SECTION 217.53. Pipe Design

(a) Flow Design Basis. An owner must use the requirements of this section to design a gravity collection system.

(1) A collection system must be designed to transport the peak flow from the service area, plus infiltration and inflow. The design must minimize inflow and infiltration. Flow calculations must be included in the engineering report.

(2) The flow calculations must include the details of the average flow, the flow peaking factor, and the infiltration and inflow.

(3) The flow calculations must include the flow expected in the wastewater treatment facility immediately upon completion of construction and at the end of a 50-year period following construction.

(b) Gravity Pipe Materials.

(1) An owner must identify in the engineering report the proposed gravity collection system pipe with its appropriate American Society for Testing and Materials (ASTM), American National Standards Institute (ANSI), or American Water Works Association (AWWA) standard numbers for both quality control (dimensions, tolerances, pipe stiffness, dimensional ratio, etc.) and installation (bedding, backfill, etc.).

(2) The selection of gravity collection system pipes must be based on:

(A) the characteristics of the wastewater conveyed;

(B) the possibility of septic conditions;

(C) the ability to minimize inflow and infiltration;

(D) any external forces;

(E) any groundwater conditions;

(F) the internal pressures; and

(G) the abrasion and corrosion resistance of the pipe material.

(c) Joints for Gravity Pipes.

(1) The specifications for joints for gravity pipes must include the materials and methods used in making joints.

(2) Materials used for gravity pipe joints must prevent inflow, infiltration, and root entrance. A joint must:

(A) include rubber gaskets;

(B) include polyvinyl chloride (PVC) compression joints;

(C) include high density polyethylene compression joints;

(D) be welded; or

(E) be heat fused.

(3) The specifications must include ASTM, AWWA, ANSI, or other appropriate national reference standards for the pipe joints.

(d) Separation Distances.

(1) Collection system pipes must be installed in trenches separate from water supply trenches.

(2) Wherever possible, a collection system pipe must be located below a water supply pipe. If a collection system pipe cannot be located below a water supply pipe, the owner must justify in the engineering report why it is not possible to locate the collection system pipe below the public water supply pipe.

(3) Wherever possible, collection system pipes and manholes must be located at least nine feet from all water supply pipes. If a collection system pipe or manhole cannot be located at least nine feet away from a water supply pipe, the owner must justify in the engineering report why it is not possible to provide at least nine feet of separation. Table C.1. in Figure: 30 TAC §217.53(d)(3) provides a reference to paragraphs in this subsection that apply if a collection system pipe or manhole cannot be located at least nine feet away from a water supply pipe.

Attached Graphic

(4) If a collection system pipe is located above a water supply pipe and runs parallel to the water supply pipe, each portion of the collection system pipe within nine feet of the water supply pipe must be encased. The casing pipe must be constructed of at least 150 per square inch (psi) pressure class pipe that:

(A) encases the entire length of collection system pipe that is within nine feet of the water supply pipe;

(B) is sealed at both ends with cement grout or a manufactured seal;

(C) is at least two nominal sizes larger than the wastewater collection pipe; and

(D) is supported by spacers between the collection system pipe and the encasing pipe at a maximum of five-foot intervals.

(5) If a collection system pipe crosses above a water supply pipe, each portion of the collection system pipe within nine feet of the water supply pipe must either be encased in a casing pipe according to subparagraph (A) of this paragraph, or must be constructed using at least 150 psi pressure class pipe according to subparagraph (B) of this paragraph.

(A) A casing pipe for a collection system pipe that crosses above a water supply pipe must be constructed of at least 150 psi pressure class pipe that is:

(i) sealed at both ends with cement grout or a manufactured seal;

(ii) at least two nominal sizes larger than the wastewater collection pipe; and

(iii) supported by spacers between the collection system pipe and the encasing pipe at a maximum of five-foot intervals.

(B) A collection system pipe that crosses above a water supply pipe must be constructed of at least 150 psi pressure class, corrosion-resistant, non-brittle pipe and must use manufacturer-approved adapters. Gasketed joints, compression joints, and other non-bonded joints must be designed to seal at atmospheric pressure.

