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OCR for page 85
85
F3 [kN] [kip]
0.16
PeE1.2 - e3=-0.0225m
PeE1.4 - e3=0.0225m
0.03 PeE1.3 - e3=-0.0225m
PeE1.5 - e3=0.0225m
0.12 MoE1.4 - e3=0.01m
MoE1.1 - e3=-0.01m
0.02 PeE - steplike
MoE - radial
0.08
0.01
0.04
u3 [in] 0.12 0.08 0.04 0 0.04 0.08 0.12 F1 [kip]
0 0
u3 [mm] 4 3 2 1 0
0 0.2 0.4 0.6 F1 [kN]
0.4
0.02
0.8
0.04
1.2
0.06
1.6
u1 [mm] [in]
Figure 73. Loaddisplacement curves for inclined-eccentric loading with different loading directions
utilizing data from Perau (1995) and Montrasio (1994).
the analysis also reveals that the gain of capacity is relatively provides detailed examples for the calculations performed for
small, and, for vertical load levels greater than or equal to 0.3, each analysis. Sections G.5 and G.6 relate to the utilization
the effect of loading direction is negligible. of Goodman's (1989) method, and Section G.7 relates to the
utilization of Carter and Kulhawy's (1988) method in the
traditional way (i.e., using Equation 82a). This section sum-
3.8 Uncertainty in the Bearing marizes the results of the analyses for the examined methods:
Capacity of Footings in/on Rock the semi-empirical mass parameters procedure developed by
Carter and Kulhawy (1988) and the analytical method pro-
3.8.1 Overview
posed by Goodman (1989).
The ratio of the measured/interpreted bearing capacity to The consistency of the rocks in the database, the types
the calculated shallow foundation bearing capacity (the bias ) of foundation, and the level of knowledge of the rock were
was used to assess the uncertainty of the selected design categorized, when applicable, while examining their influ-
methods for the 119 case histories of database GTR-UML ence on the bias. In addition, histograms and PDFs of the
RockFound07. Section 1.7 details the methods of analysis bias obtained by the different methods are presented and
selected for the bearing capacity calculations. Appendix G discussed.
OCR for page 86
86
F1/F10= 0.08
F1/F10= 0.13
F1/F10= 0.145
0.12
F1/F10= 0.26
F1/F10= 0.33
F1/F10= 0.41
0.1 F1/F10= 0.55
F1/F10= 0.08
F1/F10= 0.13
0.08 F1/F10= 0.15
F1/F10= 0.26
F1/F10= 0.33
F2/ F10
F1/F10= 0.5
0.06
0.04
0.02
0
-0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 0.12
M3/(F10·b2)
Figure 74. Influence of loading direction on capacity in the case of inclined-eccentric loading (Lesny, 2001).
3.8.2 Carter and Kulhawy's (1988)
100000
Semi-Empirical Bearing qL2 = 16.14 (qult)0.619
Capacity Method (n = 119; R2 = 0.921)
qL2 = 36.51 (qult)0.600 (Revised)
3.8.2.1 Presentation of Findings
Interpreted Foundation Capacity qL2 (ksf)
10000 (n = 119; R2 = 0.917)
qL2 = qult
Carter and Kulhawy's (1988) method is described in Sec-
tion 1.7.6 and its application is demonstrated in Section G.7
1000
in Appendix G. Table E-2 of Appendix E presents the calculated
bearing capacity values and the associated bias for each of
the 119 case histories of database UML-GTR RockFound07
(Table E-2 includes all 122 original cases and the excluded 100
3 cases as noted). The relationships between the bearing
capacities (qult) calculated using the two Carter and Kulhawy
(1988) semi-empirical procedures (Equation 82a and the 10 58 Footing cases
revised relations given by Equation 82b) and the interpreted 61 Rock Socket cases
119 All cases with
bearing capacity (qL2) are presented in Figure 75. Equation 109a revised equation
provides the best fit line generated using regression analysis 1
of all data using Equation 82a and results in a coefficient of 0.01 0.1 1 10 100 1000 10000 100000
determination (R2) of 0.921. Equation 109b represents the Carter and Kulhawy (1988) Bearing Capacity qult (ksf)
best fit line generated using regression analysis of all data using Figure 75. Relationship between calculated bearing
Equation 82b for calculating the bearing capacity and results capacity (qult) using two versions of Carter and
in a coefficient of determination (R2) of 0.917. Kulhawy (1988) and interpreted bearing capacity (qL2 ).
