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

   1. (1) An owner must design a wastewater collection system to handle the transport of the peak dry weather flow from the service area, plus infiltration and inflow.
   2. (2) The flow calculations must include the details of the average dry weather flow, the dry weather flow peaking factor, and the infiltration and inflow.
   3. (3) The flow calculations must include the flow expected in the facility immediately upon completion of construction and at the end of its 50-year life.

2. (b) Gravity Pipe Materials.

   1. (1) An owner must identify in the 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, etc.) and installation (bedding, backfill, etc.).

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

      1. (A) the characteristics of the wastewater conveyed;
      2. (B) the character of industrial wastes;
      3. (C) the possibility of septic conditions;
      4. (D) the exclusion of inflow and infiltration;
      5. (E) any external forces;
      6. (F) any groundwater;
      7. (G) the internal pressures; and
      8. (H) the abrasion and corrosion resistance of the pipe material.

3. (c) Joints for Gravity Pipe.

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

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

      1. (A) include rubber gaskets;
      2. (B) include polyvinyl chloride (PVC) compression joints;
      3. (C) include high density polyethylene compression joints
      4. (D) be welded;
      5. (E) be heat fused; or
      6. (F) include other types of factory-made joints.
   3. (3) The technical specifications must include ASTM, AWWA, ANSI, or other appropriate national reference standards for the joints.

4. (d) Separation distances between public water supply pipes and wastewater collection system pipes or manholes.

   1. (1) Collection system pipes must be installed in trenches separate from public water supply trenches.
   2. (2) Collection system pipes must be no closer than nine feet in any direction to a public water supply line.

   3. (3) If a nine-foot separation distance cannot be achieved, the following guidelines will apply.

      1. (A) If a collection system parallels a public water supply pipe the following requirements apply.

         1. (i) A collection system pipe must be constructed of cast iron, ductile iron, or PVC meeting ASTM specifications with at least a 150 pounds per square inch (psi) pressure rating for both the pipe and joints.
         2. (ii) A vertical separation must be at least two feet between the outside diameters of the pipes.
         3. (iii) A horizontal separation must be at least four feet between outside diameters of the pipes.
         4. (iv) A collection system pipe must be below a public water supply pipe.

      2. (B) If a collection system pipe crosses a public water supply pipe, the following requirements apply:

         1. (i) If a collection system is constructed of cast iron, ductile iron, or PVC with a minimum pressure rating of 150 psi, the following requirements apply:

            1. (I) A minimum separation distance is six inches between outside diameters of the pipes.
            2. (II) A collection system pipe must be below a public water supply pipe.
            3. (III) Collection system pipe joints must be located as far as possible from an intersection with a public water supply line.

         2. (ii) If a collection system pipe crosses under a public water supply pipe and the collection system pipe is constructed of acrylonitrile butadiene styrene (ABS) truss pipe, similar semi-rigid plastic composite pipe, clay pipe, or concrete pipe with gasketed joints, the following requirements apply:

            1. (I) A minimum separation distance is two feet.

            2. (II) If a collection system pipe is within nine feet of a public water supply pipe, the initial backfill around the collection system pipe must be:

               (-a-) sand stabilized with two or more 80 pound bags of cement per cubic yard of sand for any section of collection system pipe within nine feet of a public water supply pipe.

               (-b-) installed from one quarter of the diameter of the collection system pipe below the centerline of the collection system pipe to one pipe diameter (but not less than 12 inches) above the top of the collection system pipe.

         3. (iii) If a collection system crosses over a public water supply pipe, one of the following procedures must be followed:

            1. (I) Each portion of a collection system pipe within nine feet of a public water supply pipe must be constructed of cast iron, ductile iron, or PVC pipe with at least a 150 psi pressure rating using appropriate adapters.

            2. (II) A collection system pipe must be encased in a joint of at least 150 psi pressure class pipe that is:

               (-a-) centered on the crossing;

               (-b-) sealed at both ends with cement grout or manufactured seal;

               (-c-) at least 18 feet long;

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

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

   4. (4) Public water supply pipe and collection system manhole separation.

