes several months to several years for an
80% consolidation, although depending on the soil properties
of soft ground, and needs much labor, time, and cost for
ground maintenance and control. One leading reason is that
several piling operations must be performed step by step
until as-planned height is reached because when a given
piling structure is at once laid on the soft ground,
surrounding ground layers or structures built up on them are
deformed or displaced due to its sliding destruction, and
another reason is that long times are needed to make the soft
ground stable with respect to settlement or subsidence,
sliding destruction, and nature changes. Another defect of
these methods is that much difficulty is involved in their
application to ground found at residential quarters and
marshland, along rivers, and at slopes, or ground disturbed
as by earthquakes. In view of design, management and
construction control considerations, the amount and time of
subsidence may be theoretically estimated or predicted but,
in many instances, practice differs from theory.
Accordingly, whenever the movement of the piling structure
placed on the soft ground is observed for a certain period,
what form and condition the ground subsides in must be found.
However, there is still no effective design or control
procedures capable of being practiced within a short period
of time.
A particular problem with the piling method is that it
is pnmafaceeffective for temporarily bringing ground disturbed
by sliding destruction, earthquakes, or the like back to the
old condition provided that the height of the soiling
structure thereon lies within a certain range, but it is not
suited for permanently recovering or enhancing the strength
of disturbed ground, or preventing seismic disasters.
A piling structure laid on soft ground must be normally
stable with respect to piling loads, and dynamic loads and
vibrations produced by traffic loads. In addition to such
piling, and traffic loads, the piling structure must also
resist vibrations produced by an earthquake.
_ -3- 2 19 a 2 1 2
An essential requirement for achieving the stabilization
of such a piling structure is to improve the property of the
soil forming the piling soil structure and the foundation
without disturbing them, so that the strength needed for
design and construction can be instantaneously created,
thereby constituting a homogeneous pre-consolidated (over-
consolidated) ground layer in which the piling structure is
united with the foundation.
A primary object of the present invention is to provide
a consistent solution to the following five problems the
aforesaid conventional piling methods involve in connection
with (1) consolidation or construction time, (2) construction
environment, (3) design management, and construction control,
(4) restoration of ground disturbed by sliding destruction,
and ground or structures damaged by an earthquake, and (5)
applicability to prevention of ground and structures against
disaster.
The present invention has been accomplished with the
aforesaid object in mind, and is basically characterized by
improving the nature of the soil of the ground to be
reconstructed while the ground is not disturbed at all, using
an instantaneous consolidation method, so that the ground can
be insi~ reconstructed within 24 hours in an instantaneous
operation on the basis of four basic principles for soft
ground improvement, viz., (1) consolidation and dehydration,
(2) drainage, (3) solidification, and (4) replacement. The
present invention is also applicable to (1) every soft ground
inclusive of natural stratified ground, man-made ground, and
ground disturbed by destruction, (2) free creation of the
required strength over the range needed in view of design and
construction according to a ground improvement plan, (3)
restoration and reinforcement of ground disturbed by
destruction, and (4) prevention of ground and structures
again seismic disasters. Thus, the present invention
provides a technique that makes use of the effective soil
covering stress of the original ground to reconstruct it into
-4- 219021~
a stable ground layer capable of resisting piling loads,
traffic loads, and vibration loads additionally applied to
the original ground.
In the dra~ings,
FIG. 1 illustrates a process wherein using a specific
impregnation machine, a specifically formulated impregnation
material is injected into ground by a consolidation
impregnation method controlled according to the design and
construction standards, so that there can be obtained a
homogeneously stable yet complex ground zone in which a
solidified portion is united with an over-consolidated
portion obtained by compression effect due to in-situ
dehydration and drainage by consolidation, and post-injection
in-situ replacement and solidification effect.
FIG. 2 illustrates a process wherein by effecting the
aforesaid consolidation impregnation method using the
impregnation material being injected as a load in place of a
piling or other load, a soft and viscous ground zone or a
loose sandy ground zone can be destroyed to form crevices
therein.
