Sliplining with HDPE: Liner Design for Rehabilitation Projects requires specification of a PE4710 pipe grade with a hydrostatic design basis of 11.0 MPa at 23°C determined in accordance with ASTM D2837 and ISO 9080. Pipe is manufactured to outside-diameter-controlled dimensions under ASTM F714; resin cell classification is verified under ASTM D3350. Typical pipe-grade values include density of 0.941–0.959 g/cm³ per ASTM D1505 and melt flow rate of 0.05–0.15 g/10 min at 190°C/2.16 kg per ASTM D1238 or ISO 1133-1. Production-scale pipe extrusion for HDPE sliplining liners typically occurs on grooved-barrel single-screw machines with length-to-diameter ratios of 30:1 to 36:1 and melt temperatures of 210–230°C; batch-to-batch resin viscosity variation within the melt-flow range affects fusion soak behavior and should be recorded from the resin lot certificate.

The choice between fully structural and semi-structural liner design is not semantic. A fully structural liner must carry soil, groundwater, and live load through the host pipe defects; a semi-structural liner relies on the existing host for long-term stiffness and only spans localized gaps, missing mortar, or circumferential cracks. In a deteriorated host with 15 mm radial deformation, a nominal annular clearance of 25 mm on diameter can be consumed almost entirely before the liner reaches the pull destination. Therefore clear opening must be measured with a mandrel or CCTV laser profiler, and the host pipe must be cleaned to remove protruding laterals, fracture edge steel, and calcite deposits. HDPE liner wall surfaces are not designed to resist point-cutting from sharp metallic debris.

Carbon black content of 2.0–3.0% by weight under ASTM D3350 imparts UV protection for temporary outdoor storage. Sustained contact with strong oxidizing acids or hydrocarbon-solvent plumes at elevated temperature is outside the normal design envelope. Maximum continuous operating temperature is generally limited to 60°C; above this, long-term creep strength and buckling modulus require separate derating.

What limits pull-in force when a liner is drawn through a misaligned joint?

A misaligned joint converts a straight pull into a capstan-friction condition. Local curvature at the joint multiplies the incoming tension according to T₂ = T₁ e^(μθ), where θ is the joint angle in radians. For an 11.25° offset joint, θ is approximately 0.196 rad. With a friction coefficient of 0.50, tension increases by 10.3% at each joint. A 300 m pull containing 12 such joints at the same friction coefficient produces a multiplier of e^(12 × 0.196 × 0.50) = 3.24. Straight-section friction adds directly. A 450 mm OD DR 21 liner weighs approximately 27.4 kg/m; over 300 m with μ = 0.50, the straight frictional force is 0.50 × 27.4 kg/m × 9.81 m/s² × 300 m = 40.3 kN. Multiplying by the joint-loss factor gives approximately 131 kN total pull force.

Safe pull stress for HDPE liner during short-duration insertion is commonly limited to 6.9 MPa at 23°C. The wall cross-section of a 450 mm OD DR 21 liner is approximately 28,850 mm², giving an allowable pull force near 199 kN. The allowable value should be treated as a maximum alarm limit, not a routine operating target. Winch speed is typically limited to 0.05–0.15 m/s, with water or biopolymer lubrication used to hold the friction coefficient between 0.25 and 0.50. Load cells with automatic winch cutoff should be set to 95% of calculated safe pull. When the host contains bends greater than 11.25°, HDPE can be cold-bent to a radius of 25–40 times OD; beyond that, a segmented pull or internal roller guide is required. Insertion practice is detailed in ASTM F585.

Buckling Resistance Across a Deteriorated Host with Groundwater Load

External groundwater pressure, not traffic loading, frequently controls wall thickness in sliplining liner design. For an unconstrained circular liner, the critical elastic buckling pressure is calculated from P_c = 2E / (1 − ν²) × 1/(DR − 1)³. Using a long-term flexural modulus of 758 MPa and Poisson’s ratio of 0.45, the numerator coefficient is 1901 MPa. The long-term modulus must be used because HDPE creeps under sustained external load. The 50-year modulus is often 50–60% of the short-term secant value, and using short-term data can overpredict allowable groundwater head by a factor of 1.5–2.0.

Table 2 compares calculated pressures for 450 mm OD liners. After applying an ovality factor of 0.7 and a safety factor of 2.0, a DR 26 liner is limited to 4.3 m of groundwater head. This threshold is lower than many field groundwater elevations above a deteriorated sewer crown. Designers should assume no confinement unless the annulus is fully grouted and the host retains enough stiffness to transfer external pressure. In partially deteriorated host pipe with missing wall sections or soil voids, the constrained buckling model should not be used unless continuous grout/soil contact is verified. The AWWA M55 method and ASTM F585 insertion practice provide conservative external pressure checks.

Field observations from ungrouted sliplining trials show that DR 26 HDPE liners can collapse during installation when the host has large voids and active groundwater inflow. The design response is not to add temporary internal pressure if the host is open at the pulling shaft; it is to select a heavier wall or grout in controlled lifts before annular water pressure becomes continuous. For gravity storm and sanitary rehabilitation, external pressure calculations should also account for the additional fluid density of cementitious grout, which is 1.8–2.1 g/cm³ before set. A 2.1 m column of grout at 2.1 g/cm³ exerts approximately 0.043 MPa, matching the allowable pressure for DR 26 after ovality and safety factors.

If the annulus is pressure-grouted without longitudinal restraint

Annular grout pressure acts on the liner before the grout gains strength. The maximum injection pressure must remain below the same buckling limit used for groundwater. For DR 21 liner, an injection pressure of 0.08 MPa approaches the calculated allowable pressure of 0.083 MPa. Therefore staged grouting with discrete lifts of 0.5–1.0 m vertical rise is required, and crown pressure transducers should be monitored continuously during placement. The liner is also subject to buoyancy before the grout sets. A 450 mm OD DR 21 liner displaces 159 kg/m of water and weighs 27 kg/m; the resulting net buoyancy is 1.29 kN/m upward. Internal water ballast, anti-float straps, or staged grouting must provide stable net downward force until the grout reaches initial set.

Longitudinal thermal movement is unlikely to be fully restrained. With a coefficient of thermal expansion of 1.6 × 10⁻⁴ m/m/°C, a 200 m liner subjected to a 20°C temperature rise changes length by 0.64 m. Terminal manhole connections and end seals should accommodate this movement rather than rigidly lock the liner unless thermal anchors are specifically designed. Grout compressive strength should reach at least 3.5 MPa before external loads are transferred to the liner as a composite structure.

Because butt fusion welding is the rate-limiting step in sliplining, joining procedures follow ASTM F2620 with heater surface temperature maintained at 204–232°C. Pipe-face trimming is performed with a rotary planer, and drag pressure is measured and compensated before each weld. Data-logged fusion machines record heater temperature, soak time, and interfacial pressure. A single cold-fusion defect in a 300 m string cannot be economically repaired after insertion, so the fusion log is reviewed before the string is pulled into the host. Pulling heads are attached with a fused reducer or mechanical clamp that distributes pull force over the full circumference. After insertion, the liner is inspected for axial scratches; scratches deeper than 10% of wall thickness require cutout or repair by a qualified fusion technician. Ambient conditions below 5°C or above 40°C require shelter, preheat, or cooling as specified by the fusion machine manufacturer. Published data for inline ultrasonic inspection of HDPE butt fusion joints is limited, so procedure qualification relies on destructive bend tests of representative welds and visual bead geometry.