installation and instrumented load testing of deep soil ... · and schedule considerations , it was...

Installation and Instrumented Load Testing of Deep Soil Mixing Columns

Kenneth R. Bell1, P.E., PhD, F. ASCE, Joseph E. Baka2, P.E., M. ASCE, andMahi Galagoda3, P.E., PhD, M. ASCE

1 Chief Engineer – Geotechnical and Hydraulic Engineering Services, Bechtel Power Corporation,5275 Westview Drive Frederick, Maryland 21703 [email protected] Engineer – Geotechnical and Hydraulic Engineering Services, Bechtel Oil, Gas, and Chemical,3000 Post Oak Blvd., Houston, Texas 77056 [email protected] Geotechnical Engineer, Bechtel Oil, Gas, and Chemical, Houston, 3000 Post Oak Blvd.,Texas 77056 [email protected]

ABSTRACT: Due to very poor upper soil conditions, it was elected to support theprocess plant facilities for a Liquefied Natural Gas Plant owned by Atlantic LNG inTrinidad on a grid of Deep Soil Mixing (DSM) columns. The DSM columns, whichwere generally constructed in a grid pattern under all foundations, were installed fromthe ground surface down to the underlying hard, clayey soils, generally referred to asthe Old Marine Sediments (OMS). The depth to the OMS was typically 10 to 12meters. The DSM columns were installed using instrumented triple-shaft drillingequipment.

During the entire installation process real-time measurements were taken andrecorded regarding depth of penetration, penetration and withdrawal speed, shaftrotation speed, and slurry injection rates. Also recorded during installation was theenergy measurement and what was referred to as the tool-bar loading. The energymeasurement in combination with the tool-bar loading was used to verify that theDSM had penetrated into the underlying OMS as required by design.

The load testing of the DSM columns was performed to both verify the quality ofthe installation procedure and to confirm the actual settlements of the mat foundationsconstructed on the DSM columns would be within predicted levels. It was alsoelected to perform an instrumented load test of the DSM column in an effort todetermine both the vertical and horizontal stress distribution through the column.

INTRODUCTION

Given Trinidad’s large natural gas reserves, the country has elected to exportnatural gas as liquefied natural gas (LNG). The Trinidadian facility converting gas to

LNG is known as Atlantic LNG (ALNG). This facility, including expansion to fourprocess areas (“trains”) was completed in December, 2005. This paper deals with theconstruction and testing of the improved soils/foundation elements for the processplant of Train 4. Figure 1 below is a photograph taken in July 2005, showing the sitearea.

SITE CONDITIONS

The majority of the land area for the site was reclaimed using hydraulic fill materialthat was pumped from dredging for the ship channels. The site area prior toreclamation consisted mainly of tidal mud flats overlying several meters of soft claysand loose sands, which were underlain by stiff to very stiff clays. This upper soilprofile of soft, clayey soils that are a combination of dredged materials, mud flats,and soft alluvial materials are locally referred to as Recent Marine Sediments (RMS).Typical Standard Penetration N-values in the RMS ranged from 2 to 6 blows per footwhile friction readings from Cone Penetrometer Tests (CPTs) in the RMS typicallyhad readings of 0.2 to 0.5 bars. The RMS at some locations also contained a loose tomedium dense silty sand layer. Underlying the soft clays, at about a depth of 6 to 10

Figure 1: ALNG facility near completion of Train 4

meters, is a very stiff to hard clay referred to at this site as Old Marine Sediments(OMS). Typical strength values in the OMS as measured by a Pocket Penetrometertypically exceeded 200 kPa while CPT frictional readings ranged from about 1.2 to1.5 bars. A transition zone of weathered OMS would typically be found between theRMS and OMS stratum. In addition to the low bearing capacity and predicted high

settlements associated with the soft clays, there was additional concern regardingliquefaction of the sandy layers within the RMS, and potential lateral spreadingduring a seismic event. As a result of the soft soil conditions, it was quicklydetermined that it would not be possible to support any structures on shallowfoundations without first improving the site conditions using some method of groundimprovement or place of structures on deep foundations. After reviewing both costand schedule considerations, it was elected to use ground improvement using DSMfor the Train 4 area.

