Aspectos Teóricos e Práticos Do Compaction Grouting

Digest 2007, December 2007 1129-1140

Theoretical and Practical Aspects of Compaction Grouting1 Fatih TUNDEMR* ABSTRACT

Although not applied in Turkey, compaction grouting has been used widely in many parts of the world and especially in the USA in foundation engineering for reducing the void ratio of soils, relevelling of settled buildings, underpinning of footings and slabs, controlling settlements, and for mitigation of liquefaction potential of the soils in the last 10 to 15 years. In this study, a theoretical approach regarding compaction grouting has been presented, injection parameters and rheological properties of the grout has been correlated and practical applications, especially for increasing liquefaction resistance, have been discussed.

1. INTRODUCTION

In 1980, the ASCE Grouting Committee [1] defined compaction grouting as the grouting technique where a soil-cement grout material with less than 25 mm (1 inch) slump consisting of sufficient silt sizes to provide plasticity together with sufficient sand sizes to develop internal friction is injected at very high pressures without entering into soil pores but remaining in an expanding mass around the injection point and thus giving controlled compaction of loose soils around or giving controlled displacement for lifting of overlying structures (Figure 1).

Being one of deep compaction techniques for loose or soft soils, use of compaction grouting is becoming widespread due to its applicability to compact very deep, local soil layers effectively, its compact equipment which enables the procedure to be applied at narrow zones even at the basements of the buildings, formation of relatively less waste and pollution at the end of the procedure and due to its least vibration effects on the existing structures. Especially within the past 15 years, the procedure has also been used to mitigate soil liquefaction, although experience with this type of application is limited. Boulanger and Hayden [3] presented an extensive case histories report about successful densification of deep, loose fill deposits using compaction grouting as an effective means for increasing the liquefaction resistance. Baker [4] investigated the use of ompaction grouting for in-place densification of loose, liqueifiable soils below two existing dam embankments and Orense, et al., [5] researched the treatment of liquefiable soils beneath the existing building foundations by compaction grouting technique.

* Middle East Technical University, Ankara, Turkey - [email protected] Published in Teknik Dergi Vol. 18, No. 1 January 2007, pp: 4069-4080

Theoretical and Practical Aspects of Compaction Grouting

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Figure 1. Schematic view of compaction grouting [2]

2. THEORATICAL APPROACH

Critical point in the application of the compaction grouting procedure is the deposition of the grout in an expanding globular mass in the vicinity of the point of injection. In case the used grout is overly mobile (low viscosity), hydraulic fracturing of the adjacent soil can occur, resulting in loss of control of the compaction procedure. Furthermore this can result in damage to the overlying structures or to the underground facilities around. Compaction grouting has been successfully used in virtually all types of soil, although special considerations must be provided for work in soft clays where slow drainage will likely cause high pore pressures that require special tretment.

Graf [6], presented the theoretical description of the process hypothesizing that the injected masses would be generally spherical, and would densify the injected soil radially in all directions. The first report describing the actual mechanism and providing data derived from the excavation and removal of full scale test injections was made by Brown and Warner [7]. Since this early work, a large number of injected masses have been exposed and evaluated in research demonstrations in connection with actual or proposed projects and the grouting parameters affecting the procedure, lateral forces exerted upon adjacent sub-structures, investigation of changes in the treated soil density have been tried utilizing standard penetration or cone penetrometer tests before and after grout injection.

If the soil is assumed to be a homogeneous and isotropic material, the grout pressures within the soil mass will dissipate at a spherical boundary, centered at the tip of the injection pipe. Stresses and strains caused by the grouting procedure will be zero at this boundary (neutral boundary). The state of stress shown in Figure 2 will exist for the grout material and for the soil.

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Figure 2. State of stress for the grout material and for the soil

Strains for the soil and the grout material is shown in Figure 3.

For a homogeneous, linear, elastic and isotropic material, volumetric strain is the volume of grout divided by the soil volume within the neutral boundary and can be formulated as seen in equation 1.

nb

gv V

V= (1)

where:

Vg= volume of grout

Vnb= volume of soil within the neutral boundary

Figure 3. Strains for the grout material and the soil


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If we define the soil bulk modulus as v

gb

PE

= then

b

gv E

P= (2)

Substituting equation 1 into equation 2, the following expression is obtained.

b

g

nb

g

EP

VV

= (3)

The increase in the density of the soil mass ( ) can be expressed as:

nb

m

V

=

where m is the introduced mass.

