Downward Pipe Movement (AutoPIPE)

(D-15)

The vertical support provided by a soil against downward movement of a buried pipe may be expressed in terms of conventional bearing capacity theory. The pipeline is assumed to act as a cylindrically shaped continuous strip footing. Thus, the ultimate soilreaction (P1) for all soil types can be determined from the expression:

where

Refer to Table D-11 below for the proper effective unit weight equation relative to the location of the water table.

Ng,

Nc,

Nq = bearing capacity factors (dimensionless). Refer to Figure D-11.

P1 = ultimate soil resistance to downward pipe displacement, assuming K2 equals zero (F/L).

d = outside diameter of pipe (L).

c = cohesion (or Su) of soil below pipe (F/L2).

H = depth from ground surface to bottom of pipe (L).

= effective unit weight of the soil, not the backfill (F/L3).

AutoPIPE Reference Information

Page 1 of 3Downward Pipe Movement

11/6/2014mk:@MSITStore:C:\Bentley\AutoPIPE%20V8i%20SELECTseries5%20Maintenance%20...

Figure D-11

Figure D-11 was taken from Leonards, Foundation Engineering, McGraw-Hill, 1962 (after Terzaghi and Meyerhof). Either the Meyerhof or the Terzaghi curves can be used for dense, stiff soils. Whereas the Terzaghi local shear curves should be used for loose, compressible soils.

If the angle of internal friction () is zero, as for saturated clay in undrained shear, the first and third terms in Equation D-15 become very small and only the cohesion contributes materially to the bearing capacity. Thus, for all practical purposes in a saturated clay:

(D-16)

For this case, values of Nc with respect to the depth of embedment and pipe diameter have been developed as shown in Figure D-12. In this chart, the breadth (B) is equal to the outside diameter of the pipe (d). Figure D-12 is after Skempton (1951), and was taken from Poulos and Davis' Pile Foundation Analysis and Design, Wiley, 1980.

Page 2 of 3Downward Pipe Movement

11/6/2014mk:@MSITStore:C:\Bentley\AutoPIPE%20V8i%20SELECTseries5%20Maintenance%20...

Figure D-12

The displacement required to mobilize the full soil resistance is generally considered to be on the order of 10% to 15% of the outside diameter of the pipe. On this basis, the displacement which corresponds to P1 can be expressed as a function of the pipe diameter, and it lies in the following range:

(D-17)

Therefore, the value of K1 can be estimated to be in the range:

(D-18)

Page 3 of 3Downward Pipe Movement

11/6/2014mk:@MSITStore:C:\Bentley\AutoPIPE%20V8i%20SELECTseries5%20Maintenance%20...