micro piles by andrew brengola

8/8/2019 Micro Piles by Andrew Brengola

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Figure 1: Typical mini-drill

Micropiles are small diameter (gener-ally less than 12 inches), cast-in-place drilled or driven piles, whichare composed of grout and steel reinforce-ment. Installable in almost any type ofground where piles are required, micropiledesign loads range from 3 tons to 500+ tons(SeeFigure 4 on page 20).

Micropiles can offer a viable alternativeto conventional piling techniques, particu-

larly in restricted access or low headroom(Figures 1 & 2), vibration-sensitive areas,

where subsurface obstructions may result inpremature refusal or where other pile typesrequire expensive installation. Also knowas minipiles, pin piles, needle piles or rootpiles, micropiles were pioneered in the early1950s by Fernando Lizzi of Italy. Micropiletechnology spread to the United Kingdom,

West Germany and Switzerland in the early1960s and into North America in the earlyto mid 1970s.

CompositionSteel reinforcement may consist of a per-

manent steel casing, central reinforcing bar(s),and/or a steel reinforcing cage. The steel pipesused for a permanent casing are the same high-grade steel pipes used as casings for oil wells.These pipes are usually API N80, with a yieldstrength of 80,000 psi. The pipe out-sidediameters range from 5.5-inches to 11f-inches, with five common increments be-

tween those sizes. Wall thicknesses gener-ally are in the range of 0.4 to 0.6-inches.Special high-strength machined flush jointthreads are typically used to join the pipesections. Reinforcing bars used inside thepipes are high-strength steel, grade 75, 95 or150, with diameters from 0.5 to 3.5 inches.Grout generally consists of water and ce-ment with a ratio ranging from 0.40 to0.50 by weight. Sand is sometimes addedto increase strength and reduce cost. Otheradditives may consist of water reducers, plas-ticizers, and/or initial set retarding agents.

Micropile InstallationInstallation of micropiles are typicall

formed by drilling or driving a hole thoverburden soils and into rock or nadeposited soils capable of providinequate frictional bonding capacity (3). The two most common forms of ading the borehole are rotary drilling or percussive drilling. Rotary drilling uand/or water as a flushing medium

move drill cuttings from the drill hole. drill hole is advanced, air/water is puthrough the drill string and exits at thbit, flushing drill cuttings to the grounface. Rotary percussive drilling advancdrill hole by driving a steel casing inground. Rotary percussive drilling candisplacement method whereby a driveis used at the bottom of the casing, or adisplacement method whereby air is useflushing medium.

After advancing the borehole to the bof the bond zone, grout is tremie-placed

Micropiles

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Versatile Foundation Alternative for Challenging ProjeBy Andrew F. Brengola, P.E.

Figure 2: Mini-drill moving to micropile loc

STRUCTURE magazine December 2005


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the bottom of the drill hole. For piles foundedin soils, the drill casing is then retracted from

the bond zone and the pile continually toppedoff with grout. For high-capacity piles foundedin rock, the pipes penetration depth may beeither to the top of the rock or full length intoa drilled rock socket. If the pipe terminates atthe top of the rock, a drill string is advancedthrough the center of the pipe to drill the rockbond zone to a diameter less than the insidediameter of the pipe. If the pipe penetratesfor the full length into a drilled socket, apermanent drill bit, with a diameter largerthan that of the pipe, is fitted to the end ofthe pipe to advance the casing into the rock.

Grout ports in the bit allow for the tremieprocess, and thus, more grout can be pumpedin if needed. Additional concentric steelpipes and/or all-thread bars may be placed inthe drill hole together, providing additionalsteel for increased capacity.

Down The Hole Hammers (DTHH)are air-powered percussion tools capable ofpenetrating the hardest types of rock foundin the United States. These hammers areplaced at the bottom of the drill string andrequire compressed air to actuate the drillpercussion and flush cuttings up to thesurface. A replaceable high-strength carbidebutton bit is fitted to the bottom of thehammer. The DTHH allows drilling at arate of approximately 1 foot per minute, asopposed to 1 foot per 10 minutes with otherhammers. DTTHs are suitable for use onprojects that require the pile to penetrateobstructions such as rubble, debris, boulders,granite blocks, or concrete.