DEEP SOIL MIXING DESIGN

The design of the DSM was based on improving the soil properties of the uppersoils and the transferring of foundation loading to the underlying hard clays. DSMcolumns were arranged in a grid pattern to help reduce the potential for liquefactionof the upper sand layers by both improving the properties of the sandy soils and byconfining the liquefiable soils with the grid pattern. The grid dimensions weredesigned to provide the selected replacement ratio that satisfied the design intentions.DSM grid layout and dimensions are shown in Figures 2 and 3 below.

Figure 2: Preliminary Plan of Cellular Grid System for Train 4 Process Area

Figure 3: Typical Cellular Systems and overlapping of Soil-Cement Columns

The spacing and density of the DSM grid pattern was based on the loading on the matfoundations to be constructed above the DSM and a post construction settlementcriterion of 50mm total settlement and 10mm differential settlement between adjacentmats and lateral constraint if soils inside the DSM grid liquefied due to dynamic(earthquake) excitation The DSM, which was performed by Raito, Inc. undercontract to Bechtel, was installed using a triple mixing machine as shown below inFigure 4.

Figure 4: Deep Soil Mixing Equipment

The soil mixing was performed by injecting and mixing the grout during the initialdownward operation of the augers and continued until the soil mix column wasextended down into the OMS layer. The design depth requirement for the DSM wasbased on having a minimum penetration into the OMS of 1 meter and with aminimum undrained shear strength of the 100 kPa at the tip of the DSM column.Once the tips of the triple auger reach the required penetration the augers were liftedtwo meters and then redrilled to the original depth to insure mixing of the hard clayeysoils. After the bottom mixing was completed the augers were withdrawn and thenthe equipment was moved down along the line to install additional columns. Duringthis process, alternating columns were installed, allowing for some initial set beforetwo adjacent columns were drilled. The diameter of each auger was approximately900mm with an overall length of a single column approximately 2000 mm thatincludes the overlap between the augers as shown in Figure 3.

.QUALITY CONTROL MONITORING DURING INSTALLATION

The DSM equipment can be controlled and monitored from both inside theoperators cab on the drill rig or remotely from a control room where the grout mixingwas also monitored. Critical quality control was established using continuousmonitor and real-time electronic recording of several key installation parameters. Asindicated above, one of the key elements in the design of the DSM was to insure thatthe columns were installed a minimum of two meters into the OMS. It was originallyplanned, based on the initial trial columns, to determine the required embedment hadbeen obtained by measuring a “Power” factor for the drilling equipment. The Powerfactor is a Raito proprietary mathematical relationship resulting from a combinationof the auger rotation speed, rate of penetration of the augers, and the amperage on thedrive motor. A required Power factor was determined to indicate the top of the OMS.

STATIC LOAD TESTING

The project specifications required that the load carrying capabilities of the DSMcolumns be verified by full-scale load testing. In order to satisfy this requirement twofull scale loads tests were conducted on a cross pattern of four DSM columns. Thetesting was conducted in a similar manner to a static pile load test following theASTM D1143 (ASTM) test procedure. After the columns were installed a 2.4m x2.4m concrete block was constructed over the cross section and used to distribute thejacking load applied to the test columns. Additional columns had also been installedto serve as anchors for the load testing. The load testing proved to be more thansatisfactory when recorded movements at twice the design load were less than 4mm.

INSTRUMENTED LOAD TESTING

Background

Although the load testing was deemed to be very successful, it still was unclear howthe loading on the columns transferred; was the load transferred in shear resistancefrom the mat foundation to the surrounding soil, or was all the loading being taken inend bearing directly in the underlying OMS. In order to determine how the load wastransferred, Raito, Inc. contracted with Lymon C. Reese and Associates (LCRA) toprepare instrumentation and load testing plan. The principal objectives of theinstrumentation plan were to provide data regarding a) the distribution of imposedloading with both depth within the columns and b) radial distance from the center ofthe load point.

