Let us get down in the dirt, or at least “backfill.” A recent discussion on ASPE Connect’s “Open Forum” was entitled “Minimum Pipe Cover when Calculating Invert Under Building Slab for PVC Sanitary.” The post related to the conflicting information received when reviewing inverts with plumbing contractors. So the question became: “When running horizontal drains (whether they be sanitary or storm) underneath a building slab, is there a typical minimum burial depth or pipe cover required? Or is it acceptable to locate the piping tight to the underside of the slab?”
So, where shall we begin the discussion? As with any design project, one must deal with the specifics of that project and the site conditions. No code, standard or manufacturer’s installation instruction can cover or address all of the potential complexities of those specific site and project conditions. As a design professional, one must consider the information provided by the codes, standards and manufacturers’ installation instructions while applying sound engineering judgment.
After giving the question some thought, my initial response was: Interesting question that has many challenging inputs and differing solutions. Both the ASTM standards and the manufacturers' installation instructions are intended to protect the installed material from potential damage. But, ultimately, it is the Engineer-Of-Record (EOR) who has liability for the design. The contractor is expected to install the system in accordance with the Contract Documents using appropriate means and methods. While conflicting information between various members of the design/construction team may occur, it is the EOR who must make the final decision to which the contractors must comply.
The question of minimum backfill cover is related to the protection of the piping material from potential physical damage. So, one must look at the loading potentially that could be applied to the piping material. This loading can be complicated, as one must consider both the final installed loading along with potential loading imposed by the construction process, contractor equipment and fieldwork.
PVC adds additional challenges as it can flex as well as distort. Hence, the backfill needs to be such as to maintain the piping slope, alignment and shape. Therefore, the backfill needs to provide a firm and solid base upon which the piping can rest. That backfill also needs to assure that any imposed loading cannot cause the piping to move or deform.
PVC piping can certainly be installed directly below the slab. This is provided there is no direct contact with the slab, which could be abrasive to the material and no imposed load is transferred to the installed piping. It also requires that no construction load, such as equipment passing over the piping, is allowed. Additionally, the type of backfill material needs to be considered. Where piping materials can settle or shift when subjected to an imposed load are not practical or an appropriate selection.
It is the contractors' responsibility to appropriately install the system and to protect that installed work throughout the construction process, not the EOR. The EOR's design is intended to provide for a long-term, workable and durable system when fully installed in accordance with the Contract Documents.
As an example, I had to review and recommend corrective actions for a design/build contractor that had installed PVC in a medical office building. The plumbing contractor had installed PVC throughout, backfilling with "grits." Grits are small granular gravel that shifts and moves under imposed loading. As is typical, the minimum cover was 18 inches to allow for the upturn to closet arms, etc. The piping left the building with about 4.5 feet of cover. The contractors did nothing special to protect the installed sanitary system. Additionally, it did not appear that any bedding or compacted fill had been placed around the piping in accordance with the manufacturers’ installation instructions. Heavy construction equipment was allowed to continually drive over the area, including the trenching the enclosed the sanitary piping. As a result, the building drain flexed and formed "belles" (changes in slope) in its run. These belles held waste products and toilet tissue, resulting in frequent stoppages. Needless to say, the owner was not happy. The design/build contractor wanted to know what went wrong.
The answer was the inappropriate use of grits as an acceptable backfill material; as well as not protecting the trenched areas from seeing equipment-imposed loading. This loading allowed the piping to flex and yes, deform. The backfill needs to be such that it can be compacted and will not allow the piping material to flex, shift or deform under any imposed loading, installation construction traffic or building imposed loading.
As this facility was in operation, a large section of the building drain was abandoned and rerouted out the side of the building to reconnect with the building sewer. The design/build contractor was then provided with a set of guide specifications to include in their master specifications. Part of the reason they had allowed grit was because the local purveyor had them in their specification. However, the building sanitary specification followed the ASTM for the first couple of feet and then completed the backfill with grits. Exterior sewers generally are at least covered by 5 feet of backfill.
