settlement of shallow foundations - civil engineeringbartlett/cveen5305/ch. 8 and 9 -...

© Steven F. Bartlett, 2011

Ch. 9 Lecture Notes○

Ch. 8 and 9 (Salgado)○

Reading Assignment

Other Materials

Problems 9-5, 9-6, 9-8 (use Schmertmann's method), 9-16 (b) (use Myerhof's method)

○

Homework Assignment 8

Settlement of Shallow FoundationsWednesday, August 17, 2011 12:45 PM

Ch. 8 and 9 - Settlement


Introduction to Shallow FoundationsWednesday, August 17, 2011 12:45 PM



Square footing

(Note that any horizontal loading to the footing that is not through the center of the footing causes an overturning moment.)

Rectangular footing

(Note that the moment is shown in this figure. The overturning moment causes eccentricity in the loading configuration that effects the resultant load and the pressure distribution that develops along the base of the footing.

Loadings to Shallow FoundationsWednesday, August 17, 2011 12:45 PM



Strip footing

Strip footing with columns and strip footing with continuous wall panel

Loadings to Shallow Foundations (cont.)Wednesday, August 17, 2011 12:45 PM



Excavation and compaction of bearing level for foundation.

Note that the footing is placed below the surface at a depth below frost penetration.

Reinforcement for footing.

Construction of FootingsWednesday, August 17, 2011 12:45 PM



Forming of footings

Forming of footings

Construction of Footings (cont.)Wednesday, August 17, 2011 12:45 PM



Pouring of concrete

Finished footing with anchor bolts




Attaching steel columns to anchor bolts

Tightening of nuts on anchor bolts




Proximity to Buried Utilities and Other FoundationsWednesday, August 17, 2011 12:45 PM



Note that this lecture deals with calculating the immediate settlement.

For calculating the consolidation settlement, see methods described in CVEEN 3310 or Salgado p. 389.

Comparison of Settlement of Sands and ClaysWednesday, August 17, 2011 12:45 PM



Pressure Distribution Under Footing Versus DepthWednesday, August 17, 2011 12:45 PM



Sand has inverted parabolic shape Clay has parabolic shape

Influence of soil type on contact pressure (assumption of rigid footing)

Influence of footing flexibility on contact pressure (non-rigid footing)

Contact Pressure at Base of FootingWednesday, August 17, 2011 12:45 PM



http://civil-engg-world.blogspot.com/2008/12/contact-pressure-distribution_24.html


The fact that the contact pressure changes as a function of footing flexibility is called soil-structure interaction.

Contact Pressure (cont.)Wednesday, August 17, 2011 12:45 PM





qb Q = Wsoil + Qtop + Wftg

©Evert C. Lawton

Contact Pressure - Uniform Distribution Incorporating Weight of Footing and Overlying SoilWednesday, August 17, 2011 12:45 PM



Show your calculations here:

Contact Pressure - Uniform Distribution Incorporating Weight of Footing and Overlying Soil - ExampleWednesday, August 17, 2011 12:45 PM



Saint-Venant's Principle

The geometry of the loading has negligible effect on the calculated stresses so long as the distance from the point where the stresses are desired to the loading is much larger than the scale of the load.

Q = footing load (force)E = Young's modulus

= Poisson's ratior= radius of footing

Immediate Settlement from Elastic Theory (Point Load)Wednesday, August 17, 2011 12:45 PM



The above chart is for a uniform circular load:

Settlement (Eq. 9-16 Salgado)

z = qb/E * Iz

Or use chart above.

Immediate Settlement from Elastic Theory (Flexible Circular Load)Wednesday, August 17, 2011 12:45 PM



I is determined from Eq. 9.19

The settlement at the center of a flexible rectangular load can be calculated by superimposing the settlements at the corners of four rectangles with the total area equal to the desired area.

Immediate Settlement from Elastic Theory (Flexible Rectangular Load)Wednesday, August 17, 2011 12:45 PM



Settlement of Rigid FootingsWednesday, August 17, 2011 12:45 PM



Method for Estimating Young's Modulus E from CPT data○

Myerhof and Fellenius (1985)○

E = k qc

where: qc = uncorrected CPT tip stress

k = 1.5 for silts and sand = 2 for compacted sand = 3 for dense sand = 4 for sand and gravel

Jamiolkowski (1988)○

E=[M(v+1)(2v-1)]/(v-1)

Method of Estimating EWednesday, August 17, 2011 12:45 PM



Meyerhof (1965)

W = footing settlement, LR = reference length (1 m, 3.28 ft, 39.4 in), PA = reference stress (100 kPa, 1 tsf).

©Evert C. Lawton

Depth of 1B

0

Zone ofInfluence=

1B for sq. ftg.

2B for strip ftg.

Method for calculating N60 bar

SPT-Based MethodsWednesday, August 17, 2011 12:45 PM



2B for sq. or circular footings

4B for rectangular footing with L/B > 10

For lowest y-coordinate for Iz0 = 0 (y-axis intercept)

For y-coordinate of break point (i.e., y-coordinate for Izp max)

For x-coordinate for zf = 0

For x-coordinate of break point

For Iz between zf and zfp (i.e., bottom part)

For Iz between z0 and zfp (i.e., top part)

CPT Based MethodsWednesday, August 17, 2011 12:45 PM



Ei = 2.5qc for young NC sandEi = 3.5qc for aged NC sandEi = 6.0qc for overconsolidated silca sand

C1 from Eq. 9.43 Salgado = C1 = 1-0.5('v|zf=0 / q) > 0.5C2 from Eq. 9.44 or usually set to 1 (see note below )

For an example - see course website

CPT Based MethodsWednesday, August 17, 2011 12:45 PM


Steven F. Bartlett, 2010

Elastic Theory

Numerical Approacha/z

Note: Influence factor values from this chart must be double to account for the right side of the embankment.

I =(influence factor)

(z = depth below ground surface (i.e., depth below base of embankment)

Stresses Under Embankment and SlopesThursday, March 11, 2010 11:43 AM



BlankWednesday, August 17, 2011 12:45 PM


settlement of shallow foundations - civil engineeringbartlett/cveen5305/ch. 8 and 9 -...

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