地下室设计深基坑中英文对照外文翻译文献

中英文对照外文翻译

(

文档含英文原文和中文翻译

)

Deep Excavations

ABSTRACT

：

All

major

topics

in

the

design

of

in-situ

retaining

systems for deep excavations in urban areas are outlined. Type of wall, water

related problems and water pressures, lateral earth pressures, type of support,

solution

to

earth

retaining

walls,

types

of

failure,

internal

and

external

stability problems.

KEYWORDS

: deep excavation; retaining wall; earth pressure;

INTRODUCTION

Numbers

of

deep

excavation

pits

in

city

centers

are

increasing

every

year. Buildings, streets surrounding excavation locations and design of very

deep basements make excavations formidable projects. This chapter has been

organized in such a way that subjects related to deep excavation projects are

summarized in several sections in the order of design routine. These are types

of in-situ walls, water pressures and water related problems. Earth pressures

in cohesionless and cohesive soils are presented in two different categories.

Ground

anchors,

struts

and

nails

as

supporting

elements

are

explained.

Anchors are given more emphasis compared to others due to widespread use

observed in the recent

years. Stability of retaining systems are discussed as

internal

and

external

stability.

Solution

of

walls

for

shears,

moments,

displacements

and

support

reactions

under

earth

and

water

pressures

are

obtained making use of different methods of analysis. A pile wall supported

by anchors is solved by three methods and the results are compared. Type of

wall

failures,

observed

wall

movements

and

instrumentation

of

deep

excavation projects are summarized.

1. TYPES OF EARTH RETAINING WALLS

1.1 Introduction

More than several types of in-situ walls are used to support excavations.

The criteria for the selection of type of wall are size of excavation, ground

conditions,

groundwater

level,

vertical

and

horizontal

displacements

of

adjacent

ground

and

limitations

of

various

structures,

availability

of

construction, cost, speed of work and others. One of the main decisions is the

water-tightness

of

wall.

The

following

types

of

in-situ

walls

will

be

summarized below;

1. Braced walls, soldier pile and lagging walls

2. Sheet-piling or sheet pile walls

3. Pile walls (contiguous, secant)

4. Diaphragm walls or slurry trench walls

5. Reinforced concrete (cast-in-situ or prefabricated) retaining walls

6. Soil nail walls

7. Cofferdams

8. Jet-grout and deep mixed walls

9. Top-down construction

10. Partial excavation or island method

1.1.1 Braced Walls

Excavation proceeds step by step after placement of soldier piles or so

called

king

posts

around

the

excavation

at

about

2

to

3

m

intervals.

These

may be steel H, I or WF sections. Rail sections and timber are also used. At

each level horizontal waling beams and supporting elements (struts, anchors,

nails) are constructed. Soldier piles are driven or commonly placed in bored

holes

in

urban

areas,

and

timber

lagging

is

placed

between

soldier

piles

during the excavation. Various details of placement of lagging are available,

however,

precast

units,

in-situ

concrete

or

shotcrete

may

also

be

used

as

alternative

to

timber.

Depending

on

ground

conditions

no

lagging

may

be

provided in relatively shallow pits.

Historically

braced

walls

are

strut

supported.

They

had

been

used

extensively before the ground anchor technology was developed in 1970

?

s.

Soils with some cohesion and without water table are usually suitable for this

type of construction or dewatering is

accompanied if required and allowed.

Strut support is commonly preferred in narrow excavations for pipe laying or

similar

works

but

also

used

in

deep

and

large

excavations

(See

Fig

1.1).

Ground anchor support is increasingly used and preferred due to access

for

construction

works

and

machinery.

Waling

beams

may

be

used

or

anchors

may be placed directly on soldier piles without any beams.

1.1.2 Sheet-piling or Sheet Pile Walls

Sheet pile is a thin steel section (7-30 mm thick) 400-500 mm wide. It is

manufactured

in

different

lengths

and

shapes

like

U,

Z

and

straight

line

sections (Fig. 1.2). There are interlocking watertight grooves at the sides, and

they

are

driven

into

soil

by

hammering

or

vibrating.

Their

use

is

often

restricted

in

urbanized

areas

due

to

environmental

problems

like

noise

and

vibrations.

New

generation

hammers

generate

minimum

vibration

and

disturbance,

and

static

pushing

of

sections

have

been

recently

possible.

