Chapter 13 Use of Jet Grouting in Deep Excavations

Dazhi Wen

Land Transport Authority, Singapore

ABSTRACT

Jet grouting is an increasingly used technique for in situ soil improvement in many types of soils, especially in soft clays such as the Singapore marine clay. Among many of its applications, one of the common uses is to improve the ground for deep excavations. In this chapter, quality assurance and quality control for jet grouting works, properties of jet

grouted piles, design and construction issues associated with the use of jet grouted slabs at

the base of excavations are presented. These topics are discussed with reference to jet grouting applications in the construction of the cut & cover tunnels at Race Course Road and in the station construction by cut & cover method at Clarke Quay station during the

construction of the North East Mass Rapid Transit Line in Singapore.

1. INTRODUCTION

Jet grouting is an increasingly used technique for in situ soil improvement in many types of soil. It is a general term used to describe various grouting techniques in which high-pres- sure air, water and cementing grout are injected into the ground at high velocity. Depending on the system used, the in situ soil may be mixed with the grout, partly mixed and partly removed, or completely replaced. Typically, a single tube system is to mix the grout with in situ soil and form a grouted pile in the ground. A double tube system injects high-velocity cementing grout and compressed air simultaneously. By introducing the compressed air, the double tube system is able to produce jet grouted piles (JGPs) about twice as large as the piles by the single tube system. The in situ soil is lift up to the ground surface by the com-

pressed air. A triple tube system injects air, water and grout simultaneously into the drilled hole. The injection of both water and compressed air permits more soil to be removed from the cutting area and the in-situ soil can be replaced completely by grout during the process of triple tube jet grouting works. As the injection of grout and air/water are separate, the size

of the grouted pile by the triple tube system is usually larger for the same soil-type than the

357

358 Chapter 13

size produced by single tube or double tube systems. Typically, the diameter of a jet grout

pile by the triple tube system in Singapore marine clay is 1600-2000 mm. Figure 1 shows

the typical arrangement of air, water and grout jets in the three grouting systems.

The application of jet grouting technique in geotechnical engineering falls generally

into three categories:

�9 Strengthening of ground as excavation support or underpinning support.

�9 Temporary or permanent stabilization of soil.

�9 Groundwater control.

For the application in excavation, the JGPs are typically installed at the base of the

excavation. The improved soil layer acts as a temporary strut is to reduce the deflection of

the retaining walls. For underpinning the JGPs are installed under the structures, some-

times at an angle from the ground surface to enhance the bearing capacity of the original

ground. Examples of temporary stabilization can be found at launching or receiving shafts

for tunnel boring machines. For groundwater seepage control, JGPs are installed at the

base of embankment to cut off water flow.

2. QUALITY CONTROL FOR JET GROUTING WORKS

Quality assurance and quality control are critical components of a successful jet grouting

program to ensure that subsurface soils are consistent with design assumptions and that the

design parameters are met or exceeded throughout the project.

2.1. Quality assurance Quality assurance begins with a full-scale trial on site. The trial is to establish operation

parameters for subsequent working pile installation to achieve the design geometry and the

. . . . . : : . . . . .

Grout

Single tube jet grouting system

Air Grout Air I

Air Water Air

Grout

Double tube jet grouting system

Triple tube jet grouting system

Figure 1. Typical arrangement of jets.

Use of Jet Grouting in Deep Excavations 359

quality and strength characteristics of the JGPs. The operating parameters include air,

water and/or grout flow and pressure together with monitor rotation and withdrawal speed.

Typical grouting parameters for jet grouting in the Singapore marine clay during the con-

struction of the Mass Rapid Transit (MRT) North East Line (NEL) projects in Singapore is listed in Table 1.

The targeted diameter of the JGPs was generally 1.6-2.0 m in the NEL projects. The

purpose of the jet grouting was to strengthen the very soft to soft marine clays in deep

excavations and in launching and receiving shafts of tunnel boring machines.

To verify the geometry of the trial piles, indirect methods are often used. This is espe-

cially so for applications in deep excavations as exposure of the trial piles is impractical.

The indirect methods include coting, cone penetration tests (CPTs), standard penetration

tests (SPTs) or cross-hole geophysical tests. Retrieved core samples are laboratory tested

to confirm that satisfactory unconfined compressive strengths and stiffness (Young's mod-

ulus) are achieved.