      1. (A) Unless collection system manholes and the connecting collection system pipe are watertight, as supported by leakage tests showing no leakage, they must be installed a minimum of nine feet of horizontal clearance from an existing or proposed public water supply pipe.
      2. (B) If a nine-foot separation distance cannot be achieved, the requirements in paragraph (3) of this subsection apply.

5. (e) Building laterals and taps. Building laterals and taps on an installation must:

   1. (1) include a manufactured fitting that limits infiltration;
   2. (2) prevent protruding service lines; and
   3. (3) protect the mechanical and structural integrity of a wastewater collection system.
6. (f) Bore or tunnel for crossings. The spacing of supports for carrier pipe through casings must maintain the grade, slope, and structural integrity of a pipe as required by subsection (k) of this section.

7. (g) Corrosion potential.

   1. (1) If a pipe or an integral structural component of a pipe will deteriorate when subjected to corrosive internal conditions or if a pipe or component does not have a corrosive resistant liner installed by the pipe manufacturer, the report must demonstrate the structural integrity of a pipe during the minimum 50-year design life cycle.
   2. (2) A pipe must have an appropriate lining if the corrosion analysis indicates that corrosion will reduce the functional life of the pipe to less than 50 years.

8. (h) Odor Control.

   1. (1) An owner shall determine if odor control measures are necessary to prevent a wastewater collection system from becoming a nuisance, based upon the potential of the wastewater collection system to generate hydrogen sulfide.
   2. (2) A potential odor determination must include the estimated flows immediately following construction and throughout a system's 50-year expected life cycle.

9. (i) Active Geologic Faults.

   1. (1) An owner shall identify any active faults within the area of a collection system and minimize the number of collection system lines crossing faults.

      1. (A) Where an active fault crossing is unavoidable, the report must specify design features that protect the integrity of a wastewater collection system in the event of movement of the fault.

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

         1. (i) joints that provide maximum deflection, as required in subsection (m) of this section; and
         2. (ii) manholes on each side of the fault so that a portable pump may be used in the event of a wastewater collection system failure.
   2. (2) An owner shall not install a collection system service connection within 50 feet of an active fault.

10. (j) Capacity Analysis.

    1. (1) An owner must ensure that a wastewater collection system's capacity is sufficient to serve the estimated future population, including institutional, industrial, and commercial flows.
    2. (2) An owner must include in the report the calculations that demonstrate that the hydraulic capacity of a collection system includes the peak flow of domestic sewage, peak flow of waste from industrial sites, and maximum infiltration rates.
    3. (3) A collection system must be designed to prevent a surcharge in any pipe at the expected peak flow.
    4. (4) The minimum diameter allowed for a gravity pipe is 6.0 inches.
    5. (5) Connecting storm water drains to a collection system is prohibited.
    6. (6) An owner may use the data from an existing collection system. In the absence of existing data, a design must use data from a similar system or as described in paragraph (7) of this subsection.

    7. (7) New collection systems.

       1. (A) The sizing of pipe for a new collection system must be based on an engineering analysis of initial and future flows.
       2. (B) A new collection system design must be sized for the peak flow, which is based on the estimated daily sewage flow contribution as shown in Figure: 30 TAC §217.32(a)(3), Table B.1 of this title (relating to Organic Loadings and Flows).

11. (k) Structural Analysis.

