TESTI N G OF PI LE LES S Chandrashekhar Damodare Sr.Engineer (Quality Control)
INTRODUCTION
Pile foundations are one of the types of deep foundations. They are used in case of soft/silty /clayey type of soils. As in such case of soils going for normal foundations may not be possible either from economical point of view or may not be possible at all in case you want to construct near seashore etc. These are the foundations which transfer load to greater depths, where there is strong strata / load reaching that point is minimal.
NEED FOR PILE TESTING
Testing of the piles is very much vital as well as important activity during pile foundation. The testing can give better insight of the characteristics of the casted piles . The load transfer mechanism from a pile to the surrounding ground is complicated and could not yet be fully ascertained, although application of pile foundations is in practice over many decades. Pile transfers axial loads either substantially by skin friction along its shaft or substantially by the end bearing. Piles are used where either of the above load transfer mechanism is possible depending upon the subsoil stratification at the particular site
TESTING OF PILES PILE TESTING
Pile Integrity Test
Dynamic testing
Pile load test
Vertical Load test (Compression)
Pull out Test
Maintained Load test
C.R.P.METHOD
Cyclic method
Lateral Load test
I.S.CODES FOR PILE TESTING Sr.no.
Method of the test
Relevant I.S.Code
Remark
1
Pile Load Test I.S.2911(Part 4)
IN-SITU-TEST
2
Dynamic tests on pile I.S.2911
Using Modified Hille Hi lley’ y’s s Fo Form rmul ula a
3
Pile Integrity test
NDT testing WITH I.S.14893-2001 LOW STRAIN
NECESSARY I NFORMATI ON f or Test in g of PI LE
:
Pile type including material and reinforcement details, group of piles, if any. Method of driving with driving record or installation. Pile depth(s) and details of cross -section(s). Type of test desired. Layout of the pile(s) — space available around and position in the group for single pile test.
Continued……………… Safe load and ultimate load capacity, and the method(s) on which based. Availability and provision of type of piles or anchors or kentledge for reaction Nature of loading/loading plan with a particularly mention of pile(s) whichmay be free standing when scour is expected; Depth of water table and soil strata details with soil test results
PILE LOAD TESTS
Pil e load t est is the most direct method for determining the safe loads on piles including its structural capacity with respect to soil in which it is installed. It is considered more reliable on account of its being in - -situ situ test than the capacities computed by other methods, such as st at ic for m ula, dynam ic f orm ulae and penet rat ion t est dat a .
There are widely varying practices followed for load tests on piles. Particularly, the difficulties regarding the establishment of an acceptable criterion, for determining the ultimate and safe bearing capacity of piles, and predicting the pile group behavior from the test data obtained from individual pile.
TYPE OF LOADING CONDITIONS IN PILE LOAD TEST A) VERTICAL LOADING (COMPRESSION)
B) LATERAL LOADING
C) PULL OUT Test
There are two types of tests for each type of loading
I ni t ial Load Test ― A test pile is tested to determine the load carrying capacity of the pile by loading either to its ultimate load or to twice the estimated safe load. Rout ine Test — It is carried out on a working pile with a view to check whether pile is capable of taking the working load assigned to it
VERTICAL LOAD TEST
In this type of test, compression load is applied to the pile top by means of a hydraulic jack against rolled steel joist or suitable load frame capable of providing reaction and the settlement is recorded by suitably positioned dial gauges.
Vertical load method again divided in to three methods as following: A) Maintained load Method
B) Cyclic Method
C) CRP Method
TERMINOLOGY
Kent ledge - Dead-weight used for applying a test load on piles. Test Pile - A pile which is meant for initial test. Work ing Pile - A pile forming part of foundation of a structural system which may be used for routine load test. Saf e Load - It is a load on a pile derived by applying a factor of safety on ultimate load capacity of pile as determined by load test.
