Micropile Installation with Spoil Control at Logan Airport - Nicholson ...

The final pile drilling depths were determined in the field by the Owner’s engineer usingthe information summarized in Table 2 based on drilling rates and the design loads. TheGroup C materials were added during project construction when the diabase material wasencountered unexpectedly.Table 2. Bond Parameters for Each Idealized GroupingLoadingConditionC/C Allowable Skin Allowable Skin Allowable Skin Allowable SkinPile Ø Spacing Friction Group A Friction Group B Friction Group C Friction Clay(in) (ft) kip/sq. ft. kip/lft kip/sq. ft. kip/lft kip/sq. ft. kip/lft psf plfComp.Ten.Comp.Ten.121299>= 5 2.4 7.54 6.0 18.85 14.0 43.984 2.0 6.28 6.0 18.85 14.0 43.983 1.8 5.65 6.0 18.85 14.0 43.98>= 5 1.9 5.97 6.0 18.85 14.0 43.98 350.0 1099.004 1.6 5.02 6.0 18.85 14.0 43.98 300.0 942.003 1.4 4.40 6.0 18.85 14.0 43.98 260.0 816.40>= 4 2.4 5.65 6.0 14.14 14.0 32.993 2.0 4.71 6.0 14.14 14.0 32.99>= 4 1.9 4.48 6.0 14.14 14.0 32.99 350.00 824.673 1.6 3.77 6.0 14.14 14.0 32.99 300.0 706.86Load Test ProgramPile load tests are used routinely on micropile projects to confirm the relatively largedesign loads. The micropile test program consisted of three pile load tests; two incompression and one in tension. All tests were on non-production 12-in piles. Tworeaction piles were installed for each test pile.Test piles were installed at two locations, one on the north side of the structure and oneon the south side. Compression tests were conducted at both locations with the northcompression test pile founded in rock and the south compression test pile founded inglacial till. A second test pile was installed at the north location within the glacial till forthe tension test.Figure 2 shows thelocations of the testpiles as well as thegeneral layout of thesite. A typical loadtest set-up is shownin Figure 3.A center holehydraulic jack,reacting against a testframe supported bytwo adjacent reactionpiles, was used toapply the test load."Sister bars",installed adjacent tothe steel reinforcingNORTHTESTPILE SITENORTHEASTSTAIRTOWERFigure 2 Test Pile LocationsCENTERCOREVEHICLE BRIDGESOUTHEASTSTAIRTOWERSOUTHTESTPILE SITE5

ar and fitted with vibrating wire strain gages were employed to measure the pile load inbearing strata. The load applied to the top of the pile was measured using a calibratedpressure gauge installed on the hydraulic jack. Dial gauges, attached to an independentreference frame, were used to measure the displacement at the top of the pile.Displacement at the pile tip was measured using a telltale.Figure 3. Pile Load Test Set-up.Compression Load Test Procedure.The south compression test pile bonded in the glacial till was loaded in 107 kipincrements up to 200% of the design load (860 kips). This load was held until the telltaleshowed less than 0.01 inch of settlement per hour at the pile tip consistent with theMassachusetts State Building Code. The pile was unloaded in four increments until noload remained on the pile. The north compression test pile was tested similarly but wasbonded in rock.Tension Load Test ProcedureThe tension test pile was bonded in glacial till and loaded in 36 kip increments up to200% of design load (300 kips). The load was held until the pile demonstrated movementat the top of the pile less than 0.01 inch over 1 hour.Load Test ResultsThe load test results are summarized in Table 3.6

Table 3. Results from pre-production load tests.Net Movement (1) for Maximum Load at Bond Zone atPile Design Max Test PileTest LoadMaximum Test Load (2)LoadingDiameter Load Load Location Top of Pile Telltale atAllowable Actual Min Req'd= Butt Pile Tipin kips kips in in in kips kipsComp 12 430 1002 North 0.22 0.09 0.27 800 645Comp 12 430 859 South 0.53 0.23 0.27 700 645Tension 12 150 300 South 0.03 NA 0.5 NA NA1. Mass. Building Code (MBC) requires that top of pile movement be less than theoretical compression (assuming all theload applied to the butt is transmitted to the tip) + 0.15 in + 0.01 * pile diameter (similar to Davisson, except that 0.01 is0.008 for Davisson). Conservatively this is a net movement of 0.15 inch + 0.01 * pile diameter. MBC also requires that netsettlement at two times design load be less than 0.5 inch for both tension and compression tests.2. MBC requires that at least 150% of the design load reaches the "bearing stratum" at maximum test load.Production Pile InstallationProduction piles were installed from inside and outside the structure ranging between 90and 140 ft in length. The approach used on the interior of the structure was to make anexcavation for the pile cap and to install the piles from within these excavations as shownin Figures 4 and 5. The Comacchio CM1200s and Casagrande M9 rigs employed werespecially outfitted with short masts. Casing lengths were limited to 5-ft for this work.There were as many as four rigs working simultaneously under the low headroomconditions. Pile installation times varied significantly with an average of 1.5 piles per rigshift except when the diabase dikes were encountered unexpectedly. Within the diabaseit took three to five days per pile with as much as 8 hrs to drill one foot.Figure 4. Interior Drilling within Footing Excavation7

