2. Site and Environment Flashcards
4 types of soils
sand - granular; low plasticity; good bases for building foundations
gravel - granular; low plasticity; good bases for building foundations
Silts - smaller than sands and grave; bigger than clay; granular but slightly plastic
clays - smaller particles than silts; has cohesion or tensile strength; swells when absorbs water; shrinks when it dries
hardpan
unbroken mixture of clay, sand, and gravel
standard penetration test (SPT)
measure of the density of granular soils and the consistency of some clays
- a 2 in samplers is driven into the bottom of the borehole by a 140 lbm hammer falling 30 in.
- the number of blowsrequried to drive the cylinder 12 in is recorded
- minimum of 4 borings are taken
test pits
- trenches dug on site to inspect soil
- limited to 10 ft.
auger borings
auger bit drilled into site
usually used for sand or clay
cannot penetrate hardpan or bedrock
wash borings
2 in to 4 in diameter pipe is used though which a water jet is maintained to force up soil
analysis is difficult because material mixes together
useful for soils too hard for auger tests
good up to 100 ft.
soils are classified according to
Unified Soil Classification System
TO increase bearing capacity, decrease settlement, or do both, the following soil treatments are used:
- drainage: reduces moisture content of soil, thus increasing load carrying capacity; reduces hydrostatic pressure
- fill: is existing soil is unsuitable for building, it is removed and engineered soil is brough in; compacted before construction begins; proctor test used to determine a standard for compatction (there is an optimum relationship with a fill’s density and moisture content)
- compaction: existing soil can simply be compacted to provide adequate base; tool used to compact is a sheepsfoot roller
densification: may involve vibration, dropping of heavy weights; pounding piles into ground and filling voids with sand; technique used depends on the grain size of soil - surcharging: surchargins is the preloading of the ground with fill materials to cause consolidation and settlements before building; after settlement, fill is removed and construction begins
- mixing: instead of replacement of soil a layer of gravel or sand may be placed and mixed in
earthwork
excavating soil for construction of a building foundation, water and sewer lines, and other buried items; includes modifying sites countours
unshored sides of soil shold be no steeper than…
their natural angle of repose or not greater than a slope of 1.5 horizontal to the vertial
grading
the modification of the contours of the site according to the grading plan
-finish grading is the final removal of soil prior to landscaping to paving
common method used for shoring and bracing during excavation
- soldier beams and lagging
- braced excavation
underpinning
method used to temporarily support existing foundations while they are being repaired or strengthened or when they are being extended to a lower level
-needle bemas supported by the adjacent grade and hydraulic jacks are used to temporarily support building while new foundation is constructed
subsurface drainage
minimum slope of 1/4 in per foot is recommended
-perforated drain tile should be laid around the footings at least 6 in below the floor slab to collect water and carry it away to storm drain
Surcharging
battered wall
simply a type of retaining wall using a material, such as stones or brick slightly angled to support the adjacent earth.
bentonite
an expansive type of clay than can push foundations and floor slabs upward when it gets wet.
when buildings are build on benotnite what foundation would they have?
drilled piers with grade beams
to prevent the upward movement cause by bentonite when it gets wet, drillled piers are used to support the building weight on bedrock or stable soil below the bentonite, grade beams span continuously between the piers and transmit buildin loads from the superstructure to the piers
raft foundation
Floating raft is type of land-based foundation that protects against settlement and the liquefaction of soft soil due to seismic activity
used to distribute a building load over a large area of low bearing capacity soil
coarse grain soils
and fine grain soils
coarse: san and gravel
fine: sands and silts and clays
sheet piling or sheeting
consists of vertical planks of wood, steel, or precast concrete that are placed tightly against one another and driven into the earth to form a solid wallbefore excavation begins
slurry wall
complicated and expensive form of excavation support that is usually economical only if it becomes the permanent foundation of the building
see page 42 in FBC
precast slurry walls
wall is prestressed and is produced in section on a plant
- slurry for precast walls is a mixture of water, bentonite clay, and portalnd cement
- before a wall section is lowered into slurry, its face is coated with a compund that prevents the clay-cement slurry from adhering to it
advantages of a precast slurry wall over site cast slurry wall
- better surface quality
- more accurate wall alignment
- better water tightness
- thinner wall
- better structural efficiency
soil mixing
a technique of adding a modifying substance to soil and blending it in place by means of augers or paddles rotating on the end of a vertical shaft
-used to remediate soil
bracing types
crosslot bracing
rakers
tie backs
proctor test determines ….
the optimal desnity of a s psecific sample of soil.