FIG. 3 illustrates a process wherein with a further
continued injection of the impregnation material on the same
breaking criteria, the ground zone is successively destroyed
to cause quantitative and qualitative growth of the crevices,
so that the impregnation material starts to flow while the
crevices are filled therewith, thereby creating a sheet form
of fluid body in an oblique or vertical direction.
FIG. 4 illustrates a process where while the fluid body
flows through passages and grows, ground portions contiguous
to the breaking interfaces are insitu subjected to forcibly
rapid loading and dehydration actions in a transverse
direction.
FIG. 5 illustrates a process wherein by the in-situ loading
and dehydration actions of the fluid body, pore water is
entrained, simultaneously with the injection of the
impregnation material, from the ground to be consolidated
into the fluid body, and then dynamically discharged, and
2190212
during the injection of the impregnation material, the water
is discharged mainly through water discharge passages formed
by boundaries between the ground to be consolidated and the
fluid body, and then discharged into underground, and ground-
surface sand layers together with water separated from theimpregnation material. FIG. 6 illustrates a process where by a chain effect of
the fluid body on in-situ loading, dehydration and drainage, the
ground to be improved can be instantaneously consolidated
without being disturbed at all, resulting in a successive
ground strength increase, and the fluid body itself is
solidified within 24 hours in its as-injected state to create
an in-situ solidified replacement skeleton structure in the
ground to be consolidated.
FIG. 7 is diagram showing the relation between
consolidation yield stress and pre-consolidation stress.
FIG. 8 illustrates in section a plan for the creation of
a road with a low piling structure laid thereon.
FIG. 9 a table showing the nature of the soil forming
the ground to be reconstructed, and a profile diagram showing
the effective soil covering stress profile of a piling
structure.
FIG. 10 is a diagram showing the relation between
penetration resistance values obtained by Swedish sounding
tests performed at a depth of up to 5 meters and undrained
shearing strength values (for the reconstructed ground)
obtained by vane shear tests.
FIG. 11 is diagrams for making estimation of the effect
on ground improvement by cone penetration tests using a
portable cone penetrometer.
FIG. 12 is a diagram showing the relation between
consolidation yield s~ress Pc and pre-consolidation stress Pc',
both in tf/m2, to thereby illustrate the first results of the
reconstructed peat ground.
FIGS. 13a and 13b are diagrams showing the second
results of the reconstructed peat ground.
FIGS. 14a and 14b are diagrams showing the third
results of the reconstructed peat ground.
- 219~212
-
FIG. 15 is a table showing the constituent of the soil
forming the ground to be reconstructed, and a diagram showing
a plan for reconstructing a road with a low piling structure
laid thereon.
FIG. 16 is a diagram providing an illustration of how a
piling structure subsides.
FIG. 17 is a diagram showing the effective soil covering
stress profile of piled ground (clay of marine origin), and
the results of ground improvement.
FIG. 18 is a diagram showing the relation between pre-
consolidation stress Pc' and undrained shearing strength Cu
(of clay of marine origin).
FIG. 19 is a diagram showing the effective soil covering
stresses of the original ground (clay of river origin) and
piled ground, the consolidation yield stress of the original
ground, and the consolidation yield stress of the original
ground (clay of river origin) upon reconstructed.
FIG. 20 is a diagram providing an illustration of how
the ground subsides during reconstruction, and after
reconstruction.
FIG. 21 is a diagram showing the void ratio of undrained
shearing strength of the ground before and after
reconstruction.
The present invention will now be explained at great
length with reference to the accompanying drawings.
When earth, or a piling structure is laid on the surface
of land or structures are built up on the surface of land,
newly added loads are transmitted through the ground.
Stresses produced by the transmission of the loads through
the ground often cause ground settlement or subsidence, and
if the loads exceed a certain limitation, the ground will
then be destroyed.