General requirements and preparation required for Vertical Load methods
Pr eparat ion of PI LE HEAD - The pile head should be chipped off to natural horizontal plane till sound concrete is met. The projecting reinforcement should be cut off or bent suitably and the top finished smooth and level with plaster of Paris or similar synthetic material where required. A bearing plate with a hole at the centre should be placed on the head of the pile for the jacks to rest.
Application of Load ( Not for CRP Method) The test should be carried out by applying a series of vertical downward incremental load, each increment being of about 20 percent of safe load on the pile. For testing of raker piles, it is essential that loading is along the axis.
The reaction may be obtained from the following
a) Kent ledge placed on a platf orm suppor t ed clear of t he test t est pi le. I n case of load test below under - pinned st ructu re, t he existin g existing st ruct ure if h aving adequate w eight and suit able const ructi on ma ruct ion m ay serv e as kentledge . The cent centrre e of gr avity of t he kentledge should coincide w it h t he axis of t he pile and t he load applied by t he j ack should also be coaxial w it h t his pile. b) Anchor piles w it h centr e - t o - centr e dist ance from t he test pile not less t han3 ti mes t im es t he test pile shaft diameter subj ect t o minim mi nim um of 2 m . I f t he anchor piles are perm anent w orking orkin g piles, it should be ensured t hat t he residual uplift is w it hin lim it s. Care should be exercised tto o ensur e tth hat at t he dat um bar suppor t s are no t affected p of t he soil. aff ect ed by heaving u up c) Rock anchors w it h distance fr om t he nearest edge of t he piles at rock l evel being 2 t im es t he t est pile shaft diameter or 1.5 m w hichever is gr eat er great
SETTLEMENT
Settlement shall be recorded with minimum 3 dial gauges for single pile and 4 dial gauges of 0.01 mm sensitivity for groups. The dial gauges shall be placed symmetrically and at equal distances from the pile(s) and normally held by datum bars resting on immovable supports at a distance of 3 D (subject to minimum of 1.5 m) from the edge of the piles, where D is the pile stem diameter of circular piles or diameter of the circumscribing circle in the case of square or non -circular PILES
e sa e oa on s ng e p e or t e n t a t est shoul d be least of t he following:
a) For piles up to and including 600 mm diameter i) Two-thirds of the final load at which the total displacement attains a value of 12 mm unless otherwise required in a given case on the basis of nature and type of structure in which case, the safe load should be corresponding to the stated total displacement permissible. ii) 50 percent of the final load at which the total displacement equal to 10 percent of the pile diameter in case of uniform diameterpiles and 7.5 percent of bulb diameter in case of under -reamed piles.
For piles pil es m ore t han 600 m m diamet er
i) Two-third of the final load at which the total displacement attains a value of 2.5 percent of pile diameter . ii) Load corresponding to a total displacement of 1.5 percent of pile diameter. iii) 50 percent of the final load at which the total displacement equal to 10 percent of the pile diameter in case of uniform diameter
The safe load on groups of piles for initial test
For piles up to and including 600 mm diameter i) Final load at which the total displacement attains a value of 25mm unless otherwise required in a given case on the basis of nature and type of structure. ii) Two-third of the final load at which the total displacement attains a value of 40 mm.
Continued……. For piles more than 600 mm diameter i) Final load at which the total displacement attains a value of 3 percent of pile diameter unless otherwise required in a given case on the basis of nature and type of structure, and ii) Two-third of the final load at which the total displacement attainsa value of 5 percent of pile diameter.