The pre-production quality assurance measures form the basis for quality control dur-

ing production grouting. It may be assumed that in comparable soil conditions, the same

jet grouting parameters produce the same JGP dimension, properties and spoil return. For

the triple tube jet grouting works at Race Course Road during the construction of the NEL

projects in Singapore, the density of spoil return of 123 samples ranges from 1.01 to

1.52 g/cm 3 with an average of 1.31 g/cm 3 (Shirlaw et al., 2003). The density of grout typ-

ically ranges from 1.51 to 1.56 g/cm 3 with an average of 1.53 g/cm 3.

2.2. Quality control during production

The minimum quality control consists of reporting of operation parameters and observing

the spoil return. The technology of computerized data collection system for all jet grout-

ing parameters is available along with continuous real-time observation.

The selected operation parameters should preferably be automatically controlled and monitored throughout construction (see Table 2). Reduced flow or increased withdrawal speed will produce smaller jet grouted geometry.

Table 1. Typical grouting parameters used in MRT NEL projects in Singapore

Operating parameters Units Triple tube grouting parameters

Water pressure MPa 35-45 Air pressure MPa 0.7-1.5 Grout pressure MPa 7-11 Water flow rate 1/min 75-150 Air flow rate m3/min 1 Grout flow rate 1/min 62-105 Lifting speed or withdrawal speed min/m 8-10 Rotation speed rev/min 5-10

360 Chapter 13

Table 2. Quality control inspection items

Drilling Batching

Jetting

Documentation

Sampling and testing

Location, drilling angle and drilling depth Preparation of grout slurry for consistency in cement content and

properties (density measurement) Operating parameters (lift speed, rotation rate, pressure and flow of water,

air and grout), observing spoil returns with density measurement Documentation for each element constructed. Construction times and

correlation to any sampling performed Retrieval of representative samples for laboratory testing

In addition to the quality control inspection items for JGP production, additional project-

specific quality control measures are generally required. This can be in the form of coring for

laboratory tests to establish the unconfined compressive strength of the cores or in situ pump-

ing tests to determine the permeability of the grouted mass. Permeability of the treated

ground can also be verified by installing piezometers on both sides of the treated ground.

The frequency of quality control tests is not fully standardized. Various standards and

manuals have different recommendations. The designer will have to specify the testing

regime based on his design and how critical the JGPs are in the overall system. A common

method is to specify a frequency of quality control testing based on the volume of treated

soil. For example, it can be specified that one core per 1000 m 3 treated soil through the full

depth of the JGPs should be taken and a minimum of three unconfined compressive

strength tests should be carded out with strain measurement to verify strength and stiffness

of the installed JGPs. The samples for the tests can be taken from the top, middle and bot-

tom of the core. The cores should be fully logged and total core recovery (TCR) or rock quality designation (RQD) can also be used as a guide to determine the quality of the JGP

mass. The location of cores is often taken at the overlapping areas of the JGPs because

these areas are typically the weak areas.

Grouting sequences and documentation form an important element in the quality con-

trol system. A systematic approach is typically adopted to ensure that all JGPs are installed

as required by design and no gaps are left in the grouted mass. One example of systematic

approach is shown in Figure 2 used in the jet grouting works at Race Course Road in

Singapore. The grouting rigs were mounted on a steel truss supported on rails founded on

diaphragm walls. The truss beam could move along the longitudinal direction of the

diaphragm walls that acted as temporary supports during the subsequent excavation for the

construction of the cut & cover tunnels.

3. PROPERTIES OF JET GROUTED PILES

The strength and stiffness properties of JGPs are often obtained from unconfined compres-

sive strength (UCS) tests on core samples taken from the grouted soil. In-situ pressuremeter

Use of Jet Grouting in Deep Excavations 361

Figure 2. Schematic approach in grouting sequences.

tests can also be conducted to determine the in-situ Young's modulus of the grouted mass.

Other tests, such as SPTs and CPTs are used to verify the quality of the grouting and to cor-

relate with UCS tests and in-situ pressuremeter tests to get the strength and stiffness of the

JGPs for design purposes.