Accordingly, when it is attempted to investigate ground
destruction or ground settlement, it is required to learn
stress occurring at a certain depth of the ground due to the
~7- 2 1 ~ ~ 2 1 2
-
weight of soil, and the magnitude of stress increased by
newly added loads such as traffic, and seismic loads.
Pressure that a horizontal plane at a certain depth of
ground receives by the weight of soil placed above it is
called soil covering pressure or stress.
When soil exists above a ground water level, on the one
hand, soil covering pressure ~z is given by
~ z = ~ x Z ttf/m2~
where ~ is the unit volume weight of soil, and Z is depth.
When soil exists below a ground water level, on the
other hand, soil covering pressure ~z'is given by
~ Z = Ysub x Z ( tf/m2)
where Ysub is the submerged unit volume weight of soil
obtained by subtracting buoyancy from the unit volume weight
of soil, and Z is depth. The pressure ~z' is a sort of
pressure transmitted directly between soil particles, and
called effective stress. The soil covering pressure (stress)
represented in terms of effective stress is then called
effective soil covering pressure (stress).
An undisturbed viscous soil sample gathered from ground
at a certain depth is subjected to consolidation testing,
while incremental loads are added thereto. Void ratios are
plotted per loading, with void ratio on an arithmetic scale
as ordinate and consolidation load on a semi-logarithmic
scale as abscissa. It is then found that at the initial
loading stage the void ratio e decreases following a gentle
curve; upon a certain load exceeded, however, it decreases
sharply from that gentle curve, and changes in a linear form.
This point of inflection, viz., consolidation load under
which soil is shifted from an elastic phase into a plastic
phase is referred to as consolidation yield stress ~. In the
case of naturally stratified, saturated ground, the
consolidation yield point is often in substantial agreement
with the maximum consolidation load the ground have received
in the past. When a load less than such load is applied to
the ground, there is little, if any, ground settlement.
This invention provides a ground improving method
capable of creating ground unlikely to subside by means of
-8- 2 1 ~0 2 1l~
-
the aforesaid instantaneous consolidation technique,
according to which impregnation of ground can be used in
place of loading such as pilling based on the consolidation
principles mentioned above.
When the consolidation yield stress ~ of soil at a given
depth of ground is equal to the soil covering pressure that
soil receives currently, that soil is referred to as soil
that has been subjected to normal consolidation. When the
consolidation yield pressure ~ is larger than the soil
covering pressure that soil receives currently, that soil is
referred to as soil that has been subjected to over
consolidation, indicating that it has so far been subjected
to load larger than that receiving currently.
It is believed that leading reasons for over
consolidation are that, in the past, soil has been exposed to
load larger than the current soil covering load, a repeated
cycle of drying or dry-wetting, a large ground water level
change, and the action of gluey substances.
At present, practically feasible means for over-
consolidating ground having a certain depth is to apply aload larger than the current soil covering pressure to that
ground by piling or other loading methods. As already
mentioned, however, it is difficult to create ground having a
high degree of over consolidation within a very short period
of time by piling or other loading methods, and it is nearly
impossible to instantaneously achieve the strength required
for design and construction over the range needed for the
ground to be over-consolidated, using available piling
methods. Nonetheless, there is still no alternative with the
exception of piling or other loading methods.
This invention provides a technique which enables over-
consolidated soil ( ground) having the required strength to be
instantaneously created at the depth, and over the range,
needed for design, using the instantaneous consolidation
method.
The application of the aforesaid means makes it possible
to create homogeneous pre-consolidated ground (over-
consolidated ground) comprising an integral structure of the
-9- ~1902~2
-
foundation and a piling structure laid thereon. The term
"pre-consolidated ground" used herein is understood to refer
to improved and reinforced ground which can resist every
external force newly added to the effective soil covering
pressure of the original ground, for instance, static loads
produced by a piling structure laid thereon or structures
built up thereon, dynamic loads produced by traffic vehicles,
and loads produced by vibrations, and earthquakes.
The present invention will now be explained in further
detail with reference to some examples.
