Manhole Flotation - American Concrete Pipe Association

41Manhole FlotationIntroductionThe proper functioning of a sewer system isdependent to a large degree on the performance of itsappurtenances, and especially its manholes. As withmany buried structures, the proper design of manholesshould take into account the effect of the water tableand its specific effect on installation and operatingconditions.Figure 1Manhole InstallationsCross Section of Extended BaseManhole InstallationThe Buoyancy ConceptFrom a fluid dynamics standpoint, the buoyant forceacting on a submerged object is equal to the weight offluid which that object displaces. In the case of a buriedstructure or manhole, this concept is applicable when ahigh ground water table or other subaqueous conditionexists. As with the design of buried pipe, flotation shouldbe checked when conditions such as the use of floodingto consolidate backfill, flood planes or future man-madedrainage changes are anticipated.ExtendedBaseCross Section of SmoothwallManhole InstallationManhole Buoyancy AnalysisVertical manhole structures of two types (Figure1) are generally constructed, and each type should beconsidered when analyzing the flotation potential. Thefirst case to be considered is a structure in which thebase does not extend past the walls of the manhole.This structure will be called a smooth-wall manholeinstallation. Smooth-wall manholes utilize the weight ofthe structure itself and the downward frictional resistanceof the soil surrounding the manhole to resist the upwardbuoyant force. Some manufacturers and designers usean extended base to provide additional resistance tobuoyant forces. These structures are constructed witha lip extending beyond the outer edges of the manholeand are termed extended base manhole installations. Anextended base manhole uses the additional weight ofsoil above the lip as well as its self-weight and frictionalforces to resist flotation.Design methods, using basic soil mechanics todetermine if a manhole is susceptible to flotation, arepresented here to aid the engineer in their design of thestructure.Shear StrengthFor an installed and backfilled structure to actuallyBaseFlush withExteriorSurface“float”, or exhibit upward movement, the buoyant forcesmust overcome both the weight of the structure and theshear between the walls and the soil. Shear strength isdefined in soils engineering to be the resistance to slidingof one mass of soil against another. Shear strengths,as typically provided, are a measure of the resistanceto sliding within a single uniform soil mass. The shearstrength of a soil has traditionally been related by twostrength parameters: internal friction and cohesion.Shear strength ( τ f ) as typically expressed byCoulomb’s equation is:American Concrete Pipe Association www.concrete-pipe.org info@concrete-pipe.org ACPA 20081DD 41 May 2008

τf c σ (tan φ)where:τf c φ σ shear strength, lbs/ft2 (kPa)cohesion, lbs/ft2 (kPa)angle of internal friction, degreesnormal stress on the sliding surface orshear plane, lbs/ft2 (kPa)Certain analyses require shear strength betweendissimilar substances, most commonly soil and concrete.This shear strength is an apparent rather than true shearresistance and is usually called sliding resistance.Sliding resistance equations typically use a coefficientof friction (f) to represent the friction between the twodissimilar materials. The equation for sliding resistanceis therefore defined as:rsliding c σ fCohesionless SoilsA uniform cohesionless soil (sands and gravels) bydefinition has a cohesion coefficient, (c), equal to zero.The sliding resistance equation, therefore, reduces to:rsliding c σ fSeveral different references are available whichaddress values for the coefficient of friction. Typicalvalues of the friction coeffficient (f) from the literaturegenerally range between .35 for silts and .55 for coarsegrained gravels against concrete. The friction coefficientis a function, however, of soil material type, manholematerial, outside manhole profile, compaction levels,and material homogeneity. The designer is cautioned tosolicit site specific materials properties and appropriatedesign values for individual projects from professionalsin the geotechnical field.Table 5.5.5B of the American Association of StateHighway and Transpor tation Officials (AASHTO)Standard Specification for Highway Bridges, 15th Edition(1992), lists friction factors for soil materials againstconcrete. The values in Table 1 are taken from Table5.5.5B of AASHTO.Normal PressureIn order to quantify the sliding resistance, it isnecessary to determine the lateral pressure on the wallsof the manhole. Assuming the top of the manhole to beat ground surface, the vertical earth pressure is equalto zero at the surface and varies as a function of the soildensity with depth to the base. Since the engineer isconcerned with flotation, we will initially look at the casewhere the ground water surface is even with the top ofthe manhole.Because a high groundwater condition is beinganalyzed, the designer must determine the effectiveweight of soil solids which are buoyed up by the waterpressure. This submerged soil weight becomes less thanthat for the same soil above water and is given by:γsub γswhere:1-1S.G.γsub effective unit weight submerged,lbs/ft3 (N/m3)γs unit weight of dry soil,lbs/ft3 (N/m3)S.G Specific Gravity of the soil,dimensionlessSpecific gravity is based only upon the solid portion ofa material. Soil is a conglomerate of minerals, all havingdiffering specific gravities, and containing voids betweenthe soil grains. For this analysis, a specific gravity for thesoil particles as they occur naturally will be used. Specificgravity ranges from 2.50 to 2.80 for most soils, with amajority of soils having a specific gravity near 2.65.Vertical manhole structures normally experience ringcompression and therefore are subject to active earthTable 1Ultimate Friction Factors and Friction Angles for Dissimilar Materials from AASHTO Table 5.5.5BInterface MaterialsFrictionAngle, δ(Degrees)FrictionFactor, ftan δ (DIM)Formed concrete or concrete sheet piling against the following soils: Clean gravel, gravel-sand mixture, well-graded rock fill with spans . 22 to 26 . 0.40 to 0.50 Clean sand, silty sand-gravel mixture, single size hard rock fill . 17 to 22 . 0.30 to 0.40 Sllty sand, gravel or sand mixed with silt or clay.17 . 0.30 Fine sand silt, nonplastic silt .14 . 0.25American Concrete Pipe Association www.concrete-pipe.org info@concrete-pipe.org2DD 41 May 2008

