horizontal pullout capacity of a group of vertical square anchor plates

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Horizontal pullout capacity of Group of Vertical Square Anchor Plate Embedded in Medium to Dense Sand

UVCE,Bangalore Page 1

Introduction

Many modern engineering structures require a foundation system that provides adequate

support by resisting loads that are imposed on the foundation of the structure or vertical and

horizontal pullout forces. Stability and support of structures is provided by transferringfoundational loads from the structures foundation through some form of anchors and then

onto the surrounding soil and terrain. In many cases these loads and forces that are

transmitted through anchors to surrounding terrain can cause the anchor to experience

subsequence uplift forces .As a result, ground anchors have been developed so as to be fixed

to structures and are embedded to sufficient ground depths to provide adequate amounts of

support within required safety limits. It is therefore the primary focus of this project and

subsequent dissertation to investigate the uplift capacity of various ground anchors in sand.

Ground anchors have in fact been utilised for thousands of years in many different forms.

Predominantly the use of these early anchors was to support lightweight structures only and it

was not until recent times with the invention of suspension bridges that anchors were used to

transfer very large loads. As a result, during the last 50 years there have been a number of

studies conducted to investigate the design and use of these ground anchors. A number of

theories have been developed to estimate the ultimate uplift capacity of different ground

anchors in sand and it is the aim of this investigation to take a predominantly physical

approach to further investigate and test this pullout phenomenon. A Basic twodimensional

situation shall provide the primary focus of this investigation, however some initial

investigation into three--dimensional effects shall be conducted and will therefore provide

opportunity for further works in this area.

Ground Anchors

As mentioned ground anchors are generally designed and constructed to resist the forces that

are applied to foundations of structures. The primary function of these anchors is to transmit

the forces acting on the foundations of a structure to the surrounding soil or terrain. This

simple form of buried anchors have been around for thousands of years and used to stabilise

structures. An early example of this is the use of anchors or stakes that are buried to stabilise

a tent.

Initially ground anchors were only used to stabilise small or lightweight structures such as

transmission towers, radar towers or tents as explained in the above example. It was not until


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the mid 19th century when the creation of large suspension bridge structures introduced the

need of ground anchors to bear and transmit very large loads to the earth. From this point in

time forward, anchors began to form an important component of many civil engineering

projects and in many of these cases be required to resist some form of uplift forces.

Ground anchors have been developed since this time into a number of different forms each

with its own strengths and specific applications. Generally anchors can be classified into five

different forms of ground anchor that are currently in use in industry and they are as follows:

plate anchors, direct embedment anchors, helical anchors, grouted anchors, and anchor piles

and drilled shafts. This project shall primarily focus on the plate anchor, pile anchors and

shall further investigate buried pipelines in sand.

Plate Anchors

Plate anchors can be constructed using a many number of materials and can be constructed in

numerous shapes. For example plate anchors may be made of steel plates, precast concrete

slabs, poured reinforced concrete slabs or timber sheets just to name a few of the most

common construction materials. Plate anchors may also be put into location in a number of

ways, the two most common methods utilised today are backfilled plate anchors or excavated

trench plate anchors. Backfilled plate anchors are put into position by excavating the ground

to the required depth, placing the anchor in position then backfilling and compacting with

good quality soil. The other popular method of deposition is to install the plate anchor in

excavated trench, then attach the anchor to tie rods that are either driven or placed through

auger holes. (Das 1990). The methodology used throughout experimentation in regards to this

project will replicate the deposition method of backfilled plate anchors.

Pile Anchors

Anchor piles can be used to support both compressive loads or resist uplift loads, or in certain

circumstances act as lateral restraints. Piles are used when the structure is subject to very

large loads or in instances where surrounding soil conditions are poor and require footings to

pass to great depths to reach acceptable soil conditions. When the soil located immediately

below a given structure is weak, the load of the structure may be transmitted to a greater

depth by piles and drilled shafts (Das 1999).Some structures, like transmission towers,

mooring systems for ocean surface or submerged platforms, tall chimneys, jetty structures,

etc., are constructed on pile foundations, which have to resist uplift loads (Chattopadhyay


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and Pise 1986). Just as with plate anchors, piles can be made from many materials and be

formed into numerous shapes. They too can be created from steel, precast concrete, poured

concrete or timber and may be formed into shapes such as square, circular or octagonal piles.