- level of moisture in the sample is a factor in determining the ideal denisty
- sol is then compacted to within a specific percentage of the proctor desnity as specified by the soils engineer so that the soil has the bearing capacity necesaary to support the building load
percolation test
measure the ability of saturated soil to absorb a given quantity of water within a specified period of time
soil testing is perfrmed on a project site an reveals a suitable bearing stratum at a depth of 100 ft. the composition of the soil is silt, clay, and sand. the water table is 10 ft below the surface. Which of the following foundation strategies is the best choice?
a friction piles
b. concrete footings and CMU foundation walls with waterproofing
c. end bearing piles
d. belled caisson
a friction piles
friction piles are a good choice in the situation described
- as the pile is driven into the soil, frictional forces willl develop between the soils and the pile, whoch allow the pile to develop load carrying capacity even though the end of the pile will not bear on soil capable of carrying the building load
- bearing stratum is too depp at 100 ft for end bearing piles or belled caissons to be a resonable or economical choice
- high water table at 10 ft below grade would make footings and concrete masonry unit foundationwalls very expensive when dewatering and waterproffing are added into the project costs
WHen to use belled caissons
-In cohesive soils the shaft bottom can be enlarged by a process known as belling
The bell is constructed after the
straight shaft portion of the caisson has been drilled to the
required founding elevation. A mechanical belling tool is
attached to the base of the drill rig Kelly bar (figure 6). The
wings of the tool are forced open as the bar of the drill rig is
turned. This process forms the bell. When the belling tool is
extracted from the hole the wings retract which traps the
excavated soil inside (figure 5). Belling results in a
substantial increase in bearing capacity while minimizing the
quantity of concrete required for the caisson. The potential
concrete savings are more significant in caissons with long
shafts.
difference between pile and caisson
a pile is distinguished froma caissson by being forcibly driven ino the earth rather than drilled and poured
-used where noncohesive soils, subsurface water conditions, or excessive depth of bearing strata make caisson impractical
end bearing pile
if a pile is driven until its tip encounters firm resistance from a suitable bearing straturm such as rock, dense sand or gravel
frictional pile
if the pile is driven only into softer material, without encountering a firm bearing layer, it may still develop a considerable load carrying capacity through frictional resistance between the sides of the pile and the soil through which it is driven
A. Belled Caisson
B. Socketed Caisson
C. End Bearing Pile
D. Friction Pile
the EPA has determined that no action is required of the level of radon detected is…
…less than 4 pCi/L (picocuries per liter)
-appropriate remedial action for concentrations over 4 pCI/L include sealing any cracks in the foundation walls or floor slab and ventilating or depressurizing the basement or crawlspace area
the limits of an excavation and building footprint are located by a surveyor and marked using
batter boards
they are temporary supports erected to hols wires or strings that indicate the excavation line for a building site
the corners of the building or limits of excavation are marked at the intersection of the lines using a …
plumb bob
batter board are preferred to corner stakes or pins because…
…they are set back from the excavation lines and will not be disturbed during construction operations
grade stakes
used to inicate how much cut or fill is required at a specific location to reach finish grade
-set by a survey crew and reset periodically as excavation is taking place to monitor progress
transit
Radon …
is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless[2] noble gas, occurring naturally as an indirect decay product of uranium or thorium.
How Radon Enters Your Home
Radon is a naturally occurring radioactive gas produced by the breakdown of uranium in soil, rock, and water. Air pressure inside your home is usually lower than pressure in the soil around your home’s foundation. Because of this difference in pressure, your home acts like a vacuum, drawing radon in through foundation cracks and other openings.
The U.S. Congress has set a long-term goal that indoor radon levels be no more than outdoor levels; about ___ pCi/L of radon is normally found in the outside air.
0.4
Active Subslab suction — also called subslab depressurization — is ….
…the most common and usually the most reliable radon reduction method. One or more suction pipes are inserted through the floor slab into the crushed rock or soil underneath. They also may be inserted below the concrete slab from outside the home. The number and location of suction pipes that are needed depends on how easily air can move in the crushed rock or soil under the slab and on the strength of the radon source. Often, only a single suction point is needed.
Passive subslab suction is …
…the same as active subslab suction except it relies on natural pressure differentials and air currents instead of a fan to draw radon up from below the home.
submembrane suction
An effective method to reduce radon levels in crawlspace homes involves covering the earth floor with a high-density plastic sheet. A vent pipe and fan are used to draw the radon from under the sheet and vent it to the outdoors.
