Unexpected defects of concrete in a completed bored pile can arise during the construction stage. Therefore, post-construction testing of bored pile concrete is an important part of the design and construction process. The Cross-hole Sonic Logging (CSL) method has been the most widely used to examine the concrete quality. This method requires some access tubes pre-installed inside bored piles prior to concreting; the required quantity of access tubes has been pointed out in few literatures and also ruled in the national standard of Vietnam (TCVN 9395:2012).
Journal of Science and Technology in Civil Engineering NUCE 2020 14 (2): 76–86 DETERMINING THE QUANTITY OF ACCESS TUBES FOR QUALITY CONTROL OF BORED PILE CONCRETE BASED ON PROBABILITY APPROACH Bach Duonga,∗ a Faculty of Hydraulic Engineering, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam Article history: Received 17/02/2020, Revised 01/03/2020, Accepted 17/03/2020 Abstract Unexpected defects of concrete in a completed bored pile can arise during the construction stage Therefore, post-construction testing of bored pile concrete is an important part of the design and construction process The Cross-hole Sonic Logging (CSL) method has been the most widely used to examine the concrete quality This method requires some access tubes pre-installed inside bored piles prior to concreting; the required quantity of access tubes has been pointed out in few literatures and also ruled in the national standard of Vietnam (TCVN 9395:2012) However, theoretical bases aiming to decide the required quantity of access tubes have not been given yet A probability approach is proposed in this paper aiming to determine the essential quantity of access tubes, which depend not only on pile diameters, magnitude of defects, but also on the technical characteristics of CSL equipment Keywords: access tubes; bored piles; CSL method; defects; inspection probability https://doi.org/10.31814/stce.nuce2020-14(2)-07 c 2020 National University of Civil Engineering Introduction Most bored piles are constructed routinely and are sound structural elements However, unexpected defects in a completed bored pile can arise during the construction process through errors in handling of stabilizing fluids, reinforcing steel cages, concrete, casings, and other factors Therefore, tests to evaluate the structural soundness, or “integrity”, of completed bored piles are an important part of bored pile quality control This is especially important where non-redundant piles are installed or where construction procedures are employed in which visual inspection of the concreting process is impossible, such as underwater or under slurry concrete placement [1] From a management perspective, post-construction tests on completed bored piles can be placed into two categories [2]: - Planned tests that are included as a part of the quality control procedure - Unplanned tests that are performed as part of a forensic investigation in response to observations made by an inspector or constructor that indicates a defect might exist within a pile Planned tests for quality control typically are Non-Destructive Tests (NDT) and are relatively inexpensive; such tests are performed routinely on bored piles Meanwhile, unplanned tests will nor∗ Corresponding author E-mail address: duongb@nuce.edu.vn (Duong, B.) 76 Duong, B / Journal of Science and Technology in Civil Engineering mally be more time-consuming and expensive, and the results can be more ambiguous than those of planned tests The most common NDT methods are the Cross-hole Sonic Logging (CSL), the Gamma-Gamma Logging (GGL), and the Sonic Echo (SE) Of these methods, the CSL method is currently the most widely used test for quality assurance of bored pile concrete For this method, vertical access tubes are cast into the pile prior to concrete placement The tubes are normally placed inside the reinforcing steel cage and must be filled with water to facilitate the transmission of high frequency compressive sonic waves between a transmitter probe and a receiver one, which are lowered the same time into each access tube Acoustic signals are measured providing evaluation of concrete quality between the tubes (Fig 1) This method has advantages that are relatively accurate and relatively low cost; by using a suitable number of access tubes, the major portion of pile shaft may be inspected In addition, the testing performance for each acoustic profile is also relatively rapid The limitation of this method is that it is difficult to locate defects outside the line of sight between tubes (a) Scheme of cross-hole sonic logging method (a) Scheme of cross-hole sonic logging method (b) Access tubes placed inside the reinforcing steel cage (b) Access tubes placed inside the (b) Access tubes reinforcing steelplaced cage inside Figure Scheme of cross-hole sonic logging method (a) Scheme of cross-hole sonic logging method the reinforcing Figure Scheme of cross-hole sonic logging method steel cage To detect potential defects by the CSL method, a required number of access tubes has to be preFromThe a management perspective, post-construction tests on completed bored piles Figure of Scheme of cross-hole sonic logging method installed number access tubes for different bored pile diameters has been recommended by can be placed intoand two categories et al [3]): of access tubes recommended in these studies different authors