Chapter 2 aSoils & Excavation

Building FoundationsBuilding Foundations

The purpose of foundations is to The purpose of foundations is to provide stability and to safely provide stability and to safely transfer building loads to the earth.transfer building loads to the earth.

Three load considerations occur:Three load considerations occur: Vertical weight of the buildingVertical weight of the buildingHorizontal forces above ground Horizontal forces above ground such as windsuch as wind

Horizontal forces below ground Horizontal forces below ground as in earthquakes.as in earthquakes.

Foundation LoadsFoundation Loads

Dead LoadDead Load Weights of building material Weights of building material

due to gravitydue to gravity Live LoadLive Load

Those type loads that can Those type loads that can change, such as weight of change, such as weight of furniture, people, wind, snow, furniture, people, wind, snow, ice, and in certain areas of the ice, and in certain areas of the world, horizontal forces below world, horizontal forces below grade, such as movement grade, such as movement caused by earthquakes. caused by earthquakes.

Foundation RequirementsFoundation Requirements

Foundations Must Be: Foundations Must Be:

Safe From Safe From Structural FailureStructural Failure

Free of SettlementFree of Settlement that does not that does not impair Function, such as unsightly impair Function, such as unsightly cracks and damage to the building that cracks and damage to the building that would allow deterioration due to would allow deterioration due to weather.weather.

Foundations Must Be Foundations Must Be Technically & Technically & Economically FeasibleEconomically Feasible

Types of SettlementTypes of Settlement

No Settlement Uniform Differential

Of the two types of settlement, Of the two types of settlement, Differential Settlement is the worst, as Differential Settlement is the worst, as structural failure often occurs when one structural failure often occurs when one part of a structure moves and another part part of a structure moves and another part doesn’t.doesn’t.

Differential settlement is due basically to Differential settlement is due basically to two conditions:two conditions:

Poor bearing capacity of soil in Poor bearing capacity of soil in differing differing areas of the building. areas of the building.

Shrinking and swelling of the soil Shrinking and swelling of the soil which which allows one part of a building to allows one part of a building to sink due to sink due to load – or forces of swelling load – or forces of swelling soil causing an soil causing an upward movement in upward movement in parts of a buildingparts of a building..

The

Leaning Tower of Pisa

One of the Most FamousExamples of

Differential Settlement

Differential SettlementBetween an

Areaway & Sidewalk

Differential SettlementNext to

A Building Column

Differential Settlement happens when Differential Settlement happens when one part of a building changes elevation one part of a building changes elevation relative to another part of the building.relative to another part of the building.

Differential settlement is usually Differential settlement is usually caused by an inconsistency in the soil caused by an inconsistency in the soil bearing, or a change in the soil volume.bearing, or a change in the soil volume.

Types of soil range from the most Types of soil range from the most stable -- from hard granular particles -- to stable -- from hard granular particles -- to clay - - - to somewhere in between – which clay - - - to somewhere in between – which may contain sound, structural particles may contain sound, structural particles and some that are not.and some that are not.

Types of SoilTypes of Soil

Soil types are identified by analysis in a Soil types are identified by analysis in a laboratory by Geotechnical Engineers.laboratory by Geotechnical Engineers.

Soil that is basically stable are comprised Soil that is basically stable are comprised of hard particles that touch and do not of hard particles that touch and do not move when subjected to loads or inclusion move when subjected to loads or inclusion of water.of water.

Soil that is mostly clay has extremely Soil that is mostly clay has extremely small particles. Water will force the small particles. Water will force the particles apart, causing the mass to swell particles apart, causing the mass to swell – in all directions. When water migrates – in all directions. When water migrates out of the mass, the soil will shrink. out of the mass, the soil will shrink.

ClaysPorous(sandy)

Soil Particle DistributionSoil Particle Distribution

Soil is rarely one type - but a mixture of Soil is rarely one type - but a mixture of different particles due to years of different particles due to years of migration of particles, caused by wind migration of particles, caused by wind and water.and water.

Distribution of soil type, particle size, Distribution of soil type, particle size, and particle strength is important in and particle strength is important in Predicting:Predicting:– Load Bearing CapacityLoad Bearing Capacity– StabilityStability– Drainage CharacteristicsDrainage Characteristics

A type of soil that is available in nearly A type of soil that is available in nearly all locations is called loam, which is a all locations is called loam, which is a rather generic name for common, ordinary rather generic name for common, ordinary soil that does not have characteristics of soil that does not have characteristics of unstability in some form. unstability in some form.

Loam is very good farm type soil, as it Loam is very good farm type soil, as it contains sand, some clay, silt, and organic contains sand, some clay, silt, and organic matter. Since it does not contain an matter. Since it does not contain an overbalance of clay or silt it is a stable overbalance of clay or silt it is a stable foundation soil when compacted to a foundation soil when compacted to a dense formation.dense formation.

