Short Answer // Theory Questions

Short Answer // Theory Questions

1. LEC06 - Pile Foundations I

   • How to choose a foundation type"
   • General Structure of a Pile
   • Applications of Pile Foundations
   • Pile Design Code:
     • Regardless of the type of Pile, they must satisfy 2 design criteria:
       • Bearing Capacity
         • NZ Building Code B1:2008 says that the ultimate axial compressive pile strength must consider:
           • Geotechnical design (shaft base resistance + shaft resistance --- Qu = Qp + Qs)
           • Static Load testing - the load that produces settlement of 0.1 pile width/bell/bulb
       • Settlement
         • Settlement must be within the allowed limit (NZ Building Code B1:2008 --- PIle Foundations <= 0.1 pile width/bell/bulb)
   • Pile Design Methods
     • Satisfy the rules:
       • Simplified Rules: (Presumed bearing capacity and zero shaft friction)
       • Code of Practice
     • Rational Design Method:
       • Based on soil/rock mechanics principles and/or empirical relationships
       • Needs adequate ground investigation, in-situ and lab tests
       • Consider geotechnical capacity and settlement
       • May require instrumented pile loading tests to verify design assumptions
       • More Economic design can be achieved
   • Classifications of Piles:
     • Based on Construction Materials:
       • Steel Piles
         • Qall.s = As x fs
           • Qall,s = allowable structural capacity of steel pile
           • As = cross-sectional area of the steel
           • fs = allowable stress of steel (approx. 0.33 - 0.5 fy), minimum yield strength
         • Advantages:
           • Easy to handle with respect to cut-off and extension to the desired length
           • Can stand high driving stresses
           • Can penetrate hard layers such as dense gravel and soft rock
           • HIgh load-carrying capacity
         • Disadvantages:
           • Relatively costly
           • High level of noise during pile driving
           • Subject to corrosion (can prevent this but coating or cathodic protection)
           • May be damaged of deflected during driving through hard layers or major obstructions
       • Concrete Piles
         • Pre-Cast concrete piles
           • Ordinary Reinforcement --- Qall,s = 300 - 3000kN
           • Pre-stressed reinforcement --- Qall,s = 7500 - 8500kN
           • Advantages
             • Can be subject to hard driving
             • Resistant to corrosion
             • Can be easily combined with a concrete superstructure
           • Disadvantages
             • Difficult to achieve proper cut off
             • Difficult to transport
         • Cast-in-situ concrete piles
           • Cased Cast-In-Situ
             • Qall,s = (As x fs)+ (Ac x fc)
               • Qall,s = allowable structural capacity of cased pile
               • As = cross-sectional area of steel
               • fs = allowable stress of steel
               • Ac = cross-sectional area of concrete
               • fc = allowable stress of concrete
             • Advantages:
               • Relatively cheap
               • Allow for inspection before pouring concrete
               • Easy to extend
             • Disadvantages:
               • Difficult to splice after concreting
               • Thin casings may be damaged during driving
           • Uncased Cast-In-Situ
             • Qall,s = Ac x fc
             • Advantages
               • Initially economic
               • Can be finished at any elevation
             • Disadvantages
               • Voids may be created if concrete of rapidly placed
               • Difficult to splice after concreting
               • In soft soils, the sides of the hole may cave in
         • Continuous Flight Auger Piles (CFA)
           • Advantages:
             • Noise and vibrations during construction are minimized
             • Eliminates splicing and cutoff
           • Disadvantages:
             • Structural integrity is less reliable
             • Soil spoils need collection and disposal
           • Also known as: auger-cast, auger-cast-in-place, auger-pressure grout piles
             • Drilling by continuous flight augers
             • Drilling to the final depth in one continuous process
             • Gradually pulling the auger and pumping concrete into the hole
             • Placing reinforcement immediately after withdrawal of auger
       • Timber Piles
         • These must comply with: NZ3603:1993 (Timber Structures Standard) or NZS 3605:2001 (Timber Piles and poles for use in buildings) and are treated as recommended by NZS 3604:2011 (Timber Framed Buildings)
         • Qall,s = Ap x fw
           • Ap = average area of cross-section of timber pile (m2)
           • fw = allowable stress of the timber (kN/m2)
       • Composite Piles
         • Pre-cast concrete pile over timber pile (economic)
         • Cast-in-Situ concrete pile over Steel H Pile - limitation in designed length)
     • Based on Soil Disturbance & Installation Method
       • Driven Piles (Displacement PIles)
         • Installation by pile driving methods --- Hammering piles into the ground using sufficient energy --- use a dynamic driving force calculated to match the structural capacity
       • Bored Piles (Non-displacement or Replacement Piles)
         • Installation by boring methods --- Digging a hole in the soil --- Installing or casting the pile in the hole
     • Based on Load Transfer Method
       • Point Bearing Piles --- Qu = Qp + Qs where Qs = 0 therefore, Qu = Qp
       • Friction Piles --- Qu = Qp + Qs where Qp = 0 therefore Qu = Qs
       • Compaction Piles

