Site I

Why Dig Down for Foundations?

Transfer building loads to the ground.
Reach bedrock or stable soil.
Get below the frost line to avoid freeze-thaw heaving.

Foundation Depth Considerations

Bedrock: Ideal but expensive.
Sheds: May not need any foundation.
Residences: Below frost line (2-4 feet typically).
Large Buildings: Deeper foundations are often needed in weaker soils.

Site Considerations

Site, earth, foundations, site selection, zoning, and regulations are all related.
There's a lot going on underneath buildings that we really can't see.

Buoyant Upload Forces

Underground water pushing up on the building.
We want to avoid these forces.

Building Loads

Dead Loads: Weight of the building (permanent).
Live Loads: Occupants, furnishings, non-permanent loads, rain, snow.
Lateral Loads: Wind (tornadoes, hurricanes), seismic forces.
Foundation-Specific Loads: Underground lateral loads on basement walls (soil weight, hydrostatic pressure).

Last Ice Age and Glacial Impact

Glaciers pushed earth material (rock) from the north during the last ice age.
The Wisconsin Glacier pushed rock to around New York City.
Glaciers deposited rock and debris, forming Long Island.

Manhattan Bedrock

Bedrock is deep in Long Island.
Skyscrapers in Manhattan are located where shallow bedrock exists (Midtown and Downtown).
Manhattan Schist: Bedrock dips down in the middle of Manhattan.
It's less expensive to build on shallow bedrock because you don't have to dig as deep.
Economic factors may also influence skyscraper placement and that theory is debated.

Building Settlement Considerations

Uniform Settlement: Desired; prevents cracks and operational issues.
Differential Settling: Undesired; causes cracks, sloped floors, and operational problems.
Settling is inevitable.
Settling is negligible on rock.
Settling can be more significant on other types of soils.

Chicago Auditorium Example

Settled 29 inches due to 100 feet of soft clay.
Built on a raft foundation (railroad ties, steel rails, concrete).
A heavier masonry facade replaced a lightweight terracotta facade, causing sagging.
Differential settlement is more common than catastrophic failure.

Palace Of Fine Arts in Mexico City

Sunk 15 feet due to clay soil and water extraction from aquifers.
Mexico City is sinking due to aquifer depletion.
Sloping floors exceeding one degree can cause vertigo.
Parts of Mexico City have sunk as much as 27 feet.

Leaning Tower Of Pisa

Constructed on soft ground (silt, clay, shells, sand).
Inadequate foundation from the beginning.
Leaned by as much as 5.5 degrees.
Remediation measures have brought it back to 4 degrees.
Counterweighting, steel wrapping, concrete rings, and soil removal have been attempted.

Earth Material Classification

Classified by particle size, organic material, and moisture content.
From largest to smallest: boulders, cobbles, gravel, sand, silt, clay.
Boulders: Lifted with two hands.
Cobbles: Lifted with one hand.
Gravel: Easily held between two fingers.
Sand: Too small to pick up easily, but particles are visible.
Silt: Particles are too small to be seen by the naked eye, but they are spherical.
Clay: The particles are too small to be seen by the naked eye, but under magnification, the particles are plate-like in shape.

Organic Soil

Peat and Topsoil are examples of organic soil.
Spongy, moves with varying water content, changes with biological activity.
Should be scraped off and stored for later landscaping use.

Soil Competency

From most to least competent: gravel, sand, silt, clay.
Silt and clay can switch places depending on organic material content.
Usually, soil is a mixture of silt and clay, or sand, silt, etc.

Clay's Unique Properties

Clay is the weird one; it has its own grain because it's plate like.
It may have thousands of times the surface area of silt for the same size particle.
It arranges itself into sheet like fabric.
Wet: Putty-like and sticky.
Dry: Brittle.
It can hold itself back during excavation.
It has a higher liquid limit (holds more water before flowing).
Expansive: expands/contracts more with changes in soil moisture.
Building weight can compress it, causing consolidation.

Behavior of Sand and Silt

Sand is stronger when a bit wet. Silt can liquefy in an earthquake if saturated.
Gravel generally stays the same wet or dry.

Frictional vs. Cohesive Soils

Clay is cohesive (relies on stickiness for strength).
Gravel and sand are frictional (rely on internal friction for strength).
Silt may or may not be cohesive, but it is less cohesive than clay.

Liquid Limit of Soil

The water content at which soil transitions from semi-solid to fluid.
As water is added, soil expands (plastic limit).
Continued water addition leads to flowing (liquid limit).
It is important to know this if building on soil to understand it's behavior when wet.

Shear Strength

The resistance of particles sliding over each other due to friction or interlocking.
Densely packed coarse-grain soils have higher shear strength.
Loosely packed coarse-grain soils have less shear strength because the grains can move easily past one another under pressure.
Smaller grain soils (sand, clay, silt) are more prone to changes with water and have lower shear strength.

