studies on soil disturbance caused by grouting in treating marine clay

7/30/2019 Studies on Soil Disturbance Caused by Grouting in Treating Marine Clay

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2nd International Conference on GROUND IMPROVEMENT TECHNIQUES: 8 - 9 October 1998, Singapore

1. Introduction

Studies on Soil Disturbance Causedby Grouting in Treating Marine ClayJG Wang, BBR Ground Engineering Pte Ltd, Singapore

B Oh, BBR Ground Engineering Pte Ltd, SingaporeS W Lim, BBR Ground Engineering Pte Ltd, SingaporeGS Kumar, Kajima Overseas Asia Pte Ltd, Singapore

Jet grouting is a method to construct in underground a column shaped solid substance bycutting and mixing in-situ soil material with a cement grout injected at a very high velocity andpressure. It is generally used to strengthen and stabilize very soft soils like Marine clay andAlluvial soils for development purpose. This jet-grouting layer can be used for several purposesdepending on the particular construction requirements. Thus it is widely used in Singapore andall over the world [I].Over the years, je t grouting technology has made great development. From early invention ofsingle tube to double tube and triple tube system. These improvements have created biggersoilcrete column size as big as 2.0m diameter. However, in Japan, there is a new successfulbreakthrough on je t grouting research which can achieve up to 5.0m diameter, they called it"SuperJet". This paper shall not give the mechanical details of je t grouting, instead it will focuson the displacement of soil due to jet grouting.The amount of pressure required to facilitate the cutting and mixing of the grout with soil is atrange of 300 -- 400 bars depending on the required radius of the grouted column. Because theinjected grout replaces the cut soil, the soil comes out to the ground through the release holes asspoil. If the number of release holes are not enough or choked, upheaval will be a natural sideproduct. If the area is confined with in, say Diaphragm Wall (D/W) or sheet-piles, the pressurebuilt up due to the introduction of the additional mass in to the soil causes the ground to heaveand at the same time causes deflection in the D/W or sheet piles [2]. There is a possibility that ifthe deflection is great it may cause significant damage to the neighboring structures especiallyold structures. So this should be studied carefully while doing the je t grouting work in a denselybuilt up area. Thus, the displacement control during je t grouting should be studied carefully.This paper studies the je t grouting effects on the Diaphragm Wall and the surrounding soil for aproject in Singapore with two basements in Marine clay. Effects of the Jet grouting, direct

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loading on the Diaphragm wall due to the je t pressure and surcharge due to upheaval, arestudied by Finite Element Method (FEM). The numerical results are compared with thereadings from the Inclinometers installed in the Diaphragm wall and surrounding soils out sidethe D/W. It is found that je t grouting pressure will cause the deflection to a limited extent. Themagnitude of the displacement depends upon the soil surrounding the wall, je t grouting typeand overburden pressure. The upheaval effect varies with construction sequence. Based on theanalysis results a displacement-control method is proposed to reduce the deflection of D/W andthe surrounding soils out side the D/W during jet grouting work. This proposed displacementpractice is effective in controlling the movement of the Diaphragm Wall.2. Project Works

Existing MRT TunnelsU Rl11 Higbrising Building

Food Center

Highway

Fig. 1 Site Location and PhotoFig.I shows the job site in consideration. Prior to the je t grouting work a Diaphragm Wall of800mm thick was constructed. This D/W has an average depth of 51m and penetrated 4 metersin to the hard soil. The je t grouting was carried out at -1 lm -- -14m into the ground to form a3m layer. A total of 1368 numbers of l.6m in diameter je t grout columns were formed within ashort tight schedule of two months. The columns are spaced at l .4m centre to centre with anoverlap of 200mm. The triple tube method was adopted in this project where water is used tocut soil at high pressure of 400bar and cement grout is ejected with annulus film of air to mixwith the soil.

.The main purpose of this layer of the grout is to act like a strut and also as an upheaval blockduring the excavation. The jet-grouting layer against the diaphragm wall can be modelled as asupporting strut to resist horizontal force during excavation. It is tightly formed together as inFig 1 to prevent upheaval of soft soil during basement construction. The layer of grout massprovides a dry and stable platform at the final level of excavation. The stiff soilcrete layer alsoholds the proposed bored pile in position. Without it, the bored pile can be easily moved in softmarine clay and even uplift during the excavation of its overburden soil.As indicated in Fig. 1, Inclinometers are installed all around the site and also in the D/W tomonitor the soil and Diaphragm Wall movements respectively. The location of the project is indowntown Singapore and the site is surrounded by many high-rising buildings also there is anexisting MRT tunnel 20m below the ground level at distance of 60m from the Diaphragm Wall.The Inclinometers and settlement markers are monitored on regular basis during je t grouting.

