20348~t :~~;~i~ ~=~-A ~~* ~~~,f*f ~~i~~~~:~~(~ ~]) 1999 ip g ~NOMINAL CAPACITY OF BORED PHC NODULAR PILES BASED ON
LOADING TEST DATA CONSIDERING LOGNORMAL DISTRIBUTION
IE~~~: O~l~l~]M.
T.
B. Karkee l
H. Ogura 2
Horiguchi 3
H. Kishida 4
1. Introduction
The correlation formulations for vertical capacity ot~
piies based on the loading test data are often based on
the analysis assuming normal distribution of data (e. g.
Horiguchi Karkee, 1995). Recent investigation on adatabase of 5 1 Ioading tests on fully instrumentedbored PHC nodular piles, covering different groundconditions throughout Japan, indicate that lognormal
distribution is better suited for such analysis. The
database consists of siow maintained tests with theincremental loads maintained for 30 minutes, and theloading cycled back to zero incrementally. The pi]e top
applied load Po, top movement So and toe movement Sp
were measured directly. Axial distribution of Po Was
obtained by strain gage measurements. Load Po Was
increased until So reached at least I Oo/o of pile nodule
diameter (D), except when creep defbrmations wereexcessive to make maintaining the load difficult.
The measured resistance (R~) from loading tests wasdefined as Po at S0=0.1D following the Japanese
Geotechnical Society standard. In few cases where
maximum So couid not reach 0.1D, R~ was estimatedby extrapolation based on the hyperbolic curve fitted
to the measured data. The details are given in Karkee
et al. (1 998). The unit shaft resistance in different gagoeinterva]s was obtained from axial distribution of R~.
Standard penetration test N-va]ues and classificationof soi] type constitute the basic information avai]able
for foundation design in practice, particularly in small
to medium size construction projects where the PHC
nodular piles are mostly used. Accordingly, correlation
formulations for the vertical resistance are given interms ofN-va]ues and simple soil type c]assification.
2. Correlation Equations
As the cement grout in bored PHC nodular pi]es isdesigned to have fair]y high strength (Karkee et al.
1998, Horiguchi Karkee 1995), the unit shaft
resistance 1: was computed based on the dril]ed ho]ediameter Db. The unit toe resistance q was based on thenodule diameter D. Soil types within different strain
gage intervals were broad]y classified as sand, clay orhumus, depending on the major content. The averageN-value and the unit sl]aft resistance in the different
interva]s are designated as N=, I\1* or Nh and IE=, T* or Threspective]y, depending on whether the soi] type isc]assified as sand, c]ay or humus. The N-va]ue Np ofthe pile toe was obtained as the average in the region
between I .OD above and I .OD be]ow the noduie near
the iower end of pi]e. The soil in the pile toe region
was most]y sandy type.
Four correlation equations for the nominal va]ues ofq, 11*, T* and 1;h, in terms of Np, N~, N,, and Nhrespectively are developed based on the following
assumptions:(a) Straight line re]ationship of the forrn y=a+bx .
(b) Distribution of the data follows log-normal
distribution given by Equations I , where ~ is themean of InY and ~ is standard deviation of InY, Y
being either q, T*, 1:, or Th, depending on the caseunder consideration. In terms ofthe mean u and thestandard deviation a of Y, the values of ~ and ~ aregiven by Equations 2 and 3 respective]y.
= [ ,~- )J
lr'l ~lnY-~
fy(Y) ~ ~ O < Y < oo (1)I~~ ~"Y exp~ = ~･ (2);~ = In p-~~2 (3)(c) The nominal values of q, Ts' Tc9 and 1:h, designatedas qn' Tsn' 1'cn' and 1:hn respectiveiy, are defined asva]ues that have at least 700/0 confidence of notexceeding the measured value.
(d) The equations for q,,, I,n' xcn' and ~h. are applicablefor certain maximum values of Ns' Nc' Nh and Np'respectively, such that the mean to nominai ratio kR
is at least I .1 as reeommended by Becker ( 1996).
Under these assumptions, the correlation equations
for the respective nominal resistances q~, Tsn' 1:cn' andTh. are given by Equations 4 to 7 (Karkee et al. 1 998).
q~ =155xNp (kPa) , Np ~30 (4)Ts'l = 24.0 + 6.0 x Nsrcn = 24.0 + 5.3 x Nc
Thn 5 O + 5 ̲･ x Nh(k Pa) , N* ~ 30(k Pa) , N. ~ 1 5(k Pa) , N/, ~ 8(5)(6)(7)
3. Measured and Computed Nominal ResistanceNominal resistance R~ of a bored PHC nodular pi]e ofgiven length at a site can be obtained from Equations 4to 7. Ifthe length ofpi]e shaft in meters in contact withsand, clay and ,humus are L.,, L. and Lh respective]y, thenominai resistance R~ in kN is given by Equation 8,'2j~~~~;~~iS;;~ h 'y7, ~~~~f~1~~~~3, ~Cf~'3~1~~~~~i~;;~ h y/. ~~~Professor., Dept. of Architecture and Environment Systems. Akita Prefectural Vniversity
Chief Research Engineer, Technoiogy Deve]opment Division, CEOTOP Corporation
Senior Managing Director, GEOTOP Corporation
Professor, Dept, ofArchitecture, Faculty ofEngineering, Science University ofTokyo
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