(6) If a collection system pipe is located below a water supply pipe and runs parallel to the water supply pipe, each portion of the collection system pipe within nine feet of the water supply pipe must either be constructed using at least 150 psi pressure class pipe according to subparagraph (A) of this paragraph, or must be encased in a casing pipe according to subparagraph (B) of this paragraph.

(A) A collection system pipe that runs parallel to and below a water supply pipe must be constructed of at least 150 psi pressure class, corrosion-resistant, non-brittle pipe that:

(i) is located at least two vertical feet below the water supply pipe;

(ii) is located at least four horizontal feet away from the water supply pipe; and

(iii) includes joints that are designed to seal at atmospheric pressure.

(B) A casing pipe for a collection system pipe that runs parallel below a water supply pipe must be constructed of at least 150 psi pressure class pipe that:

(i) is sealed at both ends with cement grout or a manufactured seal;

(ii) is at least two nominal sizes larger than the wastewater collection pipe; and

(iii) is supported by spacers between the collection system pipe and the encasing pipe at a maximum of five-foot intervals.

(7) If a collection system pipe crosses below a water supply pipe, each portion of the collection system pipe within nine feet of the water supply pipe must either be constructed using at least 150 psi pressure class pipe according to subparagraph (A) of this paragraph, or must be encased in cement-stabilized sand according to subparagraph (B) of this paragraph, or must be encased in a casing pipe according to subparagraph (C) of this paragraph.

(A) A collection system that crosses below a water supply pipe and is constructed of at least 150 psi pressure class, corrosion-resistant, non-brittle pipe must:

(i) have at least six inches of separation between the outsides of the pipes;

(ii) be centered on the crossing;

(iii) be at least 18 feet long; and

(iv) terminate at joints that are designed to seal at atmospheric pressure.

(B) A collection system pipe that crosses below a water supply pipe and is constructed of any material other than at least 150 psi pressure class, corrosion-resistant, non-brittle pipe must:

(i) have at least two feet of separation between the outsides of the pipes; and

(iii) be encased in cement-stabilized sand backfill that meets the requirements of subparagraph (D) of this paragraph.

(C) A casing pipe for a collection system pipe that crosses below a water supply pipe must be constructed of at least 150 psi pressure class pipe that is:

(i) sealed at both ends with cement grout or a manufactured seal;

(ii) at least two nominal sizes larger than the wastewater collection pipe; and

(iii) supported by spacers between the collection system pipe and the encasing pipe at a maximum of five-foot intervals.

(D) Cement-stabilized sand for encasing collection system pipes must:

(i) include at least 160 pounds of cement for every cubic yard of sand;

(ii) be installed beginning one-quarter pipe diameter below the centerline of the collection system pipe;

(iii) be installed ending one full pipe diameter above the top of the collection system pipe, or 12 inches above the top of the collection system pipe, whichever is greater.

(8) If a nine-foot separation distance between a manhole and a water supply pipe cannot be achieved, the manhole must either:

(A) have no measurable leakage during a leakage test conducted according to the requirements in §217.58 of this title (relating to Testing Requirements for Manholes); or

(B) have all portions of the manhole within nine feet of a water supply pipe encased in at least one foot of cement stabilized sand that meets the requirements of paragraph (7)(D)(i) and (ii) of this subsection.

(e) Building Laterals and Taps. Building laterals and taps must:

(1) include a manufactured fitting that prevents infiltration and root entrance;

(2) prevent service lines from protruding into the collection system pipe; and

(3) protect the mechanical and structural integrity of a collection system.

(f) Bore or Tunnel for Crossings. The spacing of supports for carrier pipe through casings must maintain the grade, slope, and structural integrity of the pipe as required by subsection (k) of this section.

(g) Corrosion Potential of Collection System Pipes.

(1) The engineering report must include calculations or other information that demonstrate the structural integrity of a pipe during the minimum 50-year design life cycle if a pipe or an integral structural component of a pipe has potential to deteriorate when subjected to corrosive internal conditions, or if a pipe or component does not have a corrosion resistant liner or protective coating installed by the pipe manufacturer.

