Follow-Up Webinar Q & A: Using Modified HDD Techniques To Mitigate Sinkhole Risks

By Abdulnasser Almadhoun, P.E., NTH Consultants & Dan Dilegge, DVM Utilities Inc. 

This project won a National Recognition Award from ACEC National

​During our last webinar, there were several questions from the audience, several of which we were not able to answer due to time constraints. Below is a recap of the questions and the answers from our speakers.

If you have not yet viewed the webinar, please click this link to view.

---

1. What does HDD mean? 
   HDD stands for a trenchless technology called Horizontal Directional Drilling
   
   
2. What caused the original collapse? Waterline? Sewer?
   It is our opinion that the combined geotechnical, hydrologic and construction material used and sewage flow conditions all lead to the piping of fine sand and silt particles from the surrounding soil which ultimately caused the pipe failure. 
   
   
3. What was the total cost of this project?
   Approximately 6.7 Million Dollars
   
   
4. How quickly was it determined that the 48 inch sewer was over designed and down sized to a 24 inch line?  
   During the design phase it was discovered that the 48-inch sewer receives flow from the upstream 24-inch sewer only. Based on hydraulic analysis, it was determined the proposed 24-inch HDPE pipe hydraulic capacity should be sufficient.
   
   
5. Why was the sewer line over designed?
   The original sewer appears to have been built as a hand mined tunnel with liner plate primary liner which needed enough space to perform the work. There may have been discussion about future capacity but it appears the minimum space was the main reason.
   
   
6. What is the difference between the Pilot Tube Guided Boring Machine and a Micro Tunnel Boring Machine? Are the similar? 
   It should be noted that HDD is not the first choice for gravity sewer applications because of the limitations of the guidance and the higher degree for error. It is most often used for pressure pipe or communication lines which are not grade critical and best suited for High Density Polyethylene (HDPE) pipe pull back.  The Pilot Tube Guided Boring machine can achieve precise line and grade but is designed to push pipe in, not pull HDPE.
   
   These are the reasons we opted to use the Pilot Tube Guided Boring Machine to get the precise line and grade and the HDD machine to accomplish the ream and pullback of the HDPE pipe.  The added restraints were the tight working area.
   
   Pilot Tube Guided Boring and Microtunneling are similar but have some differences. Although all of the trenchless equipment is expensive, microtunnel tends to be the most expensive. It also needs complex fluid separation equipment and pumps. It can achieve greater drive distances and larger diameter installations however. 
   
   We have included a few videos as a reference:
   
   Ditch Witch illustrating Common Horizontal Directional Drilling (HDD)
   https://youtu.be/I7R5z3KQ_SE
   
   Akkerman Pilot Tube Guided Boring Machine
   https://youtu.be/lGyZqUytAhk
   
   Akkerman Microtunnel Boring Machine
   https://youtu.be/xO-fKEZJNtU
   
   
7. What was the constituent of the robust engineered fill?
   The Robust backfill consisted of Flowable fill cement for the bottom of our excavations to fill any voids that opened during pipe pullback and then compacted sand.
   
   
8. How do we know that a sinkhole won't re-occur - what did you do differently so that you know that the sinkhole won't re-occur?
   
   ​* The new and existing manholes within the project area were lined. HDPE and PVC Drop pipes were installed within the manholes.
   
   ​* HDPE pipe was used to replace the collapsed sewer
   
   * ​Leaks within the Existing 48-inch sewer were grouted and the 48” pipe was completely filled with flowable fill
   
   * Sinkhole area was backfilled with robust engineered fill system
   
   * Downstream area of the collapsed pipe between MH-4 and MH-3 (approximately 175 ft) was also lined with CIPP liner. Additional downstream leaks in the vicinity of the project area were grouted.
   
   
9. Did you do any geotechnical investigation like the moisture content, permeability, and strength of the subsurface soil? 
   Yes we did during the early phase of the investigation and during design phase. A geotechnical investigation report was issued as part of the Construction Documents.