I have a contractor asking if he can place a sanitary sewer, backfill w/ 57 stone to a foot over the pipe, then backfill the rest with 4-saying he wouldn't need to compact this. It is in a future parking lot (1-2 years out). Does anyone have experience in this area?
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Can replace 57 with 4 and not compact? 3
- Thread starter kedpe
- Start date
civilperson
Structural
Not strictly true. The compaction requirement should be enforced at all levels, but the #4 gravel will achieve compaction with minimal effort.
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- #3
DarthSoilsGuy
Geotechnical
he/she isn't volunteering to wrap the entire trench with high-quality filter fabric to prevent fine migration i'm betting.
another problem with large screened stone deposits is that you have to consider what happens to any water (or wastewater) that may fill these voids. you really have to look at the site and look for potential issues and places for water to get in and out. in most cases this is not an issue, but it has been in forensic case studies too.
Lastly, are there any other utilities going next to, under, or crossing, EVER. if so, go ahead and count on some undermining of the parking lot.
another problem with large screened stone deposits is that you have to consider what happens to any water (or wastewater) that may fill these voids. you really have to look at the site and look for potential issues and places for water to get in and out. in most cases this is not an issue, but it has been in forensic case studies too.
Lastly, are there any other utilities going next to, under, or crossing, EVER. if so, go ahead and count on some undermining of the parking lot.
The Fairy Tale of 'Self Compacting Gravel' lives on.
To say nothing of the long-term problems, as hinted to above. Imagine cutting a future trench trough this mistake and maintaining the trench sides and the support under adjacent pavement, sidewalks, etc.
Sorry to sound so negative. I don't recommend or allow this sort of material any thicker than a leveling layer or underslab drainage layer, 6 inches thick MAXIMUM.
To say nothing of the long-term problems, as hinted to above. Imagine cutting a future trench trough this mistake and maintaining the trench sides and the support under adjacent pavement, sidewalks, etc.
Sorry to sound so negative. I don't recommend or allow this sort of material any thicker than a leveling layer or underslab drainage layer, 6 inches thick MAXIMUM.
- Thread starter
- #5
The contractor has clarified his request was to use a # 4 stone in lieu of the 304 limestone called out for backfill in paved areas. He says that although # 4 stone is more costly than the 304 but they are of the opinion that with # 4 stone you don't run the risk of settlement issues which saves in the long run.
- Thread starter
- #6
We have used 3/4" stone in many situatins where compaction is not practical, for example, the bottom of wet excavations, or excavations that need to be backfilled rapidly. Clen stone does not need to be compacted to a proctor, but should be tamped. Fine migration can be handled by filter fabric, but the need for fabric would depend on site conditions, and is generally not a consequense of substituting stone for granular fill.
clean stone is used as backfill in freezing weather when it would not be prudent to use granular material.
clean stone is used as backfill in freezing weather when it would not be prudent to use granular material.
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- #8
Thanks DRC1: The excavation is very wet, and he mentioned that is one of the reasons he wanted to do it. If there are no fines in 3/4" stone, where is the fine migration coming from? The earth adjacent to the trench? Why doesn't clean stone need to be compacted to a proctor?
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- #9
whenever i see crushed stone used as backfill for more than about 2 foot thick, i require that we see the backfilling process since it's more of a judgement call than running a test (even though i'm sure we could try to run a test but this way just seems more practical and straightforward). i make the contractor beat each 1-2 foot layer of aggregate with the trackhoe bucket until i can no longer see movement in the layer. if they're using smaller equipment or small compaction equipment, the lift thickness gets cut down to less than 1 foot. for uniform sized aggregate, i agree with the comments above. depending on the scenario, i will usually recommend that they get a better gradation to limit migration of adjacent fines.
i learned the lesson of compacting #57 stone the hard way back before i was an engineer and was a contractor. i did a project where we had one area that had to be backfilled about 6' on top of the footing to get up to sugrade level and a second area where a footing was undercut about 5' and backfilled similarly. the material was dumped in and leveled off. the slabs/footings around these areas were poured and building construction went on. about 3 months later (in the middle of winter), i started noticing little swirling holes showing up adjacent to the exterior footing area (the area where the footing was undercut) that was backfilled as described. i took a 7' long piece of rebar and sunk it about 1 foot off of the footing. that's when i realized the material was washing in to the backfill. at the end of the project, we had problems with cracks showing up in the masonry at these two locations. i'm fairly confident it was because there was some subsidence in the slab/thickened slab or footing that was holding the walls. the cracks were worst just outside the actual backfill area and lessened with distance away from that. the cracks did extend to probably 30-40' away from the area before they disappeared.