Also, as with plate anchors, piles can be driven into place or excavated into position. Piles are

often more expensive to use then a shallow anchor of most types, but in many cases when

piles are utilised it is because they are required for the specific situation. Pile foundations

which are deep and which cost more than shallow foundations. Despite the cost, the use of

piles often is necessary to ensure structural safety (Das 2004).


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LITERATURE REVIEW

REVIEW OF RESEARCH FINDINGS RELATED TO BEHAVIOUR OF

HORIZONTAL PULLOUT CAPACITY OF GROUP OF VERTICAL SRIP ANCHOR

PLATE EMBEDDED IN SAND

Jyant Kumar(2001). The problem of finding the horizontal pullout capacity of vertical

anchors embedded in sands with the inclusion of pseudostatic horizontal earthquake body

forces, was tackled in this note. The analysis was carried out using an upper bound limit

analysis, with the consideration of two different collapse mechanisms: bilinear and composite

logarithmic spiral rupture surfaces. The results are presented in non dimensional form to find

the pullout resistance with changes in earthquake acceleration for different combinations of

embedment ratio of the anchor (), friction angle of the soil (), and the anchor-soil interface

wall friction angle ().

The soil medium follows Mohr-Coulombs failure criterion and an associated flow rule. At

failure, the back surface of the anchor is assumed to completely separate from the adjoining

soil mass, that is, the backfill material does not exert any active thrust at all on the back

surface of the anchor.

Bilinear rupture surface


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In the above equation, W0 and W1 are the weights of the blocks OAB and OBCD,

respectively. The magnitude ofPu can then be minimized with respect to independent

admissible variations of , 1, and 2 ; for a collapse mechanism to be kinematically

admissible, the magnitudes ofV1, V2, V10, and V21 in terms ofV0 should always remain

positive. It should be mentioned that for given values ofb andH, the magnitudes ofW0 and

W1 will depend on , 1, and 2.

Composite logarithmic spiral rupture surface:


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The variation of the pullout resistance for = 0.25.




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In the above equation W0 and W2 are the respective weights of blocks OAB and OCDE, and

Worklog is the rate of workdone by the body forces (due to and h) within the logspiral

zone OBC. This value was obtained mathematically using integration as

In the above expression ro is the length of the radius vector OB. Using eqs. [2] and [3], the

magnitude ofPu can be obtained for a chosen admissible collapse mechanism; for a collapse

mechanism to be kinematically admissible the magnitudes ofV1, V10, and V2 in terms ofV0

should remain positive. The magnitude ofPu can then be minimized with respect to the

variations of and .

Based on the above two mechanism they concluded that, the pullout resistance decreases

quite substantially with increases in the magnitude of the earthquake acceleration. For values

of up to about 0.250.5, the bilinear and composite logarithmic spiral rupture surfaces

gave almost identical answers, whereas for higher values of , the choice of the logarithmic

spiral provides significantly smaller pullout resistance.

MANAS KUMAR BHOI(2008).

The ultimate uplift resistance of a group of multiple strip anchors placed in sand and

subjected to equal magnitudes of vertical upward pullout loads has been determined by

means of model experiments. Instead of using a number of anchor plates in the experiments,

a single anchor plate was used by simulating the boundary conditions along the planes of

symmetry on both the sides of the anchor plate. The effect of clear spacing (s) between the

anchors, for different combinations of embedment ratio () of anchors and friction angle ()

of soil mass, was examined in detail.


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The variation of pu,av/(d) with (/B) for different values of (s/B) for = 37.4 and

A comparison ofobtained from the present study with the theory of Meyerhof and Adams .