MAINTAINED LOAD METHOD This is applicable for both initial and routine test. In this method, each stage of loading shall be maintained till the rate of movement of the pile top is not more than 0.2mm / hr or until two hours have elapsed, whichever is earlier. If the limit of permissible displacement as given in Routin and init ial t est s are not exceeded, testing of pile is not required to be continued further. The test load shall be maintained for24 hours
Set-up for Maintained Load Test
Maintained Load Test
CRP(Constant Rate of Penetration) TEST
This method which is used for initial test is generally considered to be more suitable for determining ultimate load capacity than the maintained load test but the load/deflection characteristics are quite different from those of the maintained load test and cannot be used to predict settlement of the pile under working load conditions
PROCEDURE OF THE TEST
The load shall b e measur ed by m eans of pr essure of 0 .01 m m sensi t i v i t y load gauge. The penet rat ion ( deflect ion) should be m easur ed by m eans of dial gauges held by a datu m bar rest ing on im movable datum imm ovable suppor supportt s a t a dist ance of atleast 3 D (subject t o a minim um of 1.5 m) away aw ay from t he test pile edge w here D is defi ned in intial t est . One of t he dial gauges w ill be select ed for conducti ng t he ttest conduct ing est .
Wit h continu ous applicat ion of pressure on t he pile ttop op by opera ting of the j ack , a per son w at ch es t he r at e o f set t lem en t of t he dial gau ge against a st op w atch held in his hand and directs the pump operator t o pum p faster or slow er or at t he same rate as needed to m aintain t he prescrib ed rat e of sett lement say at every 0.25 mm set t lement, he gives an indicat i on to take readings. I m mediat ely, ot her mm ediately, oth er persons record t he pressur e gauge r eadings and ot her dial gauge readings.
The pump supplyin g t he j ack m ay be hand or m echanically operat ed . For force up tto o 200 tonnes hand pumping is convenient . I f a m echanical pump is used, it should, for pr eference, have an ‘ infi nit e variable’ delivery, cont ed eit her by a bleed valve or a variable speed driv e. contrroll olled dri ve.
Continued…….
The jack should be operated to cause the pile to penetrate at uniform rate which may be controlled by checking the time taken for small increments of penetration and adjusting the pumping rate accordingly. Readings of time, penetration and load should be taken at sufficiently close intervals to give adequate control of the rate of penetration. A rate of penetration of about 0.75 mm per minute is suitable for predominantly friction piles. For predominantly end-bearing piles in sand or gravel, rate of penetration of 1.5 mm per minute may be used. The rate of penetration, if steady, may be half or twice these values without significantly affecting the results. The test should be carried out for the penetration more than 10 percent of the diameter of the pile base
Continued……. As the test proceeds a curve between load and penetration should be drawn to determine when the ultimate load capacity has been reached.
Continued……
The curve of load versus penetration in the case of a predominantly friction pile will represent either a peak and the subsequent downward trend, or a peak and then almost a straight line
CYCLI C LOAD TEST METHOD Alternate loading and unloading shall be carried out at each stage as in Main t ain ed load t est and and each unloading stage shall be maintained for at least 15 minutes and the subsequent elastic rebound in the pile should be measured accurately by dial gauges. The test may be continued up to 50 percent over the safe load
Graphical Analysis
Assuming that there is no compression compression in the pile, plot a graph relating total elastic recovery and load at the pile top. Draw a straight line parallel to the straight portion of curve I to divide the load into two parts and thereby obtained approximate values of point resistance and skin friction. From the approximate value of skin friction, and knowing the loads on top of pile, compute the elastic compression of the pile corresponding to these loads,by the following formula: /2) L ∆ ∆ = (T - F AE where ∆ ∆ = elastic compression of pile, in cm; T = load on pile top, in kgf ; F = frictional resistance, in kgf ; L = length of the pile, in cm; A = cross-sectional area of the pile, in cm2, and E = modulus of elasticity of the pile material, in kgf/cm2.
Continued……..
Sk in Fr ict ion and Poin t Resist ance (The value should normally be measured from an exposed portion of pile stem by means of compressometer during the load test itself.) Obtain values of the elastic compression of the subgrade by subtracting the elastic compression of the pile from the total elastic recovery of pile, and plot the graph relating these new values to the corresponding loads on pile top. When elastic compression of the subgrade works out negative, the negative value shall be ignored until the value is positive.
