Sign up for a Competition at Texas WaterTM 2023

Texas WaterTM 2023 will be held April 11-14, 2023 at the George R. Brown Convention Center in Houston. If you're feeling competitive, don't miss out on signing up for a competition!

Texas AWWA hosts several competitions at Texas Water, and the winners will continue on to represent TAWWA at the AWWA ACE23 competition in Toronto, Canada in June.

TAWWA competitions include the Best-Tasting Drinking Water Competition, Hydrant Hysteria (registration has closed for this event), Meter Challenge, Pipe Tapping and Top Ops. Find out more information about each competition and register online at https://www.txwater.org/competitions_2023.cfm.

If you're not participating in a competition, please stop by to cheer on your fellow TAWWA members at the conference! The competition schedule can be found in the online conference preview posted at www.txwater.org

We hope to see you in April in Houston!

CONTINUED FROM PAGE 3 | the perfect match on the polyethylene pipe. Thus, ASTM F1962 provides a means of determining project feasibility, as well as initial design information. Such results could be further refined by competent engineering expertise, including an analysis of pipe and soil characteristics and interaction, often including the use of relatively sophisticated software tools, possibly based on the ASTM F1962 methodology. The procedure is straightforward and has been incorporated into various software tools (e.g., PPI Boreaid®) as an engineering and design aid for such projects.

The ASTM standard specifically addresses polyethylene pipe (PE), a flexible product, for which the effects of any bending stiffness on drag forces at route bends or path curvature may be ignored. Relevant material properties for PE pipe, such as safe pulling tension/stress, are also provided in the document. Because of its high ductility and flexibility and ability to be fused in continuous lengths, per the well-established ASTM

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CONTINUED FROM PAGE 20 | the perfect match standards and by experienced contractors, polyethylene pipe, particularly high density polyethylene (HDPE), is the most commonly used product for HDD projects. Although ASTM F1962 is intended for carefully designed, well-controlled maxi-HDD installations, the methodology has been extended, via appropriate assumptions and mathematical simplifications, to provide a simpler methodology that may be advantageously applied to typical, less well-controlled mini-HDD projects. This is the basis of MAB-7 (2020), MAB Guidelines for Use of Mini-Horizontal Directional Drilling for Placement of HDPE (PE4710) Pipe in Municipal Applications, as referenced in the 2022 edition of ASTM F1962.

Description

Figure 2 illustrates a typical geometry for a maxi-HDD operation, in this case corresponding to a river crossing. The indicated bore/pipe path comprises three segments spanning the pipe entry (point A) to its exit point (point D), with horizontal projected distance Lbore, equal to the sum of the three horizontal (projected) segments L2, L3 and L4. There is an additional length L1 exterior to the drilled path which allows for handling at both ends and possible other effects (path curvature, thermal contraction, stretching, etc.). The intermediate horizontal segment, L3, may be of zero length, similar to the geometry in Figure 1.

ASTM F1962 contains ten main chapters or sections, as briefly described below, supplemented by several appendices.

Scope, Referenced Documents and Terminology (Sections 1, 2 and 3)

ASTM F1962 addresses the overall planning and design, product selection and installation practices for the placement of polyethylene pipe using maxi-HDD equipment. The primary focus is on commonly used high density polyethylene (HDPE) pipe with a material designation code of PE4710. For the larger diameters typically used for such applications, the polyethylene pipe is supplied in discrete segments which are fused together in the field, with essentially no loss in tensile strength, facilitating the pulling operation.

Preliminary Site Investigation (Section 4)

Both ends of the bore path, must be investigated to confirm their feasibility for successfully completing the installation of the large, long pipeline. The drill rig and auxiliary equipment located on the pipe exit side are relatively large, and require water access, storage and mixing and pumping facilities. The pipe entry side, opposite the drill rig, must accommodate the long length of assembled (fused) pipe. In addition, there should be a detailed subsurface investigation, including test borings and soil analysis, to confirm

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Infrastructure and money challenges?

Make Every Drop

CONTINUED FROM PAGE 23 | the perfect match the general suitability for the drilling operation, and to determine the appropriate equipment and hardware.

Safety and Environmental Considerations (Section 5)

Safety is a primary concern, during any activity, including construction utilizing maxi-HDD equipment and procedures. Potential safety issues fall into two general categories:

(1) those directly related to the setup and operation of the maxi-HDD equipment, related to the machine and hydraulic operations, as well as the drilling fluid under high pressure, and (2) those associated with accidentally striking buried electric power lines or other existing pipelines. Although not considered to be hazardous materials, the proper handling and disposal of drilling fluid is also discussed to avoid possible environmental issues.

Regulations and Damage Prevention (Section 6)

Depending upon the location and extent of the operation, a variety of permits or approvals may be required, possibly from Federal, state or local jurisdictions. These may include the need to file environmental, health and safety plans, or permits for passing beneath waterways, and there may be special requirements for drilling beneath railroads.

