Drilled shafts or piers are foundation units that have the same function as pile foundations. What differentiates piers from piles is the method of installation and the materials used. While piles are generally driven into the ground through hammering, piers are installed into the ground by excavating the surrounding soil by drilling. Furthermore, piles can be constructed by steel, timber and concrete, however piers are made of concrete only. Piers are also referred to as cast-in-place piles.
Advantages of piers:
1-Since piers can carry very large loads, group piles can often be replaced by a single pier.
2-If the soil is made up of dense sand or gravel, drilling piers will usually be easier that driving piles.
3-Excessive ground vibrations are caused by hammering piles which can damage structures in the vicinity of the construction site. Drilling piers on the other hand do not cause ground vibration.
4-clays heave during pile hammering and this may lead other surrounding piles to move laterally. Such problem does not exist in piers.
5-Pile hammering creates a lot of noise.
6-The base which the piers rest upon can be easily checked and inspected.
7-The equipment used for drilling piers can be easily transported from one place to another, Hence they are time saving and economical.
8-Under-reaming the bottom of the piers will significantly increase resistance to uplift loads.
9-They provide greater resistance to lateral loads.
Disadvantages of piers:
1-This method requires close supervision.
2-Hammering piles compact soil and increase bearing capacity, this does not occur when piers are used.
3-Drilling piers is hard when ground water flow exists.
Types of drilled shaft:
These are classified according to mechanism which is used to deliver the loads from the superstructure to ground.
a-Straight shaft: These shafts are used when the soil at the surface is weak and unable to support the loads above. They transfer the load to strong soil layers or rocks. In the united states the ultimate load is assumed to be carried only by the end bearing and the contribution of side friction is neglected. However, both end bearing and side friction are considered in europe.
b-Dome shaped under-reamed piers: These consist of a straight shaft and a dome that rests on a strong soil.
c-angled under-reamed piers: The bells that are constructed at the bottom have an angle of 30° or 40° with the vertical by using special under-reaming tools.
d-straight shaft socketed into rock: These shaft are drilled into the underlying rocks. The end bearing of the rock and the side friction between the shaft and the rock are considered towards the ultimate load bearing capacity.
Fig 1: types of drilled shafts
The machines used for drilling shafts consist of mountings which are usually truck crane, tractor or skid (Fig 2) and drilling tools which vary in size and shape according to the ground conditions.
Soil or soft rock is drilled with open-helix auger. For most homogeneous soils a blade cutting edge is attached with this tool. In case of hard or stiff soils the blades can be replaced by hard-surfaced teeth. For hard rocks cast steel heavy duty augers are used (Fig 3).
Under-reaming tools also vary in diameter and angle. (Fig 4 ) Shows 30° under-reamer. The diameter of these tools are usually three times the diameter of the shaft.
Fig 2: Tractor mounted hydraulic drilling rig (image source)
Fig 3: Typed of augers. a) auger bit with cutting blade for homogeneous soil b)auger bit with hard metal cutting teeth used for stiff soils c) cast steel heavy duty auger for hard rocks.
Fig 4: 30° under-reamer
General construction methods of drilled Piers foundations:
This method is suitable for soils and rocks that exist above water table and will not cave when drilling is completed. First the hole must be drilled to the desired depth, then the concrete must be poured and the rebar cage is then placed in drilled hole.
Fig 5: Dry method of construction: a)initiating drilling b)starting concrete pour c)placing rebar cage d)completed shafting.
This method is used when the soil is likely to cave when drilling is completed. At first the hole is drilled in the same procedure as the dry method. As drilling reaches a soil layer that is liable to cave, then bentonite slurry is poured to prevent soil failure. When excavation reaches an impermeable layer of rock then casing is lowered into the hole and the slurry is taken out. It is possible to under-ream the base at this stage if needed, then rebar cage is inserted and concrete is poured into the hole. Finally the casing is pulled out. In some cases the casing is left permanently, if the casing is removed improperly, then soft soil may squeeze into the concrete or if the surrounding soil is saturated with water, then water may seep into the concrete and cause segregation.
Fig 6: Casing method a) initiating drilling b) drilling with slurry c) introducing casing d) casing is sealed and slurry is being removed from interior of casing e) drilling below casing f)under-reaming g)removing casing h) completed shaft.
Wet method of construction:
Since slurry is used to keep the borehole stable, this method is also called slurry displacement method. Borehole is drilled along with slurry and after drilling is completed, rebar cage is placed in the hole. Finally concrete will be poured into the drill hole and the slurry volume will be displaced. The type of slurry is often bentonite mixed with water or polymer slurry.
Fig 7:Wet method a) drilling to full depth with slurry b) placing rebar cage c) placing concrete d) completed shaft
1- V.N.S Murthy, 2002. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering.
2- Braja M. Das, 2002. Principles of Foundation Engineering. 7th edition.
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Mohammad Barzan BSc- Geotechnical Engineering- Koya University.
BSc- Geotechnical Engineering- Koya University.
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