Effect of Load Obliquity on Pullout Capacity of Granular Anchor Pile in Cohesionless Soil

In the present study, the experimental investigations were carried out to compare the vertical and oblique behavior of Granular Anchor Pile (GAP) and Group Piles of 2 & 4 GAP in cohesionless soil. A model testing program was designed and single GAP, 2 & 4 GAP having different ratio of length (L) to diameter (D) and spacing(S) to diameter (D) installed in cohesionless soil having a relative density of 65%. Single GAP, 2 & 4 GAP were subjected to vertical and oblique pullout loads. They were tested at an angle of load inclination of 0°, 30°, 45°, 60° and 90° . Ultimate pullout capacity of GAP and group piles were estimated from the load displacement curves by tangent intersection method. It is inferred from the experimental investigations that ultimate pullout capacity under vertical as well as oblique pullout loads decreases as angle of load inclination increases with the pile axis. The laboratory results also revealed that the ultimate pullout capacity increases with increase in L/D ratio but decreases beyond 2.75 S/D ratio.

Jaswant Singh, M.Tech. Geotech.(Hons.) Research Scholar, Dr. Anupam Mital, Dr. V.K. Arora, Professor, Department of Civil Engineering, National Institute of Technology, Kurukshetra, India

Introduction
Granular Anchor Pile (GAP)Figure 1: Granular Anchor Pile (GAP)
Foundations may be broadly classified into two categories viz. Shallow foundation and Deep Foundation. Deep foundations are preferred wherever bearing capacity of soil is less. Piles are considered under the category of deep foundation when the strata at or just below the ground surface is highly compressible and very weak to support the load transmitted by the structure. Piles can be classified according to their use for construction of pile, mode of transfer of load and method of construction etc. Anchor piles are used to provide anchorage for anchored sheet pile which provides resistance against pullout forces in structures such as transmission towers, bridge abutment and mooring system for submerged platform. Interaction between soil and group of pile for vertical and oblique pullout loads is of paramount importance. Granular Anchor Pile (GAP) is a modification of the Granular Pile which is reinforced with steel tie rod and lower of steel tie rod is fixed with a steel anchor plate to increase the resistance against the uplift forces as shown in Fig. 1. Granular Anchor Pile is an innovative foundation technique devised for resisting pullout forces and improving the soil engineering behaviour. Numerous researchers have suggested the use of granular anchor pile for controlling the detrimental volume change behaviour of expansive soils. In this experimental study, an attempt has also been made to compare the vertical and oblique behavior of single GAP, 2 & 4 GAP through estimation of pullout capacities in cohesionless soil. Experimental investigations with L/D ratio varying as 7.0, 8.50, 9.50, 10.50 & 11.50 for 50 mm diameter, L/D ratio as 4.25, 5.75, 6.50, 7.00 and 7.75 for 100 mm diameter in single GAP system and keeping as 7.00 L/D ratio with S/D ratio of 1.50, 1.75, 2.25, 2.75 & 3.25 for 100 mm diameter of 2 & 4 GAP system respectively were carried out in cohesionless sand to verify the influence of load obliquity on pullout capacity. In the laboratory test, the single GAP, 2 & 4 GAP were subjected to vertical pullout load (at load inclination 0°) and oblique pullout loads at angle of 30° , 45°, 60° and 90° with vertical axis of single pile and group piles.

Properties of Sand
Locally available fine sand was used in laboratory for the experimental investigations. The sand was found to be poorly graded sand as per Indian standard. Others properties of sand as determined in the laboratory are presented in Table 1.

Table 1: Properties of Sand
Sr. No. Property Values
1. Effective size (D10) 0.12 mm
2. Uniformity Coefficient, (Cu) 1.75
3. Coefficient of Curvature, (Cc) 0.57
4. Mean Specific Gravity, (Gs) 2.60
5. Maximum Void Ratio (emax) 0.80
6. Minimum Void Ratio (emin) 0.45
7. Relative Density (ID) 65%
8. Unit Weight of Sand (γd) 15.90 kN/m3
9. Angle of Internal Friction (Φ) 32.5o

Grading of Granular Pile Material
The pile granular material having the maximum density & ultimate pullout capacity was taken by using the best composition of mix proportion as shown in Table-2. The ratio of granular pile materials of different grading were chosen as 40% stone aggregates having the size of 5 mm, 40% with the size of 2.5 mm stone aggregates and 20% sand for determination of corresponding densities of among the various combinations which provides the best grading of the granular material. The quantity of granular pile material calculated was poured in each layer & compacted by blows spread in specified layers which provided uniform density of granular pile material as 18.19 kN/m3 which was adopted in all GAP system.

NBM&CW January 2020

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