SOIL ANCHORS ignasi.llorens@upc.edu ETSAB/UPC - 2013 PASSIVE ANCHORS - ANTECEDENTS Antecedents of passive anchors can be found in Nature. Roots feed plants and provide uplift resistance against the wind involving a volume of soil. 1 2 SOIL ANCHORS ignasi.llorens@upc.edu ETSAB/UPC - 2013 PASSIVE ANCHORS - ANTECEDENTS Definition: Soil anchors are foundations for tensile forces, generally employed to resist uplift. Antecedents and applications: roots, marine anchors, tents, engineering, bridges, textile roofs, lightweight structures, pneumatic structures, mobile homes, building, foundations, retaining walls, load tests, cranes, antennas, poles, transmission tower foundations, pipelines, equipment, militar, agricultural, cattle, etc. Typology: active, passive, shallow, deep Behaviour: mechanisms, type of anchor, soil, load, installation. Uplift evaluation: plate, shaf, earth pressure, combination, stakes, plates, empirical methods. Installation: excavation, fill, compaction and pre-loading. Examples - Limitations Selection of anchors Conclusions Data base Next events 2 3
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PASSIVE ANCHORS - ANTECEDENTS PASSIVE ANCHORS - ANTECEDENTS The rest of the equipment for the sacred tent must be made of bronze, including the pegs for the tent and for the curtain surrounding the courtyard. Exodus 27.19 (~1.400 bc). 5 6 # $%# & ' ()*+&,-./0.1 PASSIVE ANCHORS - ANTECEDENTS 7 #
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PASSIVE ANCHORS - APPLICATIONS SOIL ANCHORS: ACTIVE OR PASSIVE Active anchors are submitted to permanent pre-stressing. They have a cap and a tendon fixed to the ground with mortar. The soil is precompressed between the cap and the fixed length. When the external load is applied, the soil relaxes and the movements are kept to acceptable levels. Passive anchors are not submitted to permanent pre-stressing. They act against the soil when loaded. If unloaded, both, the anchor and the soil, remain at rest. They move more than active anchors, but they are simpler and entail fewer problems of relaxation and durability. 15 16 E PASSIVE ANCHORS - APPLICATIONS SOIL ANCHORS: ACTIVE OR PASSIVE In active anchors, the external load is supported mainly by the decompression of the soil. The anchor takes much less load and the displacement is kept to a minimum. In passive anchors, the whole external load goes to the anchor and the displacement that occurs is greater. 16 17
ACTIVE (PRE-STRESSED) ANCHORS PASSIVE ANCHORS TIPOLOGY MAIN CHARACTERISTICS Active anchors are pre-stressed by initially tensioning them against a steel bearing plate. The level of prestress is a percentage of the designworking load. When the pre-stressed anchor is externally loaded, it behaves as a much stiffer member than a dead anchor would. Production process - prefabricated - built on site Installation process - static - dynamic - driving - drilling - vibrating - blasting - boring - compacting Orientation - vertical - inclined - horizontal Material - wood - concrete - steel - other materials Roughness - smooth - rough Deployment device - fixed (no mechanism) - by rotation - by expansion Transmission joint - fixed - hinged Depth - shallow - deep Load - instantaneous - permanent - repeated - co-axial - oblique - perpendicular - eccentric Soil - granular - cohesive (frictionless) - transitional 17 18 " " #" " $ "
PASSIVE ANCHORS - TYPOLOGY PASSIVE ANCHORS - TYPOLOGY Passive anchors reaching the surface shallow Buried passive anchors deep 19 20 % " " # G 1 8 # # 4 1 C 1 # # 8 C & " "! 4 @ E H E H1:! 08 I: # 3 3 3
PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS PLATE EFFECT Uplift capacity is provided by the contribution of four mechanisms: a) Plate b) Shaft c) Earth pressure d) Self weight The plate acts against the soil as a footing turned upside down. It pushes up the material that prevents uplift. 21 22 " ( ( %!.9/0 4J4.9/0 F( BA.9,)
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PASSIVE ANCHORS SELF-WEIGHT PASSIVE ANCHORS - BEHAVIOUR Factors affecting the anchor pullout capacity: a) Anchor: type, form, weight, size, depth, inclination, roughness, stiffness and group effect. b) Soil: class, unit weight, friction, cohesion, adherence, deformability, initial stress, preconsolidation, soil-moisture, water table. c) Load: value, duration, repetition, inclination and eccentricity. d) Installation: excavation, fill, compaction and pre-loading. 25 26 ( ".. " PASSIVE ANCHORS - BEHAVIOUR The four mechanisms usually act in combination: a) Shallow block: shaft + weight b) Deep block: shaft + weight + plate c) Under reamed well: shaft + weight + plate d) Pile submitted to oblique loading: shaft + weight + earth pressure 27