Bore Path Layout and Design (Section 7)

The planned bore path, such as illustrated in Figure 2, must be consistent with the steering capability of the maxi-HDD equipment as well as the bending capability of the drill rods. The stiffness of the steel drill rods determines their allowable curvature, in order to avoid fatigue. Unnecessarily large curvatures (sharp bends) also contribute to bending stresses in the HDPE product pipe, although are generally not significant for such a flexible product. Additional route bends, beyond those shown in Figure 2, as well as possible bends in the horizontal plane, should be avoided, and will increase the required pulling forces.

Pipe Design and Selection Considerations (Section 8)

Any pipe installed by HDD is subject to loads of a different type and/or magnitude than that experienced in other construction methods, including by direct burial in a trench. In addition to external pressures due to the head of the relatively dense drilling fluid/slurry or subsequent (post-installation) soil loads, the pipe must withstand the axial tensions induced during the pullback process. While the document provides useful information for evaluating the potential for collapse under lateral pressures, either during of following the installation, the most widely used portion of ASTM F1962 is the set of formulae for estimating the pulling tensions corresponding to the leading end of the pipe reaching point A, B, C and D. These formulae account for the frictional drag acting on the pipe along the surface of the borehole, primarily due to the high buoyant weight of the HDPE pipe, within the relatively dense drilling fluid, especially for vacant pipe, but are sufficiently general to consider the possible implementation of anti-buoyancy measures to reduce these otherwise high frictional forces. (The theoretical basis for the formulae for estimating the required pulling force is provided below.) Thus, a commonly used procedure for difficult or very long installations is to fill the pipe with water to reduce the buoyancy. In the absence of ballast, the maximum total calculated tension will typically occur towards the end of the installation; e.g., at point C or D.

The net resulting peak tensile stress is required to be less than the safe pull tensile stress of the HDPE pipe. The physical properties of the PE material(s) are provided in the appendices, which allow a determination of an appropriate wall thickness for the pipe, depending on the pipe diameter and estimated peak pulling force (or tendency to collapse). Software tools such as Boreaid® (www.boreaid.com, www.ppiboreaid.com), are based on the ASTM F1962 document and model.

Implementation (Section 9)

Due to the magnitude and complexity of maxi-HDD equipment and control systems, a well-trained, experienced crew is essential to plan and execute the operation. The initial decision involves selecting the size and capacity of the machine, which, at a minimum, should be able to provide the necessary pulling force based on the estimated required pulling force for the pipe itself, with possible additional capacity for accomplishing reaming. It is important to properly use the drilling fluid for the initial pilot bore and reaming operations, and to accurately locate and track the bore path. The pipe must be securely gripped, including a swivel and possible breakaway link. As-built drawings must be provided, preferably supplemented with details of the soil characteristics and drilling operation.

Inspection and Site Cleanup (Section 10)

The HDPE pipe should not be cut prematurely, but should first be allowed to reach mechanical and thermal equilibrium, to avoid shrink back onto the bore hole. The exposed leading end of the pipe should be inspected for possible damage, and a pressure or leakage test may be required, for fluid transport applications.

Appendices

The ten main sections outlined above are supported by several appendices which provide the physical properties of the HDPE (PE4710) material as well as a means of determining the post-installation loads and pipe deflection.

Theoretical Basis for Load Estimation

The theoretical model used to develop the formulae for estimating the peak required tension assumes that the local frictional drag forces on the pipe are proportional to the local normal bearing forces applied at the pipe surface. For flexible PE pipe, with minimal bending stiffness, the considered bearing forces are those due to the dead (empty) weight of the pipe where above ground, the buoyant weight of the submerged pipe (possibly reduced by the use of ballast), or the bearing forces resulting from (previously induced) pipe tension tending to pull the pipe snugly against any curved surfaces ("capstan effect"). In addition, there is a contribution due to the drilling fluid/slurry flowing along the length of the pipe, but which is relatively low, based on the present model.

Frictional Drag Due to Weight and Buoyancy

In the absence of anti-buoyancy techniques, such as internal water ballast, the frictional drag developed within the borehole is generally much greater than that developed outside. Because of the high buoyant weight for an empty PE pipe. For such cases, the buoyant weight of the submerged pipe, in combination with the corresponding frictional characteristics, is the major factor in determining the required pull force. ASTM F 1962 provides formulae for determining the buoyant

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CONTINUED FROM PAGE 25 | the perfect match weight under various conditions. The buoyant weight is a function of the density of the drilling fluid/slurry, for which a conservatively high value is suggested for design purposes.

Capstan Effect at Bends

Although pipe stiffness effects may generally be ignored for flexible PE pipe, there is nonetheless a potentially important effect due to route bends or any path curvature that should can be significant. Tensions induced in the PE pipe as it passes any curve, become amplified because the tensile forces tend to pull the pipe against the curved surface. Such effects are independent of the pipe stiffness, pipe diameter, borehole clearance, radius of curvature or direction of curvature, and, in some cases become a major consideration due to their compounding effect. This phenomenon is referred to as the "capstan effect" as it is the principle of the capstan winch, as illustrated in Figure 3.

