2' Select Fill with asphalt versus Approach Slab

[edward1]

Why would a state DOT bridge engineer demand at least 2 feet of select fill with asphalt on the brige approach if the contractor opts not to pour an approach slab? Is the 2 feet fill a rule of "That's the way we have always done it" or is does it have something to do with impact reduction?

 

[Qshake]

undoubtedly the select fill will peform better than backfill from anywhere on the project site.

As to 2', there is already typically 1 foot of select fill under a typical concrete approach slab in most States. So another foot topped with asphalt doesn't seem unreasonable.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[hokie66]

Not a bridge engineer, but why would a contractor have the option of not building an approach slab?

 

[edward1]

Because the state DOT engineer had an arch bridge designed by a consulting engineer. The arch bridge had a minimun fill of 1.5 feet at center span and obviously more at abutments. The arch bridge cannot accomodate an approach slab. The approach slab is required by this DOT engineer on all bridges except arch bridges. The contractor asked the DOT engineer for the fill option on the rigid frame and that is where the 2 feet fill was given.

 

[cvg]

sounds like a con-arch culvert. If that is the case, it is not a bridge, there is no "abutment" and what's the point of extending the 2' of fill beyond the limits of the edge of the channel?

 

[beton1]

The asphalt will likely crack no matter what. They are probably trying to mitigate cracking by placing select fill.

 

[Qshake]

Many states have gone to a modified approach slab and some states have gone further in reducing costs on rural roads to provide asphalt approach pavements that can easily be topped with asphalt should it settle (and it will!).



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[edward1]

The answer to the question was provided today. The state DOT engineer wanted a con-arch culvert but the contractor got a lower bid from the rigid frame precaster. The state DOT engineer thought if he required more fill on the rigid frame than the con-arch that hydraulic analysis would not pass on the rigid frame due to a lower elevation of the bottom of top slab. Unfortunately for the state DOT engineer the rigid frame passed the hydraulic analysis and the contractor will install such. The best laid plans of mice and men ...."

 

[hokie66]

I understand that a relieving slab makes no sense approaching an arch. But if the contractor changed to a flat bridge, the relieving slab should follow as part of his revised proposal.

 

[edward1]

I agree with hokie66. That would also help with hydraulics. The contractor said it was cheaper for him to install fill versus pour the approach slab. What I was originally pointing out was that the con-arch was only required 1.5 feet fill but the rigid frame was required 2 feet fill.

 

[Bobber1]

Frequently we use a soil cement backfill in situations like this. Or a lean concrete mix with sand aggregate only. Maybe a one or two sack per c.y. mix. Cement content can be kept low to keep cost down and make it "diggable".