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Based on the experimental study they reported in terms of efficiency factor which was

defined as the ratio of the failure load for an intervening strip anchor of a given width (B) to

that of a single strip anchor plate having the same width. It was clearly noted that the

magnitude ofy reduces quite extensively with a decrease in the spacing between the anchors.

The magnitude of y for a given s/B was found to vary only marginally with respect to

changes in and .

ADEL HANNA(2009):

Passive earth pressure on embedded anchor plates constitutes a viable resisting force for the

design of underground structures. In the current practice, these forces are empirically

calculated, ignoring the effects of the depth of embedment and the level of consolidation of

the surrounding soil, which takes place during plate installation on the in situ stress levels.

Numerical model was developed using finite-element technique and the constitutive law of

MohrCoulomb to simulate the case of a retaining wall partially supported by an embedded

anchor plate in sand, the coefficient of Passive earth pressure (kp) acting on the anchor plate

was given by

where Pult = ultimate load, ha = height of anchor plate, H = depth of top edge of the anchorplate from the ground surface, = friction angle between soil and the anchor plate, and Pult =

ultimate load (at the maximum mobilized passive earth pressure).


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Based on the numerical study they reported that passive earth pressure acting on anchor

plates increases due to the increase of angle of shearing resistance and the overconsolidation

ratio of sand, and it decreases due to an increase of the embedment depth of anchor.

PARAMITA BHATTACHARYA(2011).

The horizontal pullout capacity of a group of two vertical strip anchor plates placed along the

same vertical plane in sand, has been determined by using the lower bound finite element

limit analysis. The effect of vertical spacing (S) between the anchor plates on the magnitude

of the total group horizontal failure load (PuT) has been determined for different

combinations of H/B, / and . The magnitude of PuT has been obtained in terms of a group

efficiency factor, , with respect to the failure load for a single vertical plate with the same

H/B.


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PuT = Pu1 + Pu2

Where,

For front and backface (RT)

Front and backface (OP)

where Pu1 and Pu2 are the pull out load (per unit length of the strip plate) for lower and

upper anchor, respectively.

Group Efficiency Factor:


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where Pu and PuT refer to the failure loads for a single and a group of two anchors,

respectively, by keeping the value of H/B to be exactly the same in both the cases.

Based on the above study they concluded that , An increase in the value of and H/B leads to

generally a very marginal increase in the value of Scr/B. On the other hand, the changes in

the value of do not seem to have any significant effect on Scr/B. An increase in the value of

, and H/B generally tend to cause some increase in the values ofmax.

HAMED NIROUMAND AND KHAIRUL ANNEAR KASSIM (2011)

Numerical analysis and laboratory works were performed to study the performance of

the ultimate uplift load capacity of the anchor plate. The behavior of square anchor plates

during vertical pullout was studied by experimental data and finite element analyses in dense

sand. Validation of the analysis model was also carried out with 100 mm x100 m square

plates in dense sand. Agreement between the uplift capacities from the experimental test sand

finite element modeling using PLAXIS 2D, based on 0.2 m computed maximum

displacements was excellent for square anchor plates. Numerical analysis using square anchor

plates was conducted based on hardening soil model.


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From above both experimental and numerical analysis they conclude that the test performed

in the glass box showed that the limited ultimate capacity of the square anchor plate model

was influenced by the embedment ratio and the sand density. Typically, the results from

PLAXIS had a good agreement with the experimental results obtained from the prediction of

the uplift load capacity on the square anchor plate. These can be used to provide the basis for

the design of such structures in the sand.


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Conclusion from previous study

The pullout resistance decreases quite substantially increases in the magnitude of the

earthquake acceleration. for values of up to about 0.25-0.5, the bilinear and compositelogarithmic spiral provides significantly smaller pullout resistance. The results compare

favourably with the existing theoretical data.