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Jet grouting has two fundamental effects, Surcharge by upheaval and Jet grouting pressure. Tocontrol the soil movement surrounding the D/W several methods have been considered, likealtering the sequence of the je t grouting columns and reducing the pressure of the grout near theDF/W so that the impact on the wall due to the je t is lessened. In order to understand theseindividual effects following three cases are simulated using Finite Element Analysis:1) Surcharge and je t grouting2) Only je t grouting. The clearance of the Diaphragm Wall from nozzles is emphasized.3) Pre-excavation and je t grouting3. Soil Conditions and ParametersSoil conditions and parameters are also given in Fig. 2. Marine clay is divided into upper layerclay and lower layer clay dependingon the consolidation. Water table is Machine Upheaval Surchargeassumed to be at l .5m from groundlevel. And also the soils under theadjoining road and the MRT zoneare improved during theirconstruction time. An embankmentwith wet soil is used to simulateupheaval effect. Working machinesare replaced by direct loading andare acting on the surface of theembankment. Jet grouted zone isconsidered to be an improved soilzone. Because of low permeability ofvery soft Marine Clay, undrainedanalysis is carried out. All materialsare assumed to follow Mohr

er .,-1 skN'm3E-2025kPaMarine cJavv1m.ver)

y-lSkN/mlE-8.lE3kPacp-23

Diaphragm Wall

JGP y-16 k:Nlm1E 2.6E5 kPa =0.30cp-18 c-125 kPa

Cover

Fill

y-16kNJmlE-4050kPa -0.35cp-220 c=S kPa

MRT Zoney=16 kN'm3E-=27000 kPa -0. 35cp-25 c-5 kPa

Mud stone (completely weathered)Mud stone

.,..=20 kN'ni y = 2 0 k N / ~E-7.2E4kPa =-0.20E-1. 2E6 kPa =-0. 20 cp-300 c=S kPaq> 35 c=S kPa

Surdarge y-20 kN'm1E l.3E3 kPa =0.30q>-30 c-5 kPa

Coulomb model. Fig.2 Soil Profile and Soil Parameters3.1 Jet-Grouting Pressure on Diaphragm WallJet-grout pressure acting on diaphragm wall injet-grouting zone is estimated to be equal to theoverburden pressure, P , in this paper. This cankeep soil above balance if the cutting soil doesnot sink down. Furthermore, because ofcutting, the soil is of fluid status and soilpressure should be isotropic. Fluid dynamiccalculation [3] has shown jet-pressuredistribution for different nozzle distances asshown in Fig. 3. The farther the distance, theless the pressure.

523

200

..cl: 150..e:::lv.iVlu-..coc:= 1000=0I-~

s o - - - - - _ . _ ~ - - - - ~ ~ - - - - - _ _ .20 30 40 50 60Nozzle Distance (cm)Fig.3 Pressure Distribution with Nozzle Distance


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3 2 Embankment Height

I I HI I

po JGP

Upheaval

!

No Friction1 -----'-""'!""" , .... -....6

....A 11,,, , ctua 1ne'/ ;'"' with friction

Fig.4 Surcharge Height and Jet-grouting PressureThe height of the embankment is determined by many factors. The key factors are jet groutingpressure, withdrawing time and overburden pressure. In principle, heaving will occur only if thejet grouting pressure is greater than the overburden pressure. The process of heaving willcontinue until equilibrium between jet-grout pressure and overburden pressure of soil isreached. Fig.4 represents the loading equilibrium diagram.

where H1 is the soil thickness below water table to the top level of et-grout columnH2 is the depth of ground level to the water tableH3 is the height of the surcharge upheaval or wet soil embankmentFis the friction contributed by the side surface of the upheaval columnPv is the overburden pressure before je t groutingPo is the jet grouting pressurey1(1=1,2,3) is the average density of soil each layer

Fig.4 also shows the frictional effect on the upheaval height. For the soils below the water table,effective stress is used. For illustration, when Po= 110 k.Pa, Pv = 66kPa and )'3 = 18 kN/m3 thenH3 = 2.4m for no friction case.4. Results and Discussion4.1 Comparison with Inclinometer ReadingsFirst of all, the deflection comparison between site observation data and FEM results is made toidentify the suitability of the computational model. Fig. 5 shows the comparisons of FEManalysis to actual readings of I-6 which is located approximately Im away from the D/W. Theinclinometer readings show a deflection of lOOmm. Generally, numerical results are inagreement with the observations. This implies that soil profile and loading conditions arecorrectly assumed.

