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Pullout Capacity of Anchor Piles
Lecturer, Department of Civil Engineering, and Former graduate students, Kigali Institute of Science,
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ABSTRACT
Experimental investigations on the behaviour of model enlarged base piles, embedded in layered sand and homogeneous sand subjected to axial pulling loads and oblique pulling loads respectively have been carried out in a model tank of size 914mm X 914mm X 762mm. Mild-steel bars of diameter 12 mm and 16 mm having embedment length to diameter ratios (L/d) 25, 30 and 20 respectively are used as model piles. Mild steel circular disc of 10mm thickness has been used for providing enlargement at the base of the piles. The base enlargement ratios (B/d) were 1, 2 and 3. Dry sand, obtained from the local market, of specific gravity 2.61, uniformity coefficient 1.1, emax= 0.8, emin= 0.49 has been used as foundation medium. The relative densities of sand used as foundation medium were dense over medium dense and medium dense over dense conditions for anchors under axial pulling loads and dense condition for anchors under oblique pulling loads respectively. The corresponding angles of shearing resistance for medium dense and dense conditions were 380,400. Based on the experimental results, the influence of the layering system and the load inclination angles have been investigated on load-displacement and ultimate resistance of enlarged based piles subjected to axial pulling loads and oblique pulling loads. Relevant conclusions are drawn from the study.
Keywords: Keywords: Anchor Piles, Sand, Pulling resistance, oblique Pulling Resistance, Ultimate Resistance.
INTRODUCTION
Anchors and anchor piles are common type of foundations for resisting uplift forces in both marine and land environment and may be broadly classified into three categories.1.Gravity anchors, 2. Plate anchors, and 3. Anchor piles. These anchors and anchor piles are used extensively depending on the magnitude and type of loading, type of structure and subsoil conditions. Among these, plate anchors are widely used for structures such as transmission line towers, underground reservoirs below water table, suspension bridges, retaining walls, offshore structures, etc., as they represent an economical alternative to gravity and other embedded anchors for resisting uplift forces. In some cases the nature of soil is not homogeneous and different layers exist. Sometimes piles are used as anchorage for guyed structures and they must be capable of resisting oblique pull. Scanty experimental results on anchor piles subjected to axial pull and oblique pull up-to the failure are available. Full-scale field-tests though highly desirable, unpredictable soil conditions and involvement of large sum of funds restrict the scope of full-scale field tests. In the absence of resources and scope of testing prototype, small-scale laboratory model tests conducted on piles in foundation medium prepared under controlled condition may serve the purpose to some extent. Therefore, necessity was felt to undertake model testing to study the behaviour of piles with base enlargement under axial pulling loads in layered sandy soil and under oblique pulling loads in homogeneous sand.
BRIEF REVIEW OF LITERATURE
A number of model tests have been conducted by Das (1975), Steward (1985), Tagaya et al (1988), Sharma and Pise (1994) and some methods of analysis have been proposed by Meyerhof and Adams (1968), Sutherland et al (1982), Chattopadhyay and Pise (1986), Rao and Kumar (1994), Manjunatha (1988), Ramesh Babu (1998) for the piles or anchors separately and that too in homogeneous media. Information on piles with enlarged bases in stratified soil is limited (Patra et al (2000))
Limited investigations on piles under oblique pulling loads are available (Yoshimi (1964), Das et at (1976), Ismael (1989), Chattopadhyay and Pise (1989)) . However investigations on pile anchors under oblique pulling loads are scanty (Mandal et al 2002).
Therefore, experimental investigations have been carried out to study the behaviour of pile anchors under axial pulling loads in layered sandy soil and under oblique pulling loads in homogeneous sand.
EXPERIMNTAL SET-UP AND TEST PROCEDURE
Experimental Setup
Model Test Tank
Tests were conducted in a specially fabricated metallic tank of size 914mm X 914mm X 762mm deep.
Foundation Medium
Dry sand obtained from local market (Kigali, Rwanda) was used as foundation medium. Selection of sand as foundation medium was made because its behaviour is free from time effects and reproducible densities can be achieved reasonable well. The specific–gravity and uniformity coefficient of the material were 2.61 and 1.1 respectively. The sand grains are sub angular and limiting void ratios were emin = 0.49, emax = 0.79 corresponding to maximum and minimum dry densities 1.754gm/cc.and 1.456gm/cc respectively. The placement densities during the test were 1.6g/cc and 1.69g/cc respectively for medium dense packing; R.D (52%) and dense packing; R.D (80%). The angles of shearing resistances for medium dense and dense packing respectively were 380 and 400.
Model Anchor Piles
Piles of one surface characteristic were used as model anchor piles. The shaft was made of mild steel solid tubes having diameter 12mm and 16mm. The corresponding embedment lengths to diameter ratio (L/d) were 25, 30 and 16 respectively. Enlargement of the diameter at the bottom of the pile shaft was provided by means of circular mild steel plates of 10 mm thick. The corresponding B/d ratios were 1, 2 and 3 respectively. A thread arrangement was made at the bottom and top of the shaft to fasten the anchor plate and pile cap.
The pile diameter of 12mm, embedment lengths 300mm, 360mm, having L/d = 25 and 30 respectively, are subjected to axial pulling loads in layered sandy soils. The corresponding B/d ratios used for the axial pulling loads were 1, 2, 3. Similarly pile diameters of 12mm and 16mm, embedment lengths 300mm and 320 mm respectively are used for oblique pulling loads in homogeneous sand. The corresponding B/d ratios were 1, 2.
