DURODRAIN uPVC SOLID WALL SEWER AND DRAIN PIPE SYSTEMS (SANS 791) SYSTEM DESIGN Plastic pipe systems, in particular PVC-U, are economical and a practical material for sewer applications.
SOIL/PIPE INTERACTION The mode of failure of a pipe can be due to circumferential or longitudinal overloading. This section addresses concerns regarding external loads which cause circumferential stress. . Flexible PVC pipes deflect under the influence of vertical external loads and the reactive support of the surrounding soil. Failures can occur due to high bending, arching stresses in the wall (more common for rigid pipes) or buckling. In the case of flexible pipes, the stiffness of the surrounding material can be more important in limiting deflection than the stiffness of the pipe itself, so controlled backfill is particularly important. Rigidity of pipe Type of trench Narrow trench: Maximum friction - least load on the pipe Wide shallow trench: Less friction - more susceptible to superimposed loads such as vehicles Wide trench - deep embankment: Least friction - greater load on the pipe The vertical load due to soil is generally the most severe from the point of view of deflection and circumferential bending stress. Under embankments of cohesionless and frictionless soil, the vertical pressure at the level of the top of the pipe could be evaluated using column theory to be: W = GHD
Flexible pipes yield more than the sidefill and therefore the load is shed to the soil. The net soil load in kN/m of pipe is thus:
Soil modulus as a function of density
DEFLECTION Vertical deflection is limited by lateral soil resistance as the pipe tends to deflect outwards laterally. The load is thereby taken in arch action rather than circumferential bending, so wall stresses are considerably less than for rigid pipe, The deflection in metres allowing for load shedding is:
The E values for DURODRAIN PVC-U Sewer pipes, which decrease with age, to be used for soil load and deflection calculations are:
For live loading, one should perform a separate calculation using the short term E value and add up the two deflections caused by soil and live loads.
Deflection of PVC-U Sewer Pipe under Load EXAMPLE: DEFLECTION Calculate the deflection and maximum wall stress of a 315 mm diameter DURODRAIN heavy duty sewer pipe (wall thickness 9.20 mm) under 3m of average soil in a 900mm wide trench. Take the soil modulus as 3MPa, soil mass 2000kg/m³ or 20kN/m³ and long term modulus 1500MPa. Take bottom support over 90°, N = 0.095, Nm = 0.13 and sidefill lag factor 1.5, soil friction angle = 12°.
LIVE LOADS FROM SOIL SURFACE Pressure on the pipe in kN/m² due to a live load of P (kN) on the surface of the soil is:
The live load per metre of pipe, W1 = w1 D, should strictly be corrected for load shedding, but it activates a higher pipe modulus, E, than the soil load does, as it is only a temporary load. The deflection in metres allowing for load shedding and external live loads is:
WALL STRESS The maximum wall stress around the circumference of a pipe is due to a combination of ring bending under vertical load and arching. At the haunch it is:
DURODRAIN® sewer pipes can withstand stresses up to l10MPa (10 x 106 N/m²) since the short term minimum tensile strength is 42MPa and the 50 year nominal strength is 25MPa. Actual strength may be considerably more and depends on stress history. EXAMPLE: WALL STRESS
DEFLECTION COMPARISON: HD (class 34) vs ND (class 51) |
Depth of cover (m) | Percentage Deflection Diameter (mm) |
|||||
---|---|---|---|---|---|---|
160mm pipe | 315mm pipe | 500mm pipe | ||||
ND | HD | ND | HD | ND | HD | |
1 | 0.586 | 0.576 | 0.669 | 0.658 | 0.733 | 0.714 |
3 | 1.095 | 1.076 | 1.541 | 1.515 | 1.738 | 1.693 |
6 | 1.285 | 1.263 | 2.181 | 2.144 | 2.543 | 2.477 |
8 | 1.313 | 1.290 | 2.383 | 2.342 | 2.821 | 2.748 |
10 | 1.322 | 1.299 | 2.495 | 2.453 | 2.988 | 2.911 |
The table above compares the percentage deflection between DURODRAIN® normal duty (class 51) and DURODRAIN heavy duty (class 34) of different diameters and depths of cover. A low Es value of 3MPa (clay) was used in the above calculations of pipe deflection.
Note: The deflection of the 160mm normal duty pipe under a 3m soil load is 1.095% compared with 1.076% for heavy duty, a difference of 1.7% in the load bearing capacity, showing that wall thickness has relatively little effect on the soil load bearing capacity. This proves that normal duty (class 51) will adequately do the job of the overdesigned heavy duty (class 34) and result in cost savings.
Effects of Soil Modulus
The table below indicates the effect of soil modulus over deflection for solid wall PVC-U pipe.
DESIGN: VELOCITY AND FLOW
The flow in pipelines, flowing full or partially full but not under pressure, is defined as gravity flow because it is maintained by the slope component of the pipeline, resulting in a hydraulic gradient which is parallel to the pipeline invert. The flow characteristics can be determined by using the adjacent flow chart which is based on Mannings' formula.
Manning's Formula: | |||||||||
V | = | 1
n |
x | R0.67S0.5 | |||||
where | V | = | velocity | ||||||
n | = | Manning roughness coefficient (0.008 - 0.010) | |||||||
R | = | hydraulic radius, in metres | |||||||
S | = | slope, in metres per metres |