Public Works Research Institute, Japan

Outline of Design Manual for Seismic Retrofitting of Existing Pile Foundations wit

h High Capacity Micropiles

Public Works Research Institute, Japan Takeshi UMEBARA Jiro FUKUI Masahiro ISHIDA

Contents of the manual : Part 1 Design Manual Part 2 Execution Manual

Design and Execution Manual for Seismic Retrofitting of Existing Pile Foundations

with High Capacity Micropiles

Chapter 1 : General Chapter 2 : MaterialsChapter 3 : SurveyingChapter 4 : Basic items for designChapter 5 : Basic items for seismic retrofittingChapter 6 : Verification at normal time, during strong wind, and during level 1 earthquakeChapter 7 : Verification during level 2 earthquake Chapter 8 : Structural details

Contents of the Design Manual

(1) Scope

(2) Basic principles for seismic retrofitting design

(3) Verification at normal time, during strong wind, and during level 1 earthquake

(4) Verification during level 2 earthquake

Points of the Design Manual

(1) Scope





This manual covers the design of seismic retrofitting of existing pile foundations using high capacity micropiles.

* This items not stipulated in this manual have to be referred to the following design specifications etc.

1) Specifications for Highway Bridges, JRA 2) Reference Documents concerning the Retrofitting of Existing Highway Bridge Foundations, JRA

Scope

(1) Scope





(2) The seismic retrofitting design of an existing pile foundation shall be performed, as necessary, with consideration two levels of design earthquake motions:

Two levels of design earthquake motions

Level 1 earthquake motion : Earthquake motion with a high probability of occurring during the service life of the existing bridge

Level 2 earthquake motion : Strong earthquake motion with a low probability of occurring during the service life of the existing bridge

Basic principles for seismic retrofitting design

(1) Scope



1) Verification items 2) Bearing capacity estimation equation



(1) Pile head reaction force ( allowable bearing capacity)

(2) Horizontal displacement ( allowable horizontal displacement)

(3) Stresses generated in each member of the foundation ( allowable stresses)

Verification Items at Normal Time, during Strong Wind, and Level 1 Earthquake

(1) Pile head reaction force ( allowable bearing capacity)

(2) Horizontal displacement ( allowable horizontal displacement)

(3) Stresses generated in each member of the foundation ( allowable stresses)

Verification Items at Normal Time, during Strong Wind, and Level 1 Earthquake

Pile head part

Pile

leng

th

Bearing layer

Steel pipes anchored part

Distribution of frictional resistance

Ultimate friction strength

Rcu

Grout

Max. skin frictional resistance fi

Ultimate push-inBearing capacity

FootingUn

anch

ored

par

tAn

chor

ed p

art St

ress

dist

ribut

ion

in p

ile b

ody

Steel pipes unanchored part

(1) Frictional resistance is considered only in the anchored part. (2) Bearing capacity of the tip is not considered.

Fundamental Assumptions of Bearing Capacity Estimation

Bearing capacity estimation equation

Ru = U Li fi

Where: Ru : ultimate bearing capacity of HMP determined by the ground (kN) U : circumference of the anchored part (m) U=D D : borehole diameter (m) Li : thickness of the layer considering the skin friction (m) fi : maximum skin friction of the layer considering the skin friction (kN/m2)

* The value of “fi” is the average value of the friction strength shown in a “Design and Execution Specification for Ground Anchor” ,JGS.

(1) Scope



(4) Verification during level 2 earthquake 1) Verification items 2) Calculation model (for calculating the sectional force, pile head reactions, and the displacement in a pile foundation)


(1) The strength of the foundation (The pile foundation does not reach the yield condition of the foundation by applying horizontal capacity of the pier.)

Even in case of the pile foundation reaches the yield condition, if the bridge pier has adequate horizontal capacity,

1) The response ductility factor and response displacement of the foundation

(2) Liquefaction assessment

(3) Each member of the foundation (sectional forces generated in each member strengths of the member)

Verification Items during Level 2 Earthquake

Calculation model

Calculation model of a pile foundation retrofitted by HMP

HMP

Existing pile

VoMoHo

KVE KVE

KHE

The pile foundation with a rigid frame structure supported by the ground resistance considering the non-linearity.

Axial resistance property of a pile


PTU

PNU

Pile head reaction P (kN)

Pile head axial disp. (m)

tan-1 KVE

Upper limit of push-in bearing capacity

Upper limit of pull-out bearing capacity

Axial resistant property

HMP

Existingpile

VoMoHo

KVE KVE

KHE

HMP

Existingpile

VoMoHo

HMP

Existingpile

VoMoHo

KVE KVE

KHE

Resistant Property at a Right Angle to the Axis of a Pile


Axial resistant property at a right angle to the axis of a pile

Horizontal displacement H (m)

tan-1 KHE

Upper limit of horizontal subgrade reaction pHU

horiz

onta

l sub

grad

e

re

actio

n p H

(kN/

m2 )

HMP

Existingpile

VoMoHo

KVE KVE

KHE

HMP

Existingpile

VoMoHo

HMP

Existingpile

VoMoHo

KVE KVE

KHE

Raito of the upper limit of horizontal subgrade reaction in each row of piles

HMP

Front row Rear row

Existing piles

Load direction

Front row of existing piles

Front row OthersSandy ground 1.0 0.5 1.0 0.5

HMP Existing piles

Front row Others

Resistant Property at a Right Angle to the Axis of a Pile

Bending moment – curvature relationship of HMP


Bending moment – curvaturerelationship of HMP

Curvature (1/m)Be

ndin

g m

omen

t M (k

N･m)

Y

Fully Plastic Moment

Y : yieldY’ : full plastic

y y’

My_HMP

My’_HMP

HMP

Existingpile

VoMoHo

KVE KVE

KHE

HMP

Existingpile

VoMoHo

HMP

Existingpile

VoMoHo

KVE KVE

KHE

Y’

Yield of a pile foundation

Displacement at the action location of the superstructure’s inertia

Horiz

onta

l sei

smic

coef

ficie

nt

YieldFoundation

Pier column

Yield of a pile foundation

The yield of a pile foundation retrofitted by HMP is generallythe time when one of the following stage is first reached.

(1) The bodies of all existing piles plasticize.(2) The bodies of all HMP plasticize.(3) The pile head reaction of a row of piles (existing piles or HMP) reaches the upper limit of the push-in bearing capacity.

Thank you for your attention.