LATERAL TEST
The test may be carried out by introducing a hydraulic jack with gauge between two piles or pile groups under test or the reaction may be suitably obtained otherwise. If it is conducted by jack located between two piles or groups, the full load imposed by the jack shall be taken as the lateral resistance of each pile or group. The loading should be applied in increments of about 20 percent of the estimated safe load.
Continued ……
The next increment should be applied after the rate of displacement is nearer to 0.1 mm per 30 minutes Displacements shall be read by using at least two dial gauges of 0.01 mm sensitivity spaced at 30 cm and kept horizontally one above the other on the test pile and the displacement interpolated at cut-off level from similar triangles where cut-off level is unapproachable and for approachable cut-off level, however, one dial gauge placed diametrically opposite to the jack shall directly measure the displacement. Where, it is not possible to locate one of the dial gauges in the line of the jack axes, then two dial gauges may be kept at a distance of 30 cm at a suitable height and the displacement interpolated at load point from similar triangles.
Continued……………
The safe lateral load on the pile, for both free head & fixed head shall be taken as the least of the following: a) Fifty percent of the final load at which the total displacement increases to 12 mm; b) Final load at which the total displacement corresponds to 5 mm; and c) Load corresponding to any other specified displacement as per performance requirements.
Set-up for Lateral Test
PULL- OUT TEST ON THE PI LES
Uplift force may preferably be applied by means of hydraulic jack(s) with gauge using a suitable pull out set up. The test pile shall have adequate steel to withstand pulling. In some cases, in order to allow for neck tension in a pull out test, it may be necessary to provide additional reinforcement in the pile s to be tested. The pull out load increments and consequent displacement reading s shall be read as in the case of vertical load test. The safe load shall be taken as the least of the following: a) For piles up to and including 600 mm diameter i) Two-thirds of the load at which the total displacement is 12 mm or the load corresponding to a specified permissible uplift ii) Half of the load at which the load displacement curve shows a clear break
Continued……………….
b) For piles more than 600 mm diameter i) Two-thirds of the load at which the total displacement is 2.5 percent of pile diameter or the load corresponding to a specifie d permissible uplift ii) Half of the load at which the load displacement curve shows a clear break (downward trend) The initial test shall be carried out up to twice the estimated estimated safe load or until the load displacement curve shows a clear break (down-ward trend). Routine test shall be carried out to one -and-a-half times the estimated safe load or 12 mm total displacement whichever is earlier
RECORDI NG OF DATA AND PRESENTATI ON
The pile test data essentially concerns three variables, namely, load, displacement and time. These are to be recorded sequentially for the tests under consideration and recorded in a suitable tabular form along with the information about the pile. The data may be suitably presented by curves drawn between the variables and safe loads shown on the graphs. Load displacement curve should be there.
Pile I nt egr it y Test
Pile integrity test is used to assess the continuity of piles. With this test, necking or bulging and uniformity of quality of concrete is checked. Piles that are doubtful for integrity are taken up for Dynamic load testing along with some piles which are ok. Typical defects identified by this test are shown below
DEFECTS IN PILES
Procedure of the Test
Method of Testing : There are Indian Standards for the pile inte grity test & the most commonly used worldwide is Pile I nt egrit y Test as per ASTM – D 5882 588 2 – 00, “ St andard Test Met hod f or Low St rain I nt egrit y Test ing of Piles” . Placement of Tr ansducers – The motion sensor should be placed at or near the pile head using a suitable, or temporary, bonding material (that is wax, Vaseline etc.) so that it is assured that it correctly measures the axial pile motion. The motion sensor is placed generally near the center of pile. Additional locations s hould be considered for piles with diameters greater than 500 mm. The low strain impact should be applied to the pile head within a distance of 300 mm from the motion sensor. Objective: To check the pile for continuity of the shaft of the pile
Continued…….