Classification of MicropilesTraditional micropiles are tremie grouted

from the bottom of the drill hole up and are

classified by the method by which the pileis grouted (Figure 4). Type A micropiles areplaced under gravity head only. Therefore,the grouting of a Type A pile is completeafter tremie grouting. Type B micropilesare pressure grouted after initial tremiegrouting is performed, and as the drill casingis removed from the bond zone of the pile.Type C micropiles use a pre-installed sleeveport pipe within the pile to apply a singleglobal injection of grout before hardening ofthe primary tremie grout. Type D micropilesuse a pre-installed sleeve port pipe within thepile for multiple injections of grout at specific

intervals within the pile bond zone. Thereare a variety of other micropile types which

have been utilized in unique applications(Figure 4). These include: pushed or driven,compaction grouted, jet grouted, and drilledend bearing micropiles.

Micropile DesignMicropiles typically derive their axial load

carrying capacity from the soil-grout or rockgrout bond within the pile bond zone. Thegeotechnical capacity is essentially equal intension or compression. In granular soils,typical ultimate bond values can range from10 to 100 psi. In clay soils, typical bond val-

ues can range from 5 to 55 psi. Rock to groutbond values can range from 25 to 450 psi orgreater. Guidelines for bond strengths canbe found in the Post-Tensioning InstitutesRecommendations for Prestressed Rock and SoilAnchors. Various formulas for estimating bondcapacity can be found in Volume II of theFederal Highway Administration documentDrilled and Grouted Micropiles: State of thePractice Review, publication No. FHWA-RD-96-016.

The internal structural design of a micro-pile is a function of pile geometry, the un-

confined compressive strength of the groutand the yield capacity of the reinforcing steel.Factors are applied to the material propertiesto limit working stresses, to account for vari-ations in material properties and to accountfor strain compatibility between the groutand high strength steel. These factors varyslightly between various codes and manuals.

A commonly used equation for determthe allowable compressive capacity of a m

pile is:Qallowable = 0.33 fc Ac + 0.4 fyA

Where,fc = 28-day unconfined compressive

strength of cement groutAc = Cross sectional area of pile groufy = Yield strength of reinforcing ste

As = Area of steel reinforcement

Some specifications and building require that a minimum of 40 percent design load be carried by the reinforcingSome building codes limit the unco

compressive strength of the grout and thstrength of steel in design calculations.

The allowable tensile capacity of a micis commonly determined by the followi

Qallowable= 0.6 fy As

In determining the area of steel to ucalculating the axial capacity of micrappropriate allowances for corrosion mprovided if the steel is not otherwise proby a suitable coating or encapsulation.

Micropiles resist lateral deflections dtheir structural integrity and to the resi

derived from the adjacent soils. The load carrying capacity of vertically drillcropiles is limited by the physical size pile, the limited size of reinforcing stethe bending strength of threaded joints exterior steel casing. A steel pipe of suflength to span downward from pile cutothe first or second casing joint is som

Figure 3: Micropile installation procedures



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pile committee of the Deep FoundInstitute and the International AssociatFoundation Drilling has published theto Drafting a Specification for Micropilespublication is helpful in preparing a pspecification which includes quality cand quality assurance procedures and isable atwww.dfi.org.

The Increase in Micropile UsSuccessful installation under difficul

straints is the primary reason for the g

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placed inside the drill casing to resist bend-ing stresses where large bending moments cancause failure to casing joints. Lateral capacityis generally estimated using computer pro-grams which use nonlinear soil models andfinite difference approximations to determinepile response to a given loading. Alternatively,lateral capacity can be estimated by manualcalculations such as those provide in NavalFacilities Engineering Command documentFoundations & Earth Structures, publicationNo. NAVFAC DM 7.02. The lateral loadcapacity of vertical micropiles is generally be-tween 5 and 15 tons; however, this resistancecan be increased by battering the piles.

Many options exist for pile to pile capconnections. In some cases the reinforcingsteel bar(s) are extended into the pile capand a bearing plate is secured to it between

two full capacity hex nuts. The bar(s) alonemay provide adequate development length totransfer the load into the pile. In other casesthe casing is extended into the pile cap totransfer the load. Pile top attachments suchas steel plates may be welded to the casing.However, special welding procedures may berequired as the carbon equivalency (CE) of theN80 pipe can be high.

QA/QCA load test is generally performed to verify

a micropile design on a project. A compres-sion or tension test can be performed to verifythe axial capacity of a micropile (Figure 5).Instrumentation such as strain gauges or telltales can be installed in test piles to verify thatthe applied test load is transferred to the bondzone and to calculate actual ultimate bond ca-pacity. For laterally loaded piles, a free or fixedhead lateral load test can be performed. Mi-cropile load tests are performed in accordance

with the American Society for Testing andMaterials (ASTM) standards for piles loaded

under static axial compression, tension and/or lateral loads. The application of load dur-ing axial testing on piles founded in rock orgranular soils is generally in accordance withthe ASTM Quick Test loading procedures. Forpiles in creep susceptible soils, longer appliedtest loads, particularly at the maximum loadincrement, are required to verify pile capacity.