The design undrained shear strength for JGPs in deep excavations in Singapore marine

clay is typically set at c u = 300 kPa with an equivalent UCS, qu of 600 kPa. The Young's

modulus is typically set at 120 MPa, i.e., 400c u or 200qu. This is believed to be a lower

bound value of the JGP mass. Actual UCS tests on cores taken from the JGPs generally

show much higher UCS, typically exceeding 1000 kPa, even for cores taken from over-

lapping areas of the JGPs. Table 3 shows some of the UCS test results on JGP core sam-

ples at Race Course Road in Singapore. The cores were taken within the middle third

diameter of the JGPs and were tested at 14 days.

The distribution of 28-day UCS for samples taken from Race Course Road is summa-

rized in Figure 3. Similar results from another site at Clarke Quay are also shown in Figure

4 (Shirlaw et al., 2000a). These results demonstrate that the typical design undrained shear

strength, Cu, of 300 kPa is a lower bound value. An upper bound value can be as high as

10,000 kPa at Race Course Road.

The failure of the JGP core samples at UCS tests indicates a brittle failure mode at a

typical strain range of 0.5-1.5%, (see Figure 5), especially for the samples showing very

high UCS values. This is in contrast of the in situ soft clay, which fails under compression

in plastic mode. It is interesting to note that when the UCS is relatively low in the range of

1.0 MPa, the brittleness is less remarkable.

362 Chapter 13

Table 3. Test results of JGP at Race Course Road

No. Bulk density Unconfined Compressive No. Bulk density (kN/m 3) strength at 14 days (kPa) (kN/m 3)

Unconfined compressive

strength at 14 days (kPa)

19.3 6280 7 14.8 978

18.8 6050 8 15.4 879

14.4 3390 9 14.2 876

18.6 4900 10 15.1 780

14.3 1491 11 14.9 667

14.8 1290

4

3

1

0 , i i i i , , i | , ,

0.6 - 1.0 - 2.0 - 3.0 - 4.0 - 5.0 - 6.0 - 7.0 - 8.0 - 9.0 - 10.0 -

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0

UCS (MPa)

Figure 3. Distribution of 28-day UCS at Race Course Road.

35

30

25

+ = ~ . r ~

20 [-

O

6 15 Z

10 --m--m-m m RI !

i i i ! i ! I i i i i i

| ! | | , , | |

Unconfined Compressive Strength (MPa)

Figure 4. Twenty-eight day UCS at Clarke Quay (after Shirlaw et al., 2000a).

11.00

10.00

9.00

8.00

7.00

6.00

�9

"~ 5.00

4.00

3.00

2.00

{ 1.00

Use of Jet Grouting in Deep Excavations 363

0.00 { , , , ,

0.00 0.50 1.00 1.50 2.00 2.50 Strain (%)

Figure 5. Stress-strain curves of JGP core samples in UCS tests.

What should also be noted is the density of the JGP samples. The treated ground does not

gain in density as compared with that of the in-situ soil. JGPs will have similar density of the

in situ soil. When checking for uplift, the density of the JGPs should not be taken higher than

that of the in situ soft clay. The samples at Race Course Road showed densities typically

close to the density of the in situ marine clay, which is generally in the range of 14-16 kN/m 3,

as shown in Figure 6. Similar results at Clarke Quay are also given in Figure 7.

The ratio of Young's modulus of the JGP core samples to the UCS strength is typically

taken as E/qu = 200. The results at Clarke Quay confirm that the ratio for design purposes

is reasonable (Figure 8).

The SPT blow counts at two locations at Race Course Road 20 days after the grouting 3 at z of the radius of the pile from the center and at the overlapping areas of three piles are

shown in Figure 9. The SPT blow counts increased from the original zero blow counts to

24 to 57 blows. SPT blow counts can therefore give a clear indication of the success of the

grouting program.

364 Chapter 13

3

2

1

l l l l l l l l

1.0- 1.4 1.4- 1.5 1.5- 1.6 1.6- 1.7 1.7- 1.8

Density (Mg/cu m)

1.8- 1.9

Figure 6. Density of JGPs at Race Course Road.