In the instantaneous consolidation method by
impregnation of ground which is practiced according to
predetermined design for ground improvement, a predetermined
amount of one impregnation material selected from the group
consisting of mortar material, cement material, and a mixture
of mortar and cement materials is injected at a predetermined
pressure into very soft, viscous ground or loose sandy ground
through a preselected array of injection points.
Simultaneously with impregnation, the soft ground is
destroyed to form crevices therein, which are then filled
with the impregnation material. In such an impregnation
process, both the amount of the material impregnated and the
impregnation pressure are controlled according to the
aforesaid design, so that the end degree of consolidation can
be instantaneously achieved.
(1) Some load is required to improve the nature of
ground itself to take effect on its consolidation, and so
piling or other loading means have been used so far in the
art. In the inventive method, however, such consolidation
effect is achieved by the impregnation material injected into
the ground, which is tantamount to the piling structure
(soil) laid on the ground, and the impregnation pressure
which is tantamount to the load corresponding to the
thickness of the soil laid on the ground. See FIGS. 1 to 5.
(2) By consolidation, the ground is so dehydrated that
pore water is discharged. For this reason, natural drainage
due to a soil load applied on the ground has conventionally
been used alone or in combination with a drain. In the
-lO- 2190212
-
inventive method, however, the impregnation material being
injected into the ground is used as dehydration and discharge
means.
Example 1 - Instantaneous consolidation method by
impreqnation of qround.
(i) Using a specific impregnation machine, a
specifically formulated impregnation material is injected
into ground by the consolidation impregnation method
controlled according to the design and construction standards
to be described later. Through the mechanisms explained in
(2)-(6) below and shown in FIGS. 2-6, there can be obtained a
homogeneously stable yet complex ground zone in which a
solidified portion is united with an over-consolidated
portion obtained by compression effect due to in-situ
dehydration and drainage by consolidation, and post-injection
in-situ replacement and solidification effect. See FIG. 1.
(ii) By effecting the aforesaid consolidation
impregnation using the impregnation material being injected
as a load in place of a piling or other load, a soft and
viscous ground zone or a loose sandy ground zone can be
destroyed to form crevices therein. See FIG. 2.
(iii) With a further continued injection of the
impregnation material on the same breaking criteria, the
ground zone is successively destroyed to cause quantitative
and qualitative growth of the crevices, so that the
impregnation material starts to flow while the crevices are
filled therewith, thereby creating a sheet form of fluid body
in an oblique or vertical direction. See FIG. 3.
(iv) While the fluid body flows through passages and
grows, ground portions contiguous to the breaking interfaces
are insi~ subjected to fo~cibly rapid loading and dehydration
actions in a transverse direction. See FIG. 4.
(v) By the in-situ loading and dehydration actions of the
fluid body, pore water is entrained, simultaneously with the
injection of the impregnation material, from the ground to be
consolidated into the fluid body, and then dynamically
discharged. During the injection of the impregnation
-11- 21i90~12
-
material, the water is discharged mainly through water
discharge passages formed by boundaries between the ground to
be consolidated and the fluid body, and then discharged into
underground, and ground-surface sand layers together with
water separated from the impregnation material. See FIG. 5.
(vi) By a chain effect of the fluid body on in-situ
loading, dehydration and drainage, the ground to be improved
can be instantaneously consolidated without being disturbed
at all, resulting in a successive ground strength increase.
On the other hand, the fluid body itself is solidified within
24 hours in its as-injected state to create an in-situ
solidified replacement skeleton structure in the ground to be
consolidated. See FIG. 6.
FIG. 7 clarifies the effect of the inventive
instantaneous consolidation method by impregnation of ground
through an accumulation of experimental data.
In FIG. 7, the numbers on the abscissa indicate a
consolidation load or, more exactly, the consolidation yield
stress Pc of the original ground found by soil testing, and
the consolidation yield stress, again found by soil testing,
of the ground improved by the application of the inventive
method of impregnation of ground. In FIG. 7, the latter
consolidation yield stress is denoted as pre-consolidation
stress Pc' to define around the former consolidation yield
stress. The numbers on the left ordinate stand for void
ratio e and a compression index Cc while the numbers on the
right ordinate represent undrained shearing strength Cu.