Instrumentation During Testing

The actual load test, as with the previous load tests, was performed at theintersection of perpendicular DSM columns. The only difference in this case was thatthe load test was conducted in a production element instead of a test column. Alsosimilar to the other load tests, additional columns with embedded steel beams wereinstalled to act as anchors for the load testing. The anchor columns were typicallyinstalled several meters below the test column section to provide adequate tensioncapacity. In order to determine the load distribution a series of six chains of straingages were installed directly under the load block at varying locations and then twoadditional chains were also installed just outside the limits of the load block, alongthe axis on one of the four intersection columns. Figure 5 shows the strain gagelocations as compared to the location of the loading block. In each of the eightlocations 7 strain gages were placed in a chain as shown in Figure 6. The depths ofeach of the strain gages below the loading block, as shown on Figure 6, were at36mm, 1730mm, 3100mm, 4470mm, 5840mm, 7220mm, and 8390mm. The bottomstrain gage was approximately 500mm above the tip of the DSM columns at8900mm. The strain gages were placed in a 64mm diameter cored hole within thecompleted columns. After the strain gages were placed the core area surrounding thestrain gages was grouted with a grout mix similar in strength to the design andmeasured strength of the soil cement mixture. This was done to insure straincompatibility between the strain gages and the rest of the columns.

The load test set up at the surface was again very similar to that of a pile load test inaccordance with ASTM D1143. The load was applied using a series of 4 hydraulicjacks and each hydraulic jack was coupled to a calibrated load cell. The hydraulicjacks were powered by a single hydraulic pump connected through a manifold toinsure equal loading on each of the four jacks. A separate pressure gage was installedin each hydraulic line between the manifold and jack to verify that equal pressure wasapplied to each jack. The deflection of the load block that was constructed over theintersection of the DSM columns was measured from two sets of reference beams and

Figure 5: Core Locations & Numbering of Instrumental Figure 6: Depth of Placement and Labeling of ElectricalStrain Columns (Source: LCRA) Bars in Instrumental Columns (Source: LCRA)

a combination of dial gages and linear potentiometers (direct current displacementtransducers, DCDTs). Two additional DCDTs were installed to monitor themovement of the reaction columns. Shown below in Figure 7 is a photograph of theload test set-up.

Figure 7: Load test set-up

All the instrumentation were connected to a Campbell Scientific Model 23Xautomated data acquisition system (ADAS) in conjunction with two multiplexers toscan all the load cells, DCDTs, and strain gages at about every 10 seconds during thetesting. The data was downloaded to a computer between each load increment inorder to be able to view real-time data plots of settlement versus load and stain.During the load testing, the total instrumentation package included the embedment of56 strain gages, the use of 4 calibrated load cells, and 6 DCDTs. Figure 8 shows theinstrumentation feeding into the data collector.

Figure 8: Instrumentation leads at data collector

Test Procedure

As indicated above, the load testing was similarly conducted to a pile load test inaccordance with ASTM D1143. The first series of loading was performed per thestandard loading procedure in 8 incremental loads of about 11,000 kgs until a totalload of 88,000 kgs was applied. The 88,000 kgs was equal to 200 percent of thedesign loading. Each load increment was held for approximately one hour and themaximum loading was held for approximately 4 hours. During the initial load testingthe settlement readings versus loading appeared to be very erratic and actuallyshowed upward movement when loaded between 33,000 kgs and 44,000 kgs.Although there was never a confirmed reason for this observation, it was believed thatthe change was due to a thunderstorm that occurred after the start of the test, whichmay have caused the reference beams to stiffen when the air suddenly cooled. Notemperature measurements were taken during the testing. Up until that point, theactual movement had been under 0.10 mm, so a change in the sag of the referencebeam could have easily affected the results of the testing. As a result of the concernwith the initial load cycle, a second load cycle was performed using the quick testingloading sequence where each load was only held for a period of 5 minutes. Theloading versus settlement curve for the second loading cycle appear more as would beexpected as shown in Figure 9.

Figure 9: Load Versus Settlement Curve

Strain Measurements During Load Testing

During the load testing, strain measurements were read in the 56 strain gages todetermine the incremental increase in strain, which could then be converted to load,with each incremental loading. Unfortunately, although all the strain gages were fullytested prior to installation, there was failure of 11 of the 56 gages during the testing,which limited the ability to fully determine the load distribution patterns as hoped.

Shown on Figures 10 through 12 shows the measured strain for columns 3, 4 and 7(see Figure 5 for location of columns) during the loading sequence for the quick loadtest for each of the strain gages in the column. Figures 13 through 15 show thevariation in measured strain for columns 3, 4, and 7 versus depth for three of the eightload cycles during the quick load test. The complete set of test results from thistesting are contained in a test report, Instrumented Tests of Soil-Cement ColumnsUnder Static Compressive Loadings – June17-18, 2003 Trinidad Atlantic LNG,Trinidad & Tobago (Reese, 2003).