(2) If the corrosion analysis indicates that corrosion will reduce the functional life of the pipe to less than 50 years based on the structural analysis in subsection (k) of this section, then the pipe must have a lining or protective coating that will extend the functional life to 50 years.

(h) Odor Control. An owner shall implement odor control measures necessary to prevent a collection system from becoming a nuisance.

(i) Active Geologic Faults.

(1) An owner shall identify all active faults within the boundaries of the collection system project and minimize the number of collection system lines crossing faults.

(A) If the crossing of a collection system over an active fault is unavoidable, the engineering report must specify design features that protect the structural integrity of a collection system in the event of movement of the fault.

(B) If a collection system line crosses an active fault line, the design must specify:

(i) joints that provide maximum flexibility; and

(ii) manholes on each side of the fault that would allow a portable pump to be used to prevent unauthorized discharge of wastewater in the event of a collection system failure.

(2) An owner shall not install a collection system service connection within 50 feet of an active fault.

(j) Capacity Requirements.

(1) An owner must ensure that a collection system's capacity is sufficient to serve the estimated future population of the area served by the project, including institutional, industrial, and commercial flows.

(2) An owner must include calculations in the engineering report that demonstrate the hydraulic capacity of a collection system, accounting for the peak flow of domestic wastewater, peak flow of wastewater from industrial sites, and maximum expected infiltration rates.

(3) An owner must ensure that the collection system has capacity to prevent a surcharge.

(4) An owner must ensure that a gravity pipe is at least 6.0 inches in diameter.

(5) Any connection between a stormwater collection system and a wastewater collection system is prohibited.

(6) An owner may use the data from an existing collection system for design purposes. In the absence of existing data, a design must use data from a system with similar characteristics, including:

(A) location;

(B) inflow and infiltration characteristics;

(C) peak flows;

(D) pipe materials;

(E) customer base; and

(F) any other characteristics required by the executive director.

(7) New collection systems.

(A) The sizing of pipe for a new collection system must be based on an engineering analysis of initial and future peak flow of domestic wastewater, peak flow of waste from industrial sites, and maximum expected infiltration rates.

(B) A new collection system design must be sized for the peak flow, which is based on the estimated daily wastewater flow contribution as shown in Table B.1. in Figure: 30 TAC §217.32(a)(3) of this title (relating to Organic Loadings and Flows for New Wastewater Treatment Facilities).

(k) Structural Analysis.

(1) An owner must ensure that a collection system is designed to have a minimum structural life of 50 years.

(2) For flexible pipe used in a collection system, which is pipe that will deflect at least 2% without structural distress, the engineering report must include:

(A) live load calculations;

(B) allowable buckling pressure determinations;

(C) prism load calculations;

(D) wall crushing determinations;

(E) strain prediction calculations;

(F) calculations that quantify long-term pipe deflection;

(G) the method of determining the modulus of soil reaction for bedding material and in-situ material;

(H) pipe diameter and material with reference to appropriate standards;

(I) modulus of elasticity;

(J) tensile strength;

(K) pipe stiffness, or ring stiffness constant converted to pipe stiffness;

(L) Leonhardt's zeta factor;

(M) trench width;

(N) depth of cover;

(O) water table elevation; and

(P) unit weight of soil.

(3) For trench installations, the design must specify a minimum stiffness requirement to ensure ease of handling, transportation, and construction. Pipe stiffness must be related to the ring stiffness constant by Equation C.1. in Figure: 30 TAC §217.53(k)(3)

Attached Graphic

(4) The owner is not required to perform the structural calculations in paragraphs (2) and (3) of this subsection, if the pipe is installed and tested in accordance with all other requirements of this subchapter and meets all of the following:

(A) the pipe is installed using an open trench design;

(B) the pipe is flexible pipe with a pipe stiffness of 46 psi or greater;

(C) the pipe is buried 17 feet or less from the ground surface;

(D) the pipe has a diameter of 12 inches or less;

(E) the modulus of soil reaction for the in-situ soil is 200 psi or greater;

(F) there are no effects on the pipe due to live loads from vehicles driving over the pipe;

(G) the unit weight of soil used for backfill is 120 pounds per cubic foot or less; and

(H) the pipe trench width is 36 inches or greater.