i have to argue with contractors all the time, so i have actually bet (bragging rights) a contactor before. the contractor was going to backfill 7' in a 10'x20' area and didn't want to compact it because he thought i was full of bs. i told him that he had to densify the backfill or they'd have problems. since he didn't believe me (and apparently had the time), he decided to first backfill as he had planned. we painted a line around the top of the backfill and then he dug it back out to within a couple of feet of the bottom. then he backfilled it again while beating in each 1-2 foot layer with the trackhoe bucket. when he got back to the top, the backfill was almost a foot lower (call it 6-8"+ since he left a little material laying on the ground and the paint line was 2 inches thick,etc). his only comment was "YIKES!"...and i never had to argue with him over densifying #57 stone backfill after that.
i learned the lesson of compacting #57 stone the hard way back before i was an engineer and was a contractor. i did a project where we had one area that had to be backfilled about 6' on top of the footing to get up to sugrade level and a second area where a footing was undercut about 5' and backfilled similarly. the material was dumped in and leveled off. the slabs/footings around these areas were poured and building construction went on. about 3 months later (in the middle of winter), i started noticing little swirling holes showing up adjacent to the exterior footing area (the area where the footing was undercut) that was backfilled as described. i took a 7' long piece of rebar and sunk it about 1 foot off of the footing. that's when i realized the material was washing in to the backfill. at the end of the project, we had problems with cracks showing up in the masonry at these two locations. i'm fairly confident it was because there was some subsidence in the slab/thickened slab or footing that was holding the walls. the cracks were worst just outside the actual backfill area and lessened with distance away from that. the cracks did extend to probably 30-40' away from the area before they disappeared.
i have to argue with contractors all the time, so i have actually bet (bragging rights) a contactor before. the contractor was going to backfill 7' in a 10'x20' area and didn't want to compact it because he thought i was full of bs. i told him that he had to densify the backfill or they'd have problems. since he didn't believe me (and apparently had the time), he decided to first backfill as he had planned. we painted a line around the top of the backfill and then he dug it back out to within a couple of feet of the bottom. then he backfilled it again while beating in each 1-2 foot layer with the trackhoe bucket. when he got back to the top, the backfill was almost a foot lower (call it 6-8"+ since he left a little material laying on the ground and the paint line was 2 inches thick,etc). his only comment was "YIKES!"...and i never had to argue with him over densifying #57 stone backfill after that.
let me clarify, i agree with the comments regarding the concerns with using the aggregate. i believe all sized aggregate should be densified to some degree. #4 doesn't take as much effort to compact as #57 stone but to be realistic, #57 stone is not that difficult to densify in the first place.
DarthSoilsGuy
Geotechnical
The only possible compaction that can occur with clean angle-faced #57 stone is the displacement caused by pushing it into the trench walls and bottom. If you pour it into a proctor mold, you can beat it all day and the only compaction you will get is from breaking the stone with the hammer into smaller pieces. Consider how a field density test would be performed on the "compacted #57 stone" and you'll see how it wouldn't matter if compaction is specified or not. Off the point... Up north here in VT, a lot of the screened stone comes from glacial deposited gravel pits where the stone has been ground down. This stone needs to be crushed and rescreened so those angled faces come back in to play.
The fine migration is from the trench walls and the soil you place on top of the stone deposit. since clean has a large empty void ratio, soils from around it can migrate into the voids and then the question is what replaces those soils. there is a judgment call for the fabric that has to take into account the site, soils, depth of stone, and [fill in the blank]....
If you have groundwater entering the trenchs, it makes it a lot harder to place & compact backfill. Some clean stone could be warrented to get to a point where fill soils can be put (while running a pump in the stone on the downslope side of the trench), but the deeper the stone trench the more potential problems that i posted before. With water entering the trench, i'm a little more sympathetic to the contractor. But, beware of the ease with which contractor's will want to haul in stone over structural fill once they get going and get a write-off on having to compact.
The fine migration is from the trench walls and the soil you place on top of the stone deposit. since clean has a large empty void ratio, soils from around it can migrate into the voids and then the question is what replaces those soils. there is a judgment call for the fabric that has to take into account the site, soils, depth of stone, and [fill in the blank]....
If you have groundwater entering the trenchs, it makes it a lot harder to place & compact backfill. Some clean stone could be warrented to get to a point where fill soils can be put (while running a pump in the stone on the downslope side of the trench), but the deeper the stone trench the more potential problems that i posted before. With water entering the trench, i'm a little more sympathetic to the contractor. But, beware of the ease with which contractor's will want to haul in stone over structural fill once they get going and get a write-off on having to compact.