technical codes(Brown [3–9] The number From a mainly management perspective, post-construction tests on completed bored obtained from experimental data and expert experiences, without any theoretical base piles • Planned tests that are included as a part of the quality control procedure Li ettwo al [10] proposed a(Brown probability be placed into categories etapproach al [3]):to determine the number of access tubes The • Unplanned tests that are performed as part of a forensic investigation in response to remarkable advantage of this approach is that the authors formulated a relatively rational manner, observations by ansizes inspector or target that indicates a defect might considering bothmade the defect probability However, theexist shape of the lanned tests that are included as a and partthe ofconstructor theencountered quality control procedure defect is assumed to be spherical and the defect is equally likely located within the pile cross section within a pile UnplannedThis tests that are performed as part of a forensic investigation in response to may lead to an over-prediction of the encountered probability and, therefore, the number of Planned tests for quality control typically are Non-Destructive Tests (NDT) and access tubes trends to be small bservations made by an inspector ortests constructor that indicates on a defect might exist are relatively such approach areis presented, performedto routinely boredprobability piles plays In this paper,inexpensive; another probability which the inspection within a pile a key role The essentialtests quantity access tubes is determined in accordance with a target inspection Meanwhile, unplanned willof normally be more time-consuming and expensive, and probability for different pile diameters and magnitudes of defect, considering the technical characterthe results can be more ambiguous than those of planned tests Plannedistics tests for quality control typically are Non-Destructive Tests (NDT) and of CSL equipment Some findings are also drawn in this paper The most common NDT methods are the Cross-hole Sonic Logging (CSL), the relatively inexpensive; such tests are performed routinely on bored piles Gamma-Gamma Logging (GGL), and the Sonic Echo (SE) Of these methods, the CSL anwhile,method unplanned tests will normally more time-consuming expensive, and is currently the most widely be used test for quality assuranceand of bored pile concrete For this method, vertical are cast into the pile prior to concrete results can be more ambiguous than access thosetubes of 77 planned tests placement The tubes are normally placed inside the reinforcing steel cage and must be The most methods are the Cross-hole Soniccompressive Logging sonic (CSL), the filled common with water NDT to facilitate the transmission of high frequency waves between a transmitter probethe and Sonic a receiver one, (SE) which are the same time mma-Gamma Logging (GGL), and Echo Oflowered these methods, the CSL Duong, B / Journal of Science and Technology in Civil Engineering Number of access tubes in literatures Table shows the recommended number of access tubes for different bored pile diameters according to different authors and technical codes Table Recommended number of access tubes for different bored pile diameters Pile O’Neil Work TCVN Tijou Turner Thasnanipan MOC diameter and Bureau 9395:2012 access tubes, tubes, is is based[3] on one one[4] access tube tube for for each each0.3 0.3 m of pilediameter diameter there [6] [8]Clearly,there access on access Reese [5]for [7]diameter.Clearly, access (mm) tubes, based is based on one access tube each 0.3mmofofpile pile Clearly,[9] there exists an an inconsistency inconsistency in in the the number number of ofaccess accesstubes tubesfor forthe thesame samepile pile diameteradopted adopted exists in number 2of for 3÷4 pilediameter 2 exists600÷750 an inconsistency the access tubes the same diameter adopted by the current practice and no probabilistic analysis has been performed to suggest 750÷1,000 2÷3and no 3÷4 2÷3 analysis3 has been performed 3÷4 2÷3 the by by thethe current practice probabilistic current practice and no probabilistic analysis has been performedtotosuggest suggestthe the 1,000÷1,500 in a 4÷5 4÷5 3÷4 number of access tubes rational manner number of of access tubes in in a rational manner number access tubes a rational manner 1,500÷2,000 4÷5 5÷7 3÷4 Theoretically, the more the number of access tubes, the more precise the 2,000÷2,500 4÷5 7÷8 3÷4 4CSL Theoretically, thethe more thethenumber Theoretically, more numberofofaccess accesstubes, tubes,the themore moreprecise precisethe theCSL CSL 2,500÷3,000 the 4÷5 8 3÷4tubes leads 4 measurement However, overly increasing increasing number ofaccess access toaahigher higher measurement However, the overly number of tubes leads to measurement However, the overly increasing number of access tubes leads to a higher cost and and may may impede impede the the flow flow of of concrete concrete during during pile pile construction construction Therefore, Therefore, aa cost cost and impede theisflow of concrete during pile construction Therefore, a It canmay be seen that there a general trend in which the number of access tubes increases with pertinent number of access tubes to ensure the reliability of CSL measurements pertinent tubes to toensure thetheand reliability the pilenumber diameter, for Work Bureau [7] O’Neill Reese [5]ofpresented, as