Ground Improvement using Micropiles - Ground Improvement - Lecture Slides, Slides of Geology

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GROUND

IMPROVEMENT

USING

MICROPILES

Contents

•^

Introduction

-^

Classification

1. Based on Design criteria2. Based on Construction type

•^

Advantages of micropiles

-^

Applications

-^

Design of micro piles using FHWA guidelines

-^

Design Example

•^

Piles are divided in two general types as

a)

Displacement piles b)

Replacement piles

Displacement piles are members that are driven

or

vibrated

into

the

ground,

there

by

displacing

the

surrounding soil laterally during installation.

Replacement piles are placed or constructed with

in

a^

previously

drilled

borehole,

thus

replacing

the

excavated ground.A Micropile is a small diameter (< 300mm), drilled andgrouted pile that is typically reinforced.

Classification

of

Micropiles

Based on Design Application Case 1

: Micropile elements, which are loaded directly &where the pile reinforcementresists the majority of theapplied load. Case 2

: Micropile elements circumscribe and internallyreinforce the soil to make areinforced soil compositethat resists the applied load.

Fig1(a):

Drilled

Micropiles

under

a

building

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Applications

-^

For Structural support

(Case 1)

a) New Foundationsb) Under pinning of existing structuresc)

Seismic retrofitting of existing structures

d) Scour protectione) Earth retention•^

In situ Reinforcement

(Case 2)

a) Slope Stabilizationb) Earth retentionc)

Ground strengthening and protection

d) Settlement reduction

Classical arrangement of root piles for underpinning

CASE 1 Micropiles (Directly Loaded)

CASE 2 Micropiles-Reticulated Pile Network with ReinforcedSoil Mass Loaded or Engaged

CASE 2 Micropile arrangements

Based on Construction type^ •The method of grouting is generally the most sensitiveconstruction control over grout/ground bond capacity.Grout-to-grout capacity varies with the groutingmethod

a)

Type A: Gravity Grout b) Type B: Pressure through Casingc)

Type C: Single Global Post Grout d) Type D: Multiple Repeatable Post Grout

•^

Type C

: This is done in two step process:

As of Type A

Prior to hardening of the primary grout, similar groutis

injected

one

time

via

a^

sleeve

grout

pipe

at

pressure of at least 1.0MPa.

-^

Type D

: This is done in two step process of grouting

similar to Type C with modifications to step 2 wherethe pressure is injected at a pressure of 2.0 to 8.0MPa:

Advantages

of

Micropiles

•^

Micropiles are often used to underpin the existing structurewhere

need

of

minimal

vibration

or

noise

is

of

prime

importance.

-^

Micropiles can be easily laid where low head room is aconstraint.

-^

Micropiles can be easily installed at any angle below thehorizontal

using

the

same

equipment

used

for

ground

anchors and grouting projects.

-^

Offer

a^

practical

and

cost-effective

solution

to

costly

alternative pile systems as well as a solution to job siteswith difficult access.

-^

Do not require large access road or drilling platforms

Outline

of

Design

steps

Review available project information

Review geotechnical data

Geotechnical design

Pile structural design

Combined geotechnical & structural designconsiderations

Additional micro pile system considerations

-^

bond nominal strength

= Grout to ground bond capacity of

pile from Table 1(a).

-^

P

G-allowable

= Allowable geotechnical bond axial load

Allowable geotechnical bond axial load capacity, P

G-

allowable

can be determined by the following equation;

Determination

of

Geotechnical

bond

capacity

P

G-allowable

bond strength

bond

ˣBond length/S.F