Town of Exeter – Standard Construction Details (Manholes, Sewers, Trenches, Roadways)

Standard Detail #4 – Typical Manhole

• Intended performance

  • Complete precast assembly must provide adequate space, strength, and leak-proof service for >25 years.

  • Structural capacity: withstand 8 tons (H-20 loading).

  • Allowable leakage: <1 gallon day⁻¹ ft⁻¹ of vertical height for the life of the structure.

• Materials & components

  • Barrels and cone sections: precast, reinforced.

  • Precast sections (barrels, cones, bases) must meet ASTM C478.

  • Flexible joint required within 48 in of structure.

  • Manhole steps: not permitted.

• Invert & shelf construction

  • Brick-paved shelf and invert sized to sewer pipe.

  • Inverts laid on long-radius curves tangent to pipe center-lines.

  • Shelf elevation = highest pipe crown; slope drains to channel.

• Frame & cover

  • Heavy-duty; clear opening 30 in.

  • Cover lettering: 3 in-high characters “SEWER” or “DRAIN.”

• Bedding material (ASTM C33 gradation)

  • Gravel / crushed stone free of clay, loam, organics.

  • Gradation limits (percent passing):
    • 100\% – 1 in screen
    • 90!\text{–}100\% – 3/4 in screen
    • 20!\text{–}55\% – 3/8 in screen
    • 0!\text{–}10\% – #4 sieve
    • 0!\text{–}5\% – #8 sieve.

  • Where stabilization ordered: stone 1.5 in – 0.5 in.

• Drop support concrete (Class A, 3000 psi)

  • Cement: 6.0 bags yd⁻³.

  • Water: 5.75 gal bag⁻¹.

  • Max aggregate: 1 in.

• Shallow manholes (<6 ft depth)

  • Use reinforced concrete slab cover with eccentric opening rated for H-20.

Standard Detail #5 – Typical House Sewer

• Pipe size

  • Minimum house service diameter: 4 in.

• Approved pipe & joint materials

  • PVC: ASTM D2729 sewer pipe/fittings.

  • Cast-iron pipe: ANSI A21 series (A21.1, 4, 6, 8, 10); joints–gaskets ANSI A21.11.

  • Ductile iron: ANSI A21.50, A21.51; joints per cast-iron spec.

• Joint requirements

  • Depend on neoprene/elastomeric gasket for watertight seal.

  • Transition between dissimilar pipes via manufactured adapters.

• Tee / wye connections

  • Use factory tee/wye if present; otherwise saddle with bolts, clamps, or epoxy per manufacturer instructions.

  • Prohibited: hammer taps, stuffing cloth, mortar smears, or other crude improvisations.

• Installation practices

  • Handling, placing, jointing per manufacturer guides.

  • Bedding: 4 in crushed stone/gravel (ASTM C33) below pipe.

  • Backfill: carefully tamp to 12 in above crown.

• Leakage testing (prior to backfill)

  • Option A: Observation-tee, inflatable plug, fill to 5 ft head.

  • Option B: Hose pipe & trench, simulate wet trench; inspect through clean-out.

  • Option C: Fluorescent dye in wet trench; inspect next downstream manhole.

  • Any observed leak ⇒ remove & relay pipe.

• Illegal connections

  • Roof leaders, footing drains, sump pumps, rain/ground water not allowed.

• Water service proximity

  • Prefer separate trench; if shared: water line above & offset per drawing.

• Bedding gradation (ASTM C33-67) – same percentages as manhole spec.

  • Stabilization (if ordered): stone 1.5 in – 0.5 in.

• Location records

  • Tee/wye coordinates filed in municipal records + ferrous “chimney” locator rod.

Standard Detail #6 – Typical Trench Section

• Ordered excavation of unsuitable sub-grade ⇒ refill with bedding material.

• Bedding material

  • Screened gravel/crushed stone (ASTM C33 #67) – identical gradation limits.

  • Engineer-ordered stabilization: stone 1.5 in – 0.5 in.

• Sand blanket

  • Clean sand: 90!\text{–}100\% passing 0.5 in sieve; <15\% passing #200.

  • May be omitted for cast-iron, ductile-iron, reinforced-concrete pipe if no stone >2 in contacts pipe.

• Suitable backfill

  • Roadways: native excavate excluding debris, pavement, organics, topsoil, wet/soft muck, peat, clay, and unsuitable ledge.

  • Cross-country: same, though engineer may allow organic soils if stability & access assured.

• Base course (if ordered)

  • Must meet Division 300 of NHDOT Standard Specs.

• Wood sheeting

  • If sheet piles extend below pipe mid-diameter, cut off \ge1 ft above crown.

  • Ordered permanent sheeting: cut \ge3 ft below finish grade, yet \ge1 ft above pipe.

• Maximum trench width W (measured 12 in above crown)

  • Pipes \le15 in Ø ⇒ W\le36 in.

  • Pipes >15 in Ø ⇒ W =24 in + outside diameter.

  • W controls pay width for ledge & ordered over-excavation.

• Cross-country backfill: mound 6 in above original grade.

• Water–sewer separation (NHDES)

  • Required: 10 ft horizontal.

  • If water line \ge18 in above & \ge3 ft lateral ⇒ ok.

  • Otherwise: install sewer in ductile-iron for 10 ft either side of crossing or relocate lines to achieve separation.

Allowable Leakage – Water Mains (Detail #7)

• Table gives maximum permissible leakage (gallons h⁻¹ 1000 ft⁻¹) for combined pipe diameters 3–54 in at test pressures 100–450 psi (≈7–31 bar).
• Convert to L h⁻¹ by multiplying by 3.785.
• If pipeline has mixed diameters, total allowable = sum for each segment size.