a rule of thumb pertinent numberofexcept ofaccess access tubes ensure reliability ofCSL CSLmeasurements measurements corresponding to a target probability is very important employed bytoseveral agencies to determine the number of access tubes, is based on one access tube for corresponding a target probability is is very important corresponding to a target probability very important each 0.3 m of pile diameter Clearly, there exists an inconsistency in the number of access tubes for the Shapes Shapes of defect defect adopted by the current practice and no probabilistic analysis has been performed pile diameter of same Shapes of defect to suggest the number of access tubes in a rational manner Assume that that defects defects are are randomly randomly located located atat the the periphery periphery of of piles piles.The Thedefect defect Assume Assume that defects are number randomly located at the periphery piles defect Theoretically, the more the of access tubes, the more preciseofthe CSL The measurement shape normally is observed with some types, which are the annulus, sector, or circular However, the is overly increasing number of access tubes leads to a annulus, higher cost and may impede shape normally observed with some types, which sector, ororcircular shape normally is observed with some types, whichare arethe theannulus, sector, circularthe flow ofasconcrete during pile construction Therefore,ofa pertinent number of access tubesistoequally ensure the segment, depicted in Fig The possibility occurrence of these types segment, as as depicted in in Fig 2 The possibility segment, depicted Fig The possibilityofofoccurrence occurrenceofofthese thesetypes typesisisequally equally reliability of CSL measurements corresponding to a target probability is very important likely However, can bebe seen that the encountered probability ofofthe the first two types likely However, itit can be seen that the encountered probability likely However, it can seen that the encountered probabilityof thefirst firsttwo twotypes typesisis is certainly greater than that of the last type, the circular segment, because the first two certainly greater than that of of thethe last type, certainly greater than that last type,thethecircular circularsegment, segment,because becausethe thefirst firsttwo two Shapes of defect types of defect readily intersect with the signal path as demonstrated in Fig 2a, b For types of of defect readily intersect with thethe signal path types defect readily intersect with signal pathasasdemonstrated demonstratedininFig Fig.2a, 2a,b.b.For For Assume that defects are randomly located at the periphery of piles The defect shape is normally more conservative purpose, thethe defect with the shape ofofcircular circular segment chosen aa more conservative the defect with the shape of segment isisischosen asas a more conservative purpose, defect with the shape circular segment chosen as observed with somepurpose, types, which are the annulus, sector, or circular segment, as depicted in Fig The the examined object in this paper (Fig 2c) possibility of occurrence ofpaper these types is2c) equally thethe examined object in in this (Fig examined object this paper (Fig 2c) likely However, it can be seen that the encountered Access tube Access tube Access tube Signal pathpath Signal Signal path Defect Defect Defect (a) Annulus Annulus Annulus (a) (a) Annulus Defect Defect Defect (b) Sector Sector (b)(b) Sector (b) Sector Defect Defect Defect (c)Circular Cicular segment (c) segment (c) Circular segment (c) Circular segment Figure Shapes of defect located at the periphery of bored pile Figure Shapes defect located theperiphery peripheryof boredpile pile Figure 2 Shapes ofof defect located atatthe the periphery ofofbored bored pile Figure Shapes of defect located 78 at Inspection probability probability Inspection probability The reliability CSL method can describedby bythe theinspection inspectionprobability, probability, The reliability of the CSL method can be described ofof thethe CSL method can bebe described by the inspection probability, Duong, B / Journal of Science and Technology in Civil Engineering probability of the first two types is certainly greater than that of the last type, the circular segment, because the first two types of defect readily intersect with the signal path as demonstrated in Fig 2(a) and 2(b) For a more conservative purpose, the defect with the shape of circular segment is chosen as size(Fig 𝑎 is2(c)) detected if it is indeed encountered; 𝑃' (𝐸) |𝑎) is the encountered prob the examined object in this paper that a defect is encountered by an inspection of a given inspection plan if a defect ( |𝐸 ) Inspection probability exists; and 𝑃+ 𝐸, ) , 𝑎 is the detection probability that an inspection detects a if a defect is indeed encountered The reliability of the CSL method can be described by the inspection probability, which is ex4.1 Encountered probability pressed as a product of the encountered probability and the detection probability: Consider a general case, where a pile has nt access tubes installed ins PI (a) = PE (Ee |a) PD (Ed |Ee , a) (1) reinforcing steel cage as shown in Fig A defect, which is indicated by the where PI (a) is the inspection probability a given defect size segment a; Ee is the event that a defect withThe a defect is area, has a for shape of the circular at the periphery of pile given size a is