Two types of soil unique to the West Two types of soil unique to the West Texas area have varying characteristics:Texas area have varying characteristics:

Caliche – actually is soil with a high content Caliche – actually is soil with a high content of calcium carbonate. Is strong when dry of calcium carbonate. Is strong when dry but unstable when wet. Good for a sub but unstable when wet. Good for a sub layer for concrete and asphalt roads when layer for concrete and asphalt roads when protected from absorbing moisture. Not a protected from absorbing moisture. Not a good base for bearing capacity under good base for bearing capacity under buildings.buildings.

Playa Lakes – Soil found in playa lakes has Playa Lakes – Soil found in playa lakes has extremely small particles, structurally extremely small particles, structurally unsound, and very unstable. It is composed unsound, and very unstable. It is composed of drainage silt over years of washing of drainage silt over years of washing downhill from someplace else moving to downhill from someplace else moving to lower levels until it ends at a playa lake.lower levels until it ends at a playa lake.

Soil InvestigationSoil Investigation

Who needs to know soil Who needs to know soil characteristics?characteristics?

When do they need to know them?When do they need to know them?

How is that knowledge obtained?How is that knowledge obtained?

SOIL BEARING CAPACITYSOIL BEARING CAPACITY Since the weight of a building ultimately Since the weight of a building ultimately

must rest on the earthmust rest on the earth, the capacity for , the capacity for the soil to resist loads – forever - is of the soil to resist loads – forever - is of major concern before any appreciable major concern before any appreciable advancement is made to formulate a advancement is made to formulate a design. design.

Geotechnical Testing laboratoriesGeotechnical Testing laboratories take take soil samples at construction sites at soil samples at construction sites at varying depths and test the soil varying depths and test the soil composition for strength and stability. composition for strength and stability. Generally, the engineers will recommend Generally, the engineers will recommend a type of foundation design for a specific a type of foundation design for a specific type of building. type of building.

Pictured herePictured here

Is a geotechIs a geotech

drilling teamdrilling team

making a testmaking a test

boring. boring.

As the augerAs the auger

extends intoextends into

the ground,the ground,

the hollowthe hollow

tube gatherstube gathers

soil at allsoil at all

depths.depths.

o Multistory buildingsMultistory buildings will require deep footings that will require deep footings that extend to a rock strata for support. extend to a rock strata for support.

o Medium height (2 to 4 story) buildingsMedium height (2 to 4 story) buildings may be built may be built on soil with spread type footings, but must be on soil with spread type footings, but must be protected from adverse conditions, such as protected from adverse conditions, such as inclusion of water and freezing.inclusion of water and freezing.

o Light load buildings (1 & 2 story)Light load buildings (1 & 2 story) generally have generally have foundations near the ground surface. These also foundations near the ground surface. These also must be protected from adverse conditions. must be protected from adverse conditions.

PRIMARY ADVERSE CONDITIONSPRIMARY ADVERSE CONDITIONS For low to medium height buildings, For low to medium height buildings,

sufficient bearing capacity can generally sufficient bearing capacity can generally be found within the soil structure, without be found within the soil structure, without the need for rock bearing.the need for rock bearing.

But even with sufficient bearing But even with sufficient bearing capacity, capacity, unstable soil can be a detriment unstable soil can be a detriment to buildings due to uneven settlementto buildings due to uneven settlement. .

Shallow footings spread to areas Shallow footings spread to areas required for bearing capacity required for bearing capacity work against work against bearing design when clay soil absorbs bearing design when clay soil absorbs moisture and swellsmoisture and swells, causing upheaval in , causing upheaval in varying parts of the building. Cracks varying parts of the building. Cracks occur and increase the potential for occur and increase the potential for structural stress.structural stress.

SOLUTIONS TO DIFFERENTIAL SETTLEMENT SOLUTIONS TO DIFFERENTIAL SETTLEMENT IN SINGLE STORY BUILDINGSIN SINGLE STORY BUILDINGS

• 1 Remove the soil in the immediate 1 Remove the soil in the immediate building area to a satisfactory depth building area to a satisfactory depth (usually around 4 feet) and install stable, (usually around 4 feet) and install stable, compacted soil.compacted soil.

• 2 Stabilize the soil:2 Stabilize the soil: Mix stabilizing ingredient into the Mix stabilizing ingredient into the

soil soil such as cement or fly ash.such as cement or fly ash. Inject lime slurry under pressure Inject lime slurry under pressure

into the into the soilsoil

• 3 Where concrete floors are to be placed, 3 Where concrete floors are to be placed, expand the soil to maximum volume by expand the soil to maximum volume by saturating it with water, then seal with a saturating it with water, then seal with a moisture barrier such as plastic sheeting.moisture barrier such as plastic sheeting.

SOIL FILL and EXCAVATIONSOIL FILL and EXCAVATION After sufficient information is known After sufficient information is known

regarding foundation type due to soil, and regarding foundation type due to soil, and preliminary corrections are made;preliminary corrections are made;

– Soil fill is added to or taken awaySoil fill is added to or taken away from the site from the site as necessary to establish proper levels of as necessary to establish proper levels of surface elevation for drainage and general surface elevation for drainage and general terrain design.terrain design.