The results produced in this investigation showed that the passive earth pressure

acting on anchor plates increases due to the increase of angle of shearing resistance and the

overconsolidated ratio of sand, and it decreases due to an increase of the embedment depth of

anchor. Design theories were developed for the case of embedded anchor plate in

overconsolidated sand. The theories developed will satisfy the design needed in terms of

allowable pullout load and/or displacement.

The magnitude of the group failure load becomes maximum corresponding to a

certain critical spacing between two anchors. The value of Scr//B has been found to exist

generally between 0.5 and 0.8 for different cases considered in this study. As compared to a

single vertical anchor, the magnitude of the group failure load for two anchors, with the same

H/B, has been found to increase up to a maximum of 43% for the different cases. For given

values of S/B and H/B, the value of for two vertical anchors increases continuosly with an

increase in the values of embedment ratio, angle of internal friction of sand and anchor-soil

interface friction angle.

It was clearly noted that the magnitude ofreduces quite extensively with a decrease

in the spacing between the anchors. The magnitude of for a given s/B was found to vary

only marginally with respect to changes in and

The test performed in the glass box showed that the limited ultimate capacity of the square

anchor plate model was influenced by the embedment ratio and the sand density. These can

be used to provide the basis for the design of such structures in the sand. There was no

overall agreement found between the results in this work and the numerical modelling by

PLAXIS 2D regarding the uplift capacity and the embedment ratio. However , close

agreements were found with the numerical modelling for particular embedment ratios and the

experimental results


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Finally we found more numerical analysis than experimental analysis from the literature

survey and hence we would conduct the experimental analysis for determining the pullout

performance of vertical strip anchors embedded in sand


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Materials and Method:

The mild steel tank of size - (1.2 m * 0.75 m* 1.5 m )Anchor plate of size-(100*100*10)mm

(150*150*10)mm are used for the Proposed study.

A coarse graded sand is used for the study.

The Properties of sand used the Proposed work is given by:

Uniformity coefficient 3.75

Coefficient of curvature 0.91

Specific Gravity 2.65

Maximum Unit Weight(max),emin 17.8 kN/m ,0.460

Minimum Unit Weight(min), emax 14.16 kN/m3,0.624

The experiments are performed on three different unit weight of soil.

Sl.no Relative Density Natural void ratio Dry density Angle of internal

friction

1 20% 0.591 16.339 -

2 50% 0.542 16.85 -

3 80% 0.493 17.41 -


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Experimental Setup of the Proposed work:

Solid Mild steel anchor plate of (100 *100)mm and (150*150) mm square are used in the

proposed experimental study. The anchors are kept in vertical position with spacing varying

at 0, 1 and 2 times the width of Anchor.

The technique of sand placement plays an important role in the process of achieving

reproducible density. After proper placement of the anchors in tank, sand was poured in the

tank by calibrating the height of fall through funnel. This technique of sand pouring is termed

as rainfall technique and this technique was reported to achieve good reproducible densities.

The sand surface was levelled carefully. This method of sand pouring gives a predetermined

dry density of loose, medium and dense sand.

Pullout load is applied to the anchor through a pulley arrangement with flexible wire attached

to connecting rod, Dial gauge readings corresponding to axial displacements were recorded.


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REFERENCES

ADEL HANNA (2011) Passive earth pressure on embedded vertical plate anchors in sand

,Acta Geotechnica 6: 21-29

HAMED NIROUMAND AND KHAIRUL ANUAR KASSIM (2011) Uplift response of

square anchor plates in dense sand ,International journal of the physical sciences

(6(16)):3939-3943

JYANTH KUMAR(2001) Seismic horizontal pullout capacity of vertical anchors in sands,

can Geotech 39:982-991

JYANTH KUMAR AND MANAS KUMAR BHOI (2009) Vertical uplift capacity of equally

spaced multiple strip anchors in sand,Geotech Geol Eng 27: 461-472

PARAMITA BHATTACHARYA (2011) Horizontal pullout capacity of a group of two

vertical strip anchors plates embedded in sand, 30:513-521


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horizontal pullout capacity of a group of vertical square anchor plates

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