encountered; Edbyisthe thechord, event that a defect with a given size a is detected if it is indeed EF, and its magnitude is represented by the height of circular segm encountered; PE (Ee |a) is the encountered probability that a defect encountered by an Consider two adjacent access tubes,isi and i + 1, being in inspection the vicinityofwith the def a given inspection plan if a defect indeed exists; and PD (Ed |Ee , a) is the detection probability that an is the chord going through the centers of the access tubes i and i + M is the inspection detects a defect if a defect is indeed encountered point of the chord AB The radius, ON, goes through the middle point, M, 4.1 Encountered probability therefore perpendicular to the chord AB Consider a general case, where a pile has nt access tubes installed inside the reinforcing steel cage as shown in Fig A defect, which is indicated by the shaded area, has a shape of the circular segment at the periphery of pile The defect is located by the chord, EF, and its magnitude is represented by the height of circular segment a Consider two adjacent access tubes, i and i + 1, being in the vicinity with the defect AB is the chord going through the centers of the access tubes i and i + M is the middle point of the chord AB The radius, ON, goes through the middle point, M, and is therefore perpendicular to the chord AB The probability of an event that the defect can Figure diagram Geometrical diagram determining be encountered by the signal path between the ac- Geometrical Figure determining encountered probabilit encountered probability cess tubes, i and i+1, can be determined as a ratio: The probability of an event that the defect can be encountered by the sign ADi and i + 1, can be determined as a ratio: between thePaccess tubes, (2) E (E e |a) = AT 𝐴+ (𝐸the = ) |𝑎)shaded where AD is the cross-sectional area of the defect indicated𝑃'by 𝐴 area in Fig 3; AT is the area of the circular segment located by the chord AB, i.e., the chord goes through the centers of two where, A is the cross-sectional area of the defect indicated by the shaded area in adjacent access tubes AT is the area of the circular segment located by the chord AB, i.e., the chor D2 0.5D −a − a access tubes of two0.5D adjacent AD = arccos through the − centers sin arccos , 150 mm ≤ a ≤ MN (3) 0.5D 0.5D 𝐷 0.5𝐷 − 𝑎 0.5𝐷 − 𝑎 : ?− 𝑠𝑖𝑛 B2𝑎𝑟𝑐𝑐𝑜𝑠 : ?CD , D2 𝐴+ = 42𝑎𝑟𝑐𝑐𝑜𝑠 AM AM 0.5𝐷 AT = arcsin − sin 20.5𝐷 arcsin (4) 0.5D150𝑚𝑚 ≤ 𝑎 ≤ 𝑀𝑁 0.5D 79 D Duong, B / Journal of Science and Technology in Civil Engineering AM = (5) MN (D − MN) MN = 0.5D − (0.5D − 150) cos π nt (6) in which, D is the pile diameter; the number of 150 in Eq (6) is the shortest distance in millimeters 2𝐷 tube to the pile from the center of𝐷access 𝐴𝑀𝐴𝑀shaft perimeter 𝐴𝑀𝐴𝑀 (4) Fig 𝐴 shows the 42𝑎𝑟𝑐𝑠𝑖𝑛 encountered different given 42𝑎𝑟𝑐𝑠𝑖𝑛 − for 𝑠𝑖𝑛 B2𝑎𝑟𝑐𝑠𝑖𝑛 : ?CD?CDof the defects with a(4) : :probability ? −? 𝑠𝑖𝑛 B2𝑎𝑟𝑐𝑠𝑖𝑛 :magnitudes = 0𝐴= 0.5𝐷 0.5𝐷 the relationship between the 8 for a D = 0.5𝐷 number of access tubes 1,000 mm bored pile Fig 0.5𝐷 indicates encounterable magnitude of the defects and the number of access tubes for different pile diameters 𝐴𝑀 −Some 𝑀𝑁)findings can be given below: (5) (5) J𝑀𝑁(𝐷 == 𝑀𝑁) J𝑀𝑁(𝐷 with the target encountered𝐴𝑀 probability, PE = − 0.9 - The encountered probability increases with the magnitude 𝜋 𝜋 of the defect The bored pile D = ( ) 𝑀𝑁 = 0.5𝐷 − 0.5𝐷 − 150 𝑐𝑜𝑠 ( ) 𝑀𝑁 0.5𝐷 − 150number 𝑐𝑜𝑠 of access tubes is three, the encountered 1,000 mm is taken in Fig.=40.5𝐷 as an − example If the (6) (6) L 𝑛L increases from 150 to 325 mm probability increases from 0.34 to 1.0, as the magnitude of𝑛defect - which, For a given magnitude of the defect and a givenof encountered a pile with a greater the pile diameter; number 150 in Eq is the shortest distance in in which, DD is is the pile diameter; thethe number of 150 in Eq 6probability, is6 the shortest distance in in diameter requires a larger number of access tubes to be able to encounter the same magnitude of the millimeters from center of access tube to 300 the pile shaft perimeter millimeters from center ofwith access tube to of the pile shaft perimeter defect From Fig 5,the forthe a defect a magnitude mm and a target encountered probability of 0.9, a bored pile D = 1,000 mm needs access tubes, meanwhile a bored pile D = 2,500 mm needs Fig shows encountered probability different magnitudes of the defects Fig tubes shows thethe encountered probability forfor different magnitudes of the defects up to access with a agiven number access tubes for a D=1,000 bored pile indicates the - For given pile diameter andtubes a given probability, the magnitude of5 the defect with a given number of of access forencountered a D=1,000 mmmm bored pile Fig.Fig indicates thethat canrelationship be encountered decreases asencounterable the number ofmagnitude access tubesof increases However, the of between magnitude of defects the number of relationship between thethe encounterable thethe defects andand themagnitude number of the defect tends to be tangent with a certain value This hints that, for a given pile diameter and a given access tubes different diameters with target encountered probability,E=0.9 PE=0.9 access tubes forfor different pilepile diameters thethe target encountered probability, encountered probability, the required numberwith of access tubes should be limited at a certainPvalue, over Some findings can be given below: which itfindings