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87
Table 37. Summary of the statistics for the ratio of measured (qL2) to calculated bearing
capacity (qult) for all foundations on rock using the Carter and Kulhawy (1988) method.
Cases n No. of sites m COV
All (measured qu) 119 78 8.00 9.92 1.240
Measured discontinuity spacing (s) 83 48 8.03 10.27 1.279
Fractured with measured discontinuity spacing (s) 20 9 4.05 2.42 0.596
All non-fractured 99 60 8.80 1066 1.211
Non-fractured with measured discontinuity spacing (s) 63 39 9.29 11.44 1.232
Non-fractured with s based on AASHTO (2007) 36 21 7.94 9.22 1.161
n = number of case histories, m = mean of biases, = standard deviation, COV = coefficient of variation
Calculated capacity based on Equation 82a
qL 2 = 16.14 ( qult )
0.619
(109a) (measured bearing capacity, qL2, to calculated bearing capacity,
qult) using Carter and Kulhawy's (1988) semi-empirical method
qL 2 = 36.51 ( qult )
0.600
(109 b) are summarized in Table 37. In Table 37, the statistics are
categorized according to the joint spacing and the source of
It can be observed in Figure 75 (and in Equations 109a and the data (i.e., measured discontinuity spacing versus spacing
109b) that the revised expression provided by Equation 82b assumed based on the specifications). In Table 38, the data
gives systematically higher resistance biases than those biases are subcategorized according to type of foundation (footings
obtained using Equation 82a. The bias mean and COV obtained versus rock sockets) and the source of the joint spacing data.
using Equation 82b for all data (n = 119) are found to be 30.29 Table 39 is a summary of the statistics for the ratio of the
and 1.322, respectively, versus 8.00 and 1.240, respectively, measured bearing capacity (qL2) to calculated bearing capacity
obtained using Equation 82a. Both relations provide close to (qult) categorized according to foundation type and rock quality
parallel lines when compared to the measured capacities. Equa- ranges for each type and all types combined.
tions 109a and 109b suggest that Equation 82b roughly predicts The distribution of the ratio of the interpreted bearing capac-
half the capacity of Equation 82a as its multiplier to match the ity to the calculated bearing capacity (the bias ) for the 119 case
measured capacity is about double. As the relations pro- histories (detailed in Table E-2 of Appendix E) is presented in
vided by Equation 82a are already consistently conservative, Figure 76. The distribution of the bias has a mean (m) of 8.00
Equation 82a is preferred over Equation 82b, and the results and a COV of 1.240 and resembles a lognormal random vari-
processed and analyzed are those obtained using Equation 82a. able. The distribution of the bias for foundations on fractured
Statistical analyses were performed to investigate the effect rock only (20 cases) is presented in Figure 77 and has an m of
of the joint or discontinuity spacing (s) either measured or 4.05 and a COV of 0.596. The distribution of the bias for the
determined based on AASHTO (2008) tables (see Section 1.8.3) foundations on fractured rock resembles a lognormal random
and the effect of the friction angle (f) of the rock on the variable and has less scatter, reflected by the smaller COV when
calculated bearing capacity. Statistics for the ratio of the bias compared with the distribution of for all 119 case histories.
Table 38. Summary of the statistics for the ratio of measured (qL2) to calculated bearing capacity (qult)
of rock sockets and footings on rock using the Carter and Kulhawy (1988) method.