In the sonic test, the top of the pile is hit with a plastic ham mer and the reflected waves are recorded by a suitable computerized equipment. From the resulting signal, or reflectogram, one can determine both length and continuity of the pile. Although a powerful tool, the sonic method also has limitations, such as: the test produces no information regarding the pile capacity, and gives only limited information about the concrete quality. The sonic system, can on only discover material impedance changes. It cannot tell anything whether these impedance changes are due to poor concrete or due to reduced cross section. Even then this test is a fast & & economical tool for determining discontinuities in the pile shaft. If pile records are available, then the results can be fine tun ed for greater & more reliable information.
Lim it at ions of t he t est
The accuracy of the length determination depends on the assumed wave velocity. This factor, which depends on the grade and age of the concrete, varies from pile to pile and may cause an error of the order of 10 percent .
Graph of velocity signal Vs. Pile length
PI LE DRI VI NG ANALYZER ( PDA)
Pile Dynam ic Test
Pile dynamic test is used to assess the load bearing capacity of the pile. This test uses a hammer of a couple of tons to drop on the pile. Displacements are measured using a high tech instrument. This data is processed to arrive at the pile dynamic load bearing capacity as per IS 2911(part-1 section-1). This test is easy to conduct and requires very few site arrangements.
Advantages
As a supplem supp lem ent t o or r eplacem ent f or st at ic t est s, dynam ic t est ing t ak es far less t im e t o perf orm and t hus r educes t he cost s.
Can give bet t er insigh insightt in t er m s of bear ing capacit y of pile
Methodology
The met hodology of t est is based on a large larg e w eight givin ga giving dynamic impact t o t he elast ic body. I t equat es t he energy of hamm er blow t o w ork done in overcoming overcom ing t he resist ance of t he founding foundin g st rat a t o th e penetr ation of t he ordinary cast in - sit u pi pilles es as w ell as grou gr outt ed mi cro p piiles. les. Allow ance is m ade for losses of energ y du e t o t he elast ic com pression pr ession of t he pil e, subsoil sub soil as w ell as losses caused by t he im pact of t he pile.
The given in t he code I .S. 2911 part 1 – sect ion 1 is used in est im atin g t he ult im ate dri vingg resist dr ivin r esist ance in tonnes. From t hat w it h a fact or of safet y as out lined in t he code t he safe load on pile can be w orked out. out .
modified Hiley’ s formula
Dynam Dyn am ic Pil e Test Met hodology hodology::
Continued…..
The in st ant aneous displacement s in clu din g rebounds of t he pile are precisely r ecorded ecord ed in an aut om at ic dat a acquisit ion syst em . This is done f or several cycles & t hen using f orm ulae as in I .S. 2911 , t he safe load capacit y of t he pile is calculated. An opt ical in st ru m ent is used f or posit ion sensit ive m easur em ent by non - contact cont inuous m easurement w it h t he inst rum ent placed aw ay f rom t he vibr at ions due t o im pact load. The syst em is based on com bin ed li ght em it t ing diode t ransm it t ers & a posit ion sensit ive det ect or. The t ransm it t er & receiver are inst alled so t hat t he light beam f orm s a reference line fr om t he t ransm it t er, receiver t o t he prism group reflector.
Conti……..
The refl ect ed light can be r eceived & r ecorded 100 t im es per second. Usin g t he ener gy t ransm it t ed t o t he pile & account ing f or t em por ar y com pr ession of t he pile, gr ound & dolly occur rin g during t he im pact loading t he ult im at e dr ivin g r esist ance is calcul at ed.
MODIFIED HILLEY ’ ’S FORMULA
R = Whn / ( s + c / 2)
(As per I.S.2911)
Where R = Ult im ate drivin g resist ance in tonnes driving W = Mass of Ram in tonnes h = Height of fr ee fall in cm considered at 80% for w inch operat ed drop n = Eff iciency of t he blow represent ing representin g rat io of energy aft er im pact t o st rik ing energy of ram s = Average final set per blow in cm c = Average sum of t empor ary elast ic comp ression com pression Applying or t he pile is t he n Applyin g t he fact or of safet y, the t he safe safe load ffor calculated
Arrangements for pile dynamic test