would becan less Some beefficient given below: 1 D=1000 D=1000 mm mm D=1500 D=1500 mm mm D=2000 D=2000 mm mm D=2500 D=2500 mm mm 0.4 0.4 n =2nt=2 t n =3n =3 0.2 0.2 t t n =4nt=4 12001200 Magnitude of defect (mm) 0.6 0.6 Magnitude of defect (mm) Encountered probability, PE Encountered probability, P E E 0.8 0.8 00 14001400 Target P =0.9 Target P =0.9 E 10001000 800 800 600 600 400 400 200 200 t 200200 400400 600 600 800 800 Magnitude of defect, a (mm) Magnitude of defect, a (mm) 10001000 Figure Encountered probability Figure Encountered probability for boredfor pilefor Figure 4 Encountered probability D = 1,000 mm bored pile D=1,000 mm bored pile D=1,000 mm 0 2 4 6 8 Number of access tubes Number of access tubes 10 10 Figure 5.Encounterable Encounterable magnitudes of the Figure magnitudes of the Figure 5.Encounterable magnitudes ofdefect the versus the number of access tubes defect versus number of access tubes defect versus thethe number of access tubes The encountered probability increases with magnitude • •The encountered probability increases with thethe magnitude of of the defect bored the defect TheThe bored pile D=1,000 mm taken Fig as example If the number of access tubes pile D=1,000 mm is is taken in in Fig as an an example If the number of access tubes is is Once again, we consider a general case where a pile has nt access tubes installed and a defect three,thethe encountered probability increases from 0.34 to 1.0, as the magnitude of three, probability increases to 1.0, as the the magnitude of the indicated by a encountered shaded area has a position as shown infrom Fig 6.0.34 Let point H be middle point of defect increases from 150 325 mm chord EF increases The segment, OL,150 going through the middle point H is perpendicular to the chord EF and defect from to to 325 mm divides the defect into two equal parts Therefore, segment OL can be used as a location segment For a given magnitude defect a given encountered probability, a pile • •For a given magnitude of of thethe defect andand a the given encountered probability, a pile withwith a greater diameter requires a larger number of access tubes to be able to encounter a greater diameter requires a larger number 80 of access tubes to be able to encounter same magnitude defect From 5, for a defect with a magnitude of 300 thethe same magnitude of of thethe defect From Fig.Fig 5, for a defect with a magnitude of 300 mm and a target encountered probability a bored D=1,000 needs mm and a target encountered probability of of 0.9,0.9, a bored pilepile D=1,000 mmmm needs 3 access tubes, meanwhile a bored pile D=2,500 mm needs up to access tubes 4.2 Detection probability Therefore, the segment OL can be used as a location segment of the defect position, it represents the relative position of the defect compared to the two adjacent access tubes i and i + Let point S be the intersection of the chord EF and the chord AB, and point Duong, B / Journal of Science and Technology in Civil Engineering T be the center of access tube i It can be seen that the segment ST represents the length of the defect position, it represents the relative position of the defect compared to the two adjacent of the secant between the defect and the sonic signal path, which is formed from the access tubes i and i + Let point S be the intersection of the chord EF and the chord AB, and point center to center of two access i and i +that Obviously, when the magnitude T be the center of access tubetubes i It can be seen the segment ST represents the length ofor thethe secant between defect changes, and the sonic path,ST which is formed from the center toThis centerhints of twothat access position of thethedefect thesignal secant changes correspondingly tubes i and i + Obviously, when the magnitude or the position of the defect changes, the secant the length of the secant ST can be used as a parameter representing the detection ST changes correspondingly This hints that the length of the secant ST can be used as a parameter capability of defect respect to theofCSL method Thus, termmethod of detection length representing thewith detection capability defect with respect to the the CSL Thus, the term of detection length is used instead of the length of the secant is used instead of the length of the secant A F O t Tube i+1 M S H T B K Tube i L E N Defect Figure Geometrical diagram determining detection probability Figure Geometrical diagram determining detection probability Let point K be the intersection of the segment OT and the perimeter of the pile The angle ω, Let point K be the intersection of the segment OT and the perimeter of the pile determined by the segment OL and the segment OK, is used as the location angle of the defect The angle by the segment OL andcross the section segment OK,and is the used as the location Since𝜔,thedetermined symmetric performance of the circular of pile access tubes are equally arranged along the reinforcing cage, the variation of the location ω, from zerosection to an angle angle of the defect Since the symmetric performance of theangle, circular cross of of π/nt radians is sufficient to describe all positions of the defect compared to that of the access tubes i pile and the access tubes are equally arranged along the reinforcing cage, the variation and i + of the location angle, length 𝜔, from zero an angle 𝜋/nt radians is sufficient to describe The detection ST can be to determined as of