Jet Grouting for Support of Excavations Near Historic Structures

Nasser Massoudi 1, M. ASCE, P.E.

1 Principal Engineer, Bechtel Power Corporation, 5275 Westview Dr., Frederick, Maryland, 21703;nmassoud@bechtel.com

ABSTRACT: Results of a recent jet grouting application for a historic structure arepresented. The project required deep excavations immediately adjacent to foundationsof the historic structure. Jet grouting was used for soil improvement andunderpinning, to enhance the performance of a 12.2-m (40-ft) deep excavationultimately supported by a concrete diaphragm wall. The effectiveness of jet groutingwith respect to maintaining ground support for the adjacent structure was documentedduring both the grouting operations and as excavation progressed, resulting in finalexcavation movements of less than 6.4 mm (0.25 in.). Results confirmed that jetgrouting was a successful alternative to the more conventional methods, helping meetthe very restrictive project movement requirements.

INTRODUCTION

Jet grouting has been in use since the 1970’s. Its application has grown rapidly inthe last decade, primarily for work in urban environments, for improving foundationsoils and support of excavations. Jet grouting can provide a component or the entireexcavation support system. The project that will be discussed herein falls into theformer category. For a project at the Virginia State Capitol in Richmond, Virginia,jet grouting was used to enhance the performance of an excavation supported by aconcrete diaphragm wall by improving the ground between the diaphragm wall andthe Capitol building foundations. Summary of design, construction, and performanceof the jet grouting operation are presented.

BACKGROUND

The Virginia State Capitol building in Richmond was designed by ThomasJefferson and is over 200 years old. It is designated a National Historic Landmark andis listed on the National Register of Historic Places. The building rests on shallowfootings made of deteriorated bricks, with contact pressures in the range of 240 to480 KPa (5 to 10 ksf ) and are considered very sensitive to disturbance caused by anyconstruction. Capitol renovation required construction of an underground extensionimmediately adjacent to the Capitol building, requiring deep excavations, about 12.2

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m (40 ft) below the ground surface. Historic preservation and protection of theCapitol building against damage during construction required establishingextraordinarily limiting movement criteria for the building, consisting of a maximumsettlement of 6.4 mm (0.25 in.) and maximum distortion between adjacent columns ofL/2000, or 2.5 mm (0.1 in.). The deep excavation was designed to be supported by a permanent concretediaphragm wall. However, the soils between the diaphragm wall and the Capitolbuilding required improvement. Jet grouting was considered a feasible approach for itallowed excavations to be made in improved soils immediately adjacent to thesensitive Capitol foundations, an undertaking that would not have been permissible insoils in their natural state. The design and construction requirements for the jetgrouting operation are listed in Table 1.

Table 1. Jet Grouting Requirements

Parameter Requirement1. Contractor Experienced, Specialty Contractor2. Grouting Method Triple Fluid Jet3. Column Diameter 0.9-1.2 m (3-4 ft)4. Column Overlap Minimum 152 mm (6 in.)5. Unconfined Compressive Strength Minimum 5.5 MPa (800 psi)6. Grouting Parameters Verification Test Column Installation7. QA/QC During Construction Grout/Column Sampling and Testing8. Foundation Movement Maximum 6.4 mm (0.25 in.)

The site soils consist of Coastal Plain deposits. A typical stratigraphy is about 6.1 m(20 ft) of primarily loose to very dense clayey and silty sand with gravel underlain bymore than 18.3 m (60 ft) of firm to very stiff low to high plasticity clay containingsand lenses. Jet grouting, however, was limited to soils in the upper 7 m (23 ft). TheCapitol building and the jet grout columns layout are shown in Figure 1.

FIG. 1. Jet grouting locations in (a) plan view and (b) cross-section.

DiaphragmWall

(a)

Jet GroutColumns

Existing Capitol Building

CapitolBuilding

Jet GroutColumns

Diaphragm Wall (b)

DeepExcav-ationDeep Excavation

b

b

CapitolFootings

10 ft

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CONSTRUCTION

Jet grouting operations started by installing several test columns to different depths,using varying grouting parameters. Samples of neat cement grout and “wet grab” mixwere obtained during test column installation for laboratory testing. Samples of thejet grout columns were also obtained using coring techniques, once the test columnshad sufficiently cured, which was typically about one week after installation.

The coring also provided an opportunity to visually observe the uniformity of themixing within the grout columns. Subsequent to coring, the columns were partially orfully exhumed for further observations and measurements of the column geometry.The mixing appeared reasonably homogeneous, except in the high plasticity clayzones where relatively large chunks of clay were present in the mix. The laboratoryunconfined compressive strength results on test column samples indicated mixedresults, with strengths varying quite considerably, in the range of about 1.4 to 6.2MPa (200 to 900 psi). The difference was mainly attributed to testing of variousmaterials containing primarily clay or primarily sand and gravel, as wells as materialswith varying degrees of mixing homogeneity.

