Engineering and Bearing Capacity: Sponge Model

 

"Dry" Soils Infiltration and Runoff Wetting and Saturation
Water Holding Capacity Percolation and Drainage Engineering and Bearing Capacity

 

Soil is regularly used as an engineering medium. This is not without its consequences. Some challenges associated with soils on flat landscapes are identified below, followed by an article about a soil with some unique soil-related challenges. Several new challenges arise when any of these factors are combined with sloping soils.

There are several mechanisms which can contribute to subsidence (lowering of the landscape surface): 

  1. Compression of water out of soil pores when a load is added (consolidation)
  2. Removal of water from pores through pumping
  3. Solubilization of salts as water leaches through the profile 
  4. Decomposition of histic horizons.


The first two reduce the volume of the soil because water occupies more resistance to compression than air, so when water is removed, the soil consolidates (settles). The rate of compression in the first case is a function of the soil hydraulic conductivity (rate of water movement through the soil), which is primarily a function of soil texture and structure. This kind of compression can occur in both saturated and unsaturated (but still mostly saturated) conditions. The saturated hydraulic conductivity is faster, and so compression will begin quickly, and then generally slow down as the soil dries. However, it can continue for centuries. The rate of compression in the second case is a function of the pumping rate and the saturated hydraulic conductivity, as water moves through pores toward the cone of depression created by the pumping. The volume of the pores is reduced as air replaces water, and air supplies less force to resist the load. (Think about the difference between an air mattress and a waterbed bladder. In order for the air matress to support weight, the air in the mattress has to be under pressure - if the valve is removed, the air escapes. The water is placed in the bladder, but the water is not pressurized. When the valve  on the top surface of the waterbed bladder is open, the water does not escape. Water has more bearing capacity than air.)


The last case results in a loss of soil, typically when histic soils (wetlands) are drained for use in agriculture, industry, or municipal development. Histic horizons have more than 20% organic carbon, thus constitute much of the total volume of the soil. Draining soils lowers the water table and introduces oxygen into the profile. Once oxygen is introduced, aerobic bacteria begin to decompose the organic materials that comprise the soil, so the soil "shrinks".

 

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One gallon carton, filled with water, sitting on a dry sponge. The dry sponge gives slightly in response to the load applied. The dry matrix of the sponge has some rigidity, but remember that the sponge expands as it wets. Thus, the dry sponge has smaller pore spaces, and so there is less danger of consolidation when a load is applied.

 

 

*Notice the height of the dry sponge is approximately 30 mm.

 

 

 When the sponge is wetted, it expands about 5 mm in height, as well as in length and width. This expansion changes the strength of the sponge matrix, weakening it substantially.

Many clay soils, particularly those dominated by smectitic and vermiculitic minerals, expand when they wet and contract when they dry. This causes stresses in structures (houses, roads, etc.) that result in cracks in plaster, tiles, and even foundations.

 

 

 

When the same load is applied to a wet sponge, all the dynamics change. Initially, the sponge compresses in response to the weight of the carton. This initial compression is greater than it was in the dry sponge because the matrix in the wet sponge is expanded - it has more air (and water) filled spaces that can be reduced in volume.

When a structure is built on wet soils, the load begins to consolidate the soil, squeezing the water out of the pores.

 

 

 

 

The load begins to shift in response to unequal forces of resistance in the sponge and loading from the carton. (The bottom is uneven, so the load is not uniformly distributed.) Few buildings have equal load distributions, so the effect of placing a building on the soil is similar. When the building begins to settle unevenly, foundations or walls may crack to relieve the stress.

Once the load shifts, it is now greater on the right side of the sponge. so that side will continue to consolidate more than the left side. This is the same concept as, "When you go down hill, you pick up speed." The uneven load creates compounding forces that cause even greater nonuniformities.

This is not unlike buildings in Mexico City, Mexico, Amsterdam, Netherlands, Venice, Italy, New Orleans, LA, Houston, TX, and others, including the Tower of Pisa.

The rate of consolidation (subsidence) is a function of the soil type (how much clay), hydraulic conductivity (water transmission through the pores is related to pore size), the applied load, as well as geologic strata and presence of water tables.

 

 

 

 Once the load is removed, the sponge recovers somewhat. The height of the sponge after the water was extruded is about 30 mm again, so the load compacted the height of the sponge about 5 mm.

 

 

 

 

 

 

This sponge dried for about 16 hours. It is not fully dry, nor fully wet. Its height is about 28 mm. It is not as rigid as the air dry sponge, and will not support the weight of the jug as you will see below.

 

 

 

 

 

 

This picture was taken just as the sponge failed. The weight of the jug shifted gradually to the back of the sponge. Then the process accelerated until the sponge failed to support the weight of the jug, and the jug fell off the sponge.

 

 

 

 

 

 

Another cause of failure is uneven wetting. This very dry sponge (same as above) supported the weight of the jug until the water was added gradually to the back of the sponge. Since the wetting pattern was not uniform, the strength in the wet part of the sponge dropped rapidly, while the strength in the dry portion of the sponge remained unchanged. This created an imbalance that led the jug to fall off the back of the sponge.

 

 

 

 

 

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Other conditions also can cause structural problems in buildings and roadways:

  1. Frost heave - This is a problem with roads and sidewalks when water moves through cracks and collects below the structure and above the soil. When water freezes, it expands. The freezing water "dries" the soil at that point, creating a potential gradient that causes water to move to rewet the soil, providing more water to freeze, and eventually building up ice layers that raise the structure. In severe cases, this can cause the structure to buckle.
  2. Trees - Large trees planted too close to houses, roads, sidewalks, etc., can send lateral roots under the foundation, causing uneven surfaces, buckling, and cracking.
  3. Clay soils with high shrink wall capacities - These soils shrink as they dry and swell as they wet. The forces generated in the soils as they wet can cause heave (upward thrust on the objects at the soil surface) and throw (uneven lateral pressure that causes vertical objects to tilt). The soils do not dry uniformly, so the damages caused by wetting are compounded as the soil dries, rewets, ... This can be managed to a degree by keeping the soil at a uniform water content.


There are often engineering solutions to soil challenges. People are rather determined creatures, and are seldom appreciative of being told, "no", about where they want to build. As a result, several methods have been developed to overcome the natural soil limitations. These methodologies are collectively called, "soil stabilization".