Lecture #13 - Geotechnical: Excavation and Challenges

Question 1

What are excavation support systems used for?

Answer 1

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

Question 2

What are some causes of soil instability?

Answer 2

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

Question 3

Describe soil frost action.

Answer 3

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

Question 4

When can the edges of excavation be sloped back/benched at low angle so that soil will not slide back into hole?

Answer 4

Construction site is sufficiently larger than area to be covered by building

Question 5

Where should steep angle of repose/maximum allowable slope be used?

Answer 5

cohesive soils such as stiff clays

Question 6

Where should shallower angle of repose/maximum allowable slope be used?

Answer 6

frictional soils such as sand and gravel

Question 7

Where must the soil be held back by some kind of excavation support? What should this support be capable of resisting?

Answer 7

constricted sites, resists pressures of earth and groundwater

Question 8

What is shoring?

Answer 8

construction used to support sides of an excavation and prevent its collapse

Question 9

What are the most common types of shoring for large excavations?

Answer 9

solider beams and lagging and sheet piling

Question 10

What are driven vertically into the earth at close intervals around an excavation site before digging begins? (with solider beams and lagging)

Answer 10

steel columns called H-piles or soldier beams

Question 11

What is the lagging? Where is it placed after the earth is removed?

Answer 11

heavy wood planks, placed against the flanges of the columns to retain the soil outside of the excavation

Question 12

What does sheet piling or sheeting consist of?

Answer 12

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

Question 13

What materials are used for sheet piling?

Answer 13

most common is steel, wood, aluminum, PVC plastic, composite polymers, precast concrete also used

Question 14

Why may shoring be left in place to become a permanent part of the building’s substructure?

Answer 14

if located close to property line and no practical way to remove it after completion of construction without disturbing the adjacent property

Question 15

Is shoring temporary? When is it removed?

Answer 15

Yes, removed as soil is replaced in excavation

Question 16

When can shotcrete be usede to stabilize excavations?

Answer 16

where soil is sufficiently cohesive to hold an adequate slope at least temporarily

Question 17

What does shotcrete help with for excavation?

Answer 17

reinforces slope, protects against erosion

Question 18

Explain soil mixing.

Answer 18

sides of excavation strengthened by blending portland cement and water with existing soil, occurs in place using rotating augers or paddles lowered into ground

Question 19

What does the soil-cement mix harden into?

Answer 19

series of underground, abutting vertical cylinders of low-strength concrete against which excavation can proceed

Question 20

What can be inserted into the soil-cement mix where bracing is required?

Answer 20

soldier piles can be inserted before the mix hardens to become part of the bracing structure

Question 21

What does soil mixed excavation support become a permanent part of?

Answer 21

subgrade construction

Question 22

What are ground or earth improvements? Name one method.

Answer 22

methods for altering the properties of soil in place, soil mixing

Question 23

Explain what soil mixing does in detail.

Answer 23

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

Question 24

What is a slurry wall?

Answer 24

more complex method of constructing complete, steel-reinforced, concrete wall in the ground, even many stories below the surface, before excavation takes place

Question 25

When does slurry wall become economical?

Answer 25

if it becomes part of the permanent foundation of the building

Question 26

How is the excavation for a slurry wall performed?

Answer 26

with a narrow clamshell bucket, operated by a crane, guided by temporary guides at the ground surface defining the edges of the wall

Question 27

How is the tendency of the earth walls to collapse counteracted?

Answer 27

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

Question 28

What happens when the trench has been excavated to its full depth?

Answer 28

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

Question 29

What is lowered into each section for slurry walls? How is concrete poured for slurry walls?

Answer 29

reinforcing steel, from the bottom up using a funnel and tube arrangement called a tremie

Question 30

What happens as concrete rises in the trench?

Answer 30

slurry is displaced and pumped out into holding tanks, where it is stored for reuse

Question 31

What happens after the concrete in one section reaches the top of the trench and has hardened?

Answer 31

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

Question 32

What happens when the concrete wall has cured to adequate strength?

Answer 32

earth rmoval begins inside the wall which serves as shoring for the excavation, wall becomes permanent part of future building foundation or substructure

Question 33

What does contiguous pier excavation support consist of?

Answer 33

cylindrical concrete piers spaced closely enough that they form a continuous wall, much like soil-mixed excavation support

Question 34

How aere the piers constructed?

Answer 34

by drilling holes into the unexcavated earth, inserting steel reinfocring and then filling the holes with concrete

Question 35

When does excavation proceeed down onw side of the piers?

Answer 35

once the concrete has hardeneed and gained sufficient strngth

Question 36

When are the piers called a tangent wall?

Answer 36

when the piers are spaced so that their edges are just about one another

Question 37

When are the piers called a secant wall?

Answer 37

when the piers are spaced more closely so that they partially overlap

Question 38

What does crosslot bracing use?

Answer 38

temporary steel wide-flange columns that are driven into the earth at points where braces will cross

Question 39

What is added as the earth is excavated down around the shoring and the columns?

Answer 39

tiers of horizontal bracing, usually of steel, are added to support walers, which are beams that span across the face of the sheeting

Question 40

What is used when the excavation is too wide for crosslot bracing?

Answer 40

sloping rakers, bearing against temporary footings

Question 41

Why are rakes and crosslot bracing an obstacle to the excavation process?

Answer 41

their presence within the excavation places limitations on earth removal methods and equipment

Question 42

What can be used instead of bracing to support the shoring while maintaing a fully open excavation?

Answer 42

tiebacks

Question 43

What is drilled at each level of walers?

Answer 43

Holes are drilled at intervals through the shoring and the surrounding soil into rock or a stratum of stable soil

Question 44

What is inserted into the holes? What is the rest of this process?

Answer 44

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

Question 45

What is step one of tieback installation?

Answer 45

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

Question 46

What is step 2 of tieback installation?

Answer 46

steel prestressing tendons are inserted into the hole and grouted under pressure to anchor them to the soil

Question 47

What is step 3 of tieback installation?

Answer 47

after the grout has hardened, the tendons are tensioned with a hydraulic jack and anchored to a waler

Question 48

How can excavation in fractured rock be done without any sheeting?

Answer 48

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

Question 49

What can vertical walls of particulate soils be stabilized by in some cases?

Answer 49

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

Question 50

What is the most common method of dewatering?

Answer 50

pumping as the water accumulates in pits, called sumps, located at low points in the excavation

Question 51

Where might it be necessary to keep groundwater from entering the excavation at all?

Answer 51

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

Question 52

How can you keep groundwater from entering the excavation at all?

Answer 52

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

Question 53

What is used to depress the water table? Explain.

Answer 53

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

Question 54

Why might well points not be practical?

Answer 54

insufficient capacity to keep excavation dry, restrictions on groundwater discharge, not reliable bc of power outages, lowering water table could adversely affect neighboring buildings

Question 55

What might be used instead of well points?

Answer 55

watertight barrier wall or cutoff wall made from sheet piling, slurry wall, soil mixed wall, or contiguous piers may be used

Question 56

What must watertight barriers resist?

Answer 56

hydrostatic pressure of surrounding water, which increases with depth, so for deeper excavations, system of bracing or tiebacks is required

Question 57

A watertight barrier only works...

Answer 57

if it reaches into a stratum of impermeable soil at its bottom, like bedrocl or water-impermeable clay

Question 58

Explain soil freezing.

Answer 58

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