However, for the geometry shown in Figure 2, with relatively shallow entry and exit angles, the associated load amplification due to this effect is not major, although for more complex paths the effect could be very important. Mini-HDD applications, for instance, tend to contain bore paths with additional curvature because of the need to avoid known obstacles or follow a curved right-of-way, as well as more subtle curvature due to path corrections characteristic or these typically less precisely controlled installations.

Hydrokinetic Surface Drag (Fluidic Drag)

The effect of the shear forces directly imparted on the surface of the pipe by the drilling fluid ("fluidic drag") has been handled in a widely disparate manner within the industry, and is sometimes considered to be a major consideration. In contrast, the convenient model employed in ASTM F1962 results in a very low magnitude effect, which is directly added to the estimated pulling forces due to the frictional drag, including the capstan effect.

Summary

The recent (2022) edition of ASTM F1962 has revised the relevant physical characteristics of the PE materials, providing the physical properties for the most recent high density polyethylene (HDPE) material, PE4710.

This material is significantly tougher than older, previously used PE products, facilitating successful completion of more complicated, difficult maxi-HDD projects. The physical properties of the PE4710 material allow greater pulling forces and also provide greater resistance to collapse.

References

Standard Guide for Use of Maxi-Horizontal Directional Drilling for Placement of Polyethylene Pipe or Conduit Under Obstacles, Including River Crossings, ASTM F1962 – 22, American Society for Testing and Materials, 2022.

MAB Guidelines for Use of Mini-Horizontal Directional Drilling for Placement of HDPE (PE4710) Pipe in Municipal Applications, MAB-7 2020, Municipal Advisory Board, Plastics Pipe Institute, 2020.

Directional Drilling Design with ASTM F1962: A Decade of Success, L. Petroff, ASCE International Pipelines Conference 2010.

Discussion of ASTM F 1962 or “How are the Pulling Load Formulas Derived and How are they Used ?”, L.M. Slavin and L Petroff, NO-DIG 2010.

Integrated Water Solutions

CONTINUED FROM PAGE 4 | letter from the texas section chair push-back to these bold initiatives as being too costly or unrealistic. Unfortunately, Mary Rhodes is no longer with us today to further champion these causes, but what a water legacy she has left behind. Fortunately, the City of Corpus Christi has new leadership in Mayor Paulette Guajardo, who is working hard to ensure that Corpus Christi continues to develop its water supplies with a strong focus on environmental stewardship, resiliency, equity, and of course financial viability.

So where does Corpus Christi go from here as it relates to developing its water supply portfolio? The eighth largest city in Texas, and regional water provider for seven counties, has a lot of pressure to deliver a viable, economical, and sustainable solution. The City, like many other cities around the country, is seriously considering a number of bold options as outlined below, and perhaps, a combination of one or more alternatives will be implemented.

Groundwater, a drinking water source used extensively by many municipalities around the State, tends to be brackish in the Texas Coastal Bend with Total Dissolved Solids (TDS) generally over 500 mg/l in this region. The TCEQ secondary MCL for TDS is 1,000 mg/l, whereas the EPA’s secondary standard is 500 mg/l. Exceeding these values would not be acceptable, and a reliance on groundwater in an already compromised aquifer where water quality will not improve over time is problematic. Then there are the issues of land subsidence when pumping groundwater—just look at Houston where some areas of the city have sunk by 8 to 10 feet.

Capturing wastewater effluent could potentially be used as an ultimate water source, but not without some challenges. Corpus Christi has been evaluating the possibilities of using wastewater effluent as a source of water for Aquifer Storage, and Recovery (ASR) as part of an Indirect Potable Reuse (IPR) approach. The technology is proven; however, the challenge is that the effluent is relatively minimal and localized, thereby requiring it to be conveyed to an aquifer, where additional treatment may need to occur prior to injection into the aquifer for storage. The next step would require the recaptured water to be pumped to a water treatment plant for advanced treatment. From an environmental standpoint, the city is contractually bound by an Agreed Order to release specific quantities of water to the bays and estuaries, and wastewater effluent is one of several resources that positively contributes to these environmental flows.

Ultimately, the most drought-proof, sustainable, environmentally-responsible, and cost-effective approach given the proximity to the Texas Gulf Coast, where an endless supply of raw water exists in our backyard, is seawater desalination. The concept of seawater desalination has been studied by experts in this region for many years. While it has some opposition, much of it unfounded, it checks most, if not all of the important criteria for Corpus Christi’s next water supply. It has been successfully used by many drought-laden cities around the world for many years.

Senator Perry, the Chair of the Water, Agriculture, and Rural Affairs Committee, is currently drafting a bill that would create a Water for Texas Fund to support water infrastructure and supply projects across Texas. One of the proposed provisions is to disburse funds specifically for desalination projects. This is exciting, promising and something that anyone who lives in Texas, especially along the Gulf Coast, is watching closely.