8-9 august 2014, koc university, istanbul/turkey...

Effect of anchor inclination on retrofitting offoundation bearing capacity and settlement

Reza Ziaie MoayedAssociate Professor, Civil Engineering Dept. Imam

Khomeini International University,Qazvin, Iran

[email protected]

Mahdi AlibolandiInstructor, Sama Technical and Vocational TrainingCollege, Islamic Azad University, Andisheh Branch,

Andisheh, [email protected]

Abstract— Shallow foundations are underpinned mainly toprevent harmful settlement, to enhance bearing capacity, or forseismic retrofit. A new jet grouted anchors is attached to theexisting superstructure, often by means of an even highlycomplex load transfer structure. Vertically oriented steel groundanchor is frequently used to restrain various types of foundationelements (spread footing, mats, slabs on grade, etc.). In this study,the effect of different orientation anchor on foundation behaviorunder upward load is investigated by finite element analysis.Meanwhile, this technique influence on settlement of foundationalso explored. Based on FEM results, anchoring could have asignificant influence on the displacement and bearing capacity ofthe shallow foundation and its efficiency depends on anchorinclination, considerably.

Keywords—foundation retrofitting; anchor; uplift; bearingcapacity component

I. INTRODUCTION

The need for foundation underpinning arises as when it isdesirable to reduce the settlement or restrain uplift ofstructures. In addition, the load-bearing capacity offoundations may require enhancement because of increasedloads due to, for instance, the construction of an additionalfloor ([1]-[3]).

During different periods, various underpinning techniqueshave been applied ([4] and [5]). Until the 1980s, the methodsused included, in particular, foundation extension bydeepening and broadening, different kinds of pile work, soilnailing, and chemical grouting ([6]-[9]). Soil or groundanchors are a lightweight foundation system designed andconstructed to resist any uplifting force. Vertical uplift forcesmay be generated by hydrostatic or overturning forces. Themethod is used in underwater applications where the structurehas insufficient dead weight to counteract the hydrostaticuplift forces. An example application of ground anchors toresist uplift forces is shown in fig. 1a. Also, existing structuresmay require additional stabilization to meet current safetystandards with respect to maximum flood and earthquakerequirements. Anchors provide additional resistance tooverturning, sliding, and earthquake loadings (Fig. 1b).

The advantage of ground anchors for tiedown structuresinclude: (1) the volume of concrete in the slab is reducedcompared to a dead weight slab; and (2) excavation and/ordewatering is reduced [10].

a

bFig. 1. Applications of ground anchor in tiedown structures: a) Uplift

slab, b) Tower foundation.

Experimental-based or numerical–theoretical-based studiesare made to find the behavior of soil anchors. Kulhawy, 1985presented an equation for evaluating uplift capacity as bellow:

Qsu = π D (KN/K0 i=1) (σ′V) i (K0)i tan[φi (δ/φ)i]Δzi (1)

Where: Δzi = thickness of layer i, D = anchor diameter,and K/Ko = stress modification factor to adjust forconstruction influences. The remaining parameters areevaluated at the mid-depth of each layer: σ′V = verticaleffective stress, δ = effective stress angle of friction for theshear surface interface, Ko = in-situ horizontal stress

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coefficient, and φ = effective stress friction angle for the soil.The anchor depth and perimeter terms are computed simplyfrom the anchor geometry, while the vertical effective stressesare computed from the effective soil unit weight [11]. Saeedy(1987) indicates that the greater the value of H/D (height todiameter of anchor) the higher the uplift resistance, but with adecreasing rate until a constant value of uplift force is reached[12]. Dickin and Laman (2007) by using physical andcomputational studies concluded that the uplift maximumresistances increase with anchor embedment ratio and sandpacking [13]. Khatri and Kumar (2011) Founded that with adecrease in anchor width the average ultimate uplift pressuredecreases quite significantly [14]. Bera and Banerjee (2013)conducted a series of model tests to investigate the influenceof relative density (Dr) of sand, the ratio of embedment depth(H) of anchor to diameter (D) of bell of anchor, anchordiameter, and types of sand on uplift capacity of bell-shapedanchor embedded in sand. They also developed an empiricalmodel to predict breakout factor (Fq) of bell-shaped anchorembedded in sand in terms of H/D ratio and relative density(Dr) [15].

