American Lifelines Alliance Soil Model - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
13

The following information references "Appendix B: Soil Spring Representation" in the American Lifelines Alliance document Guidelines for the Design of Buried Steel Pipe (http://www.americanlifelinesalliance.org/pdf/Update061305.pdf). This document provides bilinear stiffness of soil for axial, lateral, uplift and bearing. Each stiffness term has a component associated with sandy soils (subscripted q) and a component associated with clays (subscripted c). Data can be entered for pure granular soils and pure clays.

Soil stiffness for both clay and sand (cohesive and granular soils, respectively) are defined through the following user-defined parameters:

c = soil cohesion representative of the soil backfill

H = soil depth to top of pipe (this is converted by C2 to depth to pipe centerline in ALA calculations)

g = effective unit weight of soil

g = total dry unit weight of fill

Ko = coefficient of earth pressure at rest (can be calculated based on internal friction angle of soil)

f = coating-dependent factor relating the internal friction angle of the soil to the friction angle at the soil-pipe interface

φ = internal friction angle of soil

Elastic range of soil is either fixed or a function of D & H with limits based on D.

Yield Displacement Factor

Entry

Limited by

Δt (dT) – Axial

Length units

Δp (dP) – Lateral

Multiple of D

0.04(H+D/2)

Δqu (dQu) – Upward

Multiple of H

Minimum

Δqu (dQu) – Upward

Multiple of D

Δqd (dQd) – Downward

Multiple of D

Axial

Tu = peak friction force at pipe-soil interface maximum axial soil force per unit length that can be transmitted to pipe)

D = pipe OD

a = adhesion factor (for clays only)

c = soil cohesion representative of the soil backfill (undrained shear strength)

H = depth of cover to pipe centerline

= effective unit weight of soil

Ko = coefficient of earth pressure at rest

The ratio of the horizontal effective stress acting on a supporting structure and the vertical effective stress in the soil at that point. At rest indicates the pipe does not move for this calculation.

δ = interface angle of friction for pipe and soil, d = ff

f = coating-dependent factor relating the internal friction angle of the soil to the friction angle at the soil-pipe interface

Pipe Coating

f

Concrete

1.0

Coal Tar

0.9

Rough Steel

0.8

Smooth Steel

0.7

Fusion Bonded Epoxy

0.6

Polyethylene

0.6

f = internal friction angle of soil

Δt = axial displacement to develop

Tu = 0.1 inch for dense sand, 0.2 inch for loose sand, 0.3 inch for stiff clay, and 0.4 inch for soft clay

Lateral

Pu = maximum horizontal soil bearing capacity (maximum lateral soil force per unit length that can be transmitted to pipe)

Nch = horizontal soil bearing capacity factor for clay (0 for c=0)

Nqh = horizontal soil bearing capacity factor for sand (0 for j=0°)

Factor

j

x

a

b

c

d

e

Nch

H/D

6.752

0.065

-11.063

7.119

--

Nqh

20°

H/D

2.399

0.439

-0.03

1.059E-3

-1.754E-5

Nqh

25°

H/D

3.332

0.839

-0.090

5.606E-3

-1.319E-4

Nqh

30°

H/D

4.565

1.234

-0.089

4.275E-3

-9.159E-5

Nqh

35°

H/D

6.816

2.019

-0.146

7.651E-3

-1.683E-4

Nqh

40°

H/D*

10.959

1.783

0.045

-5.425E-3

1.153E-4**

Nqh

45°

H/D*

17.658

3.309

0.048

-6.443E-3

1.299E-4**

*CAESAR II limits the height/diameter (H/D) ratio to a maximum of 20 for angles at 40 to 45 degrees. The software calculates any values specified that result in a ratio that is greater than 20 as equal to 20.

**The American Lifelines Alliance standard lists the horizontal soil bearing capacity factor for sand (Nqh) as a negative value for both 40 and 45 degree angles. This results in negative yield load values. CAESAR II calculates these values as a positive value, as shown in the previous table.

Nqh can be interpolated for φ between 20°and 45°.

Vertical Uplift

Qu = maximum vertical upward soil bearing capacity (maximum vertical uplift soil force per unit length that can be transmitted to pipe)

Ncv = vertical upward soil bearing capacity factor for clay (0 for c=0)

Nqv = vertical upward soil bearing capacity factor for sand

= 0.01H to 0.02H for dense to loose sands < 0.1D

= 0.1H to 0.2H for stiff to soft clays < 0.2D

Vertical Bearing

Qd - maximum vertical bearing soil force per unit length that can be transmitted to pipe.

Nc, Nq, Ng = vertical downward soil bearing capacity factors

g = total dry unit weight of fill

Dqd = vertical displacement to develop Qd

= 0.1D for granular soils

= 0.2D for cohesive soils