2. LEC07 - Pile Foundations II

   • Design Philosophies:
     • Design must comply with two requirements for the lifetime of its service:
       • 1) Adequate safety against failure of the ground
       • 2) Adequate margin against excessive pile movement
     • 2 Design Approaches are used:
       • Global Factor of Safety Approach
       • Limit State Design Approach
   • Ultimate Capacity of Pile, Qu = Qs + Qp
     • Qp = Point Load Bearing Capacity
       • Qp = Ap x qp (where qp in = to qu in Meyerhof's equation in shallow foundations)
         • Qp in Sand
           • Qp = Ap.q.N*q <= Ap(0.5.pa.N*q.tan(phi))
         • Qp in Clay
           • Qp = Ap.cu.N*c
         • Qp in Soil
           • Qp = Qp.c` + Qp.phi` = [(Ap.q`.N*q<=Ap(0.5pa.N*q.tan.phi`))+(9Ap.c`)] --- basically the summation of Qp in clay and Qp in sand
     • Qs = Frictional Resistance of Pile
       • Qs = Summation of p.change in L.f --- and then the f changes in all the different methods
         • Qs in Sand
           • f = N.us = K.o`v.tan(soil-pile friction angle)
           • Meyerhof's
             • fav = 0.02pa(N60) - for high displacement driven piles
             • fav = 0.01paN60 - for low displacement driven piles
           • Briaud's
             • fav = 0.224pa(N60)^0.29
           • Qs = fav.p.L
         • Qs in Clay
           • wavelength method
             • fav = wavelength(o`o + 2cu)
               • cu = [(cu(1).L1) + (cu(2).L2)...]/(L)
               • o`o = [A1 + A2 ...]/(L)
           • alpha method
             • f = alpha . cu
               • alpha = from tables
           • beta method
             • f = beta . o`o
               • beta = K.tan.phi`R
                 • K = 1-sin(phi`R) --- for normally consolidated clays
                 • K = [(1 - sin(phi`R).sqrt(OCR)] --- for overconsolidated clays
           • Correlation
             • f = alpha`.fc
         • Qs in soil
     • But also keep in mind that Qu must be less than or equal to Qall,s (= Allowable structural capacity of pile)
   • Pier Foundations:
     • A raised structure typically supported by well spaces pile foundation systems
     • Types of Piers
       • Masonry piers
       • Drilled Piers
         • Concrete in Steel Shell Pier
         • Drilled Concrete Pier
         • Concrete and Steel Core in Steel Shell Pier
     • Difference between Pile and Pier
       • Method of Construction
       • Method of Load Transfer
       • Variation in Depth
       • Preference
       • Type

