Bored PilesProduction Method

Bilfinger Berger Spezialtiefbau GmbHGoldsteinstrasse 114D-60528 FrankfurtPhone: +49-69 6688-345Fax: +49-69 6688-277Email: spezialtiefbau@civil.bilfinger.comwww.foundation-engineering.bilfingerberger.com

Large Diameter Bored Piles

Large bored piles are used as foundation elements to carryconcentrated building loads into deeper, more stable soil layers.Furthermore, they can be utilized as piles for retaining walls if thereis not enough space for flat foundation.

In the same way, retaining walls for excavation pits are constructedby using multiple bored piles as bored pile wall, which later can beincorporated into the final building.

Bored piles can be easily adapted to the various load and soilrequirements due to the large variety in diameter and constructiontechniques. Single pile diameter can vary between 30 and 300 cm.Piles can be constructed in all kinds of soil condition.

In contrast to driven piles, bored piles enable the immediate In-situevaluation of drilled soil layers to revise foundation length due tochanged soil conditions.

Pont de Normandie

Maumee River Crossing

The specific pile construction method depends on the soil condition,the ground water elevations, site conditions and the length of thepile.

Construction methods can be distinguished between

• The bore hole stabilization process during excavation Fullycased, partial cased, uncased and fluid stabilized excavation canbe distinguished

• The method of installing the casingHere one can distinguish between driving the casing with a freefall hammer or weight, vibrating the casing, pneumatic installationor hydraulic installation with the help of a rotary drive, an oscillatoror rotator.

• The method of excavationHere one can distinguish between grab excavation, rotary drilling,airlifting, and flush boring.

Bridge L71 across the Heilsau

Pont de Normandie

Lilla Bommen Tunnel

Bore Hole Stabilization

Uncased bore holes can only be excavated in stable soil layers. Ingeneral a short starter casing is been used at the pile top.

If uncased excavations run through unstable soil layers, the borehole wall has to be stabilized by fluid overpressure. Water orbentonite as well as polymer suspension is used as stabilizingfluid.

Partially cased bore holes are cased in areas with unstable soilsand deeper and stable soil layers are stabilized by water, bentoniteor polymers, while rock layers do not require any support.

The casing of the borehole has to stabilize the soil and minimizethe relaxation of the surrounding ground. It is always requiredwhenever unstable soil layers cannot be stabilized with fluids andthere is a risk of soil caving.

CFA piles (Continuous Flight Auger) are drilled uncased by the useof a continuous hollow auger stem. Stabilization of the boreholewall during excavation is provided by the soil filled auger. Duringpulling of the auger stem, concrete is pumped through the stem,preventing any soil instability of the bore hole wall.

High Speed Rail Nürnberg -Ingolstadt, Tunnel Offenbau

Center for Innovative Medicine,Hamburg

Gallileo Tower, Frankfurt

Methods of casing installation

During casing installation, tip resistance and skin friction of thecasing has to be overcome.

Installation by free fall weight

Casings in single length up to 6 m are installed using a fee fallweight. Suitable soil conditions are required for such drivingpractice. During this installation method, casings with extra collarswill be driven, excavated and so-called Jumbo piles areconstructed.

Installation by vibratory hammer

Casings are installed into the ground using externally inducedvibration. Friction between casing and surrounding soil is reducedby vibration generated from an attached vibro hammer. The casingpenetrates the ground due to this vibration, the weight of thehammer and its own weight. This installation method has a limiteduse due to the soil densification inside the casing and possiblevibration damage on adjacent buildings or structures.

Driving by vibratory hammer

Driving by free fall weight

Free wall weight

Pneumatic installation by the HW method

The HW-method, named after Hochstrasser/Weise, uses apneumatically-driven swinging arm, sitting on top of the casing, todrive the casing in a horizontal back and forth movement. Endstops transfer the rotation impulse of the swinging arm into thecasing, which rotates slightly at each stroke. This considerablyreduces the skin friction between soil and casing. The casing isexcavation by the wire rope guided grab at the same time.

The casing is driven into the ground as a result of the reduced skinfriction and under the weight of the casing and swinging arm.

A major advantage of this method is that the excavating crane doesnot have to be placed right next to the casing to support thereaction forces from the swinging movements of the equipment.