Cases n No. of sites m COV
All rock sockets 61 49 4.29 3.08 0.716
All rock sockets on fractured rock 11 6 5.26 1.54 0.294
All rock sockets on non-fractured rock 50 43 4.08 3.29 0.807
Rock sockets on non-fractured rock with measured discontinuity spacing (s) 34 14 3.95 3.75 0.949
Rock sockets on non-fractured rock with s based on AASHTO (2007) 16 13 4.36 2.09 0.480
All footings 58 29 11.90 12.794 1.075
All footings on fractured rock 9 3 2.58 2.54 0.985
All footings on non-fractured rock 49 26 13.62 13.19 0.969
Footings on non-fractured rock with measured discontinuity spacing (s) 29 11 15.55 14.08 0.905
Footings on non-fractured rock with s based on AASHTO (2007) 20 11 10.81 11.56 1.069
n = number of case histories, m = mean of biases, = standard deviation, COV = coefficient of variation
Calculated capacity based on Equation 82a
OCR for page 88
88
Table 39. Summary of the statistics for the ratio of measured (qL2) to
calculated bearing capacity (qult) using the Carter and Kulhawy (1988)
method categorized by the rock quality and foundation type.
Foundation
Cases n No. of sites m COV
Type
RMR > 85 23 23 2.93 1.908 0.651
65 < RMR < 85 57 36 3.78 1.749 0.463
All
44 < RMR < 65 17 10 8.83 5.744 0.651
3 < RMR < 44 22 9 23.62 13.550 0.574
RMR > 85 16 16 3.42 1.893 0.554
Rock 65 < RMR < 85 35 24 3.93 1.769 0.451
Sockets 44 < RMR < 65 9 8 6.82 6.285 0.921
3 < RMR < 44 1 1 8.39 -- --
RMR > 85 7 7 1.81 1.509 0.835
65 < RMR < 85 22 13 3.54 1.732 0.489
Footings
44 < RMR < 65 8 5 11.09 4.391 0.396
3 < RMR < 44 21 8 24.34 13.440 0.552
n = number of case histories, m = mean of biases, = standard deviation, COV = coefficient of variation
Calculated capacity based on Equation 82a
3.8.2.2 Observations higher than those obtained using Equation 82a, with very
similar COVs. As both equations are by and large conser-
The presented findings of Carter and Kulhawy's (1988) vative, only the traditional equation (Equation 82a) was
methods for the prediction of bearing capacity suggest the used for further analysis and method evaluation.
following: 2. The method (Equation 82a) substantially underpredicts
(on the safe side) for the range of capacities typically lower
1. The bias of the estimated bearing resistances obtained using than 700 ksf. The bias increases as the bearing capacity
the revised equation (Equation 82b) are systematically decreases. This provides a logical trend in which founda-
0.15 5 0.25
119 Rock sockets and Footing cases 20 Foundation cases on Fractured Rocks
Carter and Kulhawy (1988) Carter and Kulhawy (1988)
16
mean = 8.00 mean = 4.05
COV = 1.240 4 0.2
COV = 0.596
Number of observations
Number of observations
12 0.1
3 0.15
Frequency
Frequency
lognormal
lognormal distribution
8 distribution
2 normal 0.1
normal distribution
0.05
distribution
4 1 0.05
0 0 0 0
0 4 8 12 16 20 24 28 32 36 40 44 48 52 0 1 2 3 4 5 6 7 8 9 10
Bias, = qu,meas / qu,calc Bias, = qu,meas / qu,calc
Figure 76. Distribution of the ratio of the interpreted Figure 77. Distribution of the ratio of the interpreted
bearing capacity (qL2) to the bearing capacity (qult ) bearing capacity (qL2) to the bearing capacity (qult )
calculated using Carter and Kulhawy's (1988) method calculated using Carter and Kulhawy's (1988) method
(Equation 82a) for the rock sockets and footings in (Equation 82a) for foundations on fractured rock in
database UML-GTR RockFound07. database UML-GTR RockFound07.
