The light weight of HDPE pipe makes it desirable because of the ease of handling and installation but this same benefit also makes HDPE pipe prone to flotation. All pipe products, such as concrete and corrugated metal, are prone to flotation under the right circumstances. In fact, all pipe materials and other buried structures are subject to flotation. When the uplift on the pipe or structure exceeds the downward force of the weight and load it carries, the pipe (or structure) will rise or heave. Where flotation is a possibility, proper installation and/or anchoring of the pipe is critical. This document provides an analysis on minimum cover heights required to prevent pipe flotation for HDPE pipe sizes 12”-60”. Buoyant forces due to Controlled low strength material (CLSM) will also be discussed.
Hydrostatic Uplift Due to a High Water Table
Buoyancy becomes an issue in buried pipe when the groundwater encroaches into the pipe zone. For projects where a high groundwater table or water surrounding the pipe is expected, precautions should be taken to prevent the floatation of HDPE pipe. Under the right conditions and when increased cover heights are possible, providing a minimum amount of cover will help prevent flotation. The vertical hydrostatic uplift force, U, due to the water table can easily be calculated from Equation 1 below: π 2 U = D δw (1) 4 where U = lb/linear ft of pipe D = O.D. of the pipe in question, ft. δw = unit weight of water = 62.4 lb/ft3 This hydrostatic uplift force must be balanced by soil overburden and the weight of the pipe in order to ensure that the pipe will not float. Soil loads experienced by a pipe at varying water table depths (Wsoil) can be calculated from Equation 2. Figure 1 illustrates each of the three cases seen in field installations where buoyancy becomes a concern, and also clarifies all of the parameters contained within Equation 2. Wsoil = δdryHdryD + (δsat- δw)(Hsub+ 0.1073D)D (2) where Wsoil = weight of soil overburden, lb/linear ft of pipe δdry = dry unit weight of the soil, lb/ft3 Hdry = depth of dry soil, ft. Hsub = depth of submerged soil over top of pipe, ft. δsat = saturated unit weight of the soil, lb/ft3 δsat - δw = submerged unit weight of the soil, lb/ft3
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Figure 1 Installation Conditions for Possible Flotation of HDPE Pipe
(a) Water table at pipe crown
(b) Water table exceeds pipe crown elevation
(c) Water table is at ground surface
The typical weights (Wpipe) and average outside diameters are shown in Table 1.
Table 1 Approximate Weights of Dual Wall HDPE Pipe
Nominal Diameter in. (mm) 4 (100) 6 (150) 8 (200) 10 (250) 12 (300) 15 (375) 18 (450) 24 (600) 30 (750) 36 (900) 42 (1050) 48 (1200) 60 (1500) Nominal OD in. (mm) 4.6 (117) 7.0 (178) 9.5 (241) 12 (305) 14.5 (368) 18 (457) 22 (559) 28 (711) 36 (914) 42 (1067) 48 (1219) 54 (1372) 67 (1702) Weight lb/ft (kg/m) 0.44 (0.6) 0.85 (1.3) 1.5 (2.2) 2.1 (3.1) 3.2 (4.7) 4.6 (6.8) 6.4 (9.5) 11.0 (16.4) 15.4 (22.9) 19.8 (29.4) 26.4 (39.3) 31.3 (46.6) 45.2 (67.3)
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The minimum depth of cover (H) required to resist uplift can be calculated by equating the sum of the downward forces to the sum of the upward or buoyant forces. While there are varying methods to account for soil load distribution on the pipe, for conservative minimum cover requirements, the soil load is assumed to be the soil column directly above the outside diameter of the pipe as illustrated in Figure 2(a). Therefore, minimum cover is calculated using Equations 3 and 4 below: U O WSoil + WPipe where (3)
Wpipe = weight of the pipe, lb/linear ft of pipe H = Hdry + Hsub (4)
Figure 2 Forces Affection Flotation
(a) Soil Column Loading Conditions
(b) Prism Loading Conditions...