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1. What is the recommended SDR for HDD (horizontal directional drilling) pipe?
2. If I want to install an innerduct or raceway in other than a Plenum or Riser area of a building what do I use?
3. What if I want to install a fiber optic cable in a building area that is not considered a Plenum or a Riser?
4. Is it OK to lay HDPE conduits in layers vertically with spacers and backfill with soil?
5. Should HDPE conduit (5" SDR 17) installed under the Mississippi River for power/electrical cables be pressurized to ensure it doesn't collapse?
6. What size HDPE casing should be installed using a HDD, if looking to house 4"-6"
7. A project is using 4" SDR 17 ASTM F160 HDPE conduit and installing in a microtunnel under a roadway. The conduit will contain fiber optic cable and is part of a larger pipe bundle that contains high voltage transmission power cables. The concern is the non-pressure rated conduit and whether it can withstand the grouting pressure. The calculated the maximum grouting pressure is 120 psi, but the expected pressure range is 30-50 psi. The maximum grout heat of hydration is estimated at 125F at 60 hours after placing. Is it possible to hydraulically pressurized the HDPE conduit to maintain a balanced condition to offset any unbalanced pressure condition during installation.

Any critical project should be engineered. Soil conditions, the length of the bore, the diameter of the bore hole, and many other factors should be considered and calculated to determine the wall thickness of the bore pipe. See TN-48 for an overview and TR-46 for detailed design instructions on choosing a wall thickness for short bores.
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Inside a building, only UL Listed products should be used. Areas not rated as plenum or riser spaces are classified as "general purpose." In these areas the NEC suggests that a flame retardant material be used. PPI recommends that a UL listed raceway such as UL910 be used. Riser is flame retardant and lower cost than plenum raceway.
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In areas other than Plenum or Riser, generally known as General Purpose areas Riser is the product of choice. The NEC does not provide for a general purpose raceway, only for Plenum or Riser.
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Only if a flowable fill, such as lightweight concrete, soil cement or jetting sand (be careful of buoyancy effect) is used. Trying to compact natural soil between stacked conduit is extremely difficult, regardless of whether it is HDPE or PVC. If soil is the least expensive backfill alternative, it would be better to lay down the conduit, and then compact a layer around and above the conduit, then lay the second layer of conduit and repeat.
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A better practice would be having a good clean bore hole, in a soil strata with the void completely filled with bentonite. That being said, standard conduit is not long term pressure rated. It will hold pressure but the resin is not rated for a 50 year "pressure design life." If you need to pressurize the conduit long term, using PE 4710 pressure pipe is recommended. Review TR-47 for further information.
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A 20" casing conduit, SDR 11, has a nominal OD of 21.6" with an average ID of 17.463". Each 6" conduit is SDR 13.5 with an OD of 6.625". The clearance should be sufficient if the 20" casing is a straight bore. The circle of 4 conduits would have a circumference of 10.043", making the clearance in the casing roughly 1.4".

Do your installations require spacers? High voltage power cables are typically installed with spacers to keep precise spacing between the conduits in order to prevent mutual heating extremes and to center the configuration, which makes modeling the thermal constraints a known entity. What is the recommended spacer size?

Yes, spacers should be used. Cable manufacturers should be consulted regarding proper spacing.
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The ASTM F2160 conduit resin properties are similar to PE 3608 pressure pipe short term. Conduit can take short term pressure, but is not designed to withstand 50 year design life pressures. The analysis described, internal pressure to offset the grout, consideration of buckling due to external pressures, sounds practical and well thought out. One suggestion is to consult the PPI Handbook where a modulus based on time (60 hours) and temperature can be found. That also should be considered in the analysis.
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