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The deflection of the soil at the top layer ismainly affected by superimposed loading, i.e.vehicular traffic. Soil movement at the lowerdepths is caused due to the je t grout pressureand the surcharge. All the deflections are goingoutwards. It correlates with previous backanalysis [4]. Readings from the Inclinometersin the Diaphragm Wall is generally consistentto the readings of Inclinometers in thesurrounding soils. But careful comparisonreveals that they are different within IOm fromground. Ground activity such as traffic loadingout side the diaphragm wall causes thedifference.Based on this numerical model, loading pathcalculations have been carried out:1. Jet grouting and upheaval combination:

-->0-O8::s"'O~

100

90

80

Measured deflection in-situ Exdwl8i (Ver B)I Exdwl8ja(60&10kPa)60 Exdwl 8jb(70&10kPa)

5 0 - - - - - ............- - - - - - - - - - - - - - - - - - - - - . . . . _ ...........-20 0 20 40 60 80 100 120Deflection (mm)Fig. 5 Comparison ofFEM Resultsto In-situ Observation of 1-6

This study focuses on the interaction between the upheaval and the je t grouting to revalthe upheaval effect.2. Jet grouting effect: Ifwe mobilize enough excavators to remove all the upheaval soil orremove the spoil continuously, i.e. by removing the spoil at regular intervals we keepthe upheaval to a minimum so that the limit deflection is revealed.3. Pre-excavation: Before the commencement of the jet grouting work, the whole site isexcavated to a depth of say 0.5m Im.4.2 Upheaval and Jet GroutingFig.6 is the deflection of the diaphragm wall versus loading history. In this analysis, je t grouting

100--A Jet+Surcharge+Machine . t.. '' ) I . >... . I+ Jet+Surcharge .-*: .1.. I . : ~ .

60 Jet Grouting

-100 0 100 200Deflection of Diaphragm Wall (mm)Fig. 6 The Contribution of Jet-Grouting, Surcharge and Macinery to Deflections

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pressure equivalent to 110 kPa is applied first. Then, upheaval applies the surcharge by terms ofthe increase in the height of the embankment to H3=1.8m. The considering the third stage of theloading cycle, i.e. the machinery like the excavators and the jet grouting machines working onthe top of the embankment (q = 22.5 kPa).If jet grouting alone is acting on the diaphragm wall the deflection in the wall is negligible orlimited. From the analysis, the deflection of the diaphragm wall is sensitive to the swfaceloading, surcharge and machinery, which cause most of the deflection. The deflection indiaphragm wall is determined by the constraints provided by the outside soil, the stiffness of thediaphragm wall and position of jet-grouting pressure to the diaphragm wall. The totaldisplacement shows that the displacement is localized to a limited zone. But the deformationrange is much larger than the plastic flow zone, which was computed by Mr. Akira Wada ofAGA[3]. Soil movement in the MRT zone, which is 60m away from the diaphragm wall, isinsignificant and hence can be ignored.4.3 Jet-grouting pressureAs mentioned previously if the spoil is removed immediately as it comes out from the groundthe upheaval is minimum, then the soil displacement and the deflection of the diaphragm wallshould be different from the upheaval - Jet grouting combination. As shown in the Fig. 6 thedirect contribution to the deflection from je t grouting pressure on the diaphragm wall is limitedcompared to the overall deflection due to other factors. Fig. 7 shows the deflection ofdiaphragm wall for different jet-grouting pressures only.Fig. 8 shows the fading speed of soil displacement with distance. Elastic analysis indicated thatthe fading speed should be of the order rk (where ke [0,2] depends on soil conditions, structureof diaphragm wall and so on, r is the distance from force source).

110

,.-... 25E 100..._,,- -o E::: E- 20E 90 -\Se.o ~o"""'.c -.. 00ro (\S 15- '""'0 80 ..cc.


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Unloading effect is also a very interestingtopic in displacement control during je tgrouting. Modem soil mechanics hasshown that elastic deformation andplastic deformation of soils almost occurat the same time even how small thedeformation is [5]. Elastic deformation intotal deformation decreases graduallywith larger deformations. A roughestimation of elastic deformation isaround 10%. Fig.9 extends the simulationup to after the completion of the je tgrouting or the unloading process.Because soil is assumed to be elastoplastic material, most of the deformationsare residual. As a conclusion, if the walland soils move, it is difficult to recoverthem.

-E-....ccuEcu>0E

100

I80 I

60I

40

20

\\ Diaphragm wall

\J ...

