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Grout Line
OvertureOn this, the thirteenth appointment withGrout Line, we
have an article fromGermany. The Grout Line seems to bemore
international with each issue!
This issues article is related to lique-faction and the use of
Jet-Grouting forits mitigation. The author is GiovanniSpagnoli MASc
([email protected]) who is actually
working as a researcher on liquefactionproblems at Marum
MarineGeotechnics, Bremen, Germany.
Theoretical Evaluations of LiquefactionMitigation Through Jet
Grouting
Giovanni Spagnoli, MSc
General Principles of JetGroutingThe peculiarity of jet grouting
is its abil-ity to modify the characteristics of thesoil,
exploiting the effect of fluid jets atvery high speed (hundreds of
km / h)breaking up the structure and mixingthe ground, partially
replacing it with aconsolidating fluid (e.g. water/cementgrout
mix). The working procedurescurrently in use, with different
namesassigned by specialized companies, canbe grouped into three
categories identi-fied by the number of fluids used (seeFig 1).
Fluids are injected from nozzlesplaced at the end of a battery of
rods op-erated by a motion of rotation-transla-tion, realizing
column elements oftreated soil with diameters ranging from
0.5 m to over 2 m, depending on the sys-tem, operat ing
parameters andgeotechnical characteristics of the soil.
The nature of the soil and operatingmodes affect the quality of
treatment,(composition and geotechnical charac-teristics).
The parameter most usual for thequalitative assessment is the
resistanceto simple compression ranging from afew MPa for
silty-clayey soils to tens ofMPa for those sandy-gravelly.
The volume of the slurry which co-mes to the surface during the
treatmentcan have an important influence on thediameter of the
columns, depending onthe type of soil and its permeability. Inthe
saturated cohesive soils, scarce sus-pension in relation with the
volume of introduced cement, involves lateral
spreading and uplift of soil. If claquagephenomena are excluded,
the low quan-tity of suspension means a greater vol-ume of columns.
This effect must becontained within certain limits oftolerance.
Jet Grouting MethodsCurrently there are three different meth-ods
used to perform jet grouting.
Geotechnical News, September 2008 41
THE GROUT LINE
Figure 1. Different methods of treatments.
Figure 2. Medium diameter for soiljetted with monofluid system,
inrelation of grain size of soil (G =gravel, SG = gravely sand, SL=
siltysand, LA= clayey silt) (Croce et al.,2004 Ref 3).
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These differ with the position of thenozzles along the monitor,
and with thetype and number of fluids used duringtreatment.
These methods have different de-grees of effectiveness and are
used de-pending on the type of ground to betreated [Como et al.,
2005 Ref.1].
The three methods can be distin-guished as: Monofluid; Bi-fluid;
Tri-fluid.
The first system consists of a singledisruptive and stabilizing
fluid, (onlycement). In the most common case, itworks in rotation,
with the terminal ele-ment of the battery fitted both with
jetnozzles and crown perforator. Whendrilling is completed the
drilling fluid isreplaced by the stabilizing mixture,whose flow is
diverted from the noz-zles. The treatment runs with a
graduallifting and contemporary rotation ofrods. The average
diameter varies froma maximum of 1 m for sand and gravelto a
minimum of 40 cm for clayey silt. Itis clear that on equal terms of
size, theresult is different depending on the dif-ferent mechanical
properties of the soil:the minimum values are for soil with
greater shear strength [Croce et al.,2004].
A variation of the system illustratedabove consists of linking a
coaxial jet ofair compressed to the jet of cement, inorder to limit
the spread and increasethe penetrating power and hence therange. So
we expect an outcome gener-ally better than the first case. Indeed,
theincrease in diameter with bi-fluid treat-ment can reach about
50% and is there-fore a first positive aspect of thistechnique.
The improvement with the tri-fluidsystem is even more
significant. Thedisruptive action is reserved to a coaxialwater and
compressed air jet, causing aremoval of finer soil. The cement,
in-jected simultaneously from underneathnozzles, is acting as a
stabilizer, by mix-ing with disrupted soil. The diametersobtained
come up to 2.5 m in granularloose soil or up to 1.5 m in dense
soils.
Treatable SoilThe application of this technology islimited by
the principle of disintegra-tion and partial replacement of the
soil.To the process of disintegration opposesthe resistance of the
soil, which repre-sents the only limitation to the applica-
tion of this system, unlike normalinjections whose applicability
in soils isconditioned by the size and volume ofvoids in the loose
soil. These values donot play any role for the jetting
technol-ogy.