If you have fine grained soil migration into the stone will probably be an issue, coarser material, it may be an issue. Depending on the situation, migration may or may not be a problem. The issue can be addressed by filter fabric, generally cost is not significant.
Clean stone is not compacted to a proctor because 1. Proctors are not designed to measure compaction of stone sized particles 2.difficult to accurately measure in place density 3. Since the density is rarely measured, there is no tables date etc. to correlate it to.
Fillig on top of stone can be accomplished with filter fabric or choking with 1/4 in and peastone.
Clean stone is not compacted to a proctor because 1. Proctors are not designed to measure compaction of stone sized particles 2.difficult to accurately measure in place density 3. Since the density is rarely measured, there is no tables date etc. to correlate it to.
Fillig on top of stone can be accomplished with filter fabric or choking with 1/4 in and peastone.
emmgjld
The fairy tale of non compacting gravel you mention with regards to concrete foundations on soft soils I have used for many large commerical buildings with no problems. I have placed #57 gravel as much as 8 feet thick down to virgin soil BUT the base was "over excavated" with a 2 on 1 distribution load path. I never compacted #57 stone and never had any settlement issues on many brick buildings. Pipe trenches is a different story and may need a filter fabric. Hopefully no other future work will ever get done next to it. I have seen some paper work from some stone suppliers lab here in NJ that by just dumping the stone out of the truck that will give you a relative density of around 90%. Not bad.
The fairy tale of non compacting gravel you mention with regards to concrete foundations on soft soils I have used for many large commerical buildings with no problems. I have placed #57 gravel as much as 8 feet thick down to virgin soil BUT the base was "over excavated" with a 2 on 1 distribution load path. I never compacted #57 stone and never had any settlement issues on many brick buildings. Pipe trenches is a different story and may need a filter fabric. Hopefully no other future work will ever get done next to it. I have seen some paper work from some stone suppliers lab here in NJ that by just dumping the stone out of the truck that will give you a relative density of around 90%. Not bad.
here's a simple exercise if you disbelieve you can effectively densify crushed stone. let's say you've got a job where 8' of #57 will be placed in an undercut (and assuming the undercut extended to the appropriate width outside the footing and all that). let the contractor put in the #57 stone as he wishes which is probably backdumping or using a front end loader to dump it in. then, stand on top of the backfilled area with your probe rod. work and juke your probe rod in to the stone until you get down near the handle. then, rod up and down like you're using one of those older style air pumps. as you do that, you'll notice that the rod will gradually be "refused" shallower and shallower. as the rod is backed out by this, a cone shape will form in the #57 stone. for my 3' probe rod length, i typically see at least a 6" deep cone form in uncompacted #57 stone. if it's half way spread around with the trackhoe bucket (effectively providing some amount of compaction) the depth that i can probe and the funnel depth is much shallower. now imagine if you were able to probe the full 8' depth....try it one time if you're a non-believer. i'm telling you, i learned the hard way about this. naturally, this probing technique does not work very well with larger stone like #34's but it's still very important to densify to some degree. i'm typically fine with using a trackhoe bucket as long as the lift are about 1-2' max and they beat each lift with me standing there...or they can use compaction equipment in 1' lifts.
cap4000
I will admit that there some areas where the crushed stone densifies easily. You are lucky to be in an area of anomalous materials. In the last 35 years, I have run into such materials in Colorado and surrounding states, BUT very rare. I encountered a similar crushed material in Viet Nam, south of Hue. I read about a few instances on this Forum. I consider them to be anomalies.
The vast majority of recommendations using coarse rock stabilization on this forum involved working the material into place, whether above or below the water table. True, this material usually (and I stress usually)readily achieves maximum density fairly quickly. If the material is angular and properly graded, it is usually fairly stable. If the gradation is not right, though still meeting the published gradation requirements, or if the stone and sands are rounded to subrounded, densification may or may not be achieved and stability can range from moderate to extremely low.
I will admit that there some areas where the crushed stone densifies easily. You are lucky to be in an area of anomalous materials. In the last 35 years, I have run into such materials in Colorado and surrounding states, BUT very rare. I encountered a similar crushed material in Viet Nam, south of Hue. I read about a few instances on this Forum. I consider them to be anomalies.
The vast majority of recommendations using coarse rock stabilization on this forum involved working the material into place, whether above or below the water table. True, this material usually (and I stress usually)readily achieves maximum density fairly quickly. If the material is angular and properly graded, it is usually fairly stable. If the gradation is not right, though still meeting the published gradation requirements, or if the stone and sands are rounded to subrounded, densification may or may not be achieved and stability can range from moderate to extremely low.