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PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR 51 x 255 mm 51 x 153 mm 51 x 102 mm 51 x 51 mm 33 34.9,,GL1+-.+ 4LP< F,4 5@E#.9,) > R M.).L < &S M<.T ; 4LQ < F 1
PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR Geometry: d variable; d 0 38 mm; h 1 m. Soil: tamped alluvial finegrained sand; 1,63 T/m 3 Graph of the dependence of displacement u on vertical withdrawing forces Q for foundations with a diameter of 1) 40; 2) 60; 3) 80; 4) 100; 5) 120 cm. Graph of the dependence of ultimate load Q and stress on slab diameter d. d, cm 35 36,#A.9// &+ % % " 8+- M* -**&-0/&-0-&-9 HL *-.+-,>II.90+ ; + ; 1 # + " C
PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR Proportion, perimeter and area 37 38,AGU 5B!.9/* ; (!L L ( + : " M..&. + & *& LM* 1M-/-<*-&+ + & 0& LM* GU 5! " -,) + &!L0& )).& LM,<).<+ + &!L.& */<& LM<).!!%' -.' - I.90+
PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR Variation of (H/B) cr with c u Variation of the anchor capacity factor F with the angle of friction Ø. 39 40 / + + 0 1'2 #.9,< " %+ -*EAE 5 :G4.90+ "3 M4 VV5 4 3 " F&4" &" &5 4 " >6 7 >7 6 (" " " " %+ -*(4.9,0,
PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR Time Q 1 Q u /3 Q 2 Q 3 Q 3 > Q 2 > Q 1 41 42 8!% % $8#.9)9 ( 1 1B 5#.9,- 9 9 0 9 8.L< 9 89 C9 0 (9 " ' '- 5 #31.90+ /-.--
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PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR Geometry d 80 cm d 0 38 mm h 1 m Soil Tamped alluvial finegrained sand 1,63 T/m 3 45 46 :!.9,0 8 D '- #A.9// M.
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PASSIVE ANCHORS - BEHAVIOUR PASSIVE ANCHORS - BEHAVIOUR PILES IN COHESION LESS SOIL SUBJECT TO OBLIQUE AND HORIZONTAL PULL (Y.Yoshimi, 1964) (cm) 49 50 :(!3 )* >.-'.0' F >.-' W M X.)T.-' W M-.)T <-T W M.)T <-T ++!* H 0- *- C AGU 5B!.9/* -+* :% + (,)* >W ; >.)T
PASSIVE ANCHORS INSTALLATION: PRE-LOADING PASSIVE ANCHORS INSTALLATION: PRE-LOADING 51 52 # / I.90+ " 1..; - + < <>I.90+ " ".+ * ; H F D
PASSIVE ANCHORS INSTALLATION: PRE-LOADING PASSIVE ANCHORS STABILIZER PLATE Depth: 0,91 m Class 4 soil: compact gravel and sand with a small amount of clay. 1 in 25,4 mm 1 kip 4,45 kn 53 54 / '+. ( %) 84A 5>II.9,9!, <.< H < ; <! 4:"
PASSIVE ANCHORS UPLIFT EVALUATION UPLIFT EVALUATION: PLATE EFFECT Uplift is dependent on: Anchor type: - shape - width - depth - weight - inclination - roughness - position. Soil conditions: - type - density - cohesion - adherence - friction - stiffness - moisture - groundwater table position - initial stress state (preconsolidation). Loading mode: - rate - repetition - duration - direction - amount. Installation process: - vibrated - driven - bored - grouted - drilled. Basic approaches to the evaluation of the plate effect: - cone method - parabolic failure surface - earth pressure method 55 56 PASSIVE ANCHORS UPLIFT EVALUATION UPLIFT EVALUATION: PLATE EFFECT ULTIMATE LOAD (G.G.Meyerhof & J.I.Adams, 1968) Uplift is evaluated computing the contributions of the four mechanisms involved: - plate - shaft - self weight - earth pressure For each type of anchor, the effects to be computed are indicated. Examples: Logs and tubes? plate effect Dead men? plate + shaft + self weight effects 56 Strip Circular Square Rectangular Shallow (D < D c ) Deep (D D c ) B: width or diameter; L: length; c: cohesion; D: depth; D c : critical depth; F : angel of friction of soil;?: unit weight; K u : earth pressure coefficient; W: weight of soil; W p : weight of anchor. 58
UPLIFT EVALUATION: SHAFT EFFECT UPLIFT EVALUATION: EARTH PRESSURE EFFECT Five situations are possible according to the initial and final depths in relation to the critical depth: Shallow: H D; D D c H D; D > D c Deep: H < D; D D c H < D; D > D c ; D H < D c H < D; D > D c ; D H D c Short pile in sand submitted to oblique loading: the Broms approach. Uplift resistance against lateral resistance: increasing the horizontal component up to the ultimate lateral resistance improves the uplift capacity 57 58 UPLIFT EVALUATION: SHAFT EFFECT SHAFT EFFECT ESTIMATION - SUMMARY H D D D c H D D > D c H < D D D c H < D D > D c D H < D c H < D D > D c D H D c P e D a + + P e D 2 k TAN / 2 P e D a + + P e D c (D D c / 2) k TAN P e H a + + P e H (D H / 2) k TAN P e H a + + P e (D + D c H) (D c - D + H / 2) k TAN / 2 + + P e D c (D D c ) k TAN P e H a + + P e D c H k TAN a: adherence; B: width or diameter; c: cohesion; D: depth; D c : critical depth; d: angle of friction soil-foundation; f : angel of friction of soil;?: unit weight; H: height; k: earth pressure coefficient; P e : perimeter; Q f : shaft effect 58 >