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

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

       1. (A) live load calculations;
       2. (B) allowable buckling pressure determinations;
       3. (C) prism load calculations;
       4. (D) wall crushing determinations;
       5. (E) strain prediction calculations;
       6. (F) calculations that quantify long term pipe deflection; and

       7. (G) all information pertinent to a determination of an adequate design including, but not limited to:

          1. (i) the method of determining the modulus of soil reaction for bedding material and in-situ material;
          2. (ii) pipe diameter and material with reference to appropriate standards;
          3. (iii) modulus of elasticity,
          4. (iv) tensile strength,
          5. (v) pipe stiffness or ring stiffness constant converted to pipe stiffness;
          6. (vi) Leonhardt's zeta factor;
          7. (vii) trench width;
          8. (viii) depth of cover;
          9. (ix) water table elevation; and
          10. (x) unit weight of soil.
    3. (3) The design procedure dictates a minimum pipe stiffness. 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 ring stiffness constant by the following equation:
       Attached Graphic
       

    4. (4) Pipe that meet all the requirements in this paragraph are not required to perform the structural calculations in paragraph (3) of this subsection, provided that a pipe is installed and tested in accordance with all other requirements of this subchapter:

       1. (A) open trench design;
       2. (B) flexible pipe with a pipe stiffness of 46 psi or greater;
       3. (C) buried 17 feet or less;
       4. (D) diameter of 12 inches or less;
       5. (E) modulus of soil reaction for the in-situ soil of 200 psi or greater;
       6. (F) no effects on a pipe due to live loads;
       7. (G) a unit weight of soil of 120 pounds per cubic foot or less; or
       8. (H) a pipe trench width of 36 inches or greater.
    5. (5) A design analysis for rigid pipe installations must be included in the report, including a structural analysis and any details necessary to verify that the structural strength is sufficient to withstand the expected stresses. For rigid conduits, the minimum strength for each class of pipe material and the appropriate standard must be included.

12. (l) Minimum and Maximum Slopes.

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

    2. (2) Absent site-specific data, a collection system must be designed in accordance with the minimum and maximum slopes specified in this paragraph.

       1. (A) The grades 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
          

          1. (i) The minimum acceptable "n" value for design and construction is 0.013.
          2. (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.
       2. (B) If a velocity greater than 10 feet per second will occur when a pipe flows full, based on Manning's formula, shown in the following figure, and an "n" value of 0.013, special provisions must protect against pipe and bedding displacement.
          Attached Graphic
          

13. (m) Alignment.

    1. (1) A gravity collection system must be laid with a uniform grade between manholes.
    2. (2) The report must justify any deviation from straight alignment by complying with the requirements of this section.
    3. (3) Deviation from uniform grade (e.g., grade breaks or vertical curves) without manholes and with open cut construction is prohibited.
    4. (4) The calculations for horizontal pipe curvature and the detail of the proposed curvature on the plans must be included in the report.

    5. (5) A construction method that flexes a pipe joint is prohibited, unless a joint deflection meets the least of the following:

       1. (A) equal to 5 degrees;
       2. (B) less than or equal to 80% of the manufacturer's recommended maximum deflection; or
       3. (C) 80% of the appropriate ASTM, AWWA, ANSI, or other nationally established standard for joint deflection.
    6. (6) 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.

14. (n) Inverted Siphons and Sag Pipes.

    1. (1) A sag pipe must include:

       1. (A) two or more barrels;
       2. (B) a minimum pipe diameter of 6.0 inches; and
       3. (C) the necessary appurtenances for convenient flushing and maintenance.
    2. (2) A manhole must include adequate clearance for rodding and cleaning.
    3. (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. (4) The arrangement of inlet and outlet details must divert the normal flow to one barrel.
    5. (5) A system must allow any barrel to be taken out of service for cleaning.
    6. (6) Provisions must be made to allow cleaning across each bend with equipment available to the entity operating the collection system.
    7. (7) Sag pipe must be designed to minimize nuisance odors.
    8. (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).

15. (o) Bridged Sections.

    1. (1) Pipe with restrained joints or monolithic pipe across a bridged section requires a manhole on each end.
    2. (2) A bridged section must withstand the hydraulic forces applied by the occurrence of a 100-year flood event for a collection system site, including buoyancy.
    3. (3) A bridged section must be capable of withstanding impacts from debris.
    4. (4) Bank sections must be stabilized to prevent erosion.
    5. (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.