1.9 -2.0

20-

18-

16-

14- r

=~ 12- C7"

10-

8-

6-

4-

2 -

0-

[] Entrance1 �9 Break in/out area [] Station box

i I

Jl - ' IN

[L h m'' . . . . . . . . . H I q'~ O tt~ O

il RI |l HI K |1 |

~D t"-- t"-- c ~

Bulk density (Mg/cu.m.)

e,i e,i ~ c5

�9 �9

[]

Figure 7. Density of JGPs at Clarke Quay.

4. D E S I G N ISSUES F O R J E T G R O U T E D P I L E S

It is a common practice to design JGPs as a layer of extremely strong soil. In Singapore,

the use of JGPs at the base of the excavation to treat the very soft to soft marine clay is very

popular. As shown in Figure 5, the stress and strain curves for JGPs and the natural occur-

ring marine clay are very different. The JGP samples reach the peak strength typically at

0.5-1.5% of strain, and the failure tends to be brittle. The incompatibility of strains at fail-

ure of the natural occurring soft clay and JGPs should be considered. Although the behav-

ior of JGP slab as a mass of treated ground may be different from that of a solid JGP core,

as there may be gaps and pockets of untreated soft clay in the JGP mass, the design should

take into consideration the issue of strain incompatibility by verifying the strains in the

Use of Jet Grouting in Deep Excavations 365

3~ 1 25

20 I

~ I , , I , = 15 �9

10 m m

I I I I I I I I o . . . . I IIIII I 1 . , . , , . .

O ~ O ~ ~ O O O ~ O O O O O ~ O

Ratio of E to UCS

Figure 8. Ratio of Young's modulus to UCS at Clarke Quay.

i - --A- -. At 3/4 radius from centre ]

At overlapping area of piles I

12

13

14

15

16 ,.o

17

18

19

20

21

= --A,, = At 3/4 radius from centre -----o----- At overlapping area of piles

SPT blow count per 300mm

0 20 40 60 0 20 40 60 I I

t N A

ltargeted __ . V . m . . ~

SPT blow count per 300mm

14 i i

15

16

g 17

18

19

20

21

targe ,~

grouted ~ , , ~

Figure 9. SPT blow counts vs. depth at Race Course Road at two locations.

JGPs at various stages of the excavation. Where the strains in the JGPs are outside the typ-

ical range, close monitoring should be carried out, and where necessary additional meas-

ures should be taken to prevent a brittle failure of the JGP slabs during excavation.

In addition to verifying the strains in the JGPs at different stages of the excavation

during design, all failure mechanisms should also be checked. Shirlaw et al. (2000b)

have identified all the mechanisms for JGP slabs at the base of excavation for two cases,

(Figure 10):

366 Chapter 13

1. WALLS EMBEDDED IN HARD STRATUM

-Struts

Jet grout ~rinal excavation level sbb ~/////////////////////////////////////////////~,.

Hard stratum

Ground bvel v

Forces on slab

Weight of soil above sbb

from wall = I~d fram

~ t t t t t t t wa. Uplift pres sure

Potential Causes of failure

a) Hydraulic uplift causing slab to heave and bend. Critical pressure could be on base of slab or on in- terface between high and low permeability strata below slab.

b) Swelling of soft clay causing slab to heave and bend.

c) Axial force on slab causing failure, d) Defect in slab causing failure due to axial forces.

2. WALLS NOT TAKEN TO HARD STRATUM

.- Struts / Ground level

J et grout I I v Final excavation level [ I

t J I I ~ Anchors or piles to I I h o l d ~

Hard stratum

Forces on sbb

Weight of soil above slab

load from ; ; ; ; ; ; w a l l ~ ~ load from

1' '"l""r'"'t :'''1 " ' aRtPstqLint ~lknchor ~.nchor R.estrllint

Wall Torce l 'orce a t Wall

Potential causes of fai lure

a) Hydraulic uplift (in sands) causing failure or slab in bending or shear.

b) Undrained base heave pressure (in clay) caus- ing failure of slab in bending or shear.

c) Axial force on slab,causing failure. d) Defect in slab causing failure due to the other

listed mechanisms. e) Insufficient capacity of restraints at walls of

anchors. f) Lack of bond to transfer load from slab to

walls and anchors.