OCR for page 89
89
tions on lower bearing capacity materials are provided with 3.8.3 Goodman's (1989) Analytical
a higher margin of safety while for foundations on harder Bearing Capacity
rock with higher bearing capacities, the bias is smaller than
3.8.3.1 Presentation of Findings
one (1.0) (i.e., measured capacities are lower than calculated
capacities). The bearing capacity values on the higher Goodman's (1989) method is described in Section 1.7.5
capacity sides are controlled by the strength of the foun- and its application is demonstrated in Sections G.5 and G.6
dation material (i.e., concrete), and, therefore, the results of Appendix G. Table E-3 of Appendix E presents the calculated
in that range are not necessarily translated into unsafe bearing capacity values for each of the 119 case histories. The
practice. relationship between the bearing capacity calculated using
3. Comparison of the statistics obtained for shallow foun- Goodman's (1989) analytical procedure (qult) and the inter-
dations (n = 58, m = 11.90, COV = 1.075 and number preted bearing capacity (qL2) is presented in Figure 78. Equa-
of sites = 29) with the statistics obtained for rock sockets tion 110 represents the best fit line that was generated using
(n = 61, m = 4.29, COV = 0.716 and number of sites = 49) regression analysis and resulted in a coefficient of determination
may suggest that the method better predicts the capacity (R2) of 0.897.
of rock sockets than the capacity of footings. This obser-
qL 2 = 2.63 ( qult )
0.824
vation might also suggest that the use of load-displacement (110)
relations for the tip of a loaded rock socket is not analogous
Statistical analyses were performed to investigate the effect
to the use of load-displacement relations for a shallow
of the measured and AASHTO-based joint (2007) or dis-
foundation constructed below surface; hence, the data
continuity spacing (s) and friction angle (f) of the rock on
related to the tip of a rock socket should not be employed the bearing capacity calculations. Table 40 summarizes the
for shallow foundation analyses. This observation must be statistics for the ratio of the measured bearing capacity (qL2)
re-examined in light of the varied bias of the method with to calculated bearing capacity (qult) using Goodman's (1989)
the rock strength, as is evident in Figure 75 and detailed in analytical method for the entire database. Table 41 provides the
Table 39. The varying bias of the method, as observed in statistics for subcategorization based on foundation type and
Figure 75 and described in Number 2 above, results in a available information. Table 42 is a summary of the statistics
relatively high scatter (COV = 1.240 for all cases). When the for the ratio of the measured bearing capacity (qL2) to the
evaluation is categorized based on rock quality, the scatter calculated bearing capacity (qult) categorized according to
(COV) systematically decreases to be between about 0.5 foundation type and rock quality ranges for each type.
to 0.6, as detailed in Table 39. However, the changes in
the mean of the bias with rock quality for the footings are
100000
much more pronounced than the changes for the rock qL2 = 2.16 × (qult)0.868
sockets because most of the footings were tested on rock (n = 119; R2 = 0.897)
that was of lower quality than the rock existing at the tip of qL2 = qult
Interpreted Foundation Capacity qL2 (ksf)
10000
the rock sockets. For example, of the 22 cases of the lowest
rock quality (3 RMR < 44), 21 cases involved a shallow
foundation and 1 case involved a rock socket. In contrast,
of the 23 cases of the highest quality rock (RMR 85), 1000
only 7 cases involve footings and 16 cases involve rock
sockets. The conclusion, therefore, is that the variation
in the method application is more associated with the rock 100
type/strength and its influence on the method's predic-
tion than the foundation type. This conclusion is further
confirmed by examination of the Goodman (1989) method, 10
in which the bias is not affected by rock quality and, hence,
58 Footing cases
similar statistics are obtained for the rock socket and the 61 Rock Socket cases
footing cases. 1
4. No significant differences exist between the cases for 1 10 100 1000 10000 100000
which discontinuity spacing (s) was measured in the Goodman (1989) Bearing Capacity qult (ksf)
field and the cases for which the spacing was deter- Figure 78. Relationship between Goodman's (1989)
mined based on generic tables utilizing rock description calculated bearing capacity (qult) and the interpreted
(Tables 37 and 38). bearing capacity (qL2)
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90
Table 40. Summary of the statistics for the ratio of measured (qL2) to calculated bearing capacity (qult)
of rock sockets and footings on rock subcategorized by data quality using the Goodman (1989) method.