follows: all positions of the defect compared to that of π the accessπtubes i and i +1 π (0.5D − 150) cos Detection length = nt + sin ω sin nt − ω − (0.5D − a) cos π π sin − ω cos −ω nt nt nt −ω (7) here, all parameters are the same as those in Eqs (3) to (6) Note that, a is the magnitude of defect, a = HL Fig shows the variation of the detection length with the location angle of the defect for a given bored pile Here, the pile has a diameter of 1,200 mm, the number of access tubes is assumed as 3, and the magnitude of defect is supposed to be 370 mm As a result, when the location angle, ω, varies from zero to π/3 radians, the detection length gradually increases from a value of 254 mm and reaches 81 𝜋/3 radians For a more practical side, we assume that there exists a detection thresh which a CSL test may not detect a defect In this case, a detection threshold is for instance, asand 300 mm In in Fig 7,Engineering we see that when the location angle lies in Duong, B / Journal of Science Technology Civil from 0.350 to 0.985 radians, the detection length is greater than or equal to the a maximum value of 342 mm, and then decreases down to -∞, as the location angle approaches the threshold value of π/3 radians Detection length, (mm) 600 For a more practical side, we assume that there exists a detection threshold, under which a CSL 500 test may not detect a defect In this case, a detecDetection threshold=300 mm tion threshold is assigned, for instance, as 300 mm 400 In Fig 7, we see that when the location angle lies 300 in the range from 0.350 to 0.985 radians, the detection length is greater than or equal to the detection 200 threshold w=0.350 w=0.985 w=p/3 100 When the location angle lies outside this range, a CSL test may not detect the defect This 0 0.2 0.4 0.6 0.8 1.2 1.4 issue hints at a way to determine the detection Location angle, w (radian) probability for a given magnitude of defect as: Figure Geometrical diagram determining detection probability Figure Geometrical diagram determining nD detection probability PD (Ed |Ee , a) ≈ (8) When the location angle lies outside this range, a CSL test may not nω defect This issue hints at a way to determine the detection probability fo where PD (Ed |Ee , a) is the detection probability; nD is the number of values of ω, for which the defect as:threshold; n is the total number of values of detection length is greater than ormagnitude equal to theofdetection ω 𝑛+ ω, being taken from the range of zero to π/nt 𝑃+ (𝐸, |𝐸) , 𝑎) ≈ A question arising herein is, how much is the detection threshold, so that 𝑛Za CSL test really detects defects In some literatures, the minimum detectable defect diameter is 249 mm (e.g., [11]) and 201 mm (e.g., [12]) Amir and Amir [13] presented detection thresholds with respect to different emitter frequencies and wavelengths of the ultrasonic signal as shown in Table Table Detection threshold of CSL test Technical characteristics Unit Frequency Wavelength Detection threshold kHz mm mm Values 20 210 420 30 140 280 50 84 168 100 42 84 In Table 2, the frequency of 50 kHz and wavelength of 84 mm are adopted, since these values commonly selected in practice, the detection threshold is obtained as 168 mm This detection threshold is clearly smaller than that presented above by [11, 12] For conservative purposes, a detection threshold of 200 mm is adopted for this study Fig shows the detection probability for different magnitudes of defect with a given number of access tubes for a D = 1,500 mm bored pile Some comments can be drawn: - The detection probability increases with the magnitude of defect If the number of access tubes is 3, the detection probability increases from zero to 1.0, as the magnitude of defect increases from 311 to 443 mm - For a given target detection probability, the magnitude of defect that can be detected decreases as the number of access tubes increases For a target detection probability of 0.9, the magnitude of defect that can be detected decreases from 690 down to 260 mm as the number of access tubes increases from to tubes 82 n Table 2, the frequency of 50 kHz and wavelength of 84 mm are adopted, since Fig 9threshold shows the for different magnitudes of defect w lues commonly selected in practice, the detection is inspection obtained asprobability 168 number of access tubesbyforHassan a D=2,000 mm bored pile Basically, comments is detection threshold is clearly smallergiven than that presented above respect purposes, to the inspection ill [4] and Iskander et al [5] For conservative a detectionprobability threshold are the same as those for the encount Duong, B / Journal of Science and Technology in Civil Engineering probability and the detection probability as discussed in the previous subsections mm is adopted for this study 1 Target P =0.9 Target P =0.9 I Inspection probability, PI Detection probability, PD D 0.8 0.6 0.4 n =2 t n =3 t 0.2 n =4 t 0.8 0.6 n =2 0.4 t n =3 t n =4 t 0.2 n =5 t n =5 0 n =6 t 200 400 600 800 t 0 1000 Magnitude of defect, a (mm) 200 400 600 800 1000 1200 1400 Magnitude of defect, a (mm) Figure Figure Detection probability for bored pile D=1,500 mm Detection probability for bored pile Figure 9.probability Inspection probability pile mm Figure Inspection for bored for pilebored D=2,000 D = 1,500 mm D = 2,000 mm ig shows the detection probability for different magnitudespurposes, of defectawith For illustrative casea study is considered