Figure 10. Design issues for JGP slabs for deep excavations (after Shirlaw et al., 2000b).

�9 With sufficient retaining wall embedment to hard stratum at Race Course Road to con-

trol inward movement of the wall. �9 No wall embedment into hard stratum at Clarke Quay to control basal stability for the

excavation.

The JGP slabs may not necessarily be at the base of the excavation. They can also be

constructed above the base of excavation prior to commencement of excavations to further

limit the deflection of the walls (see Figure 11). These layers are sometimes called sacri-

ficial JGP layers, as they will be removed stage by stage during the excavation.

Traditionally geotechnical design parameters are selected conservatively, i.e., design

values are selected toward the lower bound values. For the design of the sacrificial JGP

slabs, it is considered necessary to carry out sensitivity analysis for the upper bound val-

ues. This is to cater for the worst loading condition for the struts above the sacrificial JGP

layer. When the JGP slab is removed, the load taken by the JGP slab will be taken over by

Use of Jet Grouting in Deep Excavations 367

/ / 2 ~ , \ \

/ / N N \

~ -- Sacri Sacrificial JGP slabs

V Final excavation level

Figure 11. Use of sacrificial JGP slabs in deep excavations.

the struts above the JGP slab. If a lower bound value is used, this may underestimate the

loading for the struts immediately above the sacrificial JGP slab.

5. CONSTRUCTION ISSUES FOR JET GROUTED PILES

The main problem associated with jet grouting in Singapore marine clay is heave. At Race

Course Road, localized heave was reported to be in excess of 300 mm (Maguire and Wen,

1999). During the early phases of the construction of MRT lines, the maximum measured

heave was 1600 mm, Shirlaw et al. (2000a). Large lateral ground movement have also been

recorded at Race Course Road (see Figure 12). Similar lateral ground movement was also reported by Wong and Poh (2000).

During the jet-grouting process large volumes of grout, air and water is injected into the ground. Surplus material produced by the injection of these materials is expelled to the surface via the annulus between the drilling rods and the surrounding marine clay. If this

passage is restricted or blocked, the pressure inside the cavity created by the high-pressure

jetting will be built up and when the pressure exceeds the cavity expansion pressure, both

lateral and vertical ground movement will be initiated.

The hydrodynamic pressure created by the high-pressure jetting exists only within a

zone of influence, which is about 300 times that of the nozzle diameter (Covil and Skinner,

1994). For a nozzle diameter of 2 mm, the zone of influence is only 0.6 m. Therefore the jet

pressure does not control the pressure within the cavity. It is believed that the movement of

the ground (both lateral and vertical) is not a direct result of the high-pressure jetting

adopted in the jet grouting works. The ground movement during jet grouting works is a