Data on void ratio e, compression index Cc, and undrained
shearing strength Cu of the ground before and after
reconstruction are plotted with respect to the consolidation
load on the abscissa. White symbols refer to the original
ground before and after reconstruction, and ground with soil
laid on it, while black symbols refer to the ground after
reconstruction. It is here to be noted that the ground with
a soiling structure laid on it was created before seven years
with a piling structure of 4.5 meters in height.
-12- 2190~ 2
Example 2 - Peat qround
FIG. 8 shows a road widening project in section for a
given road with a new piling structure placed on an adjoining
rice field, and penetration resistance values of the ground
before and after reconstruction for the purpose of
comparison. FIG. 9 illustrates the soil constitution of the
ground to be improved, the effective soil covering stress,
consolidation yield stress, and pre-consolidation stress
profiles of the ground to be improved, and ground with a
piling structure laid on it.
(1) Soil constitution of the qround to be reconstructed, and
ranqe for makinq investiqation of how soft qround is
improved
As can be seen from FIG. 9, the ground to be
reconstructed includes a surface soil layer of 0.6 meters in
thickness, beneath which highly compressible, very soft, and
organic viscous soil composed mainly of peat or humus soil is
distributed at a thickness of about 4.0 meters, and a
relatively rigid and hard subsoil layer composed of alternate
sub-layers of sandy soil, silt soil, and viscous and sandy
soils. In view of design and construction considerations,
therefore, the very soft layer having a depth of about 4.0
meters must be reconstructed.
(2) Non-consolidated qround, normally consolidated qround,
and pre-consolidated qround
Referring here to general stratified ground, the maximum
load that the ground has so far received, for instance,
through accumulated loads of deposits constituting the ground
is consolidation yield stress ~ and, in many cases, the
profile of consolidation yield stress ~ within the ground is
in substantial agreement with that of the effective soil
covering stress of the original ground. When a fresh piling
structure is placed on the original ground, a load produced
thereby causes the profile line of consolidation yield stress
~ of the original ground to become close to that of the piled
-13- ~1 9 n2 1 2
ground layer with the progress of consolidation. Therefore,
the piled ground layer continues to subside until the profile
line of the consolidation yield stress of the original ground
coincides with that of the effective soil covering stress of
the piled ground layer. However, if the consolidation yield
stress of the improved ground obtained by the inventive
method, viz., the value of pre-consolidation stress Pc' is set
on or above the profile line of the effective soil covering
stress of the piled ground layer, any ground subsidence would
not theoretically occur. Indeed, the thus improved pre-
consolidated ground is observed to undergo no subsidence at
all, or an allowable, if any, degree of subsidence, and is
found to be effective for earthquakes as well.
Basically, such concepts underlie the present invention;
it is the inventive method of stabilizing piled ground, or
ground with a low piling structure placed on it that enables
pre-consolidation stress Pc' of the soft ground to be
reconstructed to be set on or above the profile line of the
effective soil covering stress produced by piling, traffic,
and seismic loads.
(3) Estimation of traffic loads on pilinq structure accordinq
to the invention
Loads, if produced by piling, can be well detected.
However, traffic loads, for which there is still no
established estimation method, may be measured in the present
invention, as follows.
To improve the soil of the ground to be reconstructed
while the ground is kept undisturbed, there is still nothing
but a piling method according to which the ground is
dehydrated using a piling or other load, so that pore water
can be discharged from the ground for the purpose of
consolidation. In this piling method, the following equation
(1) is used:
Cu'=Cu+ m'(Po+ U~P) (1)
where Cu' is the undrained shearing strength of the
consolidated ground,
Cu is the undrained shearing strength of the
-14- 2190212
-
original ground,
Po is the effective soil covering stress of the
original ground,
m' is a percentage of strength increase due to the
consolidation load of the piling structure, and
U is the degree of consolidation under investigation
of the piling consolidation load of the piling
structure.