The properties that affect the mecha-nism of interaction are the
permeabilityand strength. For the first point, the pa-rameter of
reference is the coefficient ofDarcy (k). If the coefficient of
perme-ability is
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times as long as a diameter of the nozzleat its exit [Shibazaki,
2003 Ref. 6].
By knowing the rate of withdrawaland the diameter of the column,
accord-ing to Tornaghi (1993 - Ref. 7) it is pos-sible to get the
volume of eroded soil perunit of time:
Ve = 10 D2 / 4. L (1)Ve = volume of eroded soils (l/m)L = rate
of withdrawal (cm/min)D = diameter (m)under the assumption that Ve
for a par-ticular soil is:
L = constant (1/D2) (2)This equation is shown in fig. 6In
practice the rate of withdrawal of
the rods varies between 10 and 80cm/min. The rate is very
important forresistance of the column. The resistancedepends on the
diameter of the columnfor a quantity of cement defined. Thebigger
the diameter of columns, thelower its resistance.
The quantity of energy introduced inthe soil is determined by
the rate of liftof rods [Como et al. 2005, Ref 1], thepressure and
the volume of cement in-jected. It is controlled with
automaticspring of a few centimeters z1 in a deter-minate lapse of
time t1.
Ens = ug qP/Vt = [J/m] (3)The jetting which comes out from
the nozzles collides against the soil.Therefore its dynamic
action finishesvery quickly in relation to its distancefrom the
exit point.
Factors which influence its range ofaction are:
1. mechanical resistance of soil
2. hydrostatic pressure created
The outflow of fluid injected fromthe nozzles occurs in the ring
betweenthe injection rods and the hole executedunder excavation.
The type of motionthat occurs is turbulent. Close to thenozzle the
flow comes with constant ve-locity v for all points of section
orientedalong the x-axis orthogonal to the planeof the nozzles.
Along the border of jetthere is an exchange of energy betweenthe
fluid mass in motion and the sur-rounding one in steady state, so
that thecentral flow assumes an average veloc-ity in the direction
x of jet, and it isslowed down. This phenomenon affectsa greater
portion of space with increas-ing distance from the nozzles. One
cannote the growth in diameter of the jetwith the distance from the
nozzles andreduction of average velocity [Croce etal., 2004 Ref.
3]. With the presence ofair (system bi-fluid) around the
cement,there is a limitation in terms of energeticexchange be-tween
the fluid inmotion and theone in steadystate. Such pro-tection acts
up to adistance beyondwhich the air isdissolved in liquidmass. The
graphin Figure 7 showsthe function of adecrease of the jetin
relation to thedis tance fromnozzle (exampleof a jet of water ina
bentonitic mud).
Definition of LiquefactionTo understand the phenomenon of
soilliquefaction, during cyclic loading, it isfirst necessary to
consider the principleof effective stress. This establishes thatin
a saturated soil, whose interstitialspaces are completely filled
with water,the effective tension acts , on a solidskeleton given
by:
= u (4)where s is the total stress acting or-
thogonally to any plan through a gen-eral point, and u is the
pore pressure inthe same point. Expressing theMohr-Coulomb criteria
resistance interms of effective tensions, shearstrength, that can
be mobilized on a nor-mal plane n subject to a normal totalstress
n, is given by:
Tn = (n - u) tg + c (5)where is the angle of shear stress of
material and c cohesion. Liquefactionis thus a decrease in the
resistance of the
Geotechnical News, September 2008 43
THE GROUT LINE
Figure 5. Effects of narrowing angle of nozzles on
dynamicpressure on jet axes (Shibazaki, 2003 Ref. 6).
Figure 6. Influence of rate of withdrawal on diameter of
col-umns.
Figure 7. Residual energy of jet depending on the distance
fromthe nozzle. En is the energy of jetting from the nozzle and
Enxis the residual energy at distance x from nozzle (Como et
al.,2005 Ref.1).