A testing bored pile umber of access tubes for a D=1,500 mm bored pile Some comments can bemm bored pile with arranged access tubes conducted by ADCOM [1], a D=1,400 4.3 Inspection probability tested at a foundation of a collective building in Hanoi, Vietnam A fatal defect The encountered probability and the subsecdetected bydetection the CSLprobability method atare a analyzed depth of separately about 3.0 in m,the and then the constru tions 4.1 and 4.2 In this subsection, a combination of two probability is considered, aimingdefect aimin excavated the soil surrounding the pile measures to the depth of the suspected to determine the inspection probability using Eq (1) perform a visually-checked work As a result, a defect in a typical shape of circ Fig shows the inspection probability different of magnitudes defect a given segment with afor magnitude about 400of mm was with exposed (see number Fig 10) Fig 11 sh of access tubes for a D = 2,000 mm bored pile Basically, comments with respect to the inspection the inspection probability proposed by this paper for a bored pile D=1,400 mm, w probability are the same as those for the encountered probability and the detection probability as has the same diameter as that of the pile tested in the field It can be seen that, f discussed in the previous subsections of 400Amm, if 3bored accesspile tubes used, thebyinspection probab For illustrative purposes, amagnitude case studyofisdefect considered testing wasare conducted [14], a D = 1,400 mm bored pile with arranged access tubes was tested at a foundation of a collective building in Hanoi, Vietnam A fatal defect was detected by the CSL method at a depth of about 3.0 m, and then the constructor excavated the soil surrounding the pile to the depth of the suspected defect aiming to perform a visually-checked work As a result, a defect in a typical shape of circular segment with0.85 a magnitude of about wastubes exposed Fig.used, 10) 11inspection shows the inspection reaches Meanwhile, ifaccess 4mmaccess tubes are the probability reaches 0.85 Meanwhile, if 4400 are(see used, theFig inspection probability is probability proposed by this paper for a bored pile D = 1,400 mm, which has the same diameter as is obtained as 1.0, defect is detected certainty is true for case the case obtained as 1.0, i.e.,i.e., the the defect is detected withwith certainty ThisThis is true for the considered considered (a) Soil occupied the pile shaft (a) Soil occupied the pile shaftshaft (a) Soil occupied the pile (b) Defect exposed after excavating (b) Defect exposed after after excavating (b) Defect exposed excavating Figure 10 Defect in shape of circular segment [14] Figure 10 10 Defect in shape of circular segment (ADCOM [1]) [1]) Figure Defect in shape of circular segment (ADCOM 1 83 TargetTarget P =0.9P =0.9 I I ility, P I bility, P I 0.8 0.8 P =0.85 P =0.85 I I (a) Soil occupied the pile shaft (b) Defect exposed after excavating Figure 10.Duong, Defect shape of circular segment (ADCOM B /in Journal of Science and Technology in Civil Engineering[1]) Target P =0.9 Inspection probability, PI I 0.8 P =0.85 I 0.6 0.4 n =2 t 0.2 n =3 t n =4 0 t 200 400 600 800 1000 Magnitude of defect, a (mm) 11 Inspection probability bored pile = 1,400 mm Figure 11.Figure Inspection probability forforbored pileDD=1,400 mm Essential quantity of access tubes in this paper that of the pile tested in the field It can be seen that, for a magnitude of defect of 400 mm, if access tubes are used, theanalyses inspectionabove, probability reaches 0.85 Meanwhile, if of access tubes are used, Based on the it can be seen that the number access tubes is an the inspection probability is obtained as 1.0, i.e., the defect is detected with certainty This is true for the important factor and strongly affects, not only on the measurement results of the CSL case considered method, but also the construction costs of bored pile foundations Particularly, in cases where there is aquantity very large number piles to be used in foundations Thus, the Essential of access tubesofinbored this paper number of access tubes needs to be addressed pertinently, so that they assure technicoBased on the analyses above, it can be seen that the number of access tubes is an important factor economical requirements in the stage of design and strongly affects, not only on the measurement results of the CSL method, but also the construction costs of bored pile foundations Particularly, in cases where there is a very large number of bored piles to be used in foundations Thus, the number of access tubes needs to be addressed pertinently, so that they assure technico-economical requirements in the stage of design This section is used to synthesize the essential quantity of access tubes for different diameters of bored piles and different magnitudes of defect The target inspection probability is assigned as 0.99 The recommended number of access tubes is indicated in Table Through this table, several comments can be drawn: - For the target inspection probability of 0.99, the detectable minimum magnitude of defect decreases with the increase of the number of access tubes to be used However, the magnitude of defect tends to be tangent with a value of approximately 200 mm, regardless of the pile diameters This value can be considered as a minimum magnitude of defect, under which the CSL test cannot detect the defect (see more in Fig 5) - With respect