Engineered fill - capacity 1

[mte12]

Hi, is there a method used for determining allowable bearing pressure or bearing capacity for sites with engineered fill.

I presume it depends on depth of engineered fill and raft slab thickness.
How does natural soil below affect?

Is there any literature that is suitable to begin with?

 

[EireChch]

it makes no difference whether its fill or natural. All that maters is the strength and compressibility parameters.

Raft slab thickness makes no difference to performance of ground below.

Again, natural soil and fill are characterized based on their parameters.

Start with reading any geotechnical text book.

 

[mte12]

Why then is soil replaced with Engineered fill?

 

[EireChch]

I mean the method for determining the allowable bearing capacity (whether thats a shear capacity calculation or settlement calculation) is the same if the material beneath the foundation is natural or engineered fill.

The materials parameters are the important thing

 

[mte12]

Is the intention to spread the load then?
In that sense, thickness of footing does make a difference to the load spread, major loads typically come from a column or wall. And if footing is much stiffer than soil, it would tend towards a monolithic element.

And surely the confinement of the soft material, covered by Engineered fill and slab, is able to carry more load.

 

[EireChch]

The question of load spread is a separate topic than is there a method of determining allowable bearing capacity / pressure for engineered fill.

allowable bearing capacity / pressure is determined by a shear capacity calculation which generally does not need to consider load spread.

Similarly for settlement calculation pressure in each layer is typically calculated using a boussinesq or similar theory which assumes the material is homogeneous isotropic. Load spread in each layer is not considered.

Thickness of footing or whether a slab is flexible or rigid is again generally not considered in a shear calculation. All you care about is footing size and depth and parameters.

Settlement at the center of a foundation and is based on a bearing pressure given by the structural. If settlement is needed at the edge or corner it can calculated by including various factors.

Re confinement - if you have a large raft foundation then you can consider the constrained modulus instead of the elastic modulus. Some people do that, some dont! how much confinement warrants constrained modulus is difficult to determine. But confinement would be essentially the same if it was a large foundation on natural ground or engineered fill. Engineered fill does not really increase the confinement. It is the rigid slab that is confining the soil.

 

[mte12]

OK thanks, perhaps I've phrased my question incorrectly.
It is the load spread that is important to me, hence considering select fill and a stiff raft foundation. So the stress on the soft layer is reduced relative to a case without fill and a flexible foundation.

At the end of the day I was looking for some literature, preferably a worked example.

 

[le99]

Engineered fill is a graded soil mixture that so designed to achieve its maximum possible density through compaction or other soil densification methods. It is a material used for ground modification to replace the weak soil layers close to the ground to minimize the settlement and gain better bearing strength.

The size of the foundation does have a profound effect on the subgrade as its influence area and depth increase. The thickness of a mat foundation does affect soil response, but the matter is somewhat muted, as it involves many factors that the geotechnical engineer does not control but the structural engineers.

The fill, or backfill, acts as a surcharge to the native soil below, which will continue to densify. and settles. The settlement is the primary concern of the geotechnical engineer.

You have touched on a huge topic that requires schooling to learn. I would avoid reading a few articles on the internet to claim familiarity with soil mechanics.

 

[ukengineer58]

I dont the the engineered fill affects the soil below. engineered fill is normally used to replace weak or inconsistent soil close to the surface - not more than a few meters or its generally cheaper to pile. Regardless of the strength of the fill you governing factor will likely be the settlement or shear failure of the lower material. Unless its rock.

 

[EireChch]

The OP needs to clarify if the fill is replacing existing soft soil or being added on top. This has a massive effect.

My discussion below refers to replacing soft soil with engineered fill.

The OP stated: It is the load spread that is important to me, hence considering select fill and a stiff raft foundation. So the stress on the soft layer is reduced relative to a case without fill and a flexible foundation.

Load spread is based on bearing capacity wedges at angles of 45 deg -phi/2. Using select fill beneath a foundation has an very minor affect on the load spread. This affect is likely offset or counteracted by the fact that select fill/or engineered fill will have an increased density so increased stress than the soft soil that is removed.

If the depth of bearing capacity wedges overlaps the underlying soft soil then you may have punching shear failure. Use the Meyerhoff and Hanna method to determine the bearing capacity based on layered soil.

 

[MotorCity]

You need a geotechnical investigation to determine the soil properties. There is no literature or worked example that can tell you the bearing capacity of the soil you have.