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ground, after reaching the condition offluidity. The total loss
of resistance isreached when the water pressure fillingthe void
becomes equal to the confiningpressure, nullifying the effective
stresstransmitted through the solid particles.Once the earthquake
has triggered theprocess of liquefaction, the mass of soilremains
in motion until it reaches a newstable condition. The relation (5)
showsthat the liquefaction takes place whenthe value T tends to
zero. It is necessaryto clarify that (5) is a general report,
ir-respective of the presence or absence ofseismic shaking. How
easily observed,the possibility of a liquefaction in-creases with
decreasing of cohesion c,of the angle of friction , of total
stress, and with increasing of pore pressureu. The cyclic actions
of an earthquakehave the effect of increasing the chancefor soil
liquefaction, causing compac-tion, which reduces the volume
ofvoids, by increasing interstitial pressureas well. This implies a
loss of resistancedue to pore pressure in the soil underundrained
conditions, that means sheardeformation under constant volume.This
is essentially due to rapid shaking,too short because it may have
started thedissipation of pore pressure accumu-lated in fluid. This
phenomenon isdeeply influenced by the number of cy-cles N of the
earthquake, the relativedensity Dr and the grain size of the
soil.The lower its relative density, the moreit is liquefiable. The
more cycles N, themore likelihood of soil liquefaction.For
evaluation of the potential for lique-faction it is common to use
the approach
to characterization of earthquake load-ing through cyclic shear
stresses. Cyclicstress ratio (CSR) represents the level ofloading
induced at different depths in asoil profile by an earthquake.
Use of Jet Grouting as a Way toMitigate Liquefaction:
CaseHistoriesJet grouting involves the creation oflarge diameter
columnar soil cementpiles to bypass the liquefiable or poorsoils
and also induced lateral compac-tion of the surrounding ground to a
lim-ited extent. The main advantage is sig-nificantly larger loads
can be carriedand transferred to more competentground thus
bypassing the potentiallyliquefiable soils.
Cooke (2000 Ref. 2) studied the useof jet grouting under an
embankmentslope at existing highway bridges tomitigate the risk of
earthquake-inducedliquefaction damage. The jet-groutedzone helped
to limit movements of theabutment by containing and limiting
theshear deformations that occurred in theliquefiable soils under
the embankmentthat were softened due to the develop-ment of excess
pore water pressuresduring shaking. The limitation of
thedeformations was dependent on thestrength and stiffness of the
jet-groutedzone, which in the cases evaluated didnot fail, and its
ability to resist the in-creased overturning forces during
shak-ing. The performance of a jet-groutedzone is highly dependent
on its strength.The strength assumed for the jet groutedmaterial
was high and resulted in nomaterial failure during shaking.
Olgun (2003 Ref. 5) also studied theeffect of jet grouting
columns on the liq-uefaction of soil due the Kocaeli earth-quake in
1999 (M=7.4). Peak groundaccelerations in this area were
approxi-mately 0.4 g.
The soils were improved to increasebearing support for shallow
founda-tions and to reduce liquefaction poten-tial of the sand
layers. Surcharge fillswith wick drains were used to improvethe
soft clays, and jet-grouted columnswere used to provide increased
bearingsupport in the clays and prevent lique-faction of the loose
sands. Jet-groutedcolumn spacings and diameters wereselected on the
basis of footing spacing,footing loads, floor slab loads,
andjudgment. A primary and secondarygrid of columns was installed
in a rect-angular pattern to provide blanket treat-ment. The
columns in the primary gridwere 0.6 m in diameter with a
cen-tre-to-centre spacing of 4 m. These col-umns extended from the
ground surfaceto a depth of 9.0 m. The secondary gridconsisted of
shorter, 2.5 m-long groutedcolumns that were installed in
betweenthe primary columns to further increasethe liquefaction
resistance of the sandstratum, extending from a depth of 6.5m to
9.0 m. In addition to the primaryand secondary grids, 0.6
m-diametercolumns were also installed at eachspread foot ing locat
ion of thesupermarket building as described inthe paper.
Olgun (2003 Ref. 5) reported that vi-sual field inspections
following theearthquake indicated that no structuraldamage occurred
and no noticeable set-tlements or ground damages were ob-served
anywhere at the site except in theuntreated portions. The untreated
areasexperienced estimated settlements of 70mm to 100 mm.
Even if significant pore pressure haddeveloped in or migrated to
soils in up-per 9 m during the earthquake, as longas the jet
grouted columns maintainedstructural integrity, their higher
stiff-ness in vertical compression shouldhave significantly reduced
post earth-quake reconsolidation settlements rela-tive to untreated
soil. According toOlgun (2003 Ref. 5), the stone columnsand
jet-grout columns most likely de-
THE GROUT LINE
44 Geotechnical News, September 2008
Figure 8. Typical treatment used under the shopping centre to
bypass liquefiablesoils (Olgun, 2003 Ref. 5).
