# Why does water lower soil bearing capacity?

Most of us know that adding water to soil lowers its bearing capacity. Anyone know exactly why it does?

Consolidation happens when the weight of the structure presses existing moisture out of voids between soil particles. That’s different. I’m not asking about that.

When a newly-installed downspout starts funneling roof runoff to soil next to the foundation of a 30-year-old home, that foundation is suddenly more likely to crack because the water has lowered soil bearing capacity. What’s the actual process that reduces the bearing capacity?

http://www.ce.washington.edu/~liquefaction/html/why/why1.html

Explanation

To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles. If we look closely at these particles, we can see that each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce contact forces between the particles - these forces hold individual particles in place and give the soil its strength.

Soil grains in a soil deposit. The height of the blue column to the right represents the level of pore water pressure in the soil.

The length of the arrows represent the size of the contact forces between individual soil grains. The contact forces are large when the porewater pressure is low.
Liquefaction occurs when the structure of a loose, saturated sand breaks down due to some rapidly applied loading. As the structure breaks down, the loosely-packed individual soil particles attempt to move into a denser configuration. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is “trapped” and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit.
Observe how small the contact forces are because of the high water pressure. In an extreme case, the porewater pressure may become so high that many of the soil particles lose contact with each other. In such cases, the soil will have very little strength, and will behave more like a liquid than a solid - hence, the name “liquefaction”.

Liquifaction is caused by strong cyclic loading, typically by earthquakes or storm surge. There’s no such loading where a downspout discharges next to a foundation or where there’s reverse grade.

Ken,

Typically the load carrying capacity of fine grained soils, like clay and silt, are influenced by the compacted effort applied and the moisture content. The graph in the attached drawing represents a standard Proctor curve.

• Point A - the density of the soil is low which represents dry uncompacted soil.
• Point B - the density of the soil is higher due partly to compaction and partly due to the addition of water, which is heaver than the air it displaced. Also the water acts as a lubricant helping the soil particles to move to a more dense configuration.
• Point O - the density of the soil is at its maximum due partly to more compaction and partly due to the addition of more water, which is heaver than the air it displaced.
• Point C - the density of the soil is starting to go down, even though more compactive effort was applied. The reason is the additional water, which is lighter than the soil, is displacing more soil. The stability or the soil’s ability to support weight, is also going down due to the buoyancy effect of water. The added water is causing the soil particles to get lighter (float) and to loose contact with each other. This reduction in soil particle contact lowers the load carrying capacity of the soil.
• Point D - the density and stability of the soil continues to go down as additional water replaced more soil particles and approaches complete water saturation and eventually turning into mud.

Liquefaction is where loosely packed, saturated soils loose their surface to surface contact from the intense shaking of an earthquake causing a sudden drop in stability.

And what Randy is indicating is that there is an Optimum Water Content.
http://www.spec-net.com.au/press/0108/ele_090108.htm

http://www.ksre.ksu.edu/library/CRPSL2/AF115.pdf

[FONT=Times New Roman][size=2][size=4]The amount of soil water is a[/size]
[size=4]critical factor in soil compaction
potential. A dry soil, which has
friction between the soil particles, is
not easily compacted. Water acts as a
lubricant between the particles,
making the soil easier to compact.

[FONT=Times New Roman][size=2][FONT=Times New Roman][size=2][size=4]However, as soil water content[/size][/size][/size][/size][/FONT][size=2]
[FONT=Times New Roman][FONT=Times New Roman][FONT=Times New Roman][size=4]increases, a point is reached[/FONT]
[FONT=Times New Roman]where most pore spaces in the
soil are filled with water, not
air. Water cannot be compressed,
so water between the
soil particles carries some of
the load of the soil, resisting
compaction. Therefore, a
very wet soil will not
compact as much as a
moderately moist soil.
[/FONT][/FONT][/size][/size][/FONT]
[/FONT][/size][/FONT]

Randy is dead on. It is all about the moisture/density ratio found for the particular soils.

Basic Soil Mechanics 101.

For most HIs, this is way over their heads, esp. the guys who used to change transmissions and mid level managers laid off who turned to HI as a viable income source.

Oh, I forgot all the so-called ‘conspectors’ who have built a couple of houses by letting their ‘sub’ dump loads of soil on a non-scarified, non-compacted site and then roll it in with a simple wheel loader b4 letting their concrete guy set forms! All, of course, without any lift, moisture and compaction control.

Same goes for foundation wall b-f which is always done wrong on a house with a bsmt!

The graph takes me back to a college summer of Proctor tests, splitting aggregate samples, asphalt testing.

For the South Africans, I’m writing a course called “Soils, Substructures, and Superstructures”. Basically it covers residential walls, foundations and the soils that support them. Here’s what I’ve written:

" Water absorbed by soil- especially fine-grained, low permeability soils like clay and silt- can lower their bearing capacity by increasing soil plasticity. Water forces soil particles farther apart, reducing friction, and acting like a lubricant, allowing the weight of the structure to consolidate soil supporting the foundation, which results in subsidence.

My theory is that it’s worth going into the details, because if an inspector knows the basic performance characteristics of materials and has some understanding of the forces that affect these characteristics, they’ll be better at figuring out what condition they’re looking at, and what else might be affected by it."

It means I have to understand things well enough to give a simple-but-effective explanation. Keeping it simple is easy; I’m terrible at math and never had a physics class.

Asphalt testing?
Do you know much about the variations in asphalt used in comp shingles?

Equilibrium moisture content.

This is the point at which voids between particles are filled with water instead of air, but not enough water to force particles apart, reducing friction?

This is the point of maximum bearing capacity?