Test Pressure (psi/bar)

3

4

6

8

10

12

14

16

18

20

24

30

36

42

48

54

450\,(31)

0.48

0.64

0.95

1.27

1.59

1.91

2.23

2.55

2.87

3.18

3.82

4.78

5.73

6.69

7.64

8.60

400\,(28)

0.45

0.60

0.90

1.20

1.50

1.80

2.10

2.40

2.70

3.00

3.60

4.50

5.41

6.31

7.21

8.11

350\,(24)

0.42

0.56

0.84

1.12

1.40

1.69

1.97

2.25

2.53

2.81

3.37

4.21

5.06

5.90

6.74

7.58

300\,(21)

0.39

0.52

0.78

1.04

1.30

1.56

1.82

2.08

2.34

2.60

3.12

3.90

4.68

5.46

6.24

7.02

275\,(19)

0.37

0.50

0.75

1.00

1.24

1.49

1.74

1.99

2.24

2.49

2.99

3.73

4.48

5.23

5.98

6.72

250\,(17)

0.36

0.47

0.71

0.95

1.19

1.42

1.66

1.90

2.14

2.37

2.85

3.56

4.27

4.99

5.70

6.41

225\,(16)

0.34

0.45

0.68

0.90

1.13

1.35

1.58

1.80

2.03

2.25

2.70

3.38

4.05

4.73

5.41

6.03

200\,(14)

0.32

0.43

0.64

0.85

1.06

1.28

1.48

1.70

1.91

2.12

2.55

3.19

3.82

4.46

5.09

5.73

175\,(12)

0.30

0.40

0.59

0.80

0.99

1.19

1.39

1.59

1.79

1.98

2.38

2.98

3.58

4.17

4.77

5.36

150\,(10)

0.28

0.37

0.55

0.74

0.92

1.10

1.29

1.47

1.66

1.84

2.21

2.76

3.31

3.86

4.41

4.97

125\,(9)

0.25

0.34

0.50

0.67

0.84

1.01

1.18

1.34

1.51

1.68

2.01

2.52

3.02

3.53

4.03

4.53

100\,(7)

0.23

0.30

0.45

0.60

0.75

0.90

1.05

1.20

1.35

1.50

1.80

2.25

2.70

3.15

3.60

4.05

Standard Detail #8 – Typical Curbed Roadway (Sheet A)

• Cross-section elements

  • Cape Cod or granite curb (typical).

  • Roadway pavement: 3 in Type “B” base + 1 in Type “E” top.

  • Sidewalk pavement: 1.5 in Type “B” base + 1 in Type “E” top.

  • Asphalt pavement depth (illustration): 4 in.

  • Gravel base: 12 in (minimum).

  • Crushed gravel layer: 6 in.

  • Longitudinal slope: \tfrac14 in ft⁻¹ (~2\%).

  • Minimum shoulder / sidewalk widths indicated: curb offset 5 ft; Cape Cod curb offset 6 ft.

Standard Detail #8 – Typical Curbed Roadway (Sheet B – Alternatives)

• General geometry

  • Numerous roadway/sidewalk/guard-rail configurations; slopes vary 1.6\% – 16\%.

  • Utility strips: typical 5 ft width.

  • Guard-rail faces aligned with curb where present; slopes outside rails 6:1, 4:1, 3:1, or 2:1.

• Design notes

  1. On multilane sections, provide 10 ft (min) panel for snow storage adjacent to curb where practical.

  2. Provide 4 ft (min) grass strip ahead of all sidewalks when feasible.

  3. Dimensions shown are desirable; may vary as field conditions warrant.

  4. Grass-strip slope may be adjusted to match adjoining property grades.

  5. Normal curb reveal: 7 in for barrier curb at sidewalk (taper 0 → 7 in in 8 ft).

  6. Slope curb on urban islands (taper 0 → 6 in in 8 ft).

Standard Detail #9 – Typical Concrete Pavement Patch

• Sequence

  1. After backfill & compaction, “saw-cut” straight edges around excavation and remove damaged pavement.

  2. Place 3000 psi concrete slab (thickness per design) with reinforcement as detailed.

  3. After concrete sets, install 3 in base course, Type “B” bituminous concrete.

  4. Upon Highway Superintendent inspection, place 1 in wearing course, Type “A.”

  5. Apply asphalt emulsion sealant around perimeter, overlapping onto base course; broadcast light sand to absorb excess.

  6. Procedure may be modified by Highway Superintendent to suit field conditions.

Common Gradation Table (ASTM C33 – Used for Manholes, Sewers, Trenches)

• Percent passing limits (identical across specs)

  • 100\% through 1 in screen

  • 90!\text{–}100\% through 3/4 in screen

  • 20!\text{–}55\% through 3/8 in screen

  • 0!\text{–}10\% through #4 sieve

  • 0!\text{–}5\% through #8 sieve

Practical / Ethical / Safety Implications

• Long-term watertight integrity minimizes groundwater infiltration/exfiltration, protecting public health & reducing treatment costs.
• Strict prohibition of makeshift connections (hammer taps, cloth stuffing) upholds workmanship quality and prevents environmental contamination.
• Mandated records (tee/wye location w/ ferrous marker) ensure future maintenance & avoid utility conflicts.
• Water–sewer separation rules safeguard potable water from cross-contamination.
• Testing protocols (hydrostatic head, dye) highlight a commitment to verify performance before concealment – ethical duty of care.

Connections to Broader Principles

• Follows AWWA/ASTM/ANSI standards – illustrates hierarchy from federal/state codes to town details.
• Use of H-20 loading aligns with AASHTO highway bridge design loads.
• Bedding & backfill gradations echo soil mechanics principles: granular support, drainage, avoidance of migration/fines.
• Leakage table embodies continuity equation & hydraulic gradient concepts: allowable exfiltration proportional to pressure & circumference.