Substituting for Vnb from equation 3,

b

g

g

m

EP

V.

= is obtained.

However the introduced mass can not be considered simply as the multiplication of the volume of grout by its unit weight. The mass introduced into volume (Vnb) which effectively raises the density of the soil within (Vnb) is the volume of the introduced grout multiplied by the density of the soil itself. To be more explicit, let us assume that the grout is injected in a balloon and that air is used instead of grout. In this case what the air and balloon would displace will be the multiplication of volume of grout (i.e. air) by the density of the soil. As can be seen, the effect of the grout density will be irrelevant. Therefore;

sgm V .= where s is the unit weight of soil at the point of injection. Therefore the increase in the density of soil mass is

b

gs E

P. = or

=

gsb

PE . (4)

For practical purposes density of soil, s , can be taken as a constant. Being a property of soil, Eb, by definition, represents the relationship between the volume and the pressure of the grout.

Compaction grouting pressures applied in various soil types and the corresponding increases in the soil densities are presented in Table 1 [8].

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Due to the assumptions regarding the homogeneity, isotropy, linearity and elasticity of the soil, required modifications should be carried out taking into account the type and depth of the soil and the strain level in order to avoid deficiencies during the application.

Table 1. Values for bulk modulus for diferent soil types

Soil Type Soil Density,

kg/m3 Grouting

Pressure, kPa Increase in Density, %

Bulk Modulus, Eb , kPa x 10

6

Peat 970 1500 0.30 4.8 Soft

clay/silty clay 1300 2000 0.20 19.5

Medium stiff clay/silty clay 1450 3500 0.15 33.8

Soft sandy silt/clay 1300 3500 0.15 30.3

Medium stiff sandy silt/clay 1450 4100 0.10 59.5

Loose siltly sand/sand 1300 4100 0.08 66.6

Medium dense siltly sand/sand 1600 5500 0.07 125.7

3. GROUT MATERIAL AND PLACEMENT INTO SOIL

The importance of the resulting grout mass and its effect on the obtained improvement degree was first reported by Warner and Brown [7]. More than 100 compaction grout test columns were formed and they were exposed by subsequent excavation. Grout materials with different mixture ratios and rheological characteristics were injected at different injection rates. As a result it was observed that very stiff grout mixtures consisting of fine sand and 12 % cement and water yielded the best results. Also it has been specified in the report that a slower pumping rate resulted in significantly higher grout takes. The same researchers emphasized the importance of limitation of clay in the grout mixture for the success of the technique. This is due to the fact that clay provides excessive mobility in the mixture and the risk of soil fracturing arises. Indeed the sand gradation shown in Figure 4 indicates zero allowance for clay size constituents. It is also suggested to use the least amount of water that will provide a very stiff plastic consistency grout. In brief, the stiffer the grout, the more effective its injection will be. Also the excessive pumping rates could result in rupture of the soil and therefore this should be strictly avoided. For this reason injection rates more than 0.06 m3 per minute are not suggested.

Placement of grout into the soil requires taking into account all field conditions (soil, structure, limitations for property rights) and generally from top to bottom and bottom to up techniques and sometimes both of them are used (Figure 5). For superficial applications and uplifting of footings settled differentially, top to bottom technique is used and for stabilization works where surface heave should be kept at minimum bottom to up technique is utilized. The advantage of top to bottom technique is that the upper layers are compacted


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first and higher grouting pressures could be used as proceeding downwards. However each grouting stage will require additional drilling and thus the increase of cost of this technique will be a disadvantage. If top to bottom grouting technique creates settlement problems at deep formations then a few bottom to up grouting operations are performed first and then top to bottom grouting is continued until the final grout mass is reached at the bottom.

Figure 4. Preferred limits for sand used for compaction grouting [9]

Figure 5. Bottom to top and top to bottom compaction grouting techniques [10]

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Spacing and location of grouting holes are of great importance. Factors such as soil type, water content, existing relative density of the soil, required improvement degree, geostatic stresses, existing structural elements etc. affects the placement of grout material into the soil and thus the obtained final improvement.

The spacing for grouting holes is around 2.5 m under the foundation center and preferably a triangular pattern is selected. Formation of one or two rows of additional grouting zones around the structure is considered to be appropriate in order to avoid damage to the structural elements around due to lateral loading. In case of high geostatic stresses (grouting at deep formations or under structural loads) horizontal spacing of grouting holes should be kept wider since higher grout volume and pressures will be required. For grouting in loose soils horizontal spacing is higher since a larger grout mass will be formed as compared to grouting in denser soils.