Ground anchored has been achieved considerable attentionin the literature. However, there is little information on theeffect of inclined anchors on shallow foundations. Thepurpose of this paper is to investigate the effect of anchoringand its inclination on the improvement of bearing capacity anddisplacement of circular foundation under the both upwardand downward loaded by FE analysis.

II. ANALYSIS PROCEDURE

In order to evaluate the effects of anchor inclination onfoundation deformation the finite element analysis performedusing Plaxis 8.5 software with axis-symmetric model. The sizeof model in X and Y direction is considered greater than 2.5Dand 5D, respectively, that satisfy Bowles recommendation[16]. Six models provided to analyze, one models forfoundation without anchor, and the others assigned to thecircular foundation with different slope of anchor (m) that are0.5, 1.0, 2.0, 4.0 and vertical (Fig. 2). The dimensions of theR, anchor and grouted length are chosen as 1, 3 and 1.25 m.

The 15-node triangular elements are used to model the soiland other volume clusters. Analysis is performed under loadcontrol by a vertical distributed load boundary conditionapplied to the soil surface below the position of the circularfoundation (Fig. 3).

The elastic-perfectly plastic Mohr-Coulomb model wasused for modeling the behavior of sand. This model involvesfive input parameters; modulus of elasticity (E) and poison’sratio ( ) for soil elasticity, angle of internal friction (φ) andeffective cohesion (C) for soil plasticity and dilatancy angle(ѱ). Table 1 and 2 presented the properties of sand and grout,respectively.

Fig. 2. Geometry of model and anchors orientation.

Fig. 3. Meshed geometry

TABLE I. SAND PROPERTIES

Material model φ (o) C (kPa) ѱ (o) γ (kN/m3) υ E(kN/m2)Mohr-Coulomb 34 1 4 17 0.3 30000

TABLE II. GROUT PROPERTIES

Material type EA (kN/m)Elastic 100000


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III. RESULTS

A. Upward load

In order to determine the ultimate uplift capacity ofanchored foundation in sand, the tangent method was used.From the load-displacement curves by this method, theultimate anchor capacity is determined at the intersecting pointof two tangent lines that pass through the starting and endportions of the curve ([17] and [18]). All models wereanalyzed and upward load-displacement curves for each modelwere plotted (Fig.3).

Fig. 4 shows that the installations of anchorage increasethe bearing capacity of foundation under the upward load,significantly. It’s obvious that the anchor with moreinclination (i.e. vertical and m=4), present the moredisplacement before failure.

Fig. 5 compares the uplift bearing capacity of anchoredcircular foundation for different anchor inclination. It’sobserved that, as the anchor inclination increases up to aboutm=2, the uplift bearing capacity increases. With furtherincrease in inclination, the bearing capacity is decreased;however, the ultimate bearing capacity of anchored foundationis more than the non-anchored foundation. Aforementionedfigure shows that among the anchored foundations, verticaland m=2 orientation of anchor have minimum and maximumeffect on increasing uplift bearing capacity, respectively.

Fig. 4. Displacement vs. stress of foundation with different anchor inclinationunder upward loading.

Fig. 5. Uplift bearing capacity of non-anchored and anchored foundation

B. Downward load

Fig. 6 indicated that anchors have not considerable effecton improvement of bearing capacity of foundation under thedownward loads. However, the anchoring decreases thefoundation settlements, noticeably. As apparent, the m=1slope is most appropriate orientation for limiting thefoundation settlement and the difference of various foundationsettlement increases as load increased.

Fig. 7 presents the settlement of non-anchored andanchored foundations with different anchor inclination under160 kPa load. As clear, by increasing the slope of anchor up tom=1, settlement decreases and after this threshold increases.

Fig. 6. Settlement vs. Stress of foundation with different anchorinclination under the downward loading.


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Fig. 7. Settlement versus non-anchored and anchored foundation withdifferent inclination of anchors.

