CHAPTER  1600.  Thrust Restraint Design and Construction.

 

 1601. Scope. This chapter of the specifications defines the placement and construction of thrust blocks.  It also gives the mix design required for the Portland Cement Concrete required in the construction of the thrust blocks.

 

 1602. Placement. Thrust blocks are required at points where the pipe changes direction such as: at all tees, elbows, wyes, caps, valves, hydrants, reducers, etc.  Thrust blocks should be constructed so that the bearing surface is in direct line with the major force created by the pipe or fitting.  The earth bearing surface should be undisturbed.

 

 1603. Concrete Mix Design. The Portland Cement Concrete mixture is five sacks of cement per cubic yard mix.  The concrete mixture will have a minimum of 28‑day compressive strength of 2,500 pounds per square inch.

 

 1604. Thrust Restraint. Pipe and fittings are most often joined with push‑on or mechanical joints.  Neither of these joints provides significant restraint against longitudinal separation other than the friction between the gasket and the plain end of the pipe or fitting.  Tests have shown that this frictional resistance in the joint is unpredictable, varying widely with installation conditions and other factors that are insignificant in other respects.  Thus, these joints must be considered as offering no longitudinal restraint for design purposes.

 

            At many locations in an underground or above ground pipeline, the configuration of the pipeline results in unbalanced forces of hydrostatic or hydrodynamic origin that, unless restrained externally, can result in separation of joints.

 

            Generally, these unbalanced hydrostatic and hydrodynamic forces are called THRUST FORCES.  In the range of pressures and fluid velocities found in waterworks or wastewater piping, the hydrodynamic thrust forces are generally insignificant in relation to the hydrostatic thrust forces and are usually ignored.  Simply stated, thrust forces occur at any point in the piping system where the direction or cross sectional area of the waterway changes.  Thus, there will be thrust forces at bends, reducers, offsets, tees, wyes, dead ends, and valves.

 

            Balancing thrust forces in underground pipelines is usually accomplished with bearing or gravity thrust blocks.  Presented herein is a general discussion of the nature of thrust forces as well as suggested approaches to the design of thrust block systems for balancing these forces.  The suggested design approaches are conservatively based on accepted principles of soil mechanics.

 

THE THRUST FORCE

 

The internal hydrostatic pressure acts perpendicularly on any plane with a force equal to the pressure (P) times the area (1) of the plane.  All components of these forces acting radially within a pipe are balanced by circumferential tension in the wall of the pipe.  Axial components acting on a plane perpendicular to the pipe through a straight section of the pipe are balanced internally by the force acting on each side of the plane. See figure 16‑2.  Consider, however, the case of a bend as shown in Figure 16‑3.

 

The forces PA acting axially along each leg of the bend are not balanced.  The vector sum of these forces is shown as T.  This is the thrust force.  In order to prevent separation of the joints a reaction equal to and in the opposite direction of T must be established.

 

Figure 16‑4 depicts the net thrust force at various other configurations.  In each case the expression for T can be derived by the vector addition of the axial forces.

 

DESIGN PRESSURE

 

The design pressure, P, is the maximum pressure to which the pipeline will be subjected, with consideration given to the vulnerability of the pipe‑soil system when the pressure is expected to be applied, In most cases this will be the test pressure of the pipe, applied shortly after installation when the pipe‑soil system is normally most vulnerable.

 

For buried pipelines, thrust restraint is achieved by transferring the thrust force to the soil structure outside the pipe.  The objective of the design is to distribute the thrust forces to the soil structure in such a manner that joint separation will not occur in unrestrained joints.

 

THRUST BLOCKS

 

One of the most common methods of providing resistance to thrust forces is the use of thrust blocks. Figure 16‑5 depicts a typical bearing thrust block on a horizontal bend.  Resistance is provided by transferring the thrust force to the soil through the larger bearing area of the block such that the resultant pressure against the soil does not exceed the horizontal bearing strength of the soil.  Design of thrust blocks consists of determining the appropriate bearing area of the block for a particular set of conditions.  The parameters involved in the design include pipe size, design pressure, angle of the bend, (or configuration of the fitting involved), and the horizontal bearing strength of the soil.

 

 1605. General criteria for bearing block design.

 

(a)        Bearing surface should, where possible, be placed against undisturbed soil.  Where it is not possible, the fill between the bearing surface and undisturbed soil must be compacted to at least 90% Standard Proctor density.

 

(b)        Block height (h) should be equal to or less than one‑half the total depth to the bottom of the block,(H/T), but not less than the pipe diameter (D').

 

(c)        Block height (h) should be chosen such that the calculated block width (b) varies between one and two time the height.

 

The required bearing block area is

 

Then, for a horizontal bend,

 

 

            where S/f is a safety factor (usually l.5 for thrust block design).  A similar approach may be used to design bearing blocks to resist the thrust forces at tees, dead ends, etc.  Typical values for conservative horizontal bearing strengths of various soil types are listed in Table 16‑6.

 

            In lieu of the values for soil bearing strength shown in Table 16‑l, a designer might choose to use calculated ranking passive pressure (P/p) or other determination of soil bearing strength based on actual soil properties.

 

 1606. Gravity Thrust Blocks may be used to resist thrust at vertical down bends.  In a gravity block, the weight of the block is the force providing equilibrium with the thrust force. The design problem is then calculating the required volume of the thrust block of a known density.  The vertical component of the thrust force is balanced by the weight of the block.

 

            It can easily be shown that T/y = PA Sin 0.  Then the required volume of the block is

 

             S/f P A Sin 0

V = _______________

G             W/m

 

            where W/m = density of the block material.  In a case such as shown, the horizontal component of the thrust force T/x = PA (l - cos 0) must be resisted by the bearing of the right side of the block against the soil.  Analysis of this aspect will follow as the above section on bearing blocks.


 

TABLE 16‑6

 

SOIL

* BEARING STRENGTH S/B(LB/FT 2)

Muck

0

Soft Clay

l,000

Silt

l,500

Sandy Silt

3,000

Sand

4,000

Sandy Clay

6,000

Hard Clay

9,000

 

            * Although the above bearing strength values have been used successfully in the design of thrust blocks and are considered to be conservative, their accuracy is totally dependent on accurate soil identification and evaluation.  The ultimate responsibility for selecting the proper bearing strength of a particular soil type must rest with the design engineer.