UPLIFT EVALUATION: EARTH PRESSURE EFFECT UPLIFT EVALUATION EFFECTS IN COMBINATION c) d) a) b) a) b) c) d) e) f) When mechanisms act in combination adjustments should be made to take into account interferences: a) to d) the plate effect prevents the shaft effect in a height equivalent to the width. e) Plate and shaft effects are independent. They are computed by addition. f) Earth pressure improves friction. 59 60 $.9/* #! #!! " # " 1F "
UPLIFT EVALUATION STAKES UPLIFT EVALUATION: EMPIRICAL METHODS PROPERTIES OF GUY STAKES AND ANCHORS. (A.Kovacs, 1975).Critical depth/width versus ϕ Shallow depth and great depth 61 62
EMPIRICAL METHODS (IFAI, 2006) EMPIRICAL METHODS (IFAI, 2006) Estimation of the pullout capacity of tent stakes based on the baseline stake: - stake diameter: 0,1 - the side of the stake is smooth - the steel stake is driven vertically - the stake is embedded 36 inches in the ground - the load is fastened at 2 inches above the ground surface - the load is pulled at 45º angle. The pullout capacity for a stake that is different from the baseline case can be estimated as the baseline capacity multiplied by adjusting factors as follows: P P b x C e x C f x C i x C l x C d < 2.500 lbs P: pullout capacity for a single stake; P b : pullout capacity for the baseline case; C e : correction factor for embedment depth; C f : correction factor for fastening height; C i : correction factor for stake inclination; C l : correction factor for load angle; C d : correction factor for stake diameter. Consistency Field identification Pullout Soil resistance Stake penetration capacity resistance (1) (lbs) Hard (very dense) Very stiff (dense) Stiff (medium dense) Medium Soft (loose) Very soft (very loose) (1) inches per blow Pullout capacity for baseline case Indented with difficulty by thumbnail Less than 0,2 2.500 Readily indented by thumbnail 0,2 0,5 1.600 Readily indented by thumb but penetrated only with great effort 0,5 1,5 800 Can be penetrated several inches by thumb with moderate effort 1,5 3 400 Easily penetrated several inches by thumb 3 6 200 Easily penetrated several inches by thumb Greater than 6 100 63 64 ( +-./ + ( E(>(
EMPIRICAL METHODS (IFAI, 2006) PASSIVE ANCHORS INSTALLATION PROCESS 65 66 > : " &
PASSIVE ANCHORS INSTALLATION PROCESS PASSIVE ANCHORS INSTALLATION PROCESS Recoverable stake It could be made with a sheet pile to withstand medium to heavy loads. It is installed dynamically and put in position pre-loading it, 67 ( > ( # 3 " ( +--/ 68
PASSIVE ANCHORS INSTALLATION PROCESS PASSIVE ANCHORS - EXAMPLES: STAKE PILE 69 70 %?+ H ", '.----- +---- 0' +----- <----.' <----- *---- C
PASSIVE ANCHORS - EXAMPLES: LOG ANCHOR PASSIVE ANCHORS - EXAMPLES 71 72 @?+ ' ;! H( 3 F9-T % A! # & %?:$
EXAMPLES: TRANSMISSION TOWER FOUNDATIONS PASSIVE ANCHORS - EXAMPLES 73 74 # % C A >F
PASSIVE ANCHORS - EXAMPLES PASSIVE ANCHORS - EXAMPLES 75 76! ( %) A + 3F9-T " 8 (
PASSIVE ANCHORS LIMITATIONS PASSIVE ANCHORS SELECTION: SHALLOW 77 78 "? # (<-.<) )--- 8D ()" " " "8 "B & #+ H :?" % F ( A! ( B! # %!! F
PASSIVE ANCHORS SELECTION: DEEP PASSIVE ANCHORS CONCLUSIONS To anchor lightweight structures it is not necessary to use heavy foundations because lightweight recoverable anchors can be used instead. They replace the contribution of the self weight involving the soil whose contributions are the lateral earth pressure, friction and weight. Several types are available according to whether they reach the surface or remain buried. Differences relate to material, shape, installation process, efficiency, depth and ultimate uplift capacity that can be roughly estimated. Installation procedures include driving or boring. More research is needed to analyse and check every aspect that influences the behaviour of every type of anchor. A standard uplift test would help on using and comparing the results. An international data base of test results would be greatly appreciated. 79 80 *?(0)"! A 8 4 %