Transcript Factors to Consider in Foundation Design Chapter # 02
Factors to Consider in Foundation Design Chapter # 02
Lec. # 03
What are the key Factors?
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Effect of Frost Line Location consideration for Spread footings Net versus Gross Soil Pressure: (Design soil pressures) Erosion Problems for structures adjacent to flowing water Corrosion Protections Water Table fluctuation
1-Effect of Frost Line
Footing should be placed below or above the frost line ?
It should be Placed below frost line
.
Possible frost heave of the building and because alternate freezing and thawing of the soil tends to maintain it in an unconsolidated or loose state
Frost or Permafrost conditions
Permafrost is a condition of permanently frozen ground where the ground temperatures are never higher than 0 0 C. Construction in these areas requires that the foundations be placed below this material and into permafrost.
Where the soil is considered thaw-stable, the foundation design is the same as in temperate regions. Thaw stable soils are granular materials like coarse sands and gravels. Pile foundations are more reliable for permanent frost areas but are much more expensive.
2-Location consideration for Spread footings
Case # 01
When footings are to be placed adjacent to an existing structure, as indicated in figure 01, the line from the base of the new footing to the bottom edge of the existing footing should be 45 0 or less with the horizontal plane.
From this requirement it follows that the distance m of figure 01 should be greater than the difference in elevation of the two footings,
zf
.
Location consideration for Spread footings Case # 02
If the new footing is lower than the existing footing There is a possibility that the soil may flow latterly from beneath the existing footing. This may increase the amount of excavation somewhat but, more important, may result in settlement cracks in the existing building. This problem is difficult to analyze; however an approximation of the safe depth be made for
Φ–c
using equation 01 & 02.
z f
may
Since σ
3
= 0 on the vertical face of the excavation. The critical pressure σ
1
would include the pressure from the existing footing.
σ 1 ≈ γ
z f +
q 0 σ 3 = 0 = σ 1 K- 2c = γ
z f
K+ q 0 √K 2c √K ---(01) ---(02) --- (03) Solving for excavation depth
zf (and
using Safety Factor), we obtained
Z f = {2c / [(SF)
γ√K]} – {q 0 / (SF) γ} --
(04)
3-Net versus Gross Soil Pressure (Design soil pressures)
The bearing capacity equations are based on gross soil pressure q ult , which is everything above the foundation level.
If the allowable pressure is based on the bearing capacity equations, the pressure is a gross pressure.
Settlements are caused only by net increase in pressure over the existing overburden pressure.
If the allowable pressure is based on settlement consideration, it is a net pressure.
4-Erosion Problems for structures adjacent to flowing water
Bridge piers, abutments, bases for retaining walls and footings for other structures adjacent to or located in flowing water must be located at a depth such that erosion or scour does not undercut the soil and cause a failure An accurate prediction of scour depth is necessary so as to use the shortest possible pile lengths
When Scouring Accelerated ?
Scour is accelerated if the foundation creates channel obstruction; To reduce scour the foundation should create a minimum obstruction to normal stream flow patterns
Approaches to avoid Scouring
Determine the foundation types Estimate the probable depth of scour ,effects, etc.
Estimate the cost of foundations for normal and various scour conditions Determine the cost versus risk and revise the design accordingly
5-Corrosion Protections
In polluted ground areas such as old sanitary landfills, shorelines, near sewer outfalls line from older industrial plants, or backwater areas where water stands over dead vegetation, there can be corrosion problems with metal foundation members as well as concrete.
Concrete is normally resistant to corrosion; However, if sulfates are present, it may be necessary to use sulfate-resistant concrete.
It may occasionally be necessary to use air entrained concrete for foundation members.
Use of treated timber piling instead of metal piling may be required where the soil has a pH much above 9.5 or below 4.0
(7 being neutral)
6-Water Table fluctuation
A lowered water table increases the effective pressure and may cause additional settlements. A raised water table may create problems for the owner from the following; Floating the structure ( making it unstable or tilting it) Reducing the effective pressure (causing excessive settlement) Creating a wet basement if the basement walls are not watertight.
SOLUTION
By introducing some type of drainage (water does not accumulate around the building walls or produce hydrostatic uplift beneath the basement).
Use of drain tile around the basement perimeter (common for residential dwellings and some larger buildings).
A sloping basement excavation that is backfilled with granular materials to the required horizontal level in combination with a well (called a sump pit) at the low point that is fitted with a pump (a sump pump system) can be used.
Home work (For Snowy areas)
1: How does insulation stop frost heave from occurring?
2: Does the soil type or ground cover (e.g., snow) affect the amount of insulation required?
3: How long foundation?
will the insulation protect the 4: What happens if the heating system fails for a time during the winter?
5: Why are greater amounts of insulation needed at the corners of the foundation?