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

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?

This question might be more appropriate for the SE Engineering site.

The thing about soils is that they vary from place to place. Some are sandy, some contain more clay than others and the thicknesses of layers is also variable. All this influences how different soils react to surface loading stresses, such as from building foundations/footings.

Immediately below the footings the soils will experience the greatest stress increases. As discussed in this document, particularly from page 10 onwards, the magnitude of stress the soil experiences from footings decreases with depth - see the effect of a point load $$Q$$, applied to the surface, on the upper right of page 12. At a depth $$z$$, the vertical stress $$\sigma_v$$ has a certain higher value. At a depth of $$2z$$, the value of $$\sigma_v$$ is lower.

One reason why the affect of surface loads decreases with depth is due to the nature of soil particles and how they lie in relation to each other. Soil particles are not uniform in size or shape, so they rarely lie directly on top of each other. There are gaps between soil particles, called pores. The pores can hold ground water.

Compaction and consolidation of soils will reduce the volume (size) of pore spaces. Foundation loadings will only affect the soil beneath them.

Because soil particles do not lie directly on top of each other, one particle may lie above two or more particles, the stress exprienced by the upper soil particle will be transferred to the other particles. Providing the soil is deep enough, the affect of this is the to, over depth, transfer vertical stresses to horizontal stresses.

Ultimately, if a soil profile is deep enough, the stress the lower reaches of the soil experience due to urban development will be small and will not affect ground water movement.

• I understood why the stress decreases significantly with depth. But my original question means "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. Heading to SE Engineering, thanks. Dec 8 '19 at 11:07

Heavy buildings can sometimes be supported by piles going down to the bedrock beneath, but a popular and cheaper solution nowadays is the use of piled-raft foundations. This consists of a slab of reinforced concrete covering the entire footprint of the building, thus spreading the load, and this slab (or raft) is supported by numerous concrete piles below it which do not go down to bedrock. Very small trickles of water underneath the structure are not a problem provided they are not trapped by it. Because the load is spread over a large area, the porosity of alluvial sediments below is little affected by the weight of the building.

• All buildings (at least apartment blocks and new constructions) have slab-beam type raft foundation here. I got the point of the spread of the load. Dec 5 '19 at 7:09