Or how 'bout this:

**" **Air can be compressed, water cannot.
Compaction is the process of increasing soil density by removing air from between soil particles so that they lie more closely together.
Consolidation is the process of increasing soil density by removing water from between soil particles so that they lie more closely together.

All soils have an equilibrium moisture content (EMC). EMC is the optimum water content for a particular soil; the point of maximum bearing capacity. At EMC, voids between particles are filled with non-compressible water instead of compressible air, but not so much water that it has forced particles farther apart.
Water absorbed by soil- especially fine-grained, low permeability soils like clay and silt- can lower their bearing capacity by increasing soil plasticity- making it more like mud. Water forces soil particles farther apart, reducing friction, and acting like a lubricant, allowing the weight of the structure to consolidate soil supporting the foundation, which results in subsidence."

Asphalt testing was for asphalt pavement materials. I had to do density/compaction tests, test for air openings/pockets in the compressed sample and then dissolve the asphalt from the sample to determine if the spec amount of ashpalt was in the pavement being laid.

Don’t know how to tests for the amounts of asphalt in shingles but from what I’ve been seeing over the past 12-15 years, it’s not the same quality or amount that was being used in the 70’s and 80’s. Most shingles now begin curling at 5-7 years; that used to happen at 13-14 years!

Just look at world oil prices over the past 15 years: 1999-2000 was \$9.50-\$10/bbl; now it’s \$90-\$100/bbl…about 1000% increase…they have a very strong reason to reduce amounts used or cut on the quality of the asphalt used. If you search online for many shingle brands and “class action lawsuit”, it will surprise how many hits come up. The most recent settlement (2 months ago) I’ve seen in Canada was “BP”, an EXXON/ESSO owned subsidiary. People only had until Sept 21 to register to become part of the class!!!

Now crack were Kenton?
Please explain what type of crack. lateral. horizontal, the pyramid cracks at corners and at what depth in the soil.
Love this topic…

Now this is not scientific in answer.
Fine particulate makeup soils (loam) composition or structural makeup.
They are moved downstream in water by gravity. The soil in that area becomes less dense. The ability to hold the existing structure with the same applied force changes.
I hope that made sense. I am tired.
As if that ever had anything to do with me making sense. HA HA HA.

Kenton you must also equate THE STRUCTURE AND GRAVITY.
The pressure that surround the home 30 years ago changed. A lot has happened if the owner/s did not maintain proper grade.
By adding the downspout and allow such a stream of water to target one small area of land will change that specific areas of soil (loam) density.
You explained it. It becomes liquid and the density of the soil makeup flows due to the amount of directed water to that area. it becomes liquid.

Certain clay’s flow throughout the soil. They are like a snake or worm and ooze between layers under compression/compaction. It is a constant movement. Water adding to the soils equation of pressure.compaction and density.

Barry Adair has several illustration of a foundation. They resembled a completed 2D image resembling IR illustration of a foundation under load.
Transferred load or pressure points onto a beams on certain points of the foundation appear as a spectrum in the rainbow. Certain colours resemble cracking.

Maybe you should talk to Mr. Adair and hypothesize your question with him.

Great information for reference. PE’s seem to always have the great information and sources.

I guess Ole Abraham had dull axes.
He could sure fell a tree though.
Great having you aboard William.

I have cut into the soil of about 40 homes to enact foundation repairs or install/repair a drainage system.
I have carved cuts into loam many more times for management/control issues. They all reveal different soil (loam) compositions.

Loam soil changes by degree in relationship to geography, bedrock and other factors or so I have unearthed.

LOAM: sand, silt and clay. minerals and organic material and sedimentation.
Air and water effects loams density and how it reacts to compaction/compression for one.
water/air and movement effect sedimentation.

Other factors; Water tables, man’s interaction with the land,IE; adding back-fill , etc…

In Montreal more companies are using sand to envelope foundations.
They use to use 1/4 aggregate. They never did this, I did, although I would hypothesize with large companies at times.

A method I contrived.
#1 The extracted soil is removed and discarded.
#2 The exposed loam is covered with geofabric. 2a. The bottom and sides of the excavated loam.
2b. The french or sub drainage line is wrapped with geofabric. I make my own.
#3 Sand is inserted into the void and filled to aprox. <6" inch for grade line.
2c. The top layer of sand it covered with geofabric. It is draped 6" inches on the outer loam side cut.

The remaining void is filled with the clients wishes. Mostly loam and grass.

This process I enacted allows a “suspect equal lateral loading” or so I assumed. More companies are doing this process.

So the question you ask Kenton is wide in scope.
If a foundation had equal forces at play then any destructive force would be easier to narrow down IMO Kenton.

You are such a great gift/reward for INACHI and the membership. You are analytical in thought and human in your relationship to the members.
Al the best.

Thanks Robert. It is great to be a part of such a great organization!

You’re right… I had it backwards, thank you!

``````                 All soils have an equilibrium moisture content (EMC). EMC is the optimum water content for a particular soil; the point of maximum bearing capacity. At EMC, voids between particles are filled with non-compressible water instead of compressible air, but not so much water that it has forced particles farther apart.
``````

Water absorbed by soil- especially fine-grained, low permeability soils like clay and silt- can lower their bearing capacity by increasing soil plasticity- making it more like mud. Water forces soil particles farther apart, reducing friction, and acting like a lubricant,** allowing the weight of the structure to cause the foundation to settle.**

Sorry Robert, there’s no real crack, this is hypothetical. I like the subject too, except I try to avoid looking at or talking about a plumber’s crack.

Doesn’t putting grass in the fill cause problems from subsidence later on as the grass decays?