Paver installation - excavation limitation

Question

I am trying to install a paver patio in my backyard but have run into some excavation issues. Everything I have read says to dig deep enough to allow for 6" of base material under the paver surface (4" of base gravel and 2" of builder's sand). The problem is, at 4" I uncovered the top of a 1-ft diameter PVC pipe running horizontally across the length of my backyard. I live in a townhome, and the pipe appears to be a drain pipe that everyone's gutter downspouts feed into. So there's definitely no chance of removing the pipe - it's there for good.

Because this is only at 4" deep, should I dig the rest of the area to the max depth needed, and just leave the section where the pipe runs at 4"? Or should I dig everything to the same 4" depth? Do I need to do something special to the pavers going on top of the area where the pipe runs? Or do I actually add material and go up, raising the surrounding areas around the space I plan to put pavers on as well? So many questions I can't seem to find any answer for through google.Any advice from experienced paver installers is greatly appreciated!

Answer 1

I have run into this many times usually with light duty drainage pipe for the gutters . I really like a solid base I use more rock and less sand but 6” for a patio is good. I would expose a little of the pipe and stop at 5”.

When packing the gravel work to the pipe from both sides but don’t pack directly over it until both sides are packed. This reduces breakage of the pipe if it is light duty drainage pipe like I have run into. I also use water while packing this helps to pack the rock into the dirt and work the fines from packing into the cracks.

I use just enough sand to level everything in reality a deep spot might be 1/2” for me with a thin spot being close to 1/4”. I get it flat with a 3’ wide rake that has teeth on 1 side and smooth edge on the other side, by working at 90 degree angles he sand flattens out quickly. By over lapping each pass with 2/2 of the rake then turning and working at 90 doing the same the low spots show up quickly after I get the sand very close I use a mist nozzle to wet it down and a last pass, wet sand is harder to work that’s why I get flat prior to wetting it down, a mist nozzle takes longer but won’t dig ruts in the sand.

A border around the area is best but is really needed if more sand is used when it rains water pulls the sand out at the edges of there is no border. A thin layer of sand won’t have the sand migration as bad even with small pavers. Last make sure to have a slight grade away from the house, yes the rock will help to create a drain field but a well packed base and poly sand will hold water quite well so having a slight grade away from the house is best.. I have put in many patios with pavers if crushed quarry or shale is used the rock locks together and can last decades don’t use crushed river it doesn’t lock together I have learned this the hard way living in Oregon where crushed river is much cheaper , I only use that for French drains where o don’t want it to pack together. Hope this helps , 4” of rock will work 5 is better but 4-1/2 with 1/2 of sand will reduce your chances of breaking the pipe and still provide a good base for your pavers.

Answer 2

Use 100% sand to embed and backfill the pipe; 1/4" gravel fines at the largest. Anything larger can point load the pipe and will lead to subsidence when the material migrates.

Material that would permit migration of fines from the native material should not be used for the replaced foundation. For example, crushed rock or a gravel material containing significant voids placed next to fine grained native material should not be used. The fine-grained material could migrate into the voids of the coarser material and result in the rock particles floating in a matrix of fine-grained material. This could possibly cause loss of support for the pipe which could further result in unequal settlement.

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Compaction of embedment. - The select material used must be a cohesionless, free-draining soil such as clean sands and gravels. Just as "percent Proctor" is used to control the compaction of cohesive soils (clays, etc.), relative density is used to control the compaction of cohesionless soils. The embedment may be compacted by any means as long as the 70 percent relative density criterion is met.

– Pipe Bedding and Backfill, United States Department of the Interior Bureau of Reclamation, PDF

Answer 3

Some native soils are really lousy foundation material. They might expand when they get wet; they might heave upward when they freeze, and the list goes on. If a soil movement event happens directly under the pavers then the pavers will move too, and that'll result in an uneven surface, gaps forming, etc. On the other hand, some native soils are actually pretty decent to build on and don't require as much preparation. A reason for the general recommendation to over-excavate and then install a base material is that it's good insurance when soil conditions are unknown. Many soil problems are related to water, so the gravel and sand provide a layer through which water can drain easily. If the water infiltrates to the soil below and still causes soil movement, the layers of gravel and sand help to temper or absorb the soil movement so that it causes less distortion of the paver surface.

With all that said, I'm going to guess that the 12 inch PVC pipe is actually a decent foundation for the pavers. It probably has some bedding under it already (we don't like gravity-flow pipelines being thrust about by soil movement either!) and the PVC isn't going to change dimension much through seasonal moisture and temperature changes.

You'll probably do fine to let the gravel layer be thin over the pipe, or even to omit the gravel entirely right over the crown of the pipe and use only sand there.

Answer 4

I would go the full 6" of rock but only use 1 inch of bedding sand. Anymore than that and you can have your pavers shift due to pressure on the corners. I like Ed's advice on packing around the pipe.

When packing the gravel work to the pipe from both sides but don’t pack directly over it until both sides are packed. This reduces breakage of the pipe if it is light duty drainage pipe like I have run into. I also use water while packing this helps to pack the rock into the dirt and work the fines from packing into the cracks.

I would just be careful to not use too much water, as you want the entirety of the base material to be as dry as possible before laying down your sand and pavers.

I am a novice so this is just my two cents, but when I installed my paver patio last summer, and I used Stanley "Dirt Monkey" Genadek's videos on paver installation as my guide.

Answer 5

Mast made a comment that I'd like to expand upon to offer some ideas and guidance.

That pipe can effectively be seen as public utility for the people whose gutters feed into this pipe. Because of that, you may be called upon by local authorities to lay bare that pipe in case of emergencies, like a blockage or a leakage. In case that happens, it'll probably need to happen fast and whoever has to do it will most likely not really care about your nicely laid patio.

Because of that, my suggestion would be to not place paver on top of that pipe, and instead put something easily removable or replaceable on that area as well as half a foot on either side. In case it's needed, you want that pipe to be easy to reach. It'll also make that pipe easily detectable, so you will run less risks of them accidentally digging 2 feet over and missing the pipe entirely, ruining more of your nice patio.

There is probably something aesthethically pleasing yet easy to remove that can be placed on this spot. I have to admit my personal knowledge on this matter is fairly limited, but I'm fairly sure that others on this site will be able to suggest something amenable to your budget and taste.