The inventive method is distinguishable over the prior
art in the following points.
(i) If a piling structure is placed on soft ground,
several months to several years are then taken for an 80%
consolidation, as already mentioned. Consequently, the
degree of consolidation U is determined depending on how many
days elapses after consolidation. For instance, given the
strength achieved by an 80% consolidation, then U = 0.8. In
the inventive instantaneous consolidation method, by
contrast, U = 1.0 for every ground irrespective of when the
degree of consolidation is investigated.
(ii) Symbol m' is the percentage of strength increase due
to the piling consolidation load. If the percentage of
strength increase is found at the time of piling, it is then
required to gather a soil sample from the ground under
investigation, and subject it to indoor soil testing, i.e.,
uniface shear (consolidation undrained) testing or triaxial
compression (consolidation undrained) testing. This is also
true of when the percentage of strength increase is measured
during piling works. Thus, it is required to perform soil
testing for each piling. No target strength is obtained
without pressurization at the consolidation load needed in
view of design. In the inventive method, on the contrary,
empirical values have been predetermined through
experimentation per ground, viz., for peat or humus ground,
clay ground of marine origin, and clay ground of river
origin, respectively. Thus, the inventive method can be
instantaneously effected using the predetermined strength
needed in view of design.
-15- 2190212
(iii) Conventional methods are performed on the basis of
the effective soil covering stress Po of the original ground.
Strictly speaking, however, much difficulty is involved in
the precise calculation of the effective soil covering stress
of the original ground. In the inventive method, on the
contrary, design, and construction precision is high because
the inventive method can be practiced on the basis of the
consolidation yield stress Pc.
~iv) In the matter of the present invention, ~P is the
consolidation yield stress (pre-consolidated stress) of the
ground improved by impregnation of ground, as already noted,
and can be calculated in the form of the target value for
compressibility improvement, as designed.
In the inventive method, it is essentially important
that
Cu'= Cu+ m(Pd-Pc) ~-- (2)
where Cu is the undrained shearing strength in tf/m2 of the
original ground,
Cu' is the undrained shearing strength in tf/m2 of the
ground upon improved,
Pc is the consolidation yield stress in tf/m2 of
the original ground,
Pd is the consolidation stress in tf/m2 needed to
obtain the undrained shearing st~ength Cu' in
tf/m2 of the ground upon improved or the pre-
consolidated stress Pc' in tf/m2 of the ground
upon improved, and
m is the percentage of strength increase due to the
piling consolidation stress.
In equation (2), the soil constants of the original
ground are defined by Cu - 1.5 tf/m2, and Pc = 5.0 tf/m2.
~ese soil constants may be found by subjecting the original
ground to consolidation testing, triaxial compression or
uniface shear testing by non-consolidation undrained testing,
uniaxial compression testing, or vane shear testing.
Accordingly, if the percentage of strength increase m and the
target value Cu' for ground improvement are determined, the
consolidation stress Pd (or the pre-consolidation stress Pc' of
-16- 219021~
the ground upon improved) can then be used to calculate the
height of the piling structure of soil as the traffic load.
From the results of experimentation using the inventive
method, shown in FIG. 12, m = 0.4 is chosen from the range of
m = 0.39 to m = 0.42.
Of importance is here how to determine the target value
Cu' for ground improvement. If a follow-up survey is run on
the current stability (the amount of road subsidence, and
whether or not surrounding ground layers or structures built
up thereon are deformed and displaced, and if so, to what
degree?) of an existing road with a low piling structure
placed on it, which is laid on peat ground identical with, or
similar to, the ground under investigation in terms of soil
constitution, soil nature, soil constants, and thicknesses of
layers, as well as on the strength and compressibility
properties of the piling structure and the foundation, it is
then possible to clarify or estimate the stability and
conditions for stability of, and problems with, the ground
under investigation.
