20_Bearing Capacity - Presumptive&Insitu

7/28/2019 20_Bearing Capacity - Presumptive&Insitu

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BC18

Net Bearing Capacity & Design Considerations:

o The bearing capacity (Qu) calculated using the foregoingequations is that of the soil at the depth of the footing.

o The load applied at the depth of the footing includes thestructural load (Qs), the weight of the footing itself (W f), andthe weight of any soil immediately above the footing (W s).Since the unit weight of a typical concrete footing is not toodifferent from soil, the weight terms are often groupedtogether approximately as follows:

W = (Wf + Ws) soilDfA = qA

where A = footing area

q = unfactored surcharge at the footing depth (D f) for a soil of unit weight soil

o The net load capacity (Qnet) then is the ultimate capacity less the weight load.

Qnet = Qu - W = (Qu - qA)

o The allowable load capacity (Qa) is the net capacity divided by a factor of safety:

sunet

a QFS

qAQ

FS

QQ

== and s

uneta q

FS

qq

FS

qq

== (FS = 3 typical)

o For design purposes : [ ]qA)FS(QQ su + and [ ]q)FS(qq su +

(Note that the structural load is factored while the weight load is not.)

o Since the footing dimensions affect the shape, depth, and eccentricity factors, sizing afooting is a generally an iterative procedure. The process is simplified for a square footing.

o Design of a footing should include the possibility of a change in the groundwater depth.

o Footings in close proximity will induce overlappingstress changes in the soil (recall pressure bulbconcept). Look at potential for differentialsettlement due to increased stresses. Footingsmay tilt or deflect, causing damage to footingand/or structure. If the anticipated settlement is toogreat then use strap or mat footings.

o BC of layered soils has many considerations:

BC depth of influence 2B, layers below 2B only affect settlement

Layered profiles without strong differences in soil properties can beapproximated using weighted average of properties and general BCequation.

Weak soil over strong soil will be controlled by soft soil properties andfailure at interface.

Strong soil over weak soil may be controlled by punching throughstrong soil or by "squeeze out" of soft soil.

2

1

2B

Wf

Ws

Qs

Qa

Df

Ws


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BC19

CBEAR program (and others) will help with layered analyses.


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BC20

Presumptive Bearing:

Refer to local building codes.Chart below from ASCE reprint of COE manual quotation of Navy NAVFAC DM7.2:

C

hoice

of Design Soil Parameters: drained vs. undrained

qu= (qc + qq + q) = cNccscdci + qNqqsqdqi + BNsdi

1. Sands: effective stress analysis w/ c', ', ' orm - cannot load fast enough to induce

pore pressures

2. Partially Saturated Clays: total stress analysis, c, , m - no excess pore pressure

3. Saturated Clays:

a. long term - effective stress analysis, c', ', '

seldom controls design, get increase in strength

b. short term - total stress analysis, cu, = 0 N = 0, use sat with Nq term

usually controls design


4/6

BC21

BC Soil Parameters: Need c, cu (= su), , u, to use in BC formulae

o Evaluate from field samples in lab: triaxial tests, direct shear tests, unconfined compression

o Educated guess for clays: correlations between plasticity or liquidity index vs. cuEducated guess for sands: correlations between relative density / unit weight vs. as f()

o Guess based on local experience, water content, previous tests (nice, warm fuzzy feeling)

o Evaluate from SPT:

Bowles tables (1996) for cu and vs. N (see insitu notes, very approximate)

Wolff (1989), see Das:211 N00054.0N3.01.27 +=

where N1 = CN Nfield and Nfield N55

Schmertmann (1975), Kulhway & Mayne (1990), see Das:

34.0

field

'3.202.12

Ntan

+= where = overburden pressure in bars and N field N55

o Evaluate from Cone Penetrometer (CPT): based on cone tip bearing pressure, qc

Schmertmann (1975):k

oc

uuN

pqsc

==

where po = z = total stress at depth of test

Nk = CPT Bearing Factor = 10-20, typically use 15

Nk lower with higher sensitivity =( )

( )remoldedu

dundisturbeu

ts

sS =

see Bowles chart (insitu notes, after Lunne & Eide, 1976)for Nk from the plasticity index, Ip = (LL PL)

Schmertmann (1975), Kulhway & Mayne (1990), see Das and chart in insitu notes:

+='

qlog38.01.0tan c where = effective overburden pressure

o Vane Shear Test (VST) for direct measurement of cu

o Iowa Borehole Shear Test (IBST) direct measurement of c u and

o Marchetti Dilatometer (DMT) for estimate of cu based on initial, lateral lift-off pressure,

and based on thrust and initial lateral lift-off pressure

o Pressuremeter Test (PMT) for cu based on limit pressure pL and shear modulus G

P

hoLuuN

psc

== where ho = total horizontal stress at depth of test

u

Pc

Gln1N += (iterative solution for cu)


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BC22

BC Based on Insitu Tests:

o Some insitu tests allow direct correlation to bearing capacity, potentially superior

o SPT is a calibrated method developed by Meyerhof (1974) and modified by Bowles (1996)and uses chart below. N used in equations is an average over range of 0.5B above footingto 2B below footing. Method includes a factor for depth (D/B). For B in feet:

Method is is based on a settlement of 25 mm.

If the allowable capacity qa is desired at a different settlement, then aa qmm25

mm,ksf,'q

=

o Das (1998) presents a conservative SPT method from Peck, Hansen and Thornburn (1974)

ft4BifKft,B

1ft,B

4

Nksf,q D

55a >

+

=

ft4BifK5.2

Nksf,q d

55a

= matsforK

4

Nksf,q d

55a

=

33.1B

D33.01Kwhere fd

+=

This chart for SIunits and D/B =0


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BC23

o Three CPT methods from Schmertmanns (1975, 1978) work at UF

First calculate the average cq in kgf/cm2 over the depth range 0-1.5B below the footing.

(Bowles would calculate cq over the depth range 0.5B-1.1B)

(1 bar = 1.019 kgf/cm2 = 100 kPa = 1.044 tsf)

Method 1: Calculate cq q25.1NN ==

Method 2: Back figure from N or Nq above and use with general BC equation

Method 3: Equations based on work by Awkati (unpublished but oft repeated, 1970)

valid for Df 1.5 ft + B/2 if B < 3 ft and Df 4 ft if B 3 ft

cohesionless soils: footingstrip)kgf/cm,q300(0052.028q1.52

cu =

(sands) footingsquare)kgf/cm,q300(0090.048q1.52

cu =

cohesive soils (clays): footingstripkgf/cm,q28.02q2

cu +=

footingsquarekgf/cm,q34.05q 2cu +=

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20_Bearing Capacity - Presumptive&Insitu