3. LEC08 - Settlement of Foundations

   • Types of Settlement
     • Elastic Settlement (during or immediately after construction)
     • Consolidation Settlement (depends on the permeability of the soil under the foundation)
     • In sand; elastic and consolidation settlement occur at the same time
     • In clay: elastic settlement occurs in short time and consolidation settlement occurs in very long period of time (considered for years after construction has occured)
   • Estimation of Settlement of Shallow Foundations
     • Effect of Water Table on Settlement of Shallow Foundations
       • Se` = Se x Cw
         • Peck et, al
           • Cw = [(1)/(0.5 + 0.5 x ((Dw)/(Df + B)))] => 1
         • Teng
           • Cw = [(1)/(0.5 + 0.5 x ((Dw - Df)/(B)))] <= 2
         • Bowels
           • Cw = 2 + (Dw / (Df + B))
     • Elastic Settlement of Shallow Foundations on Sand:
       • Se(flexible) = qo (alpha x Beta`)[(1 - us^2)/(Es)] x Is x If
       • Se(rigid) = 0.93 Se(flexible)
     • Elastic Settlement of Shallow Foundations on Clay:
       • Se = (A1 x A2 ](qo x B)/Es])
   • Estimation of Settlement of Pile Foundations
     • The settlements in piles are relatively low when compared to shallow foundations
     • Under working loads, with a factor of safety of 2 - 3, the settlement is about 1% of the pile diameter
     • The total settlement of piles under a vertical working load is Qw
   • Settlement due to Primary Consolidation
     • Consolidation settlement occurs over time in saturated clayey soil subjected to an increased load cause by construction of the foundation
   • Settlement due to Secondary Consolidation
     • At the end of primary consolidation, some settlement takes place due to the plastic adjustment of soil fabrics. This stage of consolidation is called secondary consolidation.

4. LEC09 - Geometric Design of Foundations

   • Isolated Footing:
     • Square, Rectangle etc. supporting one column or wall load
   • Combined Footing --- Supporting two or more column loads
     • Rectangular combined footing
       • When the standard footing design will extend out of the property line
       • When adjacent footings will be very close (<25cm)
       • 6 Design Steps:
         • 1) determine the area of the combined footing
         • 2) determine the location of the resultant load (taking moments about centre of column 1)
         • 3) find the length of combined footing, assuming uniform pressure, L
         • 4) check the validity of uniform load assumption
         • 5) find the extend of interior side of the combined footing, L1
         • 6) find the width of combined footing, B based on the known area
     • Trapezoid combined footing
       • 4 design steps:
         • 1) Determine the area of combined footing
         • 2) Determine the location of the resultant load
         • 3) check validity of uniform load assumption
         • 4) determine B1 and B2 by solving the system
       • Where loads are largely different in magnitude
       • Where space is tight (limited from both sides)
     • Cantilever footing (strap footing)
       • Cantilever footing construction uses a strap beam to connect an eccentrically loaded column or wall foundation to the foundation of an interior column
       • Replaces trapezoidal or rectangular combined footings when: the allowable soil bearing capacity is high and the distances between the columns are large
       • 5 Design Steps:
         • 1) determine the area of the combined footing
         • 2) determine the location of the resultant load (taking moment about centre of column 2)
         • 3) centre of gravity of two footings should coincide with the resultant load --- taking moment about centre of column 2, assuming B, Yc and hf
         • 4) Calculate L2
   • Mat or Raft Foundation
     • Supporting more than one line of column loads
     • Are used when:
       • When more than 50% of the structures footprint will otherwise be constructed as pad or strip footings
       • When the type of structure requires that the differential settlements be limited to very small values
       • When there are non-uniform soil conditions or soil where pockets of weak soil are present
     • Common types of mat Foundations:
       • a) Flat Plate
       • b) Flate plate thickened under columns
       • c) bean and slab - waffle slab
       • d) flat plates with pedestals - ribbed raft
       • e) Slab with basement walls as part of the mat
     • When water table is high, mat is placed over the piles
   • Foundation Design Considerations
     • Mat foundations can also be designed by considering shear forces and moment based on:
       • Conventional rigid method
       • Approximate flexible method

5. LEC10 - Lateral Earth Pressure and Retaining Structures

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