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creased shear strains (and thus porepressure development) in the
soils.
Yilmaz et al. (2008 Ref. 4) per-formed a study on the soil
improvementin Beydag dam against liquefaction ofalluvium at the dam
site. Peak accelera-tion on rock was estimated to be 0.32 gfor an
earthquake having magnitude of7. Liquefiable soils, which consisted
oftwo separate layers of diatomaceous siltand one layer of volcanic
ash beneaththe downstream toe of Wickiup Dam,were stabilized using
4.3 m diameter jetgrouting columns. These liquefiablestrata
extended to depths up to 26 m(Fig. 9). The dam had a square grid
ofintersecting jet grout piles at the down-stream side of upstream
wall having anarea replacement ratio of about 10%.
Depending upon the shear modulusratio, G, between jet grouted
columnand soil, it was found that stress reduc-tion coefficient
changes with area re-placement ratio. Cyclic stress ratio(CSR)
after ground improvement is cal-culated by multiplying stress
reductioncoefficient with CSR before treatment.Thus, it was
possible to calculate thearea replacement ratio required to
reachthe intended factor of safety. It wasfound that 10% area
replacement ratiomay reduce CSR at least about 50%(Yilmez et al.,
2008 Ref. 4).
ConclusionsA lot more needs to be understood re-garding the
liquefaction phenomenon.Research in this field is continuing
par-ticularly in the areas of post liquefactionresidual strength
prediction as well as
prediction of settlements induced byliquefaction. The state of
the art in theunderstanding of the liquefaction phe-nomenon is
still evolving and it is stud-ied by various researchers
worldwide.More work needs to be done in order tofully understand
liquefaction and howto mitigate or eliminate its effects. Fromthe
case history here shown it is clearthat jet grouting is a valid
treatment inorder to contain liquefaction and otherphenomena linked
with the loose ofstrength due to an earthquake, since it isalso a
cheap ground improvement tool.
Jet grouted columns do not providemeans of liquefaction
mitigation,though. This process does not densifythe soil. Orgun
showed that they do notstiffen the ground by attracting the
seis-mic shear stresses. Moreover jetgrouted columns do not reduce
shearstresses on the soft ground. They maystill act as vertical
support if there isenough bearing capacity and side resis-tance
from layers that did not liquefy. Itis possible rely on jet grouted
columnsto provide bearing support and reducesettlements if
liquefaction is limited to aspecific zone. It is clear that there
couldbe liquefaction mitigation only if theentire liquefiable zone
is treated. A totalreplacement of potentially liquefiablematerial
by jet grouting, will avoid theliquefaction likelihood.
References
1. Como G., Como G. (2005) Jetgrouting: cenni teorici, campi
diapplicazione e impiego quale opera
di sostegno. Intervento al corso diperfezionamento SUPSI di
Luganorelativo alle Opere di sostegno perscavi 102.1-R-169 /
10.10
2. Cooke H.G. (2000) Ground Im-provement for Liquefaction
Mitiga-tion at Existing Highway Bridges.PhD thesis, Department of
Civil andEnvironmental Engineering Vir-ginia Polytechnic
Institute.
3. Croce P., Flora A., Modoni G.,(2004) Jet Grouting.
Heveliusedizioni.
4. Yilmaz D., Babuccu F., Batmaz S.,Kavruk F. (2008)
Liquefaction anal-ysis and soil improvement inBeydag dam.
Geotechical and Geo-logical Engineering (2008) 26
5. Olgun C.G. (2003) Performance ofimproved ground and
reinforcedsoil structures during earthquakes case studies and
numerical analyses,PhD thesis, Department of Civil andEnvironmental
Engineering Vir-ginia Polytechnic Institute.
6. Shibazaki M. (2003) State of prac-tice of jet grouting.
Internationalconference on grouting and groundtreatment, New
Orleans.
7. Tornaghi R. (1993) Controlli ebilanci analitici dei
trattamenticolonnari. Rivista italiana digeotecnica 3/93.
Giovanni Spagnoli, MSc, Marum, Ma-rine Geotechnics
Leobenerstrasse, Bre-men, 28359 Germany,
email:[email protected]
Send your grouting papers, articles orcomments to: Paolo
Gazzarrini,fax 604-913 0106 [email protected] ,[email protected]
[email protected].
Ciao!
Geotechnical News, September 2008 45
THE GROUT LINE
Figure 9. Jet grouting application at Wickiup dam (Yilmaz et
al., 2008 Ref. 4).
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