One of the discussion contributors had several points that I offered the following responses for consideration:
- “How have you interpreted this?” Exactly as stated, the EOR needs to evaluate the specific project condition. This includes anticipating the conditions that potentially can occur during the construction process;
- “How have you evaluated the project conditions and justified the integrity of the pipe for certain depths and backfill methods and materials?” The EOR must consider how best to protect the installed piping material for installation as well as complete/operational loading and select the backfill materials that can best protect the piping;
- “Have you just gone with ASTM’s recommendation of 36 inches of cover?” Yes and no, the recommendations are not specific to the project. Beginning with 36" of cover is not practical if one wishes to avoid a lift station;
- “Some general recommendations have been given, but is past experience enough to stand behind the design?” General recommendations are only a starting point. The EOR needs to look at the overall systems a select specific backfill based on the conditions along the piping run. Shallow cover will require different backfill and protection verse areas where the cover is deeper and has less risk of equipment damage;
- “What has allowed starting at 18 inches below slab when the standard and installation instructions recommend otherwise?” Starting at 18 inches or even 6 inches is design specific, requiring specific considerations to be addressed. Standards and installation instructions are a guide, not something set in stone. After all, it is engineering and left to the discretion of the EOR;
- “Does anyone have experience with installing plastic pipe shallow and backfilling with controlled density fill that doesn’t require compaction? If so, how was the retainage of pipe strength accomplished?” The standard does not seem to include that method unless I’ve missed it. Controlled density fill is viable as a means of backfill that will remain in place. However, it is not normally the initial bedding material for the PVC piping nor the first 6 inches of cover over the piping. This needs to be closer to the ASTM/installation instruction guidance;
- “Can the contractor reasonably do their work around the site without driving vehicles and machinery over the installed pipe when it is shallow?” This is a contractor’s responsibility as part of their means and methods. They need to coordinate with the other contractors and take precautions to protect their installed systems; and
- “Do you just give up on plastic pipe and go back to cast iron?” PVC is viable as a material, just as cast iron has its uses. No one material is perfect for every use. Materials need to be selected based on application as well as location.”
One of the contributors to the discussion referred to IPC (International Plumbing Code) Section 306.3 Backfilling: “Backfill shall be free from discarded construction material and debris. Loose earth free from rocks, broken concrete and frozen chunks shall be placed in the trench in 6-inch (152 mm) layers and tamped in place until the crown of the pipe is covered by 12 inches (305 mm) of tamped earth. The backfill under and beside the pipe shall be compacted for pipe support. Backfill shall be brought up evenly on both sides of the pipe so that the pipe remains aligned. In instances where the manufacturer's instructions for materials are more restrictive than those prescribed by the code, the material shall be installed in accordance with the more restrictive requirement.”
The contributor also noted that the code does not differentiate between under a building or grade (which may not be true, as this is typically covered in Chapter 2 – Definitions), while the code does give some direction related to backfill. One should note that per code, you need a cover of 12 inches of tamped earth over the crown of the pipe. While this will work in certain areas, it is not an all-inclusive statement. Most codes defer to the manufacturers' requirements and installation instructions. "Earth" is not normally placed in trenches under structures, roadways, sidewalks, etc. Granular fill is generally the preferred backfill material. Regardless of the backfill material, compaction is always recommended to avoid settling over time and to assure the piping remains in a fixed position.
Another contributor offered comments found within the code commentary: “Piping in trenches under a slab-on-grade building need not be buried any more than what is necessary to provide for a full thickness of the concrete slab above.”
As this discussion shows, the burial depth and backfill methods and materials can quickly become complex. Adding to this complexity is the type of material or where that material is being placed within the project. These are all engineering issues that must be addressed. As an engineer, one needs to understand:
- The material type: Metallic, polymer, ceramic, glass, etc.;
- Where the material is being placed within the project: Exterior, under the slab, under roadway, under parking lot, under sidewalk, etc.;
- Soil conditions into which the piping materials will be placed;
- Is settling of the backfill material acceptable or must the final elevation be held constant? Backfill material selection: Earth, controlled density fill, compactable granular fill, sand, grits, gravel, etc.;
- The ability of the backfill material to be compacted and hold that compaction;
- Coordination between specification and the requirements of codes, standards, as well as purveyor requirements. Some of these may conflict with each other or may only apply to specific areas of the project;
- How will burial depth impact the structural integrity of the slab;
- How will the material react to the backfill in relation to the materials expansion coefficient; and
- What impact can the construction process have on the installed material relative to transfer loading onto the material?
So, as design professionals, we must carefully consider the selection of the material and the backfill needed to protect it. We must also consider the construction process, not just the final installed system.