The pile wall friction angle,(, for loose and dense packing were 250 and 280 respectively.
TEST PROCEDURE
Axial pulling loads in layered sandy soils and oblique pulling loads in homogeneous sand were applied at inclinations of 00 and 00,300,600,900 on the pile top through a double pulley (frictionless) arrangement. Sand was poured manually in the tank through the slot of the hopper moving it horizontally (rainfall technique), keeping the height of fall of sand particles constant. With a height of fall of 20 cm and 40 cm, the density of 1.6g/cc (medium dense condition) and 1.69 g/cc (Dense condition) are obtained (Patra and Pise (2001)). For layered sandy soil, half of the pile length is poured by sand at a height of fall of either 20cm or 40 cm to achieve a density of medium dense condition or dense condition. The other half of the pile length is poured by sand at a height of fall of 40cm or 20 cm to achieve a density of dense condition or medium dense condition. Similarly for pile anchors under oblique pulling loads in homogeneous sand, the density is achieved by pouring the sand at a height of fall of 40 cm. Flexible steel wire was attached to the ' S ' hook at the top of the pile cap. The arrangements were made with precision to avoid eccentricity in the application of loading. Wire was taken first through the adjustable pull (which is fitted with nut and bolt to the top solid steel channel) near the pile head and then over a second pulley to the pan where weights were put for loading in stages. For oblique pulling loads, the position of the first pulley was fixed according to the desired direction of loading. The loads were applied by dead weight in the loading pan starting from the smallest and gradually increasing in stages. Dial gauge readings were recorded when they became stable at each stage of loading. For axial pulling loads, vertical dial gauge readings were recorded corresponding to each load increment. Similarly for oblique pulling loads, vertical and lateral dial gauge readings were recorded corresponding to each load increment. Fig1. shows the schematic view of the experimental assembly.
Figure 1. The Experimental Setup
EXPERIMENTAL RESULTS AND DISCUSSION
Load - Displacement Diagrams
The basic observations from the experimental tests are applied load and the axial vertical displacement for axial pulling loads and applied load and axial, lateral and rotation for oblique pulling loads. Fig. 2 shows typical load-displacement diagram under axial pull. The load - displacement diagrams are practically linear in the early stages of the loading but afterwards they are non- linear. For all the piles axial movement or axial displacement is in the range of 2mm to 8mm was required to mobilize the ultimate resistance. For straight-shafted piles the axial displacement is in the range of 2.5mm to 3mm and for enlarged base piles 4mm to 8mm to mobilize the ultimate resistance.
Ultimate Uplift Capacity of Piles
The ultimate uplift capacity of pile anchors under axial pulling loads is determined from the pull Vs axial - displacement response. The ultimate load is taken as the load corresponding to the point where the load-displacement curve practically changes its curvature.
Ultimate Oblique-Pulling Resistance of Piles
The ultimate resistance of a pile anchors under oblique pull have been estimated from the oblique pull versus displacement diagrams. It is taken as the point at which the curve exhibits a peak or maintains a continuous displacement increase with no further increase in pull. The minimum load obtained from these diagrams is taken as the ultimate resistance.
Figure 2. Axial Pull versus Axial Displacement (L/d= 25, B/d =2)
Figure 3. Ultimate uplift capacity versus B/d ratio
Variation of Uplift Capacity with B/d ratio
Fig. 3 shows the variation of Uplift capacity of pile anchors with B/d ratio .The ultimate uplift capacity increases linearly with an increase in B/d ratio. The ultimate uplift capacity is higher for medium dense over dense condition than dense over medium dense condition
Variation of Uplift Capacity with L/d ratio
The variation of uplift capacity with L/d ratio is shown in Fig. 4. The ultimate uplift capacity increases with increase in L/d ratio from 25 to 30. For a particular value of B/d ratio, the ultimate uplift capacity is more for medium dense over dense condition than dense over medium dense condition.
Figure 4. Ultimate uplift capacity versus embedment Length
Variation ultimate oblique pull with load inclination:
The variation of ultimate oblique pull versus angle of inclination is shown in Fig. 5. It shows the inclination at which the pile anchor offers maximum/optimum resistance. The optimum resistance depends on the L/d ratio and B/d ratio.
Figure 5. Oblique pull versus angle of inclination
The values of the optimum resistances are as follows:
For 12 mm bar, L/d =25, B/d=1 Optimum resistance, q = 900
For 12 mm bar, L/d =25, B/d=2 Optimum resistance, q = 300
For 16 mm bar, L/d =20, B/d=1 Optimum resistance, q = 900
For 16 mm bar, L/d =20, B/d=2 Optimum resistance, q = 300
CONCLUSIONS
From the foregoing investigation, following conclusions are drawn:
1. Generally the load - displacement response under axial pulling loads and oblique pulling loads are nonlinear. For all pile-anchors under axial pulling loads and oblique pulling loads, the displacements in the range of 2mm to 8mm was required to mobilize the ultimate resistance. However, it is in the range of 2.5mm to 3mm for straight-shafted piles and 4mm to 8mm for enlarged base piles.
2. The uplift capacity of the pile increases with increase in length, base enlargement. The ultimate uplift capacity is higher for medium dense over dense condition than dense over medium dense condition.
3. The inclination at which the pile anchor offers maximum resistance to oblique pull depends on the embedment length to diameter ratio, L/d, base enlargement ratio, B/d, inclination of load and density of foundation medium.
REFERENCES
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