Cases n No. of sites m COV
All 119 78 1.35 0.72 0.535
Measured discontinuity spacing (s) and friction angle ( f) 67 43 1.51 0.69 0.459
Measured discontinuity spacing (s) 83 48 1.43 0.66 0.461
Measured friction angle ( f) 98 71 1.41 0.76 0.541
Fractured 20 9 1.24 0.34 0.276
Fractured with measured friction angle ( f) 12 7 1.33 0.25 0.189
Non-fractured 99 60 1.37 0.77 0.565
Non-fractured with measured s and measured f 55 37 1.55 0.75 0.485
Non-fractured with measured discontinuity spacing (s) 63 39 1.49 0.72 0.485
Non-fractured with measured friction angle ( f) 86 64 1.42 0.81 0.569
Spacing s and f, both based on AASHTO (2007) 5 3 0.89 0.33 0.368
Discontinuity spacing (s) based on AASHTO (2007) 36 21 1.16 0.83 0.712
Friction angle ( f) based on AASHTO (2007) 21 7 1.06 0.37 0.346
n = number of case histories, m = mean of biases, = standard deviation, COV = coefficient of variation
The distribution of the ratio of the interpreted measured 3.8.3.2 Observations
bearing capacity to the calculated bearing capacity () for
The presented findings of Goodman's (1989) method for
the 119 case histories in Table E-3 of database UML-GTR
the prediction of bearing capacity suggest the following:
RockFound07 is presented in Figure 79. The distribution of
has a mean (m) of 1.35 and a COV of 0.535 and resembles
a lognormal random variable. The distribution of for only 1. The method is systematically accurate, as demonstrated
the foundations on fractured rock is presented in Figure 80 by the proximity of the best fit line to the perfect match
and has an m of 1.24 and a COV of 0.276. line (measured qL2 = predicted qu) presented in Figure 78
Table 41. Summary of the statistics for the ratio of measured (qL2) to calculated bearing capacity (qult) of
rock sockets and footings on rock subcategorized by foundation type and data quality using the Goodman
(1989) method.
Cases n No. of sites m COV
All rock sockets 61 49 1.52 0.82 0.541
Rock sockets with measured friction angle ( f) 46 48 1.64 0.90 0.547
All rock sockets on fractured rock 11 6 1.29 0.26 0.202
Rock sockets on fractured rock with measured friction angle ( f) 7 5 1.23 0.18 0.144
All rock sockets on non-fractured rock 50 43 1.58 0.90 0.569
Rock sockets on non-fractured rock with measured s and measured f 26 26 1.58 0.79 0.497
Rock sockets on non-fractured rock with measured discontinuity spacing (s) 34 14 1.49 0.71 0.477
Rock sockets on non-fractured rock with measured friction angle ( f) 39 43 1. 72 0.96 0.557
Rock sockets on non-fractured rock with discontinuity spacing (s) based on
13 12 1.99 1.22 0.614
AASHTO (2007) and measured friction angle ( f)
Rock sockets on non-fractured rock with measured discontinuity spacing (s) and
8 3 1.19 0.21 0.176
friction angle ( f) based on AASHTO (2007)
Rock sockets on non-fractured rock with discontinuity spacing (s) based on
3 2 0.75 0.36 0.483
AASHTO (2007) and friction angle ( f) based on AASHTO (2007)
All footings 58 29 1.23 0.66 0.539
Footings with measured friction angle ( f) 52 23 1.27 0.69 0.542
All footings on fractured rock 9 3 1.18 0.43 0.366
Footings on fractured rock with measured friction angle ( f) 5 2 1.47 0.29 0.200
All footings on non-fractured rock 49 26 1.24 0.70 0.565
Footings on non-fractured rock with measured s and measured f 29 11 1.51 0.73 0.481
Footings on non-fractured rock with measured discontinuity spacing (s) 29 11 1.51 073 0.481
Footings on non-fractured rock with measured friction angle ( f) 47 21 1.25 0.72 0.573
Footings on non-fractured rock with discontinuity spacing (s) based on AASHTO
18 10 0.82 0.45 0.543
(2007) and measured friction angle ( f)
Footings on non-fractured rock with discontinuity spacing (s) based on AASHTO
2 1 1.10 0.13 0.115
(2007) and friction angle ( f) based on AASHTO (2007)
n = number of case histories, m = mean of biases, = standard deviation, COV = coefficient of variation
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Table 42. Summary of the statistics for the ratio of measured (qL2) to
calculated bearing capacity (qult) using the Goodman (1989) method
categorized by rock quality and foundation type.
