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30 12 th International Scientific Conference, April 20-22, 2009 Brno, Czech Republicin Germany /Dürrwang, 1977/. The set of static load tests is still opened. The author presumes thatthe results can become more exact depending on the increase of the number of tests.Limit load-settlement curve of single bored pileSet of 350 static load tests includes bored piles of 0,6 to 1,50 m diameter, length 5,0 to 30,0m. The piles were carried out by the following methods:- method of intermittent unsupported or supported excavation by either casing or stabilizing fluid,- boring with continuous flight auger (CFA).Part of tested piles was fully instrumented, e.g. the distribution of normal stress in pile shaft wasmonitored. The above-mentioned monitoring together with the load-settlement curve describes fullythe pile behaviour /Feda, 1997, Masopust, 1994/. The foundation soil was classified into 10 classesaccording to Table 1 for the purpose of this analysis. In spite of the fact that the geotechnicalconditions in the Czech Republic are rather varied, it was found that the classification given inTable 1 is sufficient for the setting up of the computing model.Computing modelThe relationship between load and deformation of bored pile especially for the settlement greaterthen 5 – 10 mm is distinctly non-linear. It was the reason for the acceptance of rheological model offoundation soil characterized by pressiometric modulus of soil deformation E s and rheologicalcoefficient of soil structure a according to Ménard /1965/. The rheological coefficient a depends onconsistency index and consolidation degree for the cohesion soils; or on relative density for thecohesionless soils and finally on compression strength and stress-strain relationship for the weakrocks. The typical values of a is given in Table 2. for the relevant classes of foundation soils.Mobilization of bored piles skin friction depending on deformation of pile shaft goes according tocurves shown in equation (1).Tab. 1 Classification and properties of foundation rocks and soilsSoilSoil descriptionTypeR 3 Moderately and slightly weathered rocks withσ C = 15 – 20 MPaR 4 Strongly weathered rocks and moderately weatheredweak soils with σ C = 5 – 15 MPaR 5 Weathered and strongly weathered weak rocks withσ C = 1,5 – 5,0 MpaC 10 Cohesive soils with I C ≥ 1,0C 75 Cohesive soils with I C ≅ 0,75C 50 Cohesive soils with I C ≅ 0,5D 9 Cohesionless soils with D r = 0,8 - 0,9D 7 Cohesionless soils with D r = 0,7D 5 Cohesionless soils with D r = 0,5Y Unsuitable soilsσ C – axial compression strength, I C - consistency index,D r – relative densityThe equation of the curves is following:q s = q s,lim [1 – (1 – s/s s,lim ) f(a) ] (1)
12 th International Scientific Conference, April 20-22, 2009 Brno, Czech Republic 31where q s,lim is the limit skin friction of the respective layer of the foundation soil according to theequation (2),s is the settlement of pile head,s s,lim is the limit pile head settlement according to equation (3),f(a) is the function value of the rheological coefficient a according to [7].The limit skin friction is:q s,lim = 0,7.m 1 .m 2 .A.tgh(D/(4.d) (2)and limit settlement of pile in the respective soil layer is:s s,lim = A.g(a,d)/(E s .d 1/2 ) (3)where A is the basic value of skin friction in the respective soil layer determined on the basis ofstatistical analysis of the field static load tests (Table 2),m 1 is the coefficient of the pile installation method,m 2 is the coefficient of the shaft insulation - if there is any,D is the distance from the pile head to the middle of the respective soil layer,d is the pile diameter in the respective soil layer,g(a,d) is the function value of the rheological coefficient a and pile diameter d according to[7].Tab.2 Values of a, f(a), A, X, ZSoil classes a f(a) A/kPa/X/MPa/Z/MPa/R3 0,66 - 0,80 3,017 – 2,500 400 3,70 2,67R4 0,66 - 0,75 3,017 – 2,670 280 2,60 1,89R5 0,66 3,017 200 1,70 1,25C10 0,50 - 0,66 4,500 – 3,017 150 1,35 0,98C75 0,50 – 0,55 4,500 – 3,985 125 0,85 0,62C50 0,50 4,500 90 0,36 0,26D9 0,66 – 0,75 3,017 – 2,670 220 1,82 1,09D7 0,66 3,017 140 1,15 0,69D5 0,55 – 0,60 3,985 – 3,524 110 0,53 0,32The tip resistance q o corresponding to the pile bottom settlement s o = 10 mm for the foundationsoils R5, C10, C75, C50, D9, D7 and D5 is given:q o = m o .(X – Z/(L/d o )) (4)The tip resistance for the weak rocks R4, R3 (according to Table1) is given:q o = m o .(X – Z/(L/d o )).t 1/2 (5)on condition that minimum depth of pile bottom fixed in the rock is t min ≥ 0,5 m,where m o is the coefficient of pile installation method,X, Z are the basic values for the tip resistance in the respective soil layer, determined on thebasis of statistical analysis of the field static load tests (Table 2),L is total length of pile,d o is pile diameter measured at the bottom.
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