Quality control and quality assurance pro-cedures should be implemented on every mi-cropile project to confirm that the productionpiles are installed in accordance with project

specifications and pile design, and that theyare consistent with the means, methods andmaterials used on the test pile. Grout cubesand specific gravity measurements should betaken to monitor grout quality. Installationrecords should be maintained to documentall pile installation aspects. A joint micro-

Figure 4: Types of micropiles and capacity ranges

Figure 5: Left - typical compression test setup; Right - loading apparatus and dial gages



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of micropile use in recent years. Site logistics and equipment accessmay prohibit the installation of other deep foundation methods, leavingmicropiles as the only feasible choice. At many sites, micropiles can bethe most economical choice.

The popularity of micropiles has resulted in more competition andthus, lower prices. Similarly, as workers have gained experience withmicropile use and improved installation methods have been developed,productivity has increased resulting in further cost reduction. Accept-ance of higher capacity micropiles has led to fewer piles installed on

individual projects, lowering the overall project cost. Over the pastdecade, costs have decreased by approximately 25 percent.

ApplicationsMicropiles work in a wide range of challenging conditions in both

soil and rock. They can carry loads in numerous difficult groundconditions, whether for new loads being added to an existing structure,for arresting structural settlement, for resisting uplift and dynamicloads, for seismic retrofits or for underpinning andslope stabilization.

Micropiles are used in rehabilitation projects orin new construction with physical constraints, suchas limited headroom and restricted access, and in

vibration or settlement sensitive areas. They havebeen installed directly through existing shallowfoundations. A typical application might involvefoundations placed close to existing footings,columns, walls, or other impediments (Figure 6).Difficult ground conditions suitable for micropilesinclude: karstic limestone geology with voids, minedrock geology with voids (including rubble-filledvoids), glacial tills, profiles with floating boulders,sites with high water tables, random urban fills, andother difficult geological characterizations.

Conclusion

Micropiles have proved successful at solving prob-lems at a wide range of difficult sites. Installationcosts are decreasing while productivity is increasing.Both will encourage the methods wide use. Higherload capacities will further their benefits and enhancetheir use. Many new applications for this advance-ment in construction techniques can be expected,both in new projects and for rehabilitation work.

Andrew F. Brengola is the New EnglandArea Manager of G. Donaldson, a divisionof Hayward Baker, based in Cumberland,

RI. He has 12 years experience inmicropiles, earth support, jet grouting, and

various ground improvement [email protected]

All photos and graphics courtesy ofHayward Baker Inc.

See Mt. Storm Power StationCase History on next page....


Figure 6: Hydraulic track drill installing battered micropiles


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CASEHist

ory

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Power Plant in West Virginia

Mt. Storm power station is founded on the shores of Mt. StormLake in northeastern West Virginia. Selective Catalytic Reduction

(SCR) units were slated for installation on the emission stacks.Because the power plant is over 3,500 feet above sea level and isfrequently exposed to high wind loads, large steel structures were

designed to support the SCR units, requiring new foundations to resist

high compressive, uplift and lateral loads. Micropiles fixed into the sideof the steep hill were designed to support these foundations.

Crews were required to work around the existing plant foundationssituated through a steep hill, and to alter the existing conditions to provide

access. Nearly half of the work was done in either low or restrictedheadroom. A mini-rig was used to install 28, 10-inch battered

piles in a low headroom area inside a diesel generator room.

The micropiles consisted of a 10 -inch surface casingand a 20 to 30-foot rock socket.

The first installation area was situated at the toe of a 2H:1V slope (Figure 7). Thegeotechnical contractor excavated bays into the slope at the pile cap locations

and installed a soil nail and shotcrete wall to support the faces of the cut.

The second area was approximately mid-way up the slope. The geotechnicalcontractor drilled 60, 22-inch diameter holes, while working from the top of an

existing precast retaining wall and from a 7.5-foot wide bench.

Once a bench was excavated at the top of the slope, the geotechnicalcontractor installed 432 battered micropiles for the new pile caps.

Two load tests were performed to two times the design load. The micropiles were tested intension to verify that all axial loads were resisted by friction only, and strain gauges were used

to accurately compute load transfer from the pile to the rock. Net pile displacement afterunloading of a 600 kip test load was 0.10 and 0.21 inches for the two test piles, respectively.

Figure 7: Section view, micropileinstallation at Mt. Storm power station


Mt.StormP

owerStation

micro piles by andrew brengola

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