4. APPLICATION AREAS

Compaction grouting has been commonly used for correcting differential settlements, for underpinning slabs and footings [11, 12, 10], for liquefaction mitigation and prevention of seismic problems [3], for compensation of superficial settlements due to tunnel excavations in soft or loose soils [13, 14], (Figure 6), for improving foundations of underground pipe lines and culverts [15] and for prevention of formation of sinkholes in dam foundations [16].

Figure 6. Compensation of settlements due to tunnel excavation by compaction

grouting [2]


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Boulanger and Hayden [3] prepared a comprehensive summary of published case histories involving compaction grouting for liquefaction mitigation. These case histories indicate that compaction grouting can be effective in increasing the standard penetration test (SPT) and cone penetration test (CPT) resistance of soils ranging from silty sands to silt. However it should be noted that long term durability of the grout mass formed in the soil as a result of compaction grouting should be sufficient. Also application of this technique in very soft clays will create excessive pore water pressures and this will result in long term settlements. Therefore these points should be taken into account during the design of grouting parameters and grout materials.

Boulanger and Hayden [3] shows the benefits of compaction grouting for mitigation of liquefaction potential in a clayey silt and a silty sand, and the effects of time on the penetration resistance of the treated soils. For this purpose CPT soundings one week, one month, and 18 months after treatment were utilized. Based on the results of the test section, it may be concluded that compaction grouting was successful in increasing the SPT and CPT resistance of the silt and sand. The level of improvement observed in the silt is particularly encouraging since few other improvement methods are as effective in this type of soil.

A large increase in the penetration resistance of the grouted silt and sand was observed. However, up to about 30 % of the average increase was lost within the following 18 months (Figure 7). This result shows that lateral stresses decrease with time. The fact that the lateral stresses decrease in long term should be taken into account in compaction grouting for liquefaction mitigation.

4.1 Equipment

A special equipment is required for compaction grouting since a very stiff cement mortar is used. Mixer should be at such a capacity that it will be able to mix the low slump cement mortar uniformly. Pump required for the compaction grouting should:

be at a capacity to pump the cement mortar with low slump value, be able to operate at high pressures (4000 7000 kPa), be able to provide a pumping rate of up to 20 m3/hour which could be adjusted during

pumping and

be of a type that could measure pressure at the injection point. Cement mortar is transmitted from grout pump to the injection point via 38-50 mm diameter pipes and due to use of stiff, high viscosity cement mortar there exist high frictional pressure losses inside the pipes. For this reason the distance between the pump and the injection point should be kept a minimum.

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4.2 Data required for design

Data required for the design of compaction grouting consists of the followings as in other grouting techniques:

site geology and geotechnical parameters (soil type, unit weight and structure), permeability of soil, temporality or permanence of the application, allowable soil or structural displacements, site access (open site or applications under the existing structures).

4.3 Grouting parameters

Grouting pressure should be observed both at the pump and at the injection point during the grouting operation. Because pressureinjected volume relationship will provide information regarding the soil conditions and the problems that may be encountered. For example a drop in pressure might mean that grout fills either a cavity or is injected into an underground structure or the soil is hydraulically fractured or the lateral resistance provided by a retaining structure is overcome.

Generally a criterion for the completion of pumping is determined. Pumping is stopped when te first of the followings is observed:

A peak pressure drop when pumping at a constant rate, this indicates that the shear strength of the soil is overcome.

Occurance of surface heave (except grouting is used for prevention of settlements). In case the maximum amount of grout determined previously was injected (occurs

rarely).

In case the grout could no longer be injected into the soil [10].

4.4 Trial injections

In large projects it is recommended to perform trial injections in the existing soil conditions. In this way necessary modifications could be made on the grouting parameters, layout of grouting holes, grouting sequence and on the rheological characteristics of the grout material. Furthermore, at the end of the trials, it should be determined whether the compaction grouting operation is the most appropriate soil improvement technique or not for that specific project.

4.5 Supervision

Supervision is a must during the grouting operation in order to perform the compaction grouting appropriately. At least the consistency of grout, injection rate, grouting pressure,


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and the amount of grout injected at each stage should be observed and recorded. This data could be used in determination of soil conditions and the efficiency of grouting operation. In relatively soft and loose soils grouting pressures will be low and the injected volumes will be high. Injected grout at the first stage will stiffen the soil somehow therefore higher pressures could be required at the second stage and amount of injected volume might be less.