Foundation
Cases n No. of sites m COV
type
RMR > 85 23 23 1.55 0.679 0.438
65 < RMR < 85 57 36 1.33 0.791 0.595
All
44 < RMR < 65 17 10 1.27 0.746 0.586
3 < RMR < 44 22 9 1.24 0.529 0.426
RMR > 85 16 16 1.59 0.809 0.509
Rock 65 < RMR < 85 35 24 1.40 0.722 0.515
Sockets 44 < RMR < 65 9 8 1.47 0.916 0.624
3 < RMR < 44 1 1 1.27 -- --
RMR > 85 7 7 1.46 0.204 0.140
65 < RMR < 85 22 13 1.22 0.896 0.738
Footings
44 < RMR < 65 8 5 1.06 0.461 0.437
3 < RMR < 44 21 8 1.24 0.542 0.437
n = number of case histories, m = mean of biases, = standard deviation, COV = coefficient of variation
and the bias of about 1.2 to 1.5 for all types of major variation of bias with rock strength resulted in a similar
subcategorization. COV only when each range of rock strength was examined
2. The consistently reliable performance of the method for separately. This observation enforces the notion of incor-
all ranges of rock strength (and hence RMR) provides a porating rock quality categorization (e.g., RMR) within the
COV of 0.535 for all cases. The variation of the bias mean bearing capacity predictive methodology when necessary.
and COV with rock quality is essentially absent, as can be 3. Similar statistics were obtained for shallow foundations
observed in Table 42. This is in contrast to the perform- (n = 58, m = 1.23, COV = 0.539) and rock sockets (n = 61,
ance of Carter and Kulhawy's (1988) method, in which the m = 1. 52, COV = 0.541). These observations suggest that
12 0.6
119 Rock sockets and Footing cases 20 Foundation cases on Fractured Rocks
0.35
40 Goodman (1989) Goodman (1989)
mean = 1.35 mean = 1.24
10 0.5
COV = 0.535 0.3 COV = 0.276
Number of observations
Number of observations
30 0.25 8 0.4
lognormal
Frequency
Frequency
0.2 distribution
lognormal 6 0.3
20 distribution
0.15 normal
distribution
normal 4 0.2
distribution
0.1
10
2 0.1
0.05
0 0 0 0
0 0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2
Bias, = qu,meas / qu,calc Bias, = qu,meas / qu,calc
Figure 79. Distribution of the ratio of the interpreted Figure 80. Distribution of the ratio of the interpreted
bearing capacity (qL2) to the bearing capacity (qult) bearing capacity (qL2) to the bearing capacity (qult)
calculated using Goodman's (1989) method for the calculated using Goodman's (1989) method for
rock sockets and footings in database UML-GTR foundations on fractured rock in database UML-GTR
RockFound07. RockFound07.