Based on field observations and laboratory test results, final grouting parameterswere established for the production phase, as shown in Table 2.

Table 2. Jet Grouting Parameters for Production Phase

Grouting Parameters Pressure DemandsLift Rate 1.3 ft/min Water 6,000 psiFlow Rate 40 gpm Air 100 psiRotation 17 rpm Grout 150 psi

A total of 110 columns were installed in a period of about 3 weeks. Typical columnlength and diameter was 7 m (23 ft) and 0.9 m (3 ft), respectively. The clay zoneswere double-cut to improve the homogeneity of the mix. Some columns were alsoreamed several times. Over 10% of production columns were cored for final testingand observations. Wet grab samples of the mix were taken as well. Results of thelaboratory unconfined compressive strength (UCS) tests are shown in Figure 2.

0

500

1,000

1,500

2,000

2,500

0 7 14 21 28 35 42 49 56Age (days)

UCS(psi)

Cored-Test ColumnWet Grab-Test ColumnCored-ProductionWet Grab-Production

Design800 psi

1 psi=6.9 KPa

FIG. 2. Laboratory compressive strength results of jet grout samples.

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As evident in Figure 2, large scatters were common in the test results, likely due tovariations in method of sampling, soil type, and homogeneity of the mix. Variationsare evident in condition and quality of the mix, shown in Figure 3, both in coredsamples and in an exhumed test column, with clay, sand and gravel, or small voidsdominating various portions of the mix. Despite the variation, observations ofdrilling, sampling, and the overall condition of the cores indicated sufficient mixingand strength. This was later corroborated by difficult excavation of a few jet groutedcolumns encountered during the diaphragm wall excavation and at other occasionswhen excavated using pneumatic drills.

FIG. 3. Jet grouted soils (a) cored samples and (b) exhumed column.

MOVEMENT MONITORING

The extraordinary and limiting movement tolerances for the project requiredmonitoring all phases of construction on a continual basis, including monitoring ofthe jet grouting. A real-time, automated instrumentation system was used formonitoring. It consisted of total station theodolites, optical prisms, in-placeinclinometers, temperature sensors, and a data acquisition system. An example ofsettlement monitoring at one of the building columns is shown in Figure 4.

-6-4-20246

38,241 38,271 38,301 38,331 38,361 38,391 38,42Tim e (day)

Vert.Movement(m

m)

Start jet grouting

End jet grouting

0 30 60 90 120 150 180

FIG. 4. Foundation movement monitoring.

(a) (b)1 ft 1 ft

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The instruments proved extremely useful in monitoring of the construction. Duringan episode of jet grouting, the instruments issued an alarm rather unexpectedly. Amovement of 4.3 mm (0.17 in.), or about 70% of the maximum allowable, wasmeasured for some foundations, although, it was unclear whether this was entirelydue to the jet grouting operation. The movements for one of the affected foundationsare shown in Figure 4.

The monitoring results prompted taking additional measures to prevent suchoccurrences from repeating, and as evident in Figure 4, further foundationmovements ceased and the unexpected movements were all but recovered. Theadditional measures included increasing the distance between columns installed inone day, limiting the number of columns that could be installed in one day per area,and more rigorous monitoring.

CONCLUSIONS

Jet grouting provided a successful ground improvement alternative and excavationretention for the renovation of the historic Virginia State Capitol building. Thesuccess is owed to detailed knowledge of the soils and the project, early involvementof an experienced, specialty contractor; calibration of grouting measures with sitespecific soils, and detailed verification and monitoring. Jet grouting was found to bemost effective in improving granular soils, although clayey soils were similarlyimproved, however, requiring greater effort. Jet grouting operations are highly site-specific and their performance must be calibrated with the site soils and verified.QA/QC during initial testing and construction is paramount, particularly monitoringof grouting near sensitive structures for movement. Caution should be exercisedwhen grouting near sensitive foundations for movements can develop unexpectedly.The latter confirms the important role of geotechnical instrumentation, especiallyreal-time monitoring, when warranted by the degree of project complexity andmovement tolerance that is specified. It goes without saying that the more restrictivethe movement tolerances, the greater the required redundancy in design, the morecomprehensive the testing and QA control, and the more extensive the monitoring.

ACKNOWLEDGEMENT

The author greatly acknowledges the support and cooperation from the VirginiaDepartment of General Services.

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