The results of follow-up surveys run so far on existing
roads with low piling structures laid on them reveal that (1)
the mean value of the undrained shearing strength is Cu' = 2.5
tf/m2 for some roads found to undergo slight subsidence and
little soil nature changes but offer no particular obstacle
to the traveling of vehicles thereon, and (2) the undrained
shearing strength is Cu' > 3.0 tf/m2 for some roads observed
to undergo little, if any, subsidence and soil nature changes
or to be kept stable. See FIGS. 12, 13, and 14.
On the basis of these results of surveys, now assume
that the reference value for the undrained shearing strength
for the roads as mentioned in (1) above is Cu' = 2.5 tf/m2.
Then, the consolidation stress Pd (pre-consolidation stress
Pc') with respect to this reference value is found by
Pd=(Cu'-Cu+ m ~Pc)/m (3)
Substitution of Cu' = 2.5 tf/m2, Cu = 1.5 tf/m2, Pc = 5.0
tf/m2, and m = 0.4 into equation (3) gives
Pd = (2.5 - 1.5 + 0.4 x 5.0)/0.4 = 7.5 tf/m2
-17- 2190212
For the stably maintained roads, too, assume that the
reference value for the undrained shearing strength is Cu' =
3.0 tf/m2. Then, the consolidation stress Pd (pre-
consolidation stress Pc') with respect to this reference value
is also found by equation (3).
That is to say, substitution of Cu' = 3.0 tf/m2, Cu = 1.5
tf/m2, ~ = 5.0 tf/m2, and m = 0.4 into equation (3) gives
Pd = (3.0 - 1.5 + 0.4 x 5.0)/0.4 = 8.75 tf/m2
In FIG. 9, curves 3 and 4 show underground stress
profiles wherein the consolidation stresses Pd (pre-
consolidation stresses Pc') as calculated above are
distributed in the ground to be reconstructed, correspondlng
to the effective soil covering stress of the original ground.
Here, assume that the unit volume weight of the piling
structure is yt = 1.8 tf/m2. For the case as set forth in (1)
above, pile height as calculated as traffic loads is 2.5
meters if pile height as planned is 1.0 meter, and for the
case as set forth in (2) above, pile height as calculated as
traffic loads is 3.2 meters if pile height as planned is 1.0
meter.
(4) Results of construction works accordinq to the
invention
In exemplary construction works shown in FIGS. 8 and 9,
the resulting effects were ascertained and estimated firstly
by Swedish sounding tests, and secondly by subjecting typical
spots to vane shearing tests, penetration tests using a
portable cone penetrometer, and Dutch double-pipe cone
penetration tests for the purpose of comparison. The results
are shown in FIG. 10. The resulting empirical equation from
the relation between the undrained shearing strength (found
by the vane shearing tests ) and penetration resistance values
Wsw obtained in the Swedish sounding tests is
Cu' = 0.04 Wsw
While design management and construction control were
effected using such penetration resistance values,
construction works were performed at Cu' > 3.0 tf/m2 for
portions of structures built up on ground, and at Cu' > 2.5
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tf/m2 for other portions. After the completion of the
construction works, neither subsidence nor changes in the
nature of the soil were observed.
(5) Influences of seismic vibrations
Much is still unknown or unsettled about earthquake
scales, and the form, type, magnitude, frequency, and time of
seismic vibrations, and so influences of all seismic
vibrations cannot be established or predicted. At least,
however, inventor's experience teaches that grounds with an
improved undrained shearing strength of Cu' > 3.0-4.0 tf/m2
have been hardly hit by earthquakes having a seismic
intensity of 4 to 5. If general piled grounds, grounds on
which low piling structures are laid and which receive
traffic loads, and piling structures and foundations of
filled-up grounds are improved such that their undrained
shearing strengths are Cu' > 3.0-4.0 tf/m2, it will then be
expected that they are not damaged by earthquakes or, if hit,
suffer from a minimum damage.