I \ \' \ Jct grouting (P= 110 kPa)

\ \ \ - - - Add surcharge (H=l.8m)' \ - - Add machine (q=22.5 kPa)

\ \\

\ \\ \

\\ \' \' '' - - - - ---- .:::.O L - - ' - J . . . . . . L ~ . . 1 . - - - ' - _ . : : : t = : : : : t : : : : : = * - - . . L - ~ : . . . : : . . . . J

0 20 40 60 80 100X Coordinates (m)Fig. 8 Fading of Soil Displacement with Distance

110- 100-IE 9000I.....r::.c...!! 80

~ 0~ >v 70 Jct pressure P=SO L:Pu....J"'O Jct pn:ssurc ~ L:Pau(,)::3 Jct p r c s . ~ u r e P= l I0 kP:.i"Ov 60 +::: Jct pn:ssurc P= 150 L:P:i

s o - - - - - - - - - ~ - - - - - - - - - - - - --5 0 5 10 15Residual Deflection (mm)

Protection "blanket". As shown in theFig. 3, jet-grouting pressure dependson the nozzle distance from thediaphragm wall. Also consideringanother scenario where light je tgrouting is applied all along thediaphragm wall with low pressure toform a protection 'blanket'.Subsequently when/standard groutingis carried out near to the diaphragmwall, the light je t grouting would actlike a barrier, thus reducing the soildisplacement and deflection . of thediaphragm wall. Fig. 9 Residual Deformation after Jet-grouting4.4 Pre-excavation before Jet-groutingAs we have seen from the above analysis, soil and diaphragm wall will move outward duringjet grouting and most of these movements are permanent or non-recoverable. In order to preventthe significant movement in the diaphragm wall, it is advisable to excavate 0.5m-- Im depth ofsoil so as to reduce the surcharge due to heaving. Also by excavating the diaphragm wall willslightly move inward, this inward deflection can reduce the magnitude of outward deflection. Inthis paper, the effects of pre-excavation of varying depth i.e. 0.5m, I Om and 1.5m on thedeflection of the diaphragm wall are computed. Loading process is assumed as follows:Pre-excavation 9 Jet grouting o Completion ofjet grouting.

The pre-excavation method is effective to reduce the magnitude of deflection of diaphragmwall. Fig. 10 shows the deflection history for the three cases. The pre-excavation has two

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effects: 1. Passive soil pressure. The diaphragm wall moves inward during excavation due tothe active soil pressure. When the je t grouting is carried out, the deflection is pushed back andsoil is into passive soil pressure zone. This pushing requires much bigger force. 2. Lessupheaval height. Upheaval or discharge volume is roughly determined for a site and jet-grouting parameters. Pre-excavation 110will reduce a little overburden pressureand produce a little more upheavalvolume, but pre-excavation reduces theupheaval height above ground. That is,the additional height of theembankment is reduced a lot. Thus, theless deflection is obtained.5. ConclusionsBased on above studies, followingunderstandings can be drawn:I) Jet grouting pressure will cause alimited deflection of the diaphragmwall. The deflection depends onmany factors such as wallproperties, soil profile, position of

~ 0->0-l-000='-00~

100

90

80

70

Pre-excavation of 0.5m Pre-excavation of I .Om60 Pre-excavation of I Smso----------------------------0 20 40Deflection (mm) 60 80Fig. 10 Pre-excavation Effect on Diaphragm Wall

je t grouting and nozzle distance. A feasible method of reducing the deflection is to increasenozzle distance from diaphragm wall. The risk for this method is that the joint of JGPcolumn with diaphragm wall may be not good as a strut wal.er. The JGP layer will lose somefunctions as a strut. Light jet grouting can be used as a protection blanket.

2) Most of the soil displacement and deflections of the diaphragm wall comes from upheaval.The contribution from working machinery during je t grouting contributes ranks the second.Thus, control or removal of upheaval can reduce most of the deflection. Avoiding heavymachinery during jet grouting can further reduce the soil displacement. It should be notedthat if the displacement or deflection occurs, it is difficult to recover them.3) Pre-excavation can reduce the soil movement. Pre-excavation of 0.5m--1.0m is advisable.

6. References[I] J.P. Welsh (1996), "State of the Art of Grouting in North America", Grouting and Deep Mixing, Proc. OfISTokyo'96, Vol. 2, pp.825-831[2] K. Jayatharan and K.K. Soh( 1997), "Excessive Movement of Retaining Structures in Singapore Soft MarineClay", Proc. Of the Int. Conf. Foundation Failure, pp.217-228[3] AGA(1998), Effect Range of Jet Grouting by Limit State Method, Personal Communication[4] Wang, J.G.(1998), Evaluation of Bending Moment Directly From Inclinometer Readings, Technical Report in

BBR Ground Engineering Pte Ltd [5] Ng, C.W.W., Bolton, M.D. and Dasari, G.R.(1994), The Small Strain Stiffness of A Carbonate Stiff Clay,Technical Report of Engineering Department ofCambridge University.

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Proceedings of the 2nd International Conference on

incorporatingPre-Conference Symposium on JET GROUTING andPost-Conference Seminar on GEOSYNTHETICS7 - 10 October 1998, Singapore

[ISBN: 981-04-0234-1]

Gold Co-sponsor: Geotechnics Holland b.v.Organiser: Cl-Premier Conference Organisation

studies on soil disturbance caused by grouting in treating marine clay

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