In lieu of the values for soil bearing strength shown in Table 1, a designer might choose to use calculated Rankine passive pressure (Pp) or other determination of soil bearing strength based on actual soil properties.

BEARING BLOCK

FIGURE 1

RESTRAINED JOINTS An alternative method of providing thrust restraint is the use of restrained joints. A restrained joint is a special push-on-type joint that is designed to provide longitudinal restraint. Restrained joint systems function in a manner similar to thrust blocks, insofar as the reaction of the entire restrained unit of piping with the soil balances the thrust forces. The thrust force must be restrained or balanced by the reaction of the restrained pipe unit with the surrounding soil. The source of the restraining forces is twofold: first, the static friction between the pipe unit and the soil, and second, the restraint provided by the pipe as it bears against the sidefill soil along each leg of the bend. Both of these forces are presumed to be functions of the restrained length (L) on each side of the bend, and they are presumed to act in the direction opposing the thrust force (i.e., directly opposing impending movement of the bend). Values of soil cohesion (Cs) and internal friction angle of the soil (ø) must be known or conservatively estimated for the soil at a particular installation. The values fc and fø are related to soil types and pipe material. Table 3 presents conservative values of these parameters for ductile iron pipe in seven general classifications of saturated soils. UNIT FRICTIONAL FORCE, FS A static frictional force acting on a body is equal in magnitude to the applied force up to a maximum value. In the conventional analysis, the maximum static friction is proportional to the normal force between the surfaces which provide the friction. The constant of proportionality, in this case called the coefficient of friction, depends upon the nature of the surfaces. Potyondy's empirical work indicates that for friction between pipe and soils, the force is also dependent upon the cohesion of the soil. Thus, Fs=APC+W tan where AP is the surface area of the pipe exterior in m2/m, C is the pipe cohesion in kg/m2, and is the pipe friction angle in degrees. The term is defined by the equation . The unit normal force (W) is given by W = 2We + Wp + Ww, where the earth load (We) is taken as the prism load on the pipe in kg/m. It is defined by the equation We = HD', where is the soil density in kg2/m3 and H is the depth of covers in meters. The earth load is doubled to account for the forces acting on both the top and the bottom of the pipe. The unit weight of the pipe and water (WP + Ww) is given in Table 2. The pipe cohesion (C) is defined by the formula C=fc·Cs. Then, UNIT BEARING RESISTANCE RS The maximum unit lateral resistance, Rs , at the bend is limited so as not to exceed a rectangular distribution of the Rankine passive soil pressure, Pp , which is generally less than the ultimate capacity of the soil to resist pipe movement. The passive soil pressure for a particular soil is given by the Rankine formula: Where: As discussed above, the full Rankine passive soil pressure, Pp, can be developed with insignificant movement in well-compacted soils. For some of the standard laying conditions for ductile iron pipe, the design value of passive soil pressure should be modified by a factor Kn to ensure that excessive movement will not occur. Therefore, Rs = KnPpD´. Empirically determined values for Kn can be found in Table 3. In this context, the value chosen for Kn depends on the compaction achieved in the trench, the backfill materials, and the undisturbed earth. Thus, for a horizontal bend, the equation is: Extraordinary installations might result in lesser loads and frictional resistance on the pipes than calculated by these equations. When such conditions exist, this must be provided for in the design.

INT Thrust Restraint Calculation for ISO Pipe Using DIPRA Design

Notes:
Above information is based upon the following:
10 bar maximum system pressure
90° horizontal bend: multiply by the following coefficients for other horizontal bends:
45° - 0.414; 22.5° - 0.199; 11.25° - 0.098
Class K9 pipe weight
Additionally, pipe must be bedded in at least 100mm of loose material.
H = depth of cover