It appears the specific question relates to sanitary sewer pipe backfill, though responses have gone into other areas with arguable relevance. While I’m not going to claim a great deal of direct experience in all types of pipes and aspects of parking lot designs, I would only add that I believe most crushed rocks of whatever description (and perhaps various angular crushed rocks in particular) have a specific “angle of repose”. Some lengths of pipes are perhaps not laid perfectly, with absolute control over the axial grade of the pipe trench to support/mate with the pipe barrel. It would follow that if such rock is just “dumped” over a circular cross-section pipe e.g. setting on a flat trench bottom, the rock backfill might well end up pretty much “reposed”, with in effect the sidewall angle of the dump pile contacting the outside arc of the pipe pretty much tangentially somewhere above the level of the bottom of the trench in the haunch area, and an initial void of some description and volume perhaps thus existing at least below the contact point in the haunch area. I believe the presence or absence of voids might well be confirmed with a careful probe rod check in the haunch area of a dumped installation.
I suspect the issues of migration of fines, and uneven pipe support etc. surrounding such local voids (if some effort is not made, by mechanical compaction and/or I will note many other means such as shovel slicing or narrow roller compaction wheels alongside the pipe to somehow move the rock more evenly up under the haunches), could well be even more important per se than how much can one compact a particular layer of crushed rock. I would add that I would suspect the overall volume of voids that might occur under the pipe with dumped rock might generally well be much larger with very large pipes, whereas it could in general be much smaller e.g. with minimum size 8” sewer pipes. Also, some types of pipes and joining systems might well have adequate strength, long-term ring stiffness, and ductility etc. to tolerate such imperfect support, and some might not. Perhaps the same could be said of any deleterious effects of voids etc. on the overlying parking lot. If however the pipe/joint system is compromised by non-uniform bedding, resulting in broken pipes or infiltration/sinkholes etc., I suspect most would agree this is not a desirable situation for either the pipeline or the parking lot.
I suspect the issues of migration of fines, and uneven pipe support etc. surrounding such local voids (if some effort is not made, by mechanical compaction and/or I will note many other means such as shovel slicing or narrow roller compaction wheels alongside the pipe to somehow move the rock more evenly up under the haunches), could well be even more important per se than how much can one compact a particular layer of crushed rock. I would add that I would suspect the overall volume of voids that might occur under the pipe with dumped rock might generally well be much larger with very large pipes, whereas it could in general be much smaller e.g. with minimum size 8” sewer pipes. Also, some types of pipes and joining systems might well have adequate strength, long-term ring stiffness, and ductility etc. to tolerate such imperfect support, and some might not. Perhaps the same could be said of any deleterious effects of voids etc. on the overlying parking lot. If however the pipe/joint system is compromised by non-uniform bedding, resulting in broken pipes or infiltration/sinkholes etc., I suspect most would agree this is not a desirable situation for either the pipeline or the parking lot.
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dirtsqueezer
Geotechnical
The FHWA has a spec for the larger rock that isn't easily dealt with using modern methods of compaction testing, that is performance-based. My last project required 4 passes per lift, (albiet using a roller), with a soils tech on hand to verify and document compaction had been performed. No testing. And these were the Feds.
One thing about 'self compacting' gravel is that you also don't know how the soil around it is going to react. When you place that #4 (which should be angular to even consider this in the first place) you want those stones to lock in with the excavated wall- this requires tamping of some sort. The surrounding soil also may be softer than your fill and might benefit from the pressure of that compacted fill. If you don't compact the backfill against the wall, the wall fill might just settle back against the fill. If you don't compact the fill at all, you don't get the benefit of densification of the wall material.
I agree with everyone else, compaction's good all around. What does it cost for a jumping jack, $20 a day? Easily accomplished on a job like that. Clean washed rock is a great solution for a wet trench, but don't skimp on the compaction. You may just be perpetuating the problem you were trying to avoid.
One thing about 'self compacting' gravel is that you also don't know how the soil around it is going to react. When you place that #4 (which should be angular to even consider this in the first place) you want those stones to lock in with the excavated wall- this requires tamping of some sort. The surrounding soil also may be softer than your fill and might benefit from the pressure of that compacted fill. If you don't compact the backfill against the wall, the wall fill might just settle back against the fill. If you don't compact the fill at all, you don't get the benefit of densification of the wall material.
I agree with everyone else, compaction's good all around. What does it cost for a jumping jack, $20 a day? Easily accomplished on a job like that. Clean washed rock is a great solution for a wet trench, but don't skimp on the compaction. You may just be perpetuating the problem you were trying to avoid.
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