Figure 5. Interior pile installation.The exterior piles were installed using a Casagrande C12 rig as shown in Figure 6. Thisrig utilized casing lengths of 15 and 20-ft and typically two piles were drilled per day.Figure 6. Exterior pile installation with Casagrande C12.8

DesilterThe desilting process makes use ofsmaller diameter (~4-in) hydrocyclones.In order to maintain the same overallflow rate it is commonly necessary toemploy several hydrocyclones in thedesilting system. Figure 9 is aphotograph of the system used. Similarto the desander, the denser solidcomponents leaving the hydrocyclone aredeposited on a screen with any materialthat passes the screen being recirculated.Many hydrocyclonesCentrifugeHydrocycloneFigure 8. The Desanding system.The final stage of treatment was acentrifuge system that employed a polymerflocculent. In concept, the operation of thecentrifuge is similar to the hydrocyclone inthat it use centrifugal forces to separatedenser materials. However, the centrifugeuses a spinning drum and thus inducesmuch more force than that obtained withthe hydrocyclone. The added polymerflocculent facilitated coagulation of theclay minerals such that the efficiency of thecentrifuge was greatly enhanced. Thecentrifuges employed at Logan is shown inFigure 10.Figure 9. The Desilting SystemFigure 10. The CentrifugesThe complete system is shownschematically in Figure 11 and aphotograph in Figure 12. Several largetanks are incorporated into the system toprovide needed storage for efficientoperation and the ability to accommodatepeak flow conditions. The effectivenessof this system in separating the spoils andin ultimately producing a clear liquid forreuse in the drilling operations is quiteremarkable. The capacity of the systemis such that it can process an incomingspoil stream of 60 gpm, per centrifuge.10

Figure 11. Schematic of the Spoil Control SystemGrout PlantsSolid Spoil RemovalDesanderDesilterCentrifugesProcessing TanksPumpsSteel LinesFigure 12. Global View of Spoil Conrol System11

Project ChallengesThe Logan Airport job provided many challenges that can be divided into two groups: i)coordination of trades working within difficult access and while trying to maintain garageoperations; and ii) unexpected surprises.The Construction Manager made significant effort to facilitate the work activities of thevarious trades by having daily coordination meetings. These meetings were very helpful,however, inevitably there were conflicts related to scheduling access to various portionsof the site such that trades could not work optimally. As is generally the case during thenegotiation stages of a project, concerns for these types of conflicts and inefficiencies fallon deaf ears and there is over-optimism regarding the coordination that is possible.A significant challenge encountered was the presence of diabase dikes that were notidentified in the subsurface exploration. These dikes were especially troublesomebecause of the external flush "lost bit" drilling technique employed. The lost bits are infact used roller bits from the oil industry. The use of these used bits is necessary becauseof the very high cost of new bits when considering a single use application. For thefounding materials expected to be encountered, namely argillite and glacial till, the usedbits were more than adequate. However, when the diabase was encountered installationoperations slowed dramatically wherein pile installation went from taking about six hoursper pile, to taking 3 to 5 days per pile. The use of down-hole percussion hammers wouldhave penetrated much faster, but the confined space conditions, existing site layout, theuncertainty of the number of piles possibly effected by the diabase, and changes neededto support operations made this option a last resort.Another impediment to production was the presence of utilities at unknown locations,which is a very common occurrence on urban construction sites. The time necessary toidentify the extent of the utilities, their owner, and to negotiate a plan for their movementor development of a foundation retrofit is often substantial. Importantly, money for theseactivities is never budgeted and therefore there is always haggling over what constitutesfair compensation; with ultimately no one feeling like they were treated fairly.SummaryThis paper provided an overview of micropile installation operations to support anexpansion to the Central Parking Garage at Logan Airport. Topics discussed included:1. The re-design of the foundation system to make use of micropiles in place ofdrilled shafts. The redesign allowed the work to be completed at the lowest levelof the garage and thereby minimize disruption of the parking operations. Thedrilled shaft approach was to be completed by drilling through the top floor andwould have required substantial bracing of the structure to accommodate thecrane loads.2. The external flush method was used to install the piles and the specific selectionof bond length was based on drilling response. The drilling penetration rate wascorrelated with specific founding materials and appropriate bond stresses.12

3. The use of a novel spoil control system to minimize the need to dispose of liquidspoils offsite. The cost associated with disposal of liquids offsite far exceeds thatfor solids. Accordingly, the spoils were treated to remove solids and the liquidwas reused in the drilling operations.4. Several project challenges were discussed including dealing with coordination ofconstruction activities and encountering unexpected bedrock and unexpectedutilities onsite.13