to the pile diameter in the range from 600 to 3,000 mm and the target inspection probability of 0.99, eight (8) access tubes can be considered as the maximum number of access tubes that can be used when the CSL method is required - Through Table 3, for a given pile diameter, a suitable number of access tubes can be selected based on the detectable minimum magnitude of defect, if a designer supposes that this magnitude of defect may adversely affect the safety degree of bored pile foundations 84 Duong, B / Journal of Science and Technology in Civil Engineering Table Detectable minimum magnitudes of defect (in mm) according to pile diameters and number of access tubes with target inspection probability of 0.99 Pile diameter (mm) nt = nt = nt = nt = nt = nt = nt = 600 750 1,000 1,200 1,500 2,000 2,500 3,000 349 375 497 596 744 992 1,241 1,489 293 305 324 372 447 571 695 819 257 265 279 288 324 397 469 541 237 242 251 257 268 312 359 406 225 228 234 239 245 262 296 329 215 216 221 225 229 237 257 282 209 209 213 214 218 223 232 251 Conclusions This paper has proposed a probability approach for determining the essential quantity of access tubes in quality control of bored pile concrete when using the CSL method The encountered probability, detection probability, and inspection probability for the CSL method are formulated Based on the inspection probability, the quantity of access tubes is recommended to designers of bored pile foundations Some findings can be given from the paper: - The quantity of access tubes depends on pile diameters, magnitude of defects needed to detect, and the technical characteristics of CSL equipment - The value of 200 mm can be considered as a minimum magnitude of defect in shape of circular segment, under which the CSL test cannot detect - Eight access tubes can be considered as the maximum number of access tubes that can be used when the CSL method is required References [1] Duong, B., van Gelder, P (2016) Calibrating resistance factors under load and resistance factor design method (LRFD) using Monte-Carlo simulation Journal of Science and Technology in Civil Engineering (STCE)-NUCE, 10(5):79–87 [2] Brown, D A., Turner, J P., Castelli, R J., Americas, P B (2010) Federal highway administration - Report No FHWA-NHI-10-016: Drilled shafts: Construction procedures and LRFD design methods Technical report, National Highway Institute, Federal Highway Administration, US Department of Transportation, Washington, D.C [3] Tijou, J C (1984) Integrity and dynamic testing of deep foundations-recent experiences in Hong Kong (1981-1983) Hong Kong Engineer, 12(9):15–22 [4] Turner, M J (1997) Integrity testing in piling practice Technical report, CIRIA report 144 Construction Industry Research and Information Association, London [5] O’Neill, M W., Reese, L C (1999) Federal highway administration - Report No FHWA-IF-99-025: Drilled shafts: Construction procedures and design methods Technical report, National Highway Institute, Federal Highway Administration, US Department of Transportation, Washington, D C [6] Thasnanipan, N., Maung, A W., Navaneethan, T., Aye, Z Z (2000) Non-destructive integrity testing on piles founded in Bangkok subsoil In Proceedings of 6th International Conference on the Application of Stress-wave Theory to Piles, AA Balkema, Rotterdam, 171–177 [7] Work Bureau (2000) Enhanced quality supervision and assurance for pile foundation works Works Bureau Technical Circular No 22/2000, Hong Kong 85 Duong, B / Journal of Science and Technology in Civil Engineering [8] MOC (2003) Technical code for testing of building foundation piles (JGJ 106-2003) Ministry of Construction, Beijing, China [9] TCVN 9395:2012 Bored pile - Construction, check and acceptance National standard of Vietnam [10] Li, D Q., Zhang, L M., Tang, W H (2005) Reliability evaluation of cross-hole sonic logging for bored pile integrity Journal of Geotechnical and Geoenvironmental Engineering, 131(9):1130–1138 [11] Hassan, K M., O’Neill, M W (1998) Final report, phase I: Structural resistance factors for drilled shafts with minor defects Technical report, Department of Civil and Environmental Engineering, University of Houston [12] Iskander, M., Roy, D., Ealy, C., Kelley, S (2001) Class-A prediction of construction defects in drilled shafts Transportation Research Record, 1772(1):73–83 [13] Amir, J M., Amir, E I (2008) Critical comparison of ultrasonic pile testing standards In Proceedings of 8th International Conference on the Application of Stress-wave Theory to PilesProceedings of 8th International Conference on the Application of Stress-wave Theory to Piles, Lisbon, Portugal, 453–457 [14] ADCOM (2008) Report on Sonic & PIT tests for bored piles at Van Khe collective building, Hanoi, Vietnam Technical report, ADCOM consultants 86 ... [7]diameter.Clearly, access (mm) tubes, based is based on one access tube each 0.3mmofofpile pile Clearly,[9] there exists an an inconsistency inconsistency in in the the number number of ofaccess accesstubes tubesfor... Theoretically, the more the number of access tubes, the more precise the 2,000÷2,500 4÷5 7÷8 3÷4 4CSL Theoretically, thethe more thethenumber Theoretically, more numberofofaccess accesstubes, tubes ,the themore... 251 Conclusions This paper has proposed a probability approach for determining the essential quantity of access tubes in quality control of bored pile concrete when using the CSL method The encountered