Lecture #13 - Geotechnical: Excavation and Challenges Flashcards
What are excavation support systems used for?
Minimize excavation area, keep sides of deep excavations stable, ensure that movements don’t cause damage to neighboring structures/utilities, soil stability to keep workers safe
What are some causes of soil instability?
increase in depth of cut, increase in water content of soil, hydrostatic pressure in ground cracks near walls of excavatoin, weight of excavated material, equipment and adjacent buildings, shock and vibration from machinery etc, soil frost action
Describe soil frost action.
phenomenon where soil freezes and expands due to low temperatures, when soil feezes, ice lenses can form in the soil, causing it to expand and push against surrounding soil particles
When can the edges of excavation be sloped back/benched at low angle so that soil will not slide back into hole?
Construction site is sufficiently larger than area to be covered by building
Where should steep angle of repose/maximum allowable slope be used?
cohesive soils such as stiff clays
Where should shallower angle of repose/maximum allowable slope be used?
frictional soils such as sand and gravel
Where must the soil be held back by some kind of excavation support? What should this support be capable of resisting?
constricted sites, resists pressures of earth and groundwater
What is shoring?
construction used to support sides of an excavation and prevent its collapse
What are the most common types of shoring for large excavations?
solider beams and lagging and sheet piling
What are driven vertically into the earth at close intervals around an excavation site before digging begins? (with solider beams and lagging)
steel columns called H-piles or soldier beams
What is the lagging? Where is it placed after the earth is removed?
heavy wood planks, placed against the flanges of the columns to retain the soil outside of the excavation
What does sheet piling or sheeting consist of?
vertical sheets of various materials that are aligned tightly against one another edge-to-edge and driven into the earth to form a solid wall, also before excavation begins
What materials are used for sheet piling?
most common is steel, wood, aluminum, PVC plastic, composite polymers, precast concrete also used
Why may shoring be left in place to become a permanent part of the building’s substructure?
if located close to property line and no practical way to remove it after completion of construction without disturbing the adjacent property
Is shoring temporary? When is it removed?
Yes, removed as soil is replaced in excavation
When can shotcrete be usede to stabilize excavations?
where soil is sufficiently cohesive to hold an adequate slope at least temporarily
What does shotcrete help with for excavation?
reinforces slope, protects against erosion
Explain soil mixing.
sides of excavation strengthened by blending portland cement and water with existing soil, occurs in place using rotating augers or paddles lowered into ground
What does the soil-cement mix harden into?
series of underground, abutting vertical cylinders of low-strength concrete against which excavation can proceed
What can be inserted into the soil-cement mix where bracing is required?
soldier piles can be inserted before the mix hardens to become part of the bracing structure
What does soil mixed excavation support become a permanent part of?
subgrade construction
What are ground or earth improvements? Name one method.
methods for altering the properties of soil in place, soil mixing
Explain what soil mixing does in detail.
Can be used to create cutoff walss to prevent water seepage into excavations, to stabilize or strengthen areas of weak soil around or under buildings, or to remediate biologically or chemically contaminated soil by adding chemicals that neutralize the contamination
What is a slurry wall?
more complex method of constructing complete, steel-reinforced, concrete wall in the ground, even many stories below the surface, before excavation takes place
When does slurry wall become economical?
if it becomes part of the permanent foundation of the building
How is the excavation for a slurry wall performed?
with a narrow clamshell bucket, operated by a crane, guided by temporary guides at the ground surface defining the edges of the wall
How is the tendency of the earth walls to collapse counteracted?
by maintaining the trench full at all times with a viscous mixture of water and bentonite clay or polymers, called a slurry, exerts pressure against the earth’s walls, holding them in place
What happens when the trench has been excavated to its full depth?
steel tubes, equal in diameter to the width of the trench, are inserted vertically at interals to divide the trench into sections of a size that can be reinforced and concreted conveniently
What is lowered into each section for slurry walls? How is concrete poured for slurry walls?
reinforcing steel, from the bottom up using a funnel and tube arrangement called a tremie
What happens as concrete rises in the trench?