pressures. The magnitude of the normal pressure, at agiven depth (d) is determined by both the effective weightof the soil pushing against the wall of the structure andthe water pressure caused by the submerged condition(Figure 2). This normal force is given by the followingequation:σ (Ka γsub γw) dwhere: σKadγw Normal stress against wall, lbs/ft2(kPa) Active lateral earth pressurecoefficient, dimensionless Depth at which normal pressureacts, ft (m) Unit weight of water, 62.4 Ibs/ft3(9.8 kN/m3)However, water exerts a negligible friction force onthe wall of the manhole. Therefore, the normal stressconsidered for friction is based on the effective verticalstress:σ (Ka γsub) dwhere: σ Effective normal stress against wall,lbs/ft2 (N/m2)Estimates of active lateral earth pressure vary withsoil type. Marston’s values included in Table 3, as wellas commonly accepted values, indicate that a lateralearth pressure coefficient of 0.33 is adequate for mostsoil materials.Sliding ResistanceThe total sliding resistance available to resistflotation at a given depth (d) is the combination of the unitresistance (rsliding) acting over the exterior circumfe 0-4,000 (96 - 192)Very Stiff . Readily indented by thumbnail .4,000-8,000 (192 - 384) Hard . Indented with difficulty by thumbnail .Over 8,000 (384)American Concrete Pipe Association www.concrete-pipe.org info@concrete-pipe.org5DD 41 May 2008

The designer is cautioned to use site specificvalues or, moreover, those determined to be valid by aprofessional skilled in the determination of geotechnicalparameters. The values shown in Table 2 may not applyto individual project conditions.With the value of (qu) defined, Rsliding is defined as:Rsliding π (Bd )qu2(H )Rsliding π (Bd ) (c) (H )As previously stated, the buoyant force (B) is equal tothe weight of water displaced by the manhole structure.This force is defined as the density of water multipliedby the volume of water displaced by the structure, andis expressed as:Bd 24W RslidingBExtended Base Manhole InstallationsBuoyancy AnalysisπF.S.buoyancy Generally, if the weight of the structure is the primaryforce resisting flotation than a safety factor of 1.0 isadequate. However, if friction or cohesion are the primaryforces resisting flotation, then a higher safety factor wouldbe more appropriate to account for the variability of thesoil properties.orB γWKeeping this relationship in mind, a design safety factorfor buoyancy is easily derived:Hwhere: γw density of water 62.4 Ibs/ft3 (9.8 kN/m3)Installations in which the base slab extends beyondthe outer face of the manhole wall (Figure 5) are treatedonly slightly differently than the smooth-wall installations.This case can be analyzed as a smooth-wall installationwith the following exceptions.1) The diameter of the base (Db) should be usedinstead of using the external diameter of the structureitself (Bd) to determine the perimeter of the failurecylinder.2) Since the failure mode is a backfill shear failure, thecoefficient of friction (f) for granular materials shouldbe determined by:Design Safety Factorf tanThis buoyant force (B) is resisted by the weight of themanhole assembly (W) and the total sliding resistance(Rsliding).For the structure to be stable:φwhere: φ angle of internal friction for the backfillmaterialFor an initial estimate, a friction coefficient maybe taken from Table 3 which gives soil frictionW Rsliding BTable 3Approximate Safe Working Values of the Constants to Be Used In Calculating the Loads on Pipesin DitchesDitch Fillw Unit Weightof Backfill(lbs/ft3)K Ratio ofLateral toVertical EarthPressuresu coefficientof FrictionAgainst Sides ofTrenches900.330.50Saturated Top Soil1100.370.40Partly Compacted Damp Yellow Clay1000.330.40Saturated Yellow Clay1300.370.30Dry sand1000.330.50Wet Sand1200.330.50Partly Compacted Top SoilAmerican Concrete Pipe Association www.concrete-pipe.org info@concrete-pipe.org6DD 41 May 2008

Figure 5WWManholeAdditionalAnchoringForceProvided BySoil Above BaseLipBdDbcoefficients developed by Marston for soils placedin a trench with a similar insitu soil.3) The additional effective weight of the backfill in thecylinder above the lip can be added as an anchoringforce.The designer is advised to use caution in the analysisof extended base structures because the parametersemployed vary greatly with material proper ties,compaction levels, and friction factors assumed betweennative and compacted soils. In the case of extendedbase structures, an alternative would be to perform theanalysis as detailed for a smooth-wall structure. Althoughgreatly conservative, this analysis can provide a quickcheck of the particular installation against buoyancy.Cones and ReducersIf a cone or reducer section is used for a manhole,the designer shall calculate the total weight accordingly.The sliding resistance for the cone or reducer sectionshall be calculated as follows:Ps Kaγsub (Hbot – Htop)Hbot – Htop2where: Rs-sliding Sliding resistance of the individualsectionRs–sliding Ps (f ) πPsHbotHtopDbotDtopDbot Dtop2 Resultant horizontal force on the individualsection Fill height to the bottom of the section Fill height to the top of the section Outside diameter of the bottom of thesection Outside diameter of the top of the sectionThis sliding resistance can then be added to thesliding resistance of the manhole above and below thesection. (Figure 6)American Concrete Pipe Association www.concrete-pipe.org info@concrete-pipe.org7DD 41 May 2008