DESIGN AND ANALYSIS OF DEEP FOUNDATION

WEEK 9 • FRICTION AND END BEARING PILES • BEARING CAPACITY ANALYSIS OF PILES USING EMPIRICAL AND DYNAMIC FORMULAE

Learning Outcomes

Student should be able to: • Explain the end bearing and skin friction contribution to pile capacity.

• Estimate the bearing capacity of a pile.

DESIGN AND ANALYSIS OF DEEP FOUNDATION PILE CLASSIFICATION BASED ON BEARING CAPACITY:

END BEARING PILES FRICTION PILES

IN GENERAL MOST PILES HAVE END BEARING AND SKIN FRICTION RESISTANCE SUBSOIL CONDITIONS DETERMINE THE CATEGORY OF PILE

DESIGN CONSIDERATION OF DEEP FOUNDATION GEOTECHNICAL ENGINEERING IS A BRANCH OF CIVIL ENGINEERING WHICH REQUIRES THE MOST OF EXPERIENCE AND JUDGEMENT OF ITS PRACTITIONER To produce satisfactory pile foundation systems that are Neither unnecessarily over-designed or dangerously inadequate For example, it is prudent to design micropile using skin Friction only if its in a limestone karstic formation due to Presence of cavities and overhang Some designers ignore end bearing contribution for bored When in doubt that the base will be properly cleaned

Pile Design

Concept Friction Pile End Bearing Pile

Summary of Pile Capacity Calculation Not more 100kPa

Factor of Safety for Design FOS for skin friction is 2 FOS for end bearing resistance is 3 Generally design the working load of the pile to be Around 80% of geotechnical capacity

TENSION PILE Piles that are designed to take uplifting load due to: Wind eg a hangar Tall chimneys (overturning) Transmission towers Jetty structures

Skin frictional resistance is much lower than those for compression 50% reduction is suggested Pull out test may be conducted Lateral force eg wave action if transmitted to pile will Destroy most of the skin friction.

Factor of Safety to be applied ie FOS = 2 Tension pile has no end bearing resistance.

To be designed for both compression and tension loads.

WAVE EQUATION

Pile Capacity -Static Analysis - Dynamic Analysis -Dynamic formulae - Wave Equation Dynamic Formulae -Engineering New formula - Hiley Main weakness- modeling of the pile as one rigid mass inadequate modeling of soil –pile interaction

History of Wave Equation Method David Victor Issacs (1931) In Australia. He reviewed the Dynamic Formulae.

Developed mathematical model based on successive Transmission and reflection of waves. Glanville et. Al (1938) Problem of concrete pile breakages at top and toe during driving.

Wave equation was used to estimate stress in pile.

Charts were developed for usage but applicable only to concrete Piles.

Smith (1960) Numerical method.

-Division of piles into springs and masses. Hammer modeled as mass With cushion spring.

-Integration of model using finite difference technique.

-Modeling of soil as combination of displacement dependent springs And velocity dependent dampers. Applied along the pile shaft They modeled the soil resistance. -Modeling of the non-linearities of soil. The soil was given a ‘yield limit’ (quake);a after this time the non-dynamic resistance Resistance was constant.

The Pile-Soil Model Standard model used in many wave equation techniques today

Goble et. Al (1980) Major steps in using stress wave theory to piles during driving And to estimate static capacity by Case Method.

Compare pile force and velocity at a given time with a time 2L/c before that .

The static and dynamic components separated.

Field application using strain gauge to measure force and Accelerometer to measure velocity.

Rausche et. al (1985) CAPWAP technique (Case Pile Wave Analysis Program) Similar instrumentation to Case Method but pile is divided into Series of elements and the wave reflection for each one are Analysed based on the time return to the top of pile.

A profile of shaft resistance distribution is obtained.