Effect of Using Equivalent Driving Energy on Small Model Driven Pile Capacity

Physical modeling is performed in order to study particular cases of the behavior of prototype and to validate theoretical and/or empirical hypotheses. However, most physical models will be constructed at much smaller scales than the prototype precisely because it is desired to obtain information about expected patterns of response more rapidly and with closer control over model details than would be possible with full-scale testing. The main problem associated with physical model tests is the stress levels and soil particle size effects. These points should be considered which require deep and thorough research when studying the behavior of small scale model piles in sand. The tests indicate that the number of blows recorded when driving the model pile is affected by pile diameter more than with pile length. As well as, the heavier hammer shows precedence in bearing capacity than the light hammer because it leads to upgrade the soil properties during pile driving.


INTRDUCTION
n scaling effect topics, the stress level and soil particles size can be considered as the most important factors affecting the model behavior.Yet, there are no clear explanations of models behavior comparing with the prototype behavior; this is may be referred to the difficulties associated with the representation of the whole model conditions in laboratory.

Points to be Considered in Model Pile Test
The following points must be taken into consideration in model pile tested inside a container: Effect of sides of container walls may strongly reduce the vertical stress with depth, to avoid side friction of walls; the ratio of the container height to the diameter must be equal or less than one (Tarnet 1999, Garnier 2001).

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Effect of horizontal deflection of the container wall should be less than (Hc/2000) to keep Ko close to its assumed value for no lateral strain (Tarnet 1999), where Hc represents height of the container.

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To avoid effect of tip resistance on diameter/width of the container, the ratio of the diameter of container to diameter of the model pile should be larger than 30 in sand (Bolton et.al., 1999).

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In shallow foundations, sand thickness of 3B below the footing is adequate in eliminating any rigid bottom boundary effect (Cerato, 2005).

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To eliminate any rigid boundary resulting from pile driving in loose sand, the bulb of stress around pile is about (7D), this distance should be considered in design (Kishida, 1967).

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Effect of Pile Dimensions on the Bearing Capacity Meyerhof (1983) concluded that the ultimate end bearing for piles in sand tends to be less for larger diameter driven piles, this state may be attributed to the decrease in reduction factor of ultimate point resistance when the pile diameter increase.The reduction in end bearing capacity has been related to the decrease in the angle of internal friction with larger diameter.So (1991) suggested that the dilation and hence the shaft resistance of a smalldiameter (model) will be greater than of large-diameter pile.

Effect of Soil Particles Size
Scale effects may be observed in a small-scale footing test and it is related to the assumed shear zone formation in the active region directly beneath the footing.Scarpelli and Wood, (1982); Muir Wood, ( 2004) noticed the phenomenon of shear zone in direct shear box.Shear zone is a group of shear bands formed in the shear surface.Shear bands are defined as narrow regions of intensely sheared material.The bigger the box, the larger the horizontal shear ruptures, the more room the soil particles have to rearrange and more room for shear zone to fully develop to a critical state.
Essentially, the shear zone has room to fully propagate in a larger shear box and is therefore a more realistic representation of the strength found in field conditions (shear bands developed have an inverse proportion with the angle of internal friction).

Behavior of Driven Piles in Sandy Soils
When a pile is driven into sand and other cohesionless soils, the soil is usually compacted by displacement and vibration, resulting in rearrangement and some crushing of the particles.The driving of pile is associated with moving large amounts of sand in vertical and radial direction.The vibrations resulted from driving a pile in sand have two effects densifying the sand (increase angle of internal friction) and increasing the value of lateral earth pressure around the pile.
In loose sand, the pile capacity is increased as a result of the increasing the relative density caused by driving.The compaction of sand extends to the surrounding soils and the increase in relative density around the pile has been presented by Robinsky and Morrison (1964).Kishida (1967) proposed a simple method of estimating the effects of driving in loose sand in vicinity of the tip; it was assumed that the diameter of the compacted zone around a pile is 7 times the diameter and angle of internal friction changes linearly with distance from the original value of 1 at a radius (r = 3.5D) to a maximum value of 2 at the pile tip.The relationship between 2 and 1 is taken to be as:- Where: (1 and 2) = angle of internal friction before and after driving process.The driving process imposes three types of motion on the soil around a pile firstly, relatively large magnitude vertical shearing along the pile shaft, secondly vibration of the soil due to the hammer blow, and radial compression of the soil around the pile.
When piles are driven into relatively dense sand, whose possess tendency to dilate, the dilation generates large normal stress against the pile shaft, after installation shearing develops between the pile shaft and the soil.Dilative sand will generate additional normal compressive stresses against pile shaft.As a result, k can be significantly greater than kₒ for very dense sand (Salgado, 2006).

EXPERIMENTAL WORK Material used and soil characterization
Karbala sand was used in present study.Standard tests were performed to determine the physical properties of the sand.The details of these properties are listed in Table (1).

Model piles details
Steel solid piles covered with cement mortar with specific weight of (7.75gm/cm 3 ) and modulus of elasticity of (1.85×10 8 kPa) (Murphy, 1950)

Model setup formulation
To simulate the pile load test in the field, a new apparatus was manufactured and described as the following:

Description of setup
Steel container is used to host the bed of soil.It was made from five separated parts.The internal dimensions of the container are (75×75×75) cm.Each part from the container is made of (6 mm) thick steel plate.A steel base was made to support the container and the loading frame weight.The axial load is applied through a hydraulic jack system.
The maximum load that can be applied is about (10 ton) according to hydraulic jack catalogue.The bed of soil is prepared with a dry unit weight of 16.5 kN/m 3 at a height of drop equal to 20 cm using the raining technique.
The driving system consists of a base plate with (86cm × 20cm) and 20mm in thickness.This plate involves three holes manufactured to be considered as focus place to penetrate the piles in the box.The steel helmet was manufactured with different grooves that are suitable for all model piles sizes that are used in the tests.
These grooves are designed to make sure the fixity of piles and as possible to reserve the vertical direction for pile penetration without tilting during the driving process.Details of setup and pile driving hammer device are shown in Figures ( 2 and 3).

ANALYSIS AND DISCUSSION OF TEST RESULTS
This study involves four models of driven piles tested to assess the effect of equivalent energy equal to (W=2.175kg  H=5.3cm) instead of (W=1.4kg H=8cm).For this group the model piles used in the test are (D=2.1cm,L=40cm, circular), (B=1.6cm,L=50cm, square), and (B/D=2.1cm,L=50cm) of square and circular shapes.
The piles are embedded in sandy layer with different lengths and diameters/widths.Piles with square and circular cross-sections under the effect of vertical static compression loads are tested.For all model tests, the failure criterion are used.All details are shown in Figure (1).

Figure ( 1 )
Figure (1) Details of model pile used in the present study.

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Figure (3) Front view of the pile driving hammer device with details.

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Figure (7) Effect of using equivalent energy on model pile behaviorfor square pile with B=2.1cm, and L=50cm.