Figure 7. Average CPT tip resistances prior to and after compaction grouting [3]

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5. CONCLUSION

Theoretical and practical aspects of compaction grouting, being one of the soil improvement (densification) techniques, have been explained, application areas, especially its use for liquefaction mitigation in the last 10-15 years have been mentioned and the soil and grouting parameters are breifly described. It has been emphasized that special precautions should be taken in case of use of this technique in soft clays due to build up of excess pore water pressures and that the penetration resistance of the tretaed soil may decrease by time as the lateral stresses reduces in the long term.

List of Symbols

x : total stress along x axis

y : total stress along y axis

z : total stress along z axis

Pg : grouting pressure

v : volumetric strain

Vg : volume of grout

Vnb : volume of soil inside the neutral boundary

x : strain in the direction of x axis

y : strain in the direction of y axis

z : strain in the direction of z axis

Eb : soil bulk modulus

: increase in the density of soil

m : mass injected into the soil

s : unit weight of soil at the injection point

References

[1] Committee on Grouting of the Geotechnical Engineering Division, 1980, Preliminary Glossary of Terms Relating to Grouting, Journal of the Geotechnical Engineering Division, ASCE, Vol. 106, No. GT7, Proc. Paper 15581, pp. 803-815.

[2] Essler, R.D., Drooff, E.R., and Falk, E., (2000), Compensation Grouting: Concept, Theory and Practice. Advances in Grouting and Ground Modification, ASCE, GSP No.104, pp. 1-15.

[3] Boulanger, R.W. and Hayden, R.F. (1995), Aspects of Compaction Grouting of Liquefiable Soil, Journal of Geotechnical Engineering, ASCE, Vol. 121, No. 12.


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[4] Baker, W.H. (1985). Embankment Foundation Densification by Compaction Grouting, Proceedings, Issues in Dam Grouting, ASCE, New York, NY, pp 104-122.

[5] Orense, R.P., Morita, Y., and Masanori, I. (2000), Assessment and Mitigation of Liquefaction Risk for Existing Building Foundation, An International Conference on Geotechnical and Geological Engineering, Melbourne, Australia.

[6] Graf, E.D., Compaction Grouting Technique, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 95, No. SM5, September, 1969.

[7] Warner, J., and Brown, D.R. (1973), Compaction Grouting, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 99, No. SM8, Proc. Paper 9908, August, 1973.

[8] Al-Alusi, H.R. (1997), Compaction Grouting: From Practice to Theory, Grouting: Compaction, Remediation and Testing, ASCE, GSP No.66, pp. 43-53.

[9] Warner, J., and Brown, D.R. (1974), Planning and Performing Compaction Grouting, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol.100, No. GT6, Proc. Paper 10606, pp. 653-666.

[10] Graf, E., (1992). Compaction Grout, 1992. Proceedings. Grouting, Soil Improvement and Geosynthetics, ASCE Geotechnical Special Publication No.30, Vol. 1, New Orleans, February 25-28, pp 275-287.

[11] Mitchell, J.K., (1981). State-of-the-Art Report on Soil Improvement, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol.96, No.SM1.

[12] Welsh, J.P., Anderson, R.D., Barksdale, R.P., Satyapriya, C.K., Tumay, M.T., and Wahls, H.E., (1987). Densification. Proceedings, Soil Improvement A Ten Year Update, ASCE Geotechnical Special Publication No.12, New Jersey, April 28.

[13] Baker, W.H., Cording, E.J., and Macpherson, H.H. (1983), Compaction Grouting to Control Ground Movements During Tunneling, Underground Space, Vol.7, pp 205-212.

[14] Harris, D.I., et al. (1994). Observations of Ground and Structure Movements for Compensation Grouting During Tunnel Construction at Waterloo Station, Geotechnique, Vol. 44, No. 4, pp 691-713.

[15] McGovern, M.S. (1996). Grouting Combination Repairs Sewer Pipe, Concrete Repair Digest, pp 94-100.

[16] Garner, S.J., Jefferies, M., and To, P. (1998). Compaction Grouting Project Completion Report, WAC Bennett Dam Sinkhole Remediation Project. BC HydroPower Supply Engineering.

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Aspectos Teóricos e Práticos Do Compaction Grouting