Pile foundations are used to support structures when the ground near the surface cannot provide the required bearing capacity or settlement represents a major concern. Different piles of varying shapes and materials are used in practice, but mostly either driven piles or drilled shafts. However, due to varying construction challenges and ever increasing demands for sustainable practices and cost saving solutions, the construction industry is pursuing foundations that feature efficient construction techniques, innovative pile configurations and novel application of materials.
Owing to their various construction advantages, helical piles are gaining popularity, especially in projects that require fast installation and quick loading of the foundation. Helical piles are typically manufactured with straight steel shafts (pipe or square section) fitted with one or more helices and are installed using mechanical torque (Perko, 2009). Currently use of helical piles have expanded to a wide range of applications such as power transmission towers, bridges and residential and commercials buildings, which involve static and cyclic compressive, uplift and lateral loading (Elsherbiny and El Naggar, 2013). Helical piles of different configurations and wide range of capacity are being developed and used in practice. For example, square shaft helical piles (Livneh and El Naggar, 2008), helical pull down micropiles and fibre reinforced helical pull down micropiles (El Sharnouby and El Naggar 2012 a and b) and large diameter helical piles (Elkasabgy and El Naggar, 2013, 2015).
Helical piles are installed into the ground by applying torque to the pile head. This installation technique produces minimal vibration, noise and soil spoils, which makes it suitable for construction in urban areas. In addition, monitoring the installation torque allows estimating the pile capacity and provides means for quality assurance/control. Given
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the installation torque, the axial pile capacity can be predicted using the following equation (Livneh and El Naggar, 2008; Hoyt and Clemence, 1989):
Pu=KtT (3 - 1) where T is the installation torque, Pu is the ultimate axial capacity and Kt is the capacity-
to-torque ratio. Perko (2009) conducted regression analysis of the results of more than 300 pile load tests and proposed the following expression for Kt:
Kt= (3 - 2)
Where: deff is effective shaft diameter and k is a curve fitting factor =1433mm0.92/m (22
in0.92/ft).
For helical piles with a single helix, the capacity is given by the resistance due the helix bearing and the shear resistance along the pile shaft. Helical piles with slender shafts can only sustain relatively small compressive loads, and low lateral loads compared to other greater diameter piles. However, different helical pile systems with large diameter shafts are developed and offer large axial and lateral capacity (Fleming et al., 2009; Abdeghany and El Naggar, 2010; El Sharnouby and El Naggar, 2012b; Elkasabgy and El Naggar, 2013). Additionally, these solutions would enhance the axial capacity of the piles owing to the increased shaft resistance, which significantly contributes to the compressive capacity.
Tapered piles of decreasing circumference with depth have been successfully used for many years as an efficient piling system. Due to their shape, additional shaft frictional resistance is induced and therefore greater axial capacity is reached. The higher compressive capacity of tapered piles compared to conventional cylindrical piles has been long recognized (e.g. Norlund 1963; Zil'berberg and Sherstnev, 1990; Wei and El Naggar, 1998; El Naggar and Sakr, 2000). The tapered configuration could increase the load carrying capacity of the pile by up to 188% compared to conventional straight shafts (Sakr and El Naggar, 2003). Furthermore, the increased sectional diameter at the top provides an increased lateral resistance compared to the regular straight piles. The capacity of tapered piles ranges between 1.5 to 2.5 times the capacities of cylindrical pile of the same average
0.92
k
eff
d
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diameter (El Naggar and Sakr, 2000). Tapered piles can be installed by drilling, driving or using torque and can be made of steel, wood, concrete or composite sections.
Wei and El Naggar (1998) found that the taper angle increases the efficiency of utilization of the pile material, especially in looser deposits where the confining pressure significantly increased the soil stiffness. The increase was attributed to transferring the load to a greater soil volume resulting from the developed soil arch compared to straight piles. In addition, the radial expansion of the soil adjacent to the pile during installation and pile loading results in higher lateral earth pressure hence greater frictional resistance compared to the straight piles. Wei and El Naggar (1998) proposed the following equation to calculate the skin friction qs along the shaft of tapered piles installed in sands:
qs=Kts Ksv’tan (3 - 3)
Kts= + (3 - 4)
where θ is the pile taper angle, v is the overburden stress, Ktsis the taper coefficient, Ks is
the coefficient of lateral earth pressure, is soil-pile interface angle, G is the sand shear modulus, = ln(rl/rm), rl is the pile radius at which the shear stresses become negligible, rm
is average pile radius and Sr is the pile settlement as a ratio of its diameter at the ultimate
load.
For tapered piles installed in frictional-cohesive soils, Kts can be given by (Khan et al.,
2008):
Kts= +
+(𝟏+𝟐𝐭𝐚𝐧 (𝜽)𝐭𝐚𝐧 (𝜽+𝜹))𝑲𝑪′
𝑺𝝈𝒗′𝐭𝐚𝐧 (𝜹) (3 - 5) where C’ is the effective cohesion.
Kurian and Srinivas (1995) investigated the compressive behavior of tapered piles in sand numerically and validated their results with laboratory testing. The results confirmed the
tan cot 1 2 tan tan
41 2 tantantan cot
r s v G tan S K
tan cot 1 2 tan tan
41 2 tantantan cot
r s v G tan S K
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efficiency of tapered piles when compared to straight shaft pile capacities. The increase in pile capacity was attributed to the direct bearing on the pile’s sides increasing the normal pressure and therefore the side frictional component of the total pile resistance(Kurian and Srinivas, 1995). Interestingly, unlike cylindrical piles, tapered piles shaft resistance continues to develop with increase in pile settlement (Kodikara and Moore, 1993). Also, Zhan et al. (2012) numerically studied the axial behavior of cast-in-situ 4m length tapered piles installed in sands using the software ABAQUS (Hibbitt et al., 2008), and concluded that a slight increase in shaft taper significantly increases the developed shaft stresses even at shallow depths as shown in Figure 3 – 1.
In the current study, an innovative pile system that combines the efficiency of the tapered section and the construction advantage of helical piles is investigated.
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Figure 3 - 1: Developed shaft friction along tapered piles in sand at 2cm displacement (after Zhan et al. 2012)