Example 3 - Clay qround of marine oriqin
One example is explained, wherein the inventive method
is applied to prevent changes of underground pipes due to
consolidation subsidence of a low piling structure laid on
ground composed of clay of marine origin and residual
subsidence thereof due to traffic loads.
(i) The nature of the soil forminq the qround to be
reconstructed
Deposits constituting the ground to be reconstructed are
mainly composed of a very soft clay layer of marine origin
and of about 10 meters in thickness, with diluvial hard clay
and sand soil distributed beneath it.
(ii) Construction history of pilinq structure and sewers
Subsidence of the piling structure placed on such ground
- after the sewers laid underground proceeded as shown in FIG.
16, and the amount of subsidence of the piling structure
19- ~1902 ~'2
reached about 170 centimeters about six years after piling.
At this time, the amount of subsidence of the sewers exceeded
the allowable amount of subsidence of 20 centimeters, posing
a serious obstacle to the function of sewerage and the
structure of the sewers.
(iii) Estimation of effect on qround improvement
The results of effect of the inventive method on low
piling structures are concisely illustrated in FIGS. 17 and
18, from which it is found that the effect on ground
improvement was achieved as initially planned.
Never until now was any subsidence observed. It is also
observed that the sewers are neither damaged nor deformed.
In the meantime, the ground was hit by earthquakes having a
seismic intensity of 3 to 4, but was kept in good condition.
Example 4 - Clay of river oriqin
One example of this inventive method is explained,
wherein the inventive method is applied to soft ground
composed of clay of river origin, on which a high piling
structure of 4.0 meters is placed to construct a railway.
(i) The nature of the soil constitutinq the qround to be
reconstructed
Deposits constituting the ground to be reconstructed
form a very soft ground layer of about 4.8 meters in
thickness and composed mainly of clay of river origin,
beneath which there is a layer composed of a gravel-
containing sand layer having a relatively high density.
(ii) Construction history of pilinq structure and sewers
A sand mat of 0.4 meters in thickness was placed on
ground, and a first piling structure having a critical height
of 1.5 meters was laid on the sand mat. After the lapse of
22 days during which they were allowed to stand, the ground
was reconstructed by the inventive method and, in three days
later during which the ground was let alone, a second piling
structure of 2.5 meters in height was laid on the ground. In
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this way, the ground was rapidly reconstructed within a total
period of 27 days.
(iii) Effective soil co~erinq pressure, consolidation
yield stress and pre-consolidation stress profiles
of the oriqinal qround and pilinq structure
As the consolidation yield stress of the original ground
is larger than the effective soil covering stress, as shown
in FIG. 19, the piling structure remains uncosolidated
because its soil covering stress is lower than its effective
soil covering stress, although the original ground is an
over-consolidated clay layer.
Thus, the consolidation yield stress (pre-consolidation
stress) of the ground to be reconstructed is distributed in a
pre-consolidation region lying above the profile line of the
effective soil covering stress due to the piling loads, so
that the ground can be stable with respect to both the loads
produced by these piling structures and both the vibration
loads produced by railway trains.
(iv) Estimation of the qround reconstructed or improved
Concisely shown in FIGS. 19, 20 and 21 are the results
the stability of the piled ground for railway tracks, which
was obtained by the application of the inventive method to
clay of river origin. From these figures, it is found that
the effect on ground improvement is achieved as initially
planned.
Never until now is any subsidence observed. The ground
has been stable with respect to earthquakes having a seismic
intensity of 3 to 5.
In terms of undrained shearing strength, the grounds
before and after reconstruction are compared with each other.
The undrained shearing strength is Cu = 1.2 to 2.2 for the
original ground, Cu = 2.1 to 2.6 for the piled ground, and Cu
= 2.7 to 5.0 for the ground reconstructed according to the
inventive method.
On the other hand, the compressibility of the original
ground and piling structure are Pc < 10 tf/m2 while the
-21- ~2190~1~
consolidation yield stress of the reconstructed ground is Pc'
> 10 to 18 tf/m2, indicating that apparent improvements in
both strength and compressibility are obtained.