– Soil fill is made and compacted in the area of theSoil fill is made and compacted in the area of the buildingbuilding to establish finish soil grade before to establish finish soil grade before preparations for installation of concrete preparations for installation of concrete foundation.foundation.

– Soil fill Soil fill is always made in “lifts”is always made in “lifts” – layers of not – layers of not more than 8” to 12” at a time, then compacted more than 8” to 12” at a time, then compacted to assure that all soil is sufficiently dense.to assure that all soil is sufficiently dense.

The compaction of soil is done The compaction of soil is done in order in order to reduce the space between particlesto reduce the space between particles of of soil as much as possible so the density of soil as much as possible so the density of the soil will resist subsequent movement. the soil will resist subsequent movement.

Water is added to the soil as compaction Water is added to the soil as compaction is done in careful amounts in order to is done in careful amounts in order to densify the soil to a percentage of densify the soil to a percentage of maximum, with optimum water contentmaximum, with optimum water content..

Compaction is done with Compaction is done with a variety of a variety of equipment specific for the soil types, equipment specific for the soil types, including prongedincluding pronged ‘sheep’s foot’ rollers, flat ‘sheep’s foot’ rollers, flat rollers, rubber-tired rollers, and vibrating rollers, rubber-tired rollers, and vibrating rollers. Hand-held compactors are used in rollers. Hand-held compactors are used in small areas.small areas.

Vibrator compactor for small areas

Backfilling in “Lifts” – Compacted & Tested

Soil Placement & Compaction

Vibratory Roller

Sheeps Foot Compactor

Compactor - Dozer Water Wagon

SheepsfootVibratory Roller

Plate TamperTrench Sheeps Foot

Walk - Behind Roller

Compaction Equipment

o Preparatory requirements by the Architect of Preparatory requirements by the Architect of a project will determine how construction a project will determine how construction procedures develop. The Architect will state procedures develop. The Architect will state in the General Conditions and Specifications in the General Conditions and Specifications how results of construction processes are how results of construction processes are verified. For instance:verified. For instance:

After placement and compaction of the soil, After placement and compaction of the soil, compaction tests will be made by the compaction tests will be made by the Geotechnical Laboratory that did the soil Geotechnical Laboratory that did the soil analysis. analysis.

Engineers will take samples of the soil after Engineers will take samples of the soil after compaction is complete, then make tests in the compaction is complete, then make tests in the lab to determine compaction percentage lab to determine compaction percentage results. results.

Reports of the compaction tests are delivered Reports of the compaction tests are delivered to the Architect, stating the moisture content to the Architect, stating the moisture content and the percentage of compaction. and the percentage of compaction.

Sample Location Selected Sample Removed & sent to the testing ‘Lab’

Sand Replaces Sample ‘Material’ ‘In Place’ Volume of Sample Determined

Maximum DensityMaximum Density

Of soil with theOf soil with the

Optimum moistureOptimum moisture

ContentContent

Density testDensity test

results forresults for

three soilthree soil

samplessamples..

EXCAVATION FOR FOUNDATIONEXCAVATION FOR FOUNDATION

After all dirt work is done, trenches are cut After all dirt work is done, trenches are cut for under floor piping such as plumbing, for under floor piping such as plumbing, electrical, fire sprinkler pipes etc., for electrical, fire sprinkler pipes etc., for installations that must be made before installations that must be made before concrete is placed.concrete is placed.

Then trenches are cut for foundation grade Then trenches are cut for foundation grade beams, spread footings, and areas where beams, spread footings, and areas where the floor slab will be made thicker for load the floor slab will be made thicker for load bearing walls. bearing walls.

The surface of excavations that will form The surface of excavations that will form concrete are sprayed with a material that concrete are sprayed with a material that will not allow the dry soil to draw water will not allow the dry soil to draw water from the concrete.from the concrete.

o Required reinforcing steel is then installed Required reinforcing steel is then installed at the proper places and secured.at the proper places and secured.

o Concrete is then placed in the excavation Concrete is then placed in the excavation under and around the reinforcing steel under and around the reinforcing steel and tamped to eliminate voids in the and tamped to eliminate voids in the structural concrete.structural concrete.

o In areas where the soil is not stiff enough In areas where the soil is not stiff enough to form the sides of beams & footings, and to form the sides of beams & footings, and in areas where appearance and finish of in areas where appearance and finish of concrete is essential, wood or steel forms concrete is essential, wood or steel forms will be set to determine the limits of the will be set to determine the limits of the finish concrete structure. finish concrete structure.

OPEN TRENCH FOR STRUCTURAL GRADE BEAM

GRADE BEAMS AFTER CONCRETE HAS BEEN PLACED

EXCAVATION FOR A SWIMMING POOL AND INTEGRAL HOT TUB