IV. CONCLUSION

Based on results obtained from the numerical study carriedout on circular foundation witch anchored with differentinclination in sand, the following conclusions are drawn:

Presence of anchor enhances the uplift bearing capacity offoundation and its efficiency depends on anchor inclination,significantly. Bearing capacity of foundation increases byincreasing the slope of anchors up to m=2 (slope: 2 vertical –1horizontal) and after this threshold decreases. However, thestrength of anchored foundations is more than the non-anchored cases.

Among the anchored foundations, vertical and m=2orientation of anchor have minimum and maximum effect onincreasing uplift bearing capacity, respectively.

Anchors have not significant effect on the improvement ofbearing capacity of foundation under the downward loads.However, the anchoring decreases the foundation settlements,prominently. By increasing the slope of anchor up to m=1(1:1), settlement decreases and after this threshold increases.

REFFERENCES

[1] J. Han and S.L. Ye, “A field study on the behavior of micropiles in clayunder compression or tension,” Canadian Geotechnical Journal, 2006a,vol. 43, pp. 19-29.

[2] J. Han and S.L. Ye, “A field study on the behavior of a foundationunderpinned by micropiles,”. Canadian Geotechnical Journal, 2006b,vol. 43, pp. 30-42.

[3] J. Ed. Lehtonen, “Underpinning – Nordic practice,” Course materialfrom Turku University of Applied Sciences, 2009, 46.

[4] J.A. Mason, and F.H. Kulhawy, “Notes on improvement andunderpinning of foundations of historic structures with reticulatedmicropiles,” Proceedings of the 2nd International Workshop onMicropiles, Ube., 1999.

[5] S. Thorburn, Introduction In Underpinning and Retention (ed. Thorburn,S. and Littlejohn, G.S.), Blackie Academic & Professional, 1993.

[6] D.A. Bruce, Insitu earth reinforcing by soil nailing. In Underpinning andRetention (ed. Thorburn, S. and Littlejohn, G.S.) Blackie Academic &Professional, 1993.

[7] D.A. Bruce, and I. Juran, Drilled and grouted micropiles. US.Department of Transportation, Federal Highway Administration reportsNo. FHWA-RD-96-016, 1997.

[8] R.A. Gould, P.R. Bedell, and J.G. Muckle, “Construction over organicsoils in an urban environment: four case histories,” CanadianGeotechnical Journal, 2002, vol. 39, pp. 345-356.

[9] H.A. Perko, “Underpinning and shoring for underground MRI ResearchFacility at Ohio State University,” Proceedings of the Spec. Sem.“Underground Construction in Urban Environments” ASCEMetropolitan Section Geotechnical Group, Geo-Institute of ASCE, 2005.

[10] Federal Highway Administration, Ground Anchors and Anchoredsystems, Report No. FHWA-IF-99-015, United States Department ofTransportation, 1999.

[11] F.H. Kulhawy, "Uplift Behavior of Shallow Soil Anchors - AnOverview,” Proceedings on the Uplift Behavior of Anchor Foundationsin Soil,” ASCE, Detroit, Michigan, 1985, pp. 1-25.

[12] H.S. Saeedy, “Stability of circular vertical earth anchors,” CanadianGeotechnical Journal, 1987, vol. 24, pp. 452-456.

[13] E.A. Dickin, and M. Laman, “Uplift response of strip anchors incohesionless soil,” Advances in Engineering Software, 2007, vol. 38, pp.618-625.

[14] V. N. Khatri, and J. Kumar, “ Effect of anchor width on pullout capacityof strip anchors in sand,” Canadian Geotechnical Journal, 2011, vol. 48,No. 3, pp. 511-517.

[15] A. K. Bera, and U. Banerjee, “Uplift capacity of a model bell-shapedanchor embedded in sand,” International Journal of GeotechnicalEngineering, 2013, vol. 7, No. 1, pp. 84-90.

[16] J. E. Bowels, Foundation Analysis and Design, fifth edition, The McGraw-Hill, 1998.

[17] A.R. Jumkis, Soil Mechanics, East-West Press Pvt. Ltd., New Delhi,1967 .

[18] S. Saran, and P.P. Rao, “Uplift behavior of horizontal plate anchors withgeosynthetics,” India Geotechnical Journal, 2002, vol. 32, pp. 235-246.


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