Observations During Load Testing

As can be seen from the attached figures, the distribution of loading was generally asexpected with a few exceptions. It was believed that prior to conducting the loadtesting a larger than observed portion of the loading from the foundation would betransferred to the bottom of column and be taken in end bearing in the OMS. In fact,all the plots indicated that near zero loading was transferred to the tip of the DSMcolumn. Even more surprising was the magnitude in the drop of the loading betweenthe highest strain gage just below the loading block and the next strain gage at a depthof about 1700mm. These observations were found to be consistent through all of thestrain gage column locations. In comparing the results as shown in Figures 10 (straingage column 3) and Figure 11 (strain gage column 4), there was significantly morestrain at the location of strain gage column 3 at the surface. The strain was likely dueto some cause of eccentric loading on the DSM columns. However, by the time theloading was down at the 1700 mm and 3100 mm depths the loading was more equallydistributed.

Conclusions

• DSM columns very effectively distribute the load to the surrounding soils andengage the entire soil mass. The cellular system used for the design of the DSMfor this project most likely also added to ability of the columns to transfer load tothe soil enclosed within the grid pattern. Most likely a single column in a similarsoil matrix would have shown more load transferred to end bearing. It should benoted that the grid pattern would transfer more loads to the column tip if theshallow soils were to liquefy during earthquake shaking.

• It appears that there is little lateral distribution of the load transfer carried tooutside the actual loaded area. This again may have been influences by theminimal movement and if there had been additional movement there would havebeen additional evidence of lateral spreading of the loading.

• The instrumentation proved that the design and as-built DSM more thanadequately provided and meet all design requirements for allowable bearing andacceptable settlements.

• Geotechnical instrumentation – related to the Quality Control monitoring andrecording, allows for improving installation and confirming installation details.For example, although the planned Quality Control monitoring initially proved tobe effective it was determined that a better method of determining the depth of theOMS was encountered was by simply measuring the force load, or tool bar load,from the auger assembly. In order to accomplish this, a calibrated load cell wasplaced above the drive motor of the augers at the location where they wereconnected to the lift cables. It was determined that the total weight of the drivesystem, which was the only downward force for the DSM system, was about17,500 kilograms. When the force of the drive system was reduce to about 1,800kilograms, the auger tips were at about the top of the OMS, or, in other words, theOMS was literally holding up the weight of the augers. The readout for the loadcell was in the control room and when the control room operator noted the loaddropped to about 1,800 kilograms, the depth would be noted and recorded and thedrilling would then continue for the additional 2 meters. The drop in themeasured loading was usually dramatic. It easily marked the top of the OMS in aclearer manner than the “Power” measurement. At some locations, it was foundthat, due to the high shear strength of the OMS, it was often difficult to obtain therequired 2 meters of penetration. Typically, at these locations, the measured loadin the drill system would drop to a near zero loading condition. Once a zeroloading condition was obtained, the drilling would continue for a maximum of tenminutes, which was accepted as a final refusal condition. As indicated above,outside the pure weight of the augers and drive motor there was no manner toforce the augers deeper upon reaching a refusal condition.

• Geotechnical instrumentation related to the static load testing providesconfirmation of design assumptions and indirectly provides data for assessing insitu DSM parameters and performance.

Figure 10: Averaged Strain Readings from Electrical Strain Bars in Column 3

Figure 13: Averaged Distribution of Strain inColumn 3 During Loading

Figure 14: Average Distribution of Strain inColumn 4 During Loading

Figure 15: Averaged Distribution of Strain in Column 7 during Loading

• The use of a “Power” rating along with the tool bar loading proved very effectiveto mark the bearing layer during the installation of the DSM.

Acknowledgements

The authors would like to thank Bechtel OG&C and Atlantic LNG 4 of Trinidad andTobago, particularly Mr. George Griesedieck, for assistance and agreement to allowthis paper to be published. In addition, we would also like to thank both Raito, Inc.and Lymon C. Reese & Associates, Inc. for their work in preparing and conductingthe load testing program.

References

ASTM International, Annual Book of Standards, Volume 04.08 Soil and RockStandard 1143-81(1994), 100 Barr Harbor Drive, West Conshohocken, PA,

Lymon C. Reese & Associates, Instrumented Tests of Soil-Cement Columns UnderStatic Compressive Loadings, June 17-18, 2003 for Raito, Inc. September 24, 2003

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