Earthwork Balance Calculations ª Subsidence

[Abacusengr]

I am preparing an estimate for grading work for a commercial retail development and the site requires significant import. The soils report says: "We have estimated that shrinkage may range between 10 and 15 percent, and a subsidence value ranging from 0.10 to 0.15 feet my be used for preliminary earthwork calculations." Can anyone give me some pointers as how to apply the subsidence value to the earthwork balance calculations? I don't want to come up short on my estimate of import!!

 

[SteveWarfle]

It is strange to be listed that way. Are you sure it doesn't say .10 to .15 per foot? That would correspond to the shrinkage values. As you know, shrinkage and expansion can impact a site tremendously. There is a primer on our website about it for anyone that would like more information.

http://www.insitesoftware.com/091999.htm

In the case of import material, you need to find the ratio of loose hauled material to in place material. Normally the quarry the material is coming from will know this.

A soils report usually refers to core samples taken on site, and since your concerns are about import, they probably don't apply.

Clear as mud.....right?

Take Care
Steve Warfle

Insite Software Incorporated
(877)746-7483

 

[cvg]

I would guess that the shrinkage refers to soil excavated from the site and moved to another location on the site would shrink 0.10 - 0.15 feet per foot when compacted as specified (90 or 95% of max).

Preparation and compaction of subgrade may also cause the ground to subside since the in-situ density may be low. Simply driving the scraper over the site a number of times may cause some compaction of the subgrade. Rolling with a sheepsfoot will almost certainly result in additional subsidence.

As stated above, a soils report is needed for the "borrow site" (probably not a quarry) to determine the following:

a)suitability for use
b)shrinkage or swell when excavated, placed and compacted
c)maximum density and optimum moisture content for compaction

However, unless you are working for the contractor, location of a borrow site and precise estimation of the quantity at the borrow site is usually left up to the contractor. Bidding and payment is usually based on accepted quantities placed on the job.

For this job, I would assume that you move existing dirt around on the site, achieve desired subgrade compaction using rollers or scrapers and then calculate some subsidence amount. This will give your subsided subgrade elevation. Then just calculate the theoretical amount of fill required to achieve the finished grade.

 

[KRSServices]

One pointer that has saved me endless hours of grief is as follows.

When estimating the earthwork, keep in mind that the excavation will be generally removed as soils with insitu densities of 100%. Borrow will be hauled in by a volume measurement or scaled. If by volume, then you must be aware and calculate the lighter density of the material in the truck box, which will be different than in the loader bucket, which will yet be different still than compacted onsite. Case in point.

We scaled our gravel utilizing a scale affixed to our loader. The truck boxes were 20 yard capacity and the loader had a (heaped) 5.5 yard bucket. In three loads, the loader would load the trucks. That means, if one used density as a constant, the loader used a 6.6 yard bucket. The material in the truck was 1.24 tonnes/yard, while in the bucket was 1.5 tonnes/yard and 1.61 compacted onsite.

Using this premise, I now caculate my borrow based upon weight, not volume. My estimated balances have been remarkably close to the final values in the contract, resulting in confidence in my methodology and cost savings in contingencies. I don't guess anymore and fiddle with "shrinkage" or settlement values, per se.

KRS Services

http://www.krs-services.com

 

[Grouser]

If you want to quantify the subsidence, strip the topsoil and have the site topo'd. Then have the site proof rolled and compacted and retopo'd.
Fluff and shrinkage will depend on the fill material and its moisture content at time of excavation. If the pay quantity is embankment compacted in place, it is determined by topo and the number of truckloads is the contractor's problem. The contractor likely is familiar with local materials and will be including these issues in his bid.
Regarding the uncertainty of the soils report, call the engineer and ask.
Good luck.

 

[treg]

In many parts of the country, subsidence refers to collapse of the surface due to underground extraction, such as mining. In your instance, the geotehnical engineer may be refering to voids caused by some other activity. You should contact the preparer of the report to understand what he means.

 

[Focht3]

I don't like the use of "subsidence" in this context. Fuzzy thinking (and writing) makes communication that much more difficult. I personally prefer "fluff" and "loss" since their geotechnical contexts are clearer.

 

[cvg]

in this area, subsidence is usually a result of long term pumping of aquifers and resulting lowering of the water table which causes the ground to subside. For example, 60 years of pumping on the west side of the city has caused localized subsidence of up to 19 feet. This in turn has caused flood control channels to begin to flow backwards, causing flooding.

 

[Focht3]

Same effect in the Houston area. It started in the Ship Channel area (southeast Houston), and was found to occur in other areas of the City as well. Whole subdivisions were lost as a result of up to 24 feet of subsidence.

 

[KRSServices]

Focht3,

Are there any reference, technical papers or historical documents I can get hold of regarding the subsidences, particularly the subdivisions. It soounds like a very interesting read with applications throughout North America in terms of establishing public policies.

Any direction will be sincerely appreciated.

KRS Services

http://www.krs-services.com