Determining Quantites for Sand Backfill 2

[asnyde92]

I am wondering what the best way to calculate backfill quantites for a trench, clay site with utilities under a road, I would assume that the side slopes would have to meet some standard and that also a box may affect the situation at some depth. I'm looking for a cubic yardage per lineal foot based on any particular depth.

 

[dicksewerrat]

Are you an engineer?

 

[asnyde92]

EIT

 

[roadspecialist]

you may have to provide a typical section of the trench showing the depth of excavation and side slopes. for clay and of course depending on the depth of excavation, the side slope would be not less than 1:1 to keep it stable or you may have to provide sheetings in case you may want to limit the trench width.

 

[Focht3]

There is no table needed; it's a simple set of calculations that should take no more than 5 minutes to do -

Yes, there are standards that have to be met. Where is the site?



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[asnyde92]

Site is a stiff clay, located in MI,

 

[Focht3]

Is the trench deeper than 5 feet? What is the groundwater depth?



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[asnyde92]

Depth of trench varies, no complete design as of yet put I would assume that the we will see nothing deeper than 20' and backfill quantites for shallow depths are of little concern. No ground water is expected.

 

[gbam]

Ok,
Lay out your trench and then assume a sideslope like above and for incremental depths, say 1', calculate the volume per linear foot. Use average end area to calculate volumes or assume a constant ground elevation and approximate the volume.

 

[Focht3]

With trenches that deep, you should assume flatter side slopes of 1.5:1 (H:V), as a minimum.



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[asnyde92]

What about 1:1 or steaper for the first say five feet of the trench to allow for the box, and then from there assume a shallower rise?

 

[Focht3]

You need to look at the OSHA (Federal and state, if any) guidelines on trench boxes and trench safety. Check out this link:

http://www.osha.gov/SLTC/trenchingexcavation/

While you may think the guidelines are excessive (many engineers do) I can assure you that your company is setting itself up for big trouble if you flout them. I have seen many excavation collapses (not in trenches.) They are sudden and very unpredictable - and trench collapse is still the single greatest cause of construction site deaths.

Please play it safe - and smart.



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[KRSServices]

Are you trying to estimate a job?

I ask, because it appears as though you may be a wee bit inexperienced for this task. The average area method will work well, but you will have to consider the compacted density of your backfill verses the hauled density. I am assuming of course that you are importing the sand. The number you are looking for, unfortunately, does not exist.

For open cut, your quantity of sand backfill increases almost exponentially with depth due to the requisite sidewall excavations. If you are using a cage, it is a little easier, but your productivity slows dramatically and there will be some questions of compaction due to the movement of the cage.

If it is possible, I would utilize the sand for bedding and the pipe zone and then utilize the native material for backfill. For road crossings, perhaps you may wish to consider unshrinkable fill (fillcrete) for backfill.

KRS Services

http://www.krs-services.com

 

[asnyde92]

This will be used as a budget figure, to account for the compaction I have been simply figuring that a compacted yard = 1.3 x Loose yard. Native backfill will not be allowed for most cases since most of the pipe runs under the road.

 

[BigH]

FOcht3 hit it right - this excavation (20 ft) needs to be properly shored and or properly sloped. This is not a mickey mouse excavation. So, your sand volume (or whatever backfill you use) is dependent on the excavation support (shored or open cut) chosen for the cut. Do a search for Trench Safety on the internet - there is a lot of information out there. Remember that excavations more than 4 to 5 ft (depends on the jurisdiction) are subject to regulations for shoring/cutting angles, ladders, etc.

 

[KRSServices]

May I enquire as to why there is an insistance on importing the sand backfill under the road versus the native material? You realize of course that your costs have just gone up because you have to dispose of the excavated material. Is the native material that unsuitable? Will this road be exposed to factors such as freezing and/or frost penetration to a depth greater than 2 feet? If so, You may wish to reconsider the use of sand, unless you intend on using it over the entire width of the road.

KRS Services

http://www.krs-services.com

 

[Zambo]

20'(6m)under a road!! Why? If a road crossing then I would propose some kind of no-dig technique, either pipe-jacking or directional drilling. If along the length of the road then there won't be much road left by the time you finish.

Zambo

 

[asnyde92]

To reply to KESServices I can simply say that your idea is not an accepted standard in these areas, all trenches under and road, residential industrial comercial, are ALWAYS backfilled with sand. Native material is then lost on site, or hualed of site. Frost is a concern, I'm replying form MI, and sand is always used over the width of the road (12" - 18" norm).

To reply ro Zambo, I am most concerned with development projects where a new road is constructed.

 

[KRSServices]

Thank you for your reply. May I enquire as to the nature of the native soils, specifically, what are they (clays, silts)?

If the trenches are backfilled with sand, how does the municipality later remediate the failures due to settlement or differential heaving?

We used to use sand as an exclusive backfill, but found it to be deterimental to the road structures after several freeze/thaw cycles. As of the last 15 years or so and depending on the soils and moisture contents (relative to groundwater tables) we have developed specs which utilize the native soils as backfill. This has proven to save not only initial project costs, but also long term maintenance. It is the value of the future maintenance costs where I have done many evaluations and had to revise existing standards as a municipal manager for several jurisdictions.

KRS Services

http://www.krs-services.com

 

[RDK]

In this part of Manitoba frost depths of 2 m are not uncommon.

I always have a fight with the regulatory people regarding backfill. They like to use sand because it's written in a book somewhere.

Native backfill if compacted to natural density will almost always act the same way as the native material in the first place. Sand or gravel backfill will either drain the local material or be a path for excess moisture to enter the material. Frost lenses are a real possibility.

I had a client who operated an airfield. He had excavated a trench and backfilled it with granular material (native material was a silty clay.) The trench heaved in winter and subsidized in the summer. This caused him to close a runway for one winter until the material could be removed and backfilled with native material.

To get in situ densities, simply use your density gauge while excavating. Take several measurements and have the same densities with the backfilled material.




Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com