Well Graded vs. Poorly Graded Soil

Poorly graded soil drains better (uniform size = more gaps).
Well graded soil compacts more for better foundation support (mixed sizes = less gaps).
Well graded soil is also a poorly sorted soil.
Poorly graded is better for drainage, well-graded is good for support.

Water Table

The elevation below which the soil is water saturated.
Easily determined for coarse-grained soils (dig a pit).
Fine-grained soils (silt, clay) may require drilling and waiting or soil sample analysis.

Performing Arts Center Example

Orchestra pit was 13 feet below the water table (expensive).
Adjacent to an existing building (challenging foundation).
Caissons needed to go down 90 feet through rock (expensive).
Low bid contractor with less experience drilling through rock wound up being a problem and costing more money in the long run.
The university didn't test the site for the amount of rock properly.
The university decided not to sue, just to go ahead and pay it.
Watertight assembly (bathtub) kept water out of the orchestra pit.

Soil Testing and Geotechnical Reports

Small buildings: rules of thumb based on typical soil content.
Bigger buildings: dig pits and load them, do test borings with drilling rigs.
Measure particle size and look for organic content. You can also gauge soil bearing capacities.
You can strike a hollow penetration sampler tube.

EPA Superfund Sites

EPA cleans up hazardous waste sites.
The EPA make a priorities list of the worst sites based on affects, and how many people are near. They EPA also charge the polluters for the cleanup.
Fund established to get the whole thing started.
Certain administrations were a little bit cozy with the polluters and didn't really go after them the way that the law had intended.
When the polluters were given a bill by the EPA, they fought it.
The US taxpayers typically wound up paying for the cleanup.

Brownfield Sites

Planning on building on a contaminated site or one believe to be contaminated.
Former disposal sites, gas stations, dry cleaners, paint factories are common examples.

Grubbing and Clearing

Taking out the trees, the plants, the stumps with heavy machinery.
If keeping trees, avoid cutting near the tree canopy and filling against the trunk to avoid rot.

Angle of Repose

The steepest angle that an excavated area can be before the soil slides back into the pit.
Steeper for cohesive soils, shallower for frictional soils
Gravel/poorly graded sand: ~34 degrees.
Weak cohesive soils/well-graded sand/silt: ~45 degrees.
Cohesive soils (clay): ~53 degrees.

Soldier Beams

Soldier beams use wide flange steel beams driven into the soil with wood boards (lagging) to keep the earth out as we go deeper and deeper and deeper.

Shoring

A wall when we're excavating to prevent the collapse of the earth around us so that so that we don't die.
One of the most common types of shoring are these soldier beams with lagging.
Another common type is called sheet piling, or sheeting. The sheets are driven into the ground directly to form a continuous underground wall.
For trenches, we will typically use reusable modular systems to hold back the earth for that little portion of time that we're building the trench.
Shoring is typically temporary, but if it's an urban site with a building immediately adjacent, we often can't remove the shoring after we're done.
If we're not using sheeting or something like that to hold it back, we can use what's called a benched excavation.

Soil Mixing

Making walls for the excavation by drilling down and mixing the soil in columns with Portland cement and water, one at a time.

Slurry Walls

Used in situations with soft earth or high groundwater table.
Produce buildable areas in locations that wouldn't be accessible through low cost excavation strategies.
Before we start excavation, we're gonna dig down all around the perimeter of the site.
Heavy mixture of this clay, soupy clay, watery stuff, this bentonite clay and water, it keeps the sides of the wall from collapsing in.
Break the perimeter into little pieces, and put barriers to separate the areas.
Put the rebar in it, and then pour concrete in the funnel, which will go down a straw displacing bentonite clay slurry mixture.

Tremie

Just the name of the funnel in the straw system that brings the concrete down to the bottom of the trench, in that slurry wall system we just saw.

Crosslot Bracing vs. Rakers

Crosslot bracing goes all the way across the excavation, and rakers only go partway across and terminate at the earth instead of going all the way to the other side of the excavation.

Tiebacks

Use grout to hold the excavation wall back by drilling holes every x feet in the horizontal members of the wall.
The holes are filled with grout and then steel rods or cables are inserted.
One end is anchored against the whaler, and then we're going to use a hydraulic jack to pull the whole thing tight.

Adjacent Buildings and Water Table

Anytime there's adjacent buildings and we start messing around with the water table, we may be asking for unintended consequences.

Excavation Below Water Table

Build a waterproof enclosure around the excavated earth so water cannot get in.
Continually pump the water out as it seeps in.
Continually pump the water out around the excavation so the water never seeps in anyway.

Deep vs. Shallow Foundations

Very tall building is more likely to have a deep foundation.
The part below the ground is known as the substructure.
And then the part below any of the spaces that transfers the load of the building to the earth will be known as the foundation.

