Stripped & Proof Rolled Natural Soil for a Mat Foundation 1

[oengineer]

I have a soil report that states for my project that "mat foundations may be supported on stripped and proof rolled natural soils".

I take this to me that if I "Compact back-fill to 95% maximum density" the soil underneath the mat foundation, I have satisfied the stripped and proof rolled natural soils requirement for the mat foundation. Is this logic accurate?

Suggestions/comments are appreciated.

 

[r13]

Did you pay the consulting fee in full, or you didn't bother to pay? I think your geotechnical consultant owe you a lot of information. The excerpt below is the beginner.

According to the Department of Transportation for the State of Ohio, proof rolling is a process where compacted soil is checked for soft areas in order to supply a balanced support system for the structure of pavement. If soft subgrade spots are located during construction, they are corrected so that the density of the subgrade is maintained.

According to Ohio's DOT, one trip with a proof roller is usually sufficient for locating soft spots. However, the agency warns that an overloaded proof roller, regardless of the soil type, may cause subgrade instability during the rolling process. On the flip side, soft areas may not be found if the roller is too light for the type of soil.

The Ohio DOT says that a 35-ton roller with a tire pressure of 120 psi should be used on soil classified as A-3, A-4, A-6 and A-7. The aforementioned tire pressure and load is indicated for use on Ohio soils. For granular soils, the DOT suggests using a 50-ton roller with 150 psi of tire pressure.

Some soft soils may be 3- to 5-feet deep. Only in rare cases is a soft spot be deeper than 5 feet. The agency adds that insufficient stability is marked by cracking, rutting or deflection on a subgrade's surface.

 

[oldestguy]

We can't comment here,since insufficient info is presented. Hell, there may be solid rock at 6" depth?

 

[oengineer]

We can't comment here,since insufficient info is presented. Hell, there may be solid rock at 6" depth?

On the drawing, it currently calls for Compact back-fill to 95% maximum density for 6" of the soil supporting the mat foundation.

Would this be sufficient info?

 

[r13]

So the consultant did provide proper instruction on site preparation. The proof roll is just an alternative, correct?

 

[oengineer]

So the consultant did provide proper instruction on site preparation. The proof roll is just an alternative, correct?

The geotechnical report provides the following information on site preparation:

[ul]
[li]All areas to support select fill should be stripped of all vegetation, topsoil, and other deleterious materials. After stripping and grubbing, the exposed subgrade should be thoroughly proofrolled in order to locate and densify any weak, compressible zones. A minimum of 5 passes of a fully‐loaded dump truck or a similar heavily‐loaded piece of construction equipment should be used for planning purposes. Proofrolling operations should be observed by the Geotechnical Engineer or his representative to document subgrade condition and preparation. Weak or soft areas identified during proofrolling should be removed and replaced with suitable, compacted on‐site clays, free of organics, oversized materials, and degradable or deleterious materials. Upon completion of the proofrolling operations and just prior to fill placement or slab construction, the exposed subgrade should be moisture conditioned by scarifying to a minimum depth of 6 in. and recompacting to a minimum of 95 percent of the maximum dry density as determined by American Society of Testing and Materials (ASTM) D698. The moisture content of the subgrade should be maintained within 2 percentage points of the optimum moisture content until permanently covered.[/li]
[/ul]


The geotechnical report provides the following information on select fill:

[ul]
[li]Materials used as select fill for final site grading preferably should be inert cohesive/semi‐cohesive sandy lean clays (CL)/clayey sands (SC) as classified according to the USCS, may be considered satisfactory for use as select fill materials at this site. Select fill materials shall have a maximum liquid limit not exceeding 40 percent, a plasticity index between 7 and 20 percent, and a maximum particle size not exceeding 4 in. or one‐half the loose lift thickness, whichever is smaller. In addition, if these materials are utilized, grain size analyses and Atterberg Limits must be performed during placement at a minimum rate of one test each per 5,000 cubic yards of material due to the high degree of variability associated with pit‐run materials. If the above listed materials are being considered for bidding purposes, the materials should be submitted to the Geotechnical Engineer for pre approval at a minimum of 10 working days or more prior to the bid date. Failure to do so will be the responsibility of the contractor. The contractor will also be responsible for ensuring that the properties of all delivered alternate select fill materials are similar to those of the preapproved submittal. It should also be noted that when using alternative fill materials, difficulties may be experienced with respect to moisture control during and subsequent to fill placement, as well as with erosion, particularly when exposed to inclement weather. This may result in sloughing of beam trenches and/or pumping of the fill materials. Soils classified as CH, MH, ML, SM, GM, OH, OL and Pt under the USCS are not considered suitable for use as select fill materials. The native clays observed in the borings are suitable for use as select fill materials. Select fill should be placed in loose lifts not exceeding 8 in. in thickness and compacted to at least 95 percent of maximum density as determined by ASTM D 698. The moisture content of the fill should be maintained within the range of 2 percentage points below to 2 percentage points above the optimum moisture content until final compaction.[/li]
[/ul]



The geotech report recommends that the mat foundation "may be supported on stripped and proof rolled natural soils, or properly prepared select fill". My intentions are to go with the "stripped and proof rolled natural soils" for the mat foundation soil preparation verses using select fill.

My goal in this thread is to confirm that I am achieving the correct mat foundation soil conditions by calling out on the construction documents to "Compact back-fill to 95% maximum density for 6 inches of the soil supporting the mat foundation".

 

[Rabbit12]

"it currently calls for compact back-fill to 95% maximum density for 6" of the soil supporting the mat foundation"

This statement doesn't make any sense. Is this the exact wording from the report or are you trying to paraphrase? Attach an excerpt from your report because you're doing a poor job of explaining what the report says. It's not uncommon to have to go back to the geotechnical engineer for clarification.

Edit: You posted the above at the same time I posted this.

 

[oengineer]

"it currently calls for compact back-fill to 95% maximum density for 6" of the soil supporting the mat foundation"

This statement doesn't make any sense. Is this the exact wording from the report or are you trying to paraphrase? Attach an excerpt from your report because you're doing a poor job of explaining what the report says. It's not uncommon to have to go back to the geotechnical engineer for clarification.

I am just paraphrasing. Please see my 3rd post in this thread regarding what the geotechnical report actually says.

 

[Rabbit12]

You need to strip the vegetation/deleterious materials/etc. and then proof roll that subgrade. Remove any poor subgrade identified by proof rolling. Then scarify to a depth of 6", moisture condition and recompact to 95% of maximum prior to placing the slab or engineered fill.

Those a pretty clear and common recommendations.

 

[r13]

Rabbit12 has the correct wording, after proof roll, a 6" soil is to be removed, then place backfill and compact to the specified density. Good soil report, as usual the case.

The one thing you might want to check with the geotechnical engineer is, some DOT allows 20 Ton roller with one pass. I wonder do you really need 3 passes as called out in the report.

 

[Rabbit12]

retired13, they won't "remove" the soil during scarifying though. It's just "plowing" the soil up and recompacting the same soil in the same place.

The only thing I might question in that report is I might scarify first, recompact and then proof roll.

 

[r13]

The proof roll is to identify/expose weak spots, and The reason for scarify layer is due to the fact the roller may create uneven surface that unsuitable to receive back fill and compact to the desirable strength level uniformly throughout the site.

I agree that if the scarify soil is deemed suitable for backfill, it could be saved and reused.

 

[Ron]

OG...you nailed it again! Exactly.