Geotechnical Design Criteria for Shallow Foundations

BRIDGES: GEOTECHNICAL DESIGN CRITERIA FOR SHALLOW FOUNDATIONS 1. Introduction An adequate foundation should support the superstructure with an adequate margin of safety against shear strength failure of the underlying ground while limiting the anticipated movements to within values commensurate with serviceability. The below design criteria are defined collectively mainly after AASHTO LRFD Bridge Design Specifications 2012.

2. Bearing resistance The factored resistance qR shall be taken as

qR = ϕb qn

(10.6.3.1.1-1)

where: ϕb = resistance factor specified in table 10.5.5.2.2-1 qn = nominal bearing resistance

Notwithstanding the above, a conservative minimum safety factor of 3.0 against a bearing capacity type failure will be adopted.

3. Foundation Depth   

The minimum foundation depth shall not be less than 1.5 m below the natural ground level or finished grade level whichever is the deepest. The foundation shall be located to bear below the maximum anticipated depth of scour, erosion or undermining (10.6.1.2) As a general rule, foundations shall not rest on too soft or weak soil. Alternatively, the unsuitable material shall be replaced and replaced with properly compacted engineered fill. (C10.6.1.1).

4. Serviceability Criteria 4.1. 

Tolerable movement

The angular distortion ω between adjacent foundations shall be as follows: (C10.5.2.2) ω ≤ 0.008 rad (1:125) ω ≤ 0.004 rad (1:250)

for simple spans for continuous spans



In addition to the requirement of AASHTO for serviceability provided above, the following criteria according to (WSDOT Geotechnical design Manual) shall also be considered in the evaluation of the impact of the calculated settlement on the supported super-structure.

Total Settlement at Pier or Abutment ΔH ≤ 25 mm 25 mm < ΔH ≤ 100 mm

Differential Settlement Over 100 Feet (30m) within Pier or Abutment, and Differential Settlement Between Piers ΔH100 ≤ 18.75 mm

ΔH > 100 mm

ΔH100 > 75 mm

4.2.

18.75 mm < ΔH100 ≤ 75 mm

Action Design and Construct Ensure structure can tolerate settlement Obtain Approval prior to proceeding with design and construction

Settlement Analysis:

4.2.1.Methodology for Cohesionless Foundation Soils   

Shallow foundation settlement resting on cohesionless soils shall be estimated using elastic theory or empirical procedures. The magnitude of elastic settlement is estimated as a function of the applied stress beneath the footing and the corrected SPT blow count or deformation modulus. According to MJ TOMLISON, a correction to the total immediate settlement shall be applied to account for the foundation rigidity (I R) and depth factor (ID). The total corrected elastic settlement (Δ corrected) can be corrected as follows: Δ

corrected

= IR . ID. Δ

calculated

4.2.2.Influence Depth For settlement calculation purposes, the depth of influence to be considered shall be the greater of the following (both measured from bottom of base of shallow foundation):  

Three times the footing width (C10.6.2.4.2) The depth where additional stress due to the foundation load is equal to or less than 20% of overburden stress.

4.2.3.Foundation Rigidity Factor (IR) 

The stress distribution used to calculate elastic settlement assumes footing is based on a totally flexible loaded area. According to ACI, he relative stiffness factor (Kr) can be used to evaluate the relative rigidity of structural foundation. The following formula (Meyerhof 1953) may be used to estimate the relative rigidity:

Kr=

EC I B E S B3

Where: EC IB = Foundation stiffness Es B3 = Soil stiffness

if Kr ≥ 1.0 then the foundation is rigid.



In case of rigid foundation the calculated total immediate settlement at the center of shallow foundation can reduced by a rigidity factor IR equal to 0.8 after Tomlinson, 1996).

4.2.4.Foundation Depth Factor (ID) A correction is applied to the calculated settlement in the form of depth factor ID. This

depends on the depth to area ration and the length to breadth ration of the foundation.

Depth factor ( I D )=

Corrected settlement for a foundation at depth D Calculated settlement for surface foundation

4.2.5.Deformation Modulus of Cohesionless Soils and Rock Granular Soil Deformation Modulus (Es) The measured SPT blow-count shall be used to obtain the modulus of elasticity (Es) and Poisson’s ratio for normally consolidated granular soil according to the empirical relationships in table C10.4.6.3-1, and could be summarized in metric units as follows: Es = 0.7 to 1.17 N MN/m2 (where N=SPT blow-count corrected)

The measured SPT blow counts shall be corrected for the overburden effect as follows: N160 = CN N60 Where: '

40/σ V ¿ CN = 0.77 log ¿ , and CN ˂ 2.0 10 ¿ Rock Mass Deformation Modulus (Em): The elastic modulus of a rock mass (Em) shall be calculated according to the following equation:

[

Em = 10 Em

RMR−10 40

]

= Elastic modulus of the rock mass (Gpa)

RMR

= Rock mass rating specified in Table 10.4.6.4-1,3