ENCE 4610Foundation Analysis and Design

Bearing CapacityOther Topics

Other Topics in Bearing Capacity

• Bearing Capacity from Field Testso SPTo CPT

• Effect of Soil Compressibility (Local and Punching Shear)

• Bearing Capacity for Foundations on Top of a Slope

• Foundations on Rock

Use of SPT and CPT Methods to Determine Bearing Capacity

• Approach I: Use SPT and CPT correlations (such as we discussed in 3610) and determine soil properties (γ, φ, c) and then apply to bearing capacity equations

• Approach II: Use a “direct” approach such as given in textbook (Murthy, 12.12 and 12.13)

• First approach is preferable as it allows more flexibility in soil type and layering structureo Note: in this course (and the vast majority of practice) the reference

standard for SPT efficiency is 60%, thus N60 = Ncor are based on this efficiency

SPT Efficiency Correction Factors(without overburden correction)

• Eh = hammer type factor• Cb = borehole diameter

factor• Cs = sampler correction

factor• Cd = rod length factor• Factors given in Murthy

(but Equation 9.6 is wrong)

• N60 = Corrected blow count to a “reference hammer” which is 60% efficient and other factors

• N = blow count from field test

6.060bsdh CCCEN=N

Note max. value

( )

kPa) Units,(SI 2100

ksf) Units,(U.S. 2 260601

≤′

=

≤′

=

=

voN

voN

N

C

C

NCN

σ

σ

Effect of Soil Compressibility

• Vesić Compressibility Factor

o G = shear modulus of soilo c = cohesion of soilo p0 = effective stress (in this

case, at a depth of Df + B/2o φ = friction angle of soilo Δ = volumetric strain in plastic

zone

• Bearing capacity equations presented until now are directed at the general shear case

• We saw that we also had local and punching shear as well

• These conditions require some consideration of the compressibility of the soil

φtanor pc

GI+

=

r

rrr I

IIΔ+

=1( )μ+=

12EG

Values of Young’s Modulus and Poisson’s Ratio

Inclusion of Soil Compressibility Factors

( )⎭⎬⎫

⎩⎨⎧

++⎥⎦

⎤⎢⎣⎡ −

= φφφsin1

2logsin07.3tan2511

53 rILB

q eCφtan1

q

qqc N

CCC

−−=

cCγCCq =

Application of Soil Compressibility Factors

• Determine the modulus of elasticity and Poisson’s ratio for the given soil (use values in earlier slide)

• Compute Ir using these values and other soil properties (c, φ, γ and compute effective stress)

• Determine critical rigidity index Ircrit (Murthy Table 12.4 or Equation 12.35)

• Compare your result of Ir with Ircrito If Ir > Ircrit, then soil is incompressible and ignore compressibility factorso If Ir < Ircrit, then soil is compressible and include compressibility factors in

bearing capacity analysis

• Compute bearing capacity equations w/compressibility factors

Bearing Capacity for Foundation at Top of a Slope• Two Approaches

o Use Vesić’s bearing capacity factor for foundations on slopeso Use Meyherhof’s method given in text (Murthy, 12.15)

• Outine of Meyerhof’s Methodo Bearing capacity equations are the same as given earlier except for the

following:• Do not use Vesić’s bearing capacity factor for foundations on slopes• Replace the main bearing capacity factors (Nc, Nγ, Nq) with factors

for slopes (Ncq, Nγq, Nqq)• Ncq, Nγq given in the following slides for two cases: foundation on top

of the slope and foundation at the base of the slope (latter not in Murthy)

• Nqq = 0 always• Handle water table same way as any other layered foundation

Meyerhof Slope FactorsTop of Slope

Meyerhof Slope Factors: Base of Slope

Example of Footings on Slopes

• Giveno Bearing wall for warehouseo Located close to slope

• Findo Size of strip footing to be

provided, ignore weight

7'

4.5 kips/ft wall length

Clay (φ = 0)γ = 100 pcfc = 1 ksf

2'20'

60º

Example of Footings on Slopes

• For this problem, b = 7 – B/2 < Hs = 20’

• From that, Ns = 0

• We thus use the top, “dashed” portion of the chart

Curve used

Example of Footings on Slopes

b b/B B FS2 0.2 10 4.4 4.6 10.22

2.5 0.28 9 4.5 4.7 9.43 0.38 8 4.6 4.8 8.53

3.5 0.5 7 4.9 5.1 7.934 0.67 6 5 5.2 6.93

4.5 0.9 5 5.3 5.5 6.115 1.25 4 5.7 5.9 5.24

5.5 1.83 3 6.2 6.4 4.276 3 2 6.9 7.1 3.16

6.5 6.5 1 7 7.2 1.6

Ncq qult• qult = cNcq + 0.5γBNγq

(Murthy Eq. 12.66)• Nγq = 1 (Murthy Eq.

12.67)• From the above, qult =

cNcq + 0.5γB (Murthy Eq. 12.68)

• FS = B qult / P• From the table, B = 2’

Required Footing Setbacks

For example problem:H/3 = 20/3 = 6.67' from 45 degree line

Other Notes on Bearing Capacity Factors

• AASHTO (2002) guidelines recommend calculating the shape factors, s, by using the effective footing dimensions, B′f and L′f. However, the original references (e.g., Vesić, 1975) do not specifically recommend using the effective dimensions to calculate the shape factors. Since the geotechnical engineer typically does not have knowledge of the loads causing eccentricity, it is recommended that the full footing dimensions be used to calculate the shape factors for use in computation of ultimate bearing capacity.

• Bowles (1996) also recommends that the shape and load inclination factors (s and i) should not be combined.

• In certain loading configurations, the designer should be careful in using inclination factors together with shape factors that have been adjusted for eccentricity (Perloff and Baron, 1976). The effect of the inclined loads may already be reflected in the computation of the eccentricity. Thus an overly conservative design may result.

Bearing Capacity on Rock• Generally, the limit-state approach used with soils is

not applied to rock.• Spread footings on rocks are generally designed

according to a “presumptive bearing capacity” approach, where a maximum q is determined based on the type and quality of the rock

• The Rock Quality Designation (RQD) is commonly used to determine the bearing capacity of foundations in rock

• For foundations on unweathered intact rocks, the rock may have greater structural strength than the concrete, and thus bearing capacity determination becomes unnecessary

Bearing Capacity on Rock: Tables

Questions?