368 Chapter 13

Deflection

-40.00 -30.00 -20.00 -10.00 0.00 10.0

/ 20.0

102.00

97.00

92.00

87.00

```4!:!:i:i:i:i:i:!:!:i:!:!:i:i:!:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:iii:.,.:iii: 82.oo ~ -

~D

77.00

72.00

67.00

62.00

Grouting zone

Figure 12. Lateral diaphragm wall movement due to jet grouting works.

result of the pressure built up to expel the sludge to the ground surface (see Figure 13).

When the annulus between the grouting tube and the ground is blocked, pressure can be

locked into the ground causing the ground to move both horizontally and vertically.

On the basis of the understanding of the cause of the ground movement during jet

grouting, measures that can be taken to reduce the pressures to expel the sludge to the sur-

face, thus to reduce the ground movement can be:

�9 Reducing the density of the sludge.

�9 Increasing the size of the annulus bydrilling a bigger hole.

�9 Using casings to prevent blockage of the sludge flow.

Shirlaw et al. (2003) showed that the diameter of the casing should be 200 mm or larger

for it to be effective. This may explain the reason why the use of 100 mm and 150 mm cas-

ing at Race Course Road was of limited success (Maguire and Wen, 1999). Other measures

implemented at Race Course Road included pre-grouting to condition the soft clays and the

use of pressure relief holes. These measures proved to be more effective. All these measures

were to ensure free flow of the sludge and to reduce the pressure locked into the ground.

Another issue that should be carefully examined is the layout of the JGPs next to the

retaining walls. Gaps between the retaining walls and the JGP slab should not exist to

ensure the effectiveness of the JGP slab in reducing the inward movement of the walls and

Use of Jet Grouting in Deep Excavations 369

in resisting the uplift force. At Clarke Quay station, shear connectors were provided at the sheetpiles to provide adequate restraints at the interface between the JGPs and the sheet- piles. As the connection is critical, the spacing of the JGPs closest to the sheetpile wall were significantly reduced. As shown in Figure 14, there would be shadows at the inter- face between the JGPs and the sheetpiles if the standard spacing of the JGPs were kept at

f Sludge flow

Grout jet

SHEET PILE - -

Grouting tube

435

h

Water o air jet P = Ys*h + Pf

Ys- Density of sludge P f - Pressure required to expel the sludge to the surface

Figure 13. Pressure required to expel the sludge to surface.

SHEAR CONNECTOR SHEAR CONNECTOR

POTE NTIALLY NOT GROUTED DUE TO 'SHADOW'

SHEET PILE - -

JGP

SPACING REDUCED TO l.Om

JGP SPACING i.4m (THE SHEAR CONNECTORS ARE POTENTIALLY NOT GROUTED DUE TO 'SHADOW)

JGP SPACING 1.0m (THE SHEAR CONNECTORS ARE GROUTED)

Figure 14. Layout of JGPs closest to sheetpiles (after Shirlaw et al., 2000a).

370 Chapter 13

1.4 m center to center. After reducing the spacing to 1.0 m center to center, the interface

would be fully grouted in the critical area of the shear connection to the sheetpiles.

6. CONCLUSIONS

There are three stages in quality assurance and quality control for jet grouting works, i.e., a

full-scale trial to establish the operating parameters, monitoring of the parameters during

working pile installation and verification tests after the installation. It can be assumed that the

same jet grouting parameters would produce JGPs of the same dimension and characteristics.

In the design of JGP slab in deep excavations, it is necessary to check the strains of the

JGP slab at different stages of the excavation. All failure mechanisms should be consid-

ered for the JGP slabs. Sensitivity analyzes are necessary to obtain the worse loading con-

ditions of the struts when sacrificial JGP slabs are used. Density of the JGPs is typically

similar to that of the in situ soft clay and should be used when checking uplift forces.

Heave and horizontal movement of the ground during grouting can be controlled if the

pressures in the cavity created by the high-pressure jetting can be controlled. There are a

few measures to prevent the built-up of the pressure in the cavity. These include the use of

a 200 mm or larger diameter polyvinyl-chloride (PVC) casing, increasing the size of the

annulus of the grout tube and the ground by drilling larger holes and reduction of sludge

density, etc. The layout of the JGPs closest to the retaining walls should be planned care-

fully to ensure that no gaps exist at the interface of the wall and the JGPs.

REFERENCES

Covil, C.S. & Skinner, A.E. (1994) Jet grouting, m a review of some of the operating parameters that form the basis of the jet grouting process, in Proceedings Grouting in the Ground, Ed. Bell, A.L., Thomas Telford.

Maguire, C. & Wen, D. (1999) Practical Aspects of Jet Grouting in Singapore Marine Clay, Proceedings of International Conference on Rail Transit, Singapore.

Shirlaw, J.N., Wen, D., Nadarajah, P., Yoon, ST & Sugawara, S. (2000a) Construction Issues Related to Jet Grouted Slabs at the Base of Excavations, Proceedings of Tunnels and Underground Structures, an International Conference, 26-29 November 2000, Singapore.

Shirlaw, J.N., Wen, D., Nadarajah, P., Yoon, S.I. & Sugawara, S. (2000b) Design Issues Related to Jet Grouted Slabs at the Base of Excavations, Proceedings of Tunnels and Underground Structures, an International Conference, 26-29 November 2000, Singapore.

Shirlaw, J.N., Wen, D., Kheng, H.Y. & Osborne, N. (2003) Controlling Heave during Jet Grouting on Marine Clay, Proceedings of the RTS Conference, Singapore.

Wong, I.H. & Poh, T.Y. (2000) Effects of jet grouting on adjacent ground and structures, J. Geotech. Geoenviron. Eng., 126(3), pp. 247-256.