(5) A design analysis for rigid pipe installations must be included in the engineering report. The design analysis must include a structural analysis and all details necessary to verify that the structural strength is sufficient to withstand the expected stresses. For rigid pipes, the minimum strength for each class of pipe material and the appropriate standard must be included.

(l) Minimum and Maximum Slopes.

(1) All collection systems must contain slopes sufficient to allow a velocity not less than 2.0 feet per second when flowing at full capacity.

(2) When site-specific data is not available, a collection system must be designed in accordance with the minimum and maximum slopes specified in this paragraph.

(A) The slopes shown in the following table are based on Manning's formula with an assumed "n factor" of 0.013 and are the minimum acceptable slopes.

Attached Graphic

(i) The minimum acceptable "n" value for design of minimum pipe slopes is 0.013.

(ii) The "n" value must take into consideration the slime, grit, and grease layers that will affect hydraulics or hinder flow as a pipe ages.

(B) If a velocity greater than 10 feet per second will occur when a pipe flows full, based on Manning's formula, shown in Equation C.2. in Figure: 30 TAC §217.53(l)(2)(B), using the "n" value for new pipe recommended by the manufacturer, the collection system must be designed to protect against pipe and bedding displacement.

Attached Graphic

(m) Alignment.

(1) Alignment Requirements. A gravity collection system must be laid with a uniform grade and straight alignment between manholes, if possible. All deviations from uniform grade and straight alignment must be justified in the engineering report.

(2) Prohibited Deviations.

(A) Deviation from uniform grade (e.g., grade breaks or vertical curves) without manholes is prohibited if the open cut construction method is used, except as provided in subparagraph (B) of this paragraph.

(B) For segmented pipe, a horizontal curve must not be accomplished by bending the pipe unless the pipe joints are welded or heat-fused. Horizontal curves must be accomplished using additional manholes or joint offset. Horizontal curves for non-segmented, welded, or heat-fused pipes must follow the manufacturer's recommendations.

(C) A construction method that bends a pipe joint is prohibited, unless the joints are offset less than the least of the following:

(i) five degrees of deflection;

(ii) 80% of the manufacturer's recommended maximum joint offset; or

(iii) 80% of the appropriate ASTM, AWWA, ANSI, or other nationally established standard for joint offset.

(3) Calculations and Plan Drawings. The calculations for horizontal curvature must be included in the engineering report. Details of the proposed curvature must be displayed on the plan drawings.

(4) Manhole Spacing. The maximum allowable manhole spacing for collection systems with horizontal curvature is 300 feet. A manhole must be at the point of curvature and the point of termination of a curve.

(n) Inverted Siphons and Sag Pipes.

(1) A sag pipe must include:

(A) two or more barrels;

(B) a minimum pipe diameter of 6.0 inches; and

(C) the necessary appurtenances for convenient and routine flushing and maintenance.

(2) A manhole must include adequate clearance for rodding and cleaning.

(3) Sag pipes must be sized and designed with sufficient head to achieve a velocity of at least 3.0 feet per second at initial and design flows.

(4) The arrangement of inlet and outlet details must divert the normal flow to one barrel.

(5) A portion of the collection system with the inverted siphon must allow any barrel to be temporarily taken out of service for cleaning.

(6) Provisions must be made to allow cleaning across each bend with equipment available to the entity operating the collection system.

(7) A sag pipe must prevent nuisance odors.

(8) Inverted siphons and sag pipes must be pressure tested according to the requirement of §217.57 of this title (relating to Testing Requirements for Installation of Gravity Collection System Pipes).

(o) Bridged Sections.

(1) Piping with restrained joints or monolithic pipe across a bridged section requires a manhole on each end.

(2) A bridged section must withstand the hydraulic forces, including buoyancy, applied by the occurrence of a 100-year flood event.

(3) A bridged section must be capable of withstanding impacts from debris.

(4) Bank sections must be stabilized to prevent erosion.

(5) Bridge supports must be designed to ensure that a pipe has adequate grade, slope, and structural integrity.

Source Note: The provisions of this §217.53 adopted to be effective August 28, 2008, 33 TexReg 6843; amended to be effective December 4, 2015, 40 TexReg 8254