Foundation Selection Factors

Shallower foundation is always better if practical because it is less expensive.
Soil bearing capacity from the geotechnical report. We use that to decide what foundation to use.
The construction schedule and inertia. The groundwater conditions, and then any effect on adjacent buildings

Spread Footings

Deeper in colder climates (North Dakota) due to frost depth.
Variations include wall footings, column footings, slab on grade, combined footings, cantilevered footings, and mat foundations.

Wall Footings

The wall will bear down and the foundation wall will bear down on the footing giving it a linear load.
This is similar when you have a crawl space or a basement below.

Column Footings

This would be used when we have point loads and these are subject to lateral loads.
To make sure that they don't slip past each other and the situation happens where everything is rigid, we brace the column footings to one another at grade, and these are known as grade beams, or tie beams.

Combined and Cantilever Footings

The first type is a cantilevered footing, and we can use that if we're up against the property line and cannot spread out the footing to far.
In a similar way combine where we may have a column footing, column footing, column footing, and so forth, And then eventually we get to the last two and we're gonna combine them.

Slab on Grade

This is only possible if we have low loads, like for a small building, and we don't have to worry about frost.

Mat Foundation

The one the Chicago Auditorium Theater had when it was sinking from the heaviness.
They do not use these before invention of deeper caissons and pilings we are gonna talk about in a little while.

Grade Beam vs Tie Beam

They both tie the columns together at the grade to lateral forces associated with earthquakes and with wind loads.
For tie beams you dont really put a wall load on top of it. Gray beams can handle bracing and vertical loads.

Frost Protected Footing

We can use insulation around the structure to avoid having to go below the frost line.

Floating Foundation

Carefully calculate the weight of the building, and you're gonna pretty carefully calculate the weight of the soil on the site that we're removing, and you're gonna make them be equal.
By removing and using the same amount of soil in what we built for the building. We didn't really add anything to the site from a soil point of view, which is going to cause settling.
They didn't work because of Frank Llyod Wright's Imperial Hotel in Tokyo was demoished after this failed.

Rubber Dampers and Hydraulic Shock Absorbers

Building code that allows to buildings and citied to withstand a tsunami in event of an earth quake.

Caissons vs. Piles

Caissons are drilled and then filled with concrete, and piles are elements that are hammered in, and piles include deep foundations.
You can go past the incompetent soil or all the way past bedrock, which give you a really high bearing capacity.

What is a Caisson

Drill a hole and then fill the hole with concrete. Caissons can be drawn with a bell on the bottom.
The bell gets distributed on top of a soil surface area.
They can only work in cohesive soil, where the earth will hold on its on its own.

Piles

Piles are driven in. It gets driven in bang, bang, bang with a pile driver, and sometimes it goes down to a predetermined depth, so we know that we want a pile to go down x feet.
They are also done as a frictional pile.
A pile is used because we are going to be in a place where their is cohesive soil and the earth doesn't hold on its own.

Pile Cap

It's pretty common where each pile is not strong enough to hold its own column above it.
So we're gonna group a bunch of piles together to give it strength to hold everything from the surface above.

Mini Piles vs. Helical Piles

Many piles are light for an existing building but you can use wheel to put them in.
You will also use a hell cat pile which is a giant screw you put into the surface.

Soil Enhancements

We're gonna just take the soil underneath a shallow foundation and enhance it to give the building strength.
Pressure with dry grout and then the soil will become stronger.
Like a rammed aggregate piers, which is the same principle but with crushed
stone compacted into drilled holes in the earth.

Underpinning a foundation

Why are we using it?
What do you need to know about underpinning?
Is the building renovation going to be done. And do you know what size building to build.
Are you weakening the building.
Three ways to do it.
Make the foundation a little bit wider. To give more contact with the soils.
Imported fill with grout.
Fill grout you can manufacture better soil that's better than other things down there.

Ways to Support the Building while you're improving the foundation

Support with with beams to lift up the building to support a new foundation.
Also the building is better and we are gonna build it down there.
New footing that's going deeper or wider
So its better for down in better soil. That is being supported in more ways.

Concrete Waterproofness

Not even close and its very dangerous for any types of leaks
Very hard to fix the leaks on water.
You can do it with liquid that the water isn't gonna bleed through.
Plastic rubber and sheet will cause more water.
But you have to have the space in place between the seams.

Keeping Basements Dry

Two things you want to keep is waterproofing and drainage
Roofwater get far away and is a way to store what is there for your building
Site that is sloping away to get more the water away and it goes up to the foundation without the building
Want the rain to go away even you are working away with the roof.
To be less inclined to remove against the building side.
What is the different side for the break in the foundation
Slope towards direction. You need to make sure and avoid anything from your foundation.
With a storm you want to make something from the bottom.
Also filter fabric is what you want to use for your soil.
Two ways to follow the capillary break by using, gravel and drainage
We need the gravel to filter with our basemen but you cant build on something that can't be built like gravel.

Liquid sheets and spray applied

Liquid you do an even portion where its better for the earth.
Where that is getting unevenly sprayed on sheets and rubber sheets that will continue.