3. Anchor Block Build

In rocky, mixed substrate deployments in the Gulf of Maine, we have successfully used a heavy concrete “deck block” that has been modified to fit a 4” diameter piece of PVC pipe. This pipe is then used as a protective mounting structure for the OWHL, allowing us to quickly remove and replace the OWHL for each deployment. The deck block is trapezoidal and weighs approximately 19 kg dry. The 4” PVC pipe is anchored to the block via concrete screws and a stainless steel washer. Our later design included an end cap to facilitate faster deployment of the OWHL in the field. The attached eye bolts provide a robust anchor point for a marker float and for using a lift bag (Figure 38).
Figure 38: Left: Anchor block showing PVC pipe and SS lifting eye-bolt. Right: Anchor block with lifting line and subsurface buoy line attached.

We have had good luck placing these blocks by lowering them from a boat with a line and then using a lift bag to move it to the final desired position. By placing the block in a slightly protected location such as under an overhang of a larger boulder or wedged into an outcropping crack, they have lasted for many field seasons.

Materials needed:

  • Deck Block
  • 4” PVC Plain End Pipe (nominal diameter) (Table 8 Item 35)
  • 4” PVC End Cap (nominal diameter)
  • 5/16” thread x 4” length Stainless Steel
  • Eye Bolt (or equivalent) (x2)
  • 5/16” thread Concrete Anchor/Lag Shield (x2)
  • 1/4” thread x 2.5” length Concrete Anchor Screws (x2)
  • 1/4″ Stainless Steel Washer (x2)

Tools Needed:

  • PVC Saw (or equivalent)
  • 3/4” (1.9 cm) Concrete Drill Bit
  • 3/16” (0.47625 cm) Concrete Drill Bit
  • Hole Saw Arbor with 1/4” (0.635 cm) bit
  • 2” (5.08 cm) Hole Saw
  • 5/16” nut driver drill bit
  • 1/2” (1.27 cm) general-purpose twist drill bit
Figure 39: An OWHL mounted inside 4 inch diameter pipe on a concrete anchor block.

3.1 Deck Block Modification

The top of the block must be cut to accept the 4” diameter pipe (Figure 40).

  • Using the masonry blade on an angle grinder:
    • Cut off approximately 1” (2.54 cm) from each side of one of the top channels (the overlap from the edge)
      • The masonry blade we used was for a 4.5” grinder and makes a 2” deep cut)
    • Use a cold chisel and hammer to break out the remaining piece of concrete

Now we will make the holes for the lag shields/concrete anchors

  • Using the 3/4” (1.9 cm) masonry drill bit:
    • Drill two 3/4” (1.9 cm) diameter x 2” (5.08 cm) deep holes approximately 2” (5.08 cm) from the edge on each side of the other channel on the top of the block
Figure 40: The modified upper surface of the anchor block.

3.2 PVC Pipe Modification

We cut several holes into the 4” diameter x 16” long PVC pipe to facilitate driving the concrete anchor screws in and to allow a line to pass through the pipe if needed.

  • Cut a length of 4” PVC DWV pipe to 16” (40.6 cm) length
  • Find the center of the pipe
    • Drill two 2” (5.08 cm) diameter holes at the center point opposing each other 180 deg.
    • Rotatepipe90degrees
    • Drill two 2” (5.08 cm) holes approximately 2.5” to 3” (6.35 to 7.62 cm) on either side of the center point of the pipe
  • Glue on the PVC end cap onto one end of the pipe
  • On the opposite end of the pipe:
    • Drill two 1/2” (1.27 cm) diameter holes 1” (2.54 cm) from the edge of the pipe
      • Oppose the 1/2” holes 180 deg. from each other
        • We aligned these to the center 2” holes above

3.3 Assembly of the Anchor Block

It is easier to install the eyebolts before we attach the 4” PVC pipe to the anchor block (Figure 41).

  • Insert a 5/16” thread concrete anchor/lag shield into the hole
  • With force, slowly twist in a 5/16” x 4” length stainless steel eye bolt into the lag shield
    • Use the cold chisel (or a screwdriver) to continue to screw in the eye bolt approximately 2” (5.04 cm) deep into the lag shield (Figure 41).
Figure 41: Inserting the eye bolts into the lag screw shield in the anchor block.

Now we can install the pipe:

  • Center the 4” PVC Pipe on the block in the newly cut wide channel
  • Using a 3/16” (0.47625 cm) concrete drill bit on a drill and through the 2” (5.08 cm) holes in the pipe:
    • Drill two pilot holes through the pipe and into the block to mark the locations of the pilot holes on the block
      • Remove the pipe
      • Continue to drill each these holes approximately 3” (7.62 cm) deep into the block
      • Replace the pipe on the block
        • The holes in the PVC pipe should align with the holes in the block. If they do not, you can easily re-drill new holes in the PVC as needed
  • Place a 1/4” stainless steel fender washer over each of the anchor screws
  • Using a drill with 5/16” driver:
    • Drive the screw and washer combo through the pipe holes and into the pilot holes in the concrete anchor block
    • DO NOT OVERDRIVE / OVERTIGHTEN THE ANCHOR SCREWS
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