How much does the weight of urban structures (buildings) affect the compaction (permeability, porosity, density) of alluvial sediments below a city?

Question

Original Question was here.

I live in Van, Turkey. Van City is situated on an alluvial plain beside Lake Van. General geological structure of the area can be seen on pages 43-44 in this conference book. One of the images is below.

According to the article in the link, sediment layers has a slope of 15-30 degrees toward the lake.

With the construction of every building, the pressure over the sediments increases. Obviously, this increase is greater in downtown area which has a lot of apartment blocks (mainly 5 to 7 story high reinforced concrete buildings). Downtown area is roughly in the middle of the plain.

I want to know that how much does the weight of urban structures (buildings) affect the compaction (permeability/porosity/density?) of alluvial sediments below a city and how does this affect the flow of underground water?

How much the stress caused by the weight of the building decreases with depth? (of course if at least a broad value can be given to help to explain the size of the change). I wonder to understand that either the effect of the weight of urban structures is generally negligible, or not.

Answer 1

In many cases, the weight of the building on the soil is roughly equal to the weight of the layer of the soil excavated for the foundation, subterranean parking, utility rooms, storage, etc.

Let's assume the buildings are made of 20cm thick concrete slabs,

$0.2*1m/2400kg.m^3= 480kg/m2$

and a live load of $100kg/m^2 \quad total=580kg/m^2 \quad 580*6floors= 3480kg/m2$

We estimate the soil weight is around $1600kg/m^2$ which is not unreasonable.

Let's assume the depth of foundation and parking 3m, excavation. $ 3*1600= 4800kg/m^2$ which is even a bit larger than the weight of the building added to the soil.

And also many sites need grading to create a flat pad for building and its surrounding landscaping and access roads. And because usually grading is done favoring cut rather fill, that takes some more weight from the top of the soil.

Usually, these kinds of jobs require a soil and geology report and one of the reasons for the report is checking into your concern.