drilled shafts ppt

8/11/2019 Drilled Shafts Ppt

1/16

DRILLED SHAFTS


2/16

CONTENTS

Introduction

Advantages

Disadvantages

Types of Drilled Shafts

Use of Casings and Drilling mud

Inspection of the Bottom of the Hole

Design Considerations

Load Bearing Capacity of a Drilled Shaft

References


3/16

INTRODUCTION

They are also no!n as Caissons" Drilled piers or Bored #iles$

They are high capacity cast in place deep foundation elements constructed !ith an auger$ A hole having the design diameter of planned shaft is first drilled to the design depth$ If the hole

assistance to remain open" casing or drilling fluid is used$ &ull'length reinforcing steel is then lo

hole and the hole is filled !ith concrete$ The finished foundation element resists compressive"

loads$ The techni%ue has (een used to support (uildings" tans" to!ers and (ridges$

They have proven to (e a cost effective" e)cellent performing" deep foundation system" that is

!ide$

They are used for large structures" !here large loads and lateral resistance are ma+or factors$


4/16

ADVANTAGES

,conomics$

,asily adapta(le to varying site conditions$

High a)ial and lateral loading capacity$

Re%uires less area as compared to traditional -AT &oundation$

A single drilled shaft may (e used instead of a group of piles and the pile cap$

Construction drilled shafts in deposits of dense sand and gravel is easier than driving piles$

Drilled shafts may (e constructed (efore completion of grading operations$

.hen piles are driven (y a hammer" the ground vi(ration may cause damage to near(y structures" !hich th

shafts avoids$

#iles driven into clay soils may produce ground heaving and (ecause previously driven piles to move latera

occur during construction of drilled shafts$

There is no hammer noise during the construction of drilled shafts" as there is during pile driving$ Because the (ase of a drilled shaft can (e enlarged" it provides great resistance to the uplifting load$

The surface over !hich the (ase of the drilled shaft is constructed can (e visually inspected $

Construction of drilled shafts generally utili*es mo(ile e%uipment" !hich" under proper soil conditions" may

economical than methods of constructing pile foundations$


5/16

DISADVANTAGES

,)tremely sensitive to construction procedures

/ot good for contaminated sites

Lac of construction e)pertise

Lac of 0ualified Inspectors


6/16

TYPES OF DRILLED SHAFTS

Drilled shafts are classified according to the !ays in !hich they are designed to transfer the str

the su(stratum$

STRAI1HT SHA&T ' It e)tends through the upper layer2s3 of poor soil" and its tip rests on a stro

soil layer or roc$ The shaft can (e cased !ith steel shell or pipe !hen re%uired$ &or such shaft

to the applied load may develop from end (earing and also from side friction at the shaft periminterface$


7/16

Types of Drilled Shafs !CONT.)

B,LL,D SHA&T ' consists of a straight shaft !ith a (ell at the (ottom" !hich rests on good (ea

(ell can (e constructed in the shape of a dome or it can (e angled$ &or angled (ells" the under

commercially availa(le can mae 45 to 67 angles !ith the vertical$ &or the ma+ority of drilled s

constructed in the United States" the entire load'carrying capacity is assigned to the end (earin

Ho!ever" under certain circumstances" the end'(earing capacity and the side friction are taen

,urope" (oth the side frictional resistance and the end'(earing capacity are al!ays taen into a


8/16

Types of Drilled Shafs !CONT.)

STRAI1HT SHA&T S8C9,T,D I/T8 R8C9 ' Straight shafts can also (e e)tended into an un

layer$ In the calculation of the load'(earing capacity of such shafts" the end (earing and the sh

developed along the shaft perimeter and roc interface can (e taen into account$


9/16

CONSTRUCTION PROCEDURES

CHICA18 -,TH8D 'In this method" circular holes !ith diameters of 4$7 ft or more are e)cava

depths of :'; ft at a time$ The sides of the e)cavated hole are then lined !ith vertical (oards" r

laggings. They are held tightly in place (y t!o circular steel rings$ After placement of the rings"

is continued for another :'; ft$ .hen the desired depth of e)cavation is reached" the (ell is e)c

&ollo!ing the completion of the e)cavation the hole is filled !ith concrete$


10/16

Co"sr#$io" pro$ed#res !CONT.)

18. -,TH8D < In this method the hole is e)cavated (y hand$ Telescopic metal shells are us

the shaft$ The shells can (e removed one section at a time as concreting progresses$ The mini

of a 1o! drilled shaft is a(out 6 ft 2=$:: m3$ Any given section of the shell is a(out : in$ 275 mm

diameter than the section immediately a(ove it$ Shafts as deep as =55 ft 245 m3 have (een ins

method$


11/16

USE OF CASINGS AND DRILLING %UD

.hen holes are driven in soft clays" the soil tends to s%uee*e in and close the hole$

In such situations" casings may (e used to eep the hole open and may have to (e driven (efo

(egins$

Holes made in gravelly and sandy soils also tend to cave in$ ,)cavation of drilled'shaft holes in these soils can (e continued either (y casing as the hole p

using drilling mud.


12/16

INSPECTION OF THE &OTTO% OF THE HOLE

The (ottom of the hole must (e inspected to ensure that the load'(earing stratum is !hat !as antic

the (ell is properly done$ &or these reasons" an inspector must descend to the (ottom of the hole$

precautions must (e o(served during this procedure>

If a casing is not already in the hole" one should (e lo!ered (y crane into it to prevent the hole

from collapsing$ The hole should (e tested for the presence of poisonous or e)plosive gases" !hich can (e don

miner?s safety lamp$

The inspector should !ear a safety harness$

The inspector should also carry a safety lamp and an air tan


13/16

DESIGN CONSIDERATIONS

&or the design of ordinary drilled shafts !ithout casings" a minimum amount of vertical steel re

al!ays desira(le$ -inimum reinforcement is =@ of the gross cross'sectional area of the shaft$

reinforcing cage having a length of a(out =: ft 24$;7 m3 is used in the top part of the shaft" and

reinforcement is provided at the (ottom$ This procedure helps in the construction process (eca

placed after most of the concreting is complete$

&or drilled shafts !ith nominal reinforcement" most (uilding codes suggest using a design conc

con the order of6$ Thus the minimum shaft diameters (ecomes

!hereDsis the diameter of the shaft fc is the : day concrete strength

Qwis the !oring load of the shaft

Agsis the gross cross sectional area of the shaft


14/16

Design Considerations (CONT.)

Depending on the loading conditions" the reinforcement percentage may sometimes to too high

use of a single rolled-steel section at the center of the pier may (e considered$

Qw (Ags! As) fc" Asfs

!hereAs is the area of the steel sectionfs is the allo!a(le strength of steel 5$7

If drilled shafts are liely to (e su(+ected to tensile loads" reinforcement should (e continued fo

length of the shaft$


15/16


16/16

drilled shafts ppt

Documents