The HW unit can therefore be utilized as a separate unit from theexcavating crane, which is an advantage when piles need to beinstalled in steep slopes or over water.

HW principle

HW swinging arm

Dike rehabilitation Emmerich

Hydraulic casing installation

The hydraulic installation method uses an oscillating motion(Oscillator) or a continuously rotating motion (Rotary Drive orRotator) of the casing to reduce the skin friction between casingand soil.

All of these machines also induce a vertical downward force into thecasing, which in combination with the casing’s own weight enablespenetration of the casing.

The casing normally consists of separate sections, which areconnected and bolted during installation.

During the casing installation, the excavating equipment is alwaysplaced right next to the casing and receives the reaction forces fromthe rotary motion.

Rotary drilling rig, GaleriaKaufhof, Alexanderplatz, Berlin

Rotator (RDM)

Oscillator (VRM)

The Method of Excavation

Grab Excavation

Excavation using a free fall or spherical grab is mostly used whenboreholes are very deep and other methods come to their limits orin combination with the HW method.

The borehole is advanced using grabs consisting of a base body,the grab shells and the closing mechanism for the shells. Onedistinguishes between single-rope and double-rope operated grabs.

The single-rope grab also uses the lifting rope as a closing rope forthe grab shells. The double-rope grab is used with two ropes, onefor grab lifting and one for closing the shells.

Performance of the grab mainly depends on the correct shellshape, which is chosen according to soil parameters.

For excavations below the groundwater level, in-the-wet drillingtools such as gravel pumps or mud cans are available for thevarious drilling diameters.

Free-fall chisels are used to drill through large obstructions such asbricks, boulders or rock layers as well as for the pile penetrationinto bedrock. Various sizes and shapes such as cross, flat, stellar,ring shapes are available. These chisels are equipped with differentblades depending on the various soil properties.

HW – Grab, long version withparallel shells

Ball grab with spherical shells

Flat chisel

Ring chisel

Rotary drilling methods

During the rotary drilling method, the excavation is advanced usingdrilling augers and buckets.

The drilling rig consists of a base carrier and the drilling unitmounted on top of the carrier. One distinguishes between shortaugers and continuous flight augers.

When drilling with short augers or buckets, the torque anddownward crowd is transferred to the drilling tools via a telescopicdrilling rod (Kelly bar). Drilling and discharging of the excavatedmaterial are alternating operations, which are performed in a swiftorder. Depending on soil conditions and pile diameter, variousaugers are used. For excavations below the ground water level,drilling or digging buckets are used. Obstructions or rock layers canbe removed with the help of core barrels. To increase the pile basearea, belling buckets or under-reamers can be employed.

When drilling with continuous flight augers, the auger isconnected directly with the rotary drive without an intermediatedrilling rod and is rotated into the ground for its full length. Afterreaching final drilling depth, the filled auger is pulled out of the borehole. Therefore the empty bore hole has to be stable enough orconcrete needs to be pumped through the hollow auger stemimmediately. Pile length is limited by the length of the auger and thesize of the drilling rig.

Sketch - drilling rig

Drilling rig with bucket

Drilling rig with continuous auger

Flush drilling / wet drilling methods

In contrast to the dry drilling method, where the soil is loosened andexcavated by the drilling tools, the wet drilling method only usesdrilling tools to loosen the material and requires a flushing streamfor transportation. Cutting tools like tri-cone bits, roller bits orunder-reamers are used.

When using the direct flushing method, bentonite or polymersuspension is pumped down to the piles tip through the hollowdrilling rod. The suspension is then raised up to the piles top in theannulus between drilling rod and bore hole wall. This method ismost suitable for smaller bore hole diameters.

When using the reversed circulation method, the flushingsuspension is poured directly into the bore hole. Using a vacuumpump, the soil/suspension mixture is then sucked through thedrilling rods to the pile’s top. Pile length up to 50 m can be drilledefficiently.

The air-lifting method also pours flushing suspension directly intothe bore hole. By inducing compressed air into the lower end of thedrilling rod below the water level, the expanding air creates anupward flow, which therefore transports the soil/suspension mixtureto the top of the pile.

Combinations of air-lifting and reversed circulation drilling can drillbore holes up to 150 m depth efficiently, depending on soilconditions.

Sketch air-lifting drilling methodfor bored pile construction

Air-lifting drilling method