slurry is displaced and pumped out into holding tanks, where it is stored for reuse
What happens after the concrete in one section reaches the top of the trench and has hardened?
the vertical pipes on either side are withdrawn and adjoinng sections can be poured, process repeated until concreting of all of the wall is completed
What happens when the concrete wall has cured to adequate strength?
earth rmoval begins inside the wall which serves as shoring for the excavation, wall becomes permanent part of future building foundation or substructure
What does contiguous pier excavation support consist of?
cylindrical concrete piers spaced closely enough that they form a continuous wall, much like soil-mixed excavation support
How aere the piers constructed?
by drilling holes into the unexcavated earth, inserting steel reinfocring and then filling the holes with concrete
When does excavation proceeed down onw side of the piers?
once the concrete has hardeneed and gained sufficient strngth
When are the piers called a tangent wall?
when the piers are spaced so that their edges are just about one another
When are the piers called a secant wall?
when the piers are spaced more closely so that they partially overlap
What does crosslot bracing use?
temporary steel wide-flange columns that are driven into the earth at points where braces will cross
What is added as the earth is excavated down around the shoring and the columns?
tiers of horizontal bracing, usually of steel, are added to support walers, which are beams that span across the face of the sheeting
What is used when the excavation is too wide for crosslot bracing?
sloping rakers, bearing against temporary footings
Why are rakes and crosslot bracing an obstacle to the excavation process?
their presence within the excavation places limitations on earth removal methods and equipment
What can be used instead of bracing to support the shoring while maintaing a fully open excavation?
tiebacks
What is drilled at each level of walers?
Holes are drilled at intervals through the shoring and the surrounding soil into rock or a stratum of stable soil
What is inserted into the holes? What is the rest of this process?
steel cables or rods, grouted tp anchor them in place, and stretched tight with hydraulic jacks (post-tensioned) before they are fastened to the walers
What is step one of tieback installation?
rotary drill bores a hole through the sheeting and into stable soil or rock, a steel pipe casing keeps the hole from caving in when it passes through noncohesive soils
What is step 2 of tieback installation?
steel prestressing tendons are inserted into the hole and grouted under pressure to anchor them to the soil
What is step 3 of tieback installation?
after the grout has hardened, the tendons are tensioned with a hydraulic jack and anchored to a waler
How can excavation in fractured rock be done without any sheeting?
by injecting grout into the ioints of the rock to stabilize it or by drilling into the rock and inserting rock anchors that fasten the blocks together
What can vertical walls of particulate soils be stabilized by in some cases?
soil nailing, soil nailing is similar to a rock anchor, a length of steel bar is inserted into a nearly horizontal hole drilled deep into the soil, grout is injected into the hole to bing the nail to the surrounding soil
What is the most common method of dewatering?
pumping as the water accumulates in pits, called sumps, located at low points in the excavation
Where might it be necessary to keep groundwater from entering the excavation at all?
where the volume of groundwater flowing into the excavation is great, or with soils, such as sand or silt that may be softened by constant seepage
How can you keep groundwater from entering the excavation at all?
by pumping water from the surrounding soil to depress the water table below the bottom of the excavation or by erecting a watertight barrier around the excavation
What is used to depress the water table? Explain.
well points, these are vertical pipes inserted into the gorund with screeened opening at the bottom that keep out soil particles while allowing water to enter
Why might well points not be practical?
insufficient capacity to keep excavation dry, restrictions on groundwater discharge, not reliable bc of power outages, lowering water table could adversely affect neighboring buildings
What might be used instead of well points?
watertight barrier wall or cutoff wall made from sheet piling, slurry wall, soil mixed wall, or contiguous piers may be used
What must watertight barriers resist?
hydrostatic pressure of surrounding water, which increases with depth, so for deeper excavations, system of bracing or tiebacks is required
A watertight barrier only works…
if it reaches into a stratum of impermeable soil at its bottom, like bedrocl or water-impermeable clay
Explain soil freezing.
array of vertical pipes similar to well points is used to circulate coolant at temperatures low enough to freeze the soil around an excavation area, resulting in a temporary but reliable barrier to groundwater