In

soft ground several sections may be driven using a template. The end product

is a watertight steel wall in soil. One side (inner) of wall is excavated step by

step

and

support

is

given

by

struts

or

anchor.

Waling

beams

(walers)

are

frequently used. They are usually constructed in water bearing soils.

Steel

sheet

piles

are

the

most

common

but

sometimes

reinforced

concrete

precast

sheet

pile

sections

are

preferred

in

soft

soils

if

driving

difficulties

are

not

expected.

Steel

piles

may

also

encounter

driving

difficulties in very dense, stiff soils or in soils with boulders. Jetting may be

accompanied during the process to ease penetration. Steel sheet pile sections

used in such difficult driving conditions are selected according to the driving

resistance rather than the design moments in the project. Another frequently

faced

problem

is

the

flaws

in

interlocking

during

driving

which

result

in

leakages

under

water

table.

Sheet

pile

walls

are

commonly

used

for

temporary

purposes

but

permanent

cases

are

also

abundant.

In

temporary

works sections are extracted after their service is

over,

and they

are reused

after

maintenance.

This

process

may

not

be

suitable

in

dense

urban

environment.

1.1.3 Pile Walls

In-situ pile retaining walls are very popular due to their availability and

practicability. There are different types of pile walls (Fig. 1.3). In contiguous

(intermittent)

bored

pile

construction,

spacing

between

the

piles

is

greater

than the diameter of piles. Spacing is decided based on type of soil and level

of design moments but it should not be too large, otherwise pieces of lumps

etc.

drop

and

extra

precautions

are

needed.

Cohesive

soils

or

soils

having

some

cohesion

are

suitable.

No

water

table

should

be

present.

Acceptable

amount of water is collected at the base and pumped out. Common diameters

are 0.60, 0.80, 1.00 m. Waling beams (usually called ?breasting beams

?

) are

Tangent

piles

with

grouting

in

between

are

used

when

secant

piling

or

diaphragm

walling

equipment

is

not

available

(i.e.

in

cases

where

ground

water exists). Poor workmanship creates significant problems.

Secant bored pile walls are formed by keeping spacing of piles less than

diameter

(S

It

is

a

watertight

wall

and

may

be

more

economical

compared

to

diaphragm

wall

in

small

to

medium

scale

excavations

due

to

cost of site operations and bentonite plant.

There

is

also

need

for

place

for

the

plant.

It

may

be

constructed

“hard

-

hard” as well as “soft

-

hard”. “Soft” concrete pile

contains low cement

content and some bentonite. Primary unreinforced piles are constructed first

and then reinforced secondary piles are formed by cutting the primary piles.

Pile construction methods may vary in different countries for all type of pile

walls

like

full

casing

support,

bentonite

support,

continuous

flight

auger

(CFA) etc. mostly reinforced concrete but sheet pile sections or steel beams

are also used.

1.1.4 Diaphragm Walls

Diaphragm wall provides structural support and water tightness. It is a

classical technique for many deep excavation projects, large civil engineering

works,

underground

car

parks,

metro

pits

etc.

especially

under

water

table.

These reinforced concrete diaphragm (continuous) walls are also called slurry

trench walls due to the reference given to the construction technique where

excavation of wall is made possible by filling and keeping the wall cavity full

with

bentonite-water

mixture

during

excavation

to

prevent

collapse

of

the

excavated vertical surfaces. Wall thickness varies between 0.50 m and 1.50 m.

The wall is constructed panel by panel in full lengths are 2 m to

10 m. Short lengths (2-2.5 m) are selected in unstable soils or under very high

surcharges.

Nowadays

depth

of

panels

water

stops

exceeded

100

m,

excavation

depths

exceeded

50

m.

Different

panel

shapes

other

than

the

conventional straight section like T, L, H, Y

, + are possible to form and used

for

special

purposes.

Panel

excavation

is

made

by

cable

or

kelly

supported

buckets

and by a recent

design

called ?cutter

?

or ?hydrofraise

?

which is a

pair of hydraulically operated rotating disks provided with hard cutting tools.

Excavation

in

rock

is

possible.

Slurry

wall

technique

is

a

specialized

technique

and

apart

from

the

bucket

or

the

frame

carrying

the

cutter

equipment

like

crawler

crane,

pumps,

tanks,

desanding

equipment,

air

lifts,

screens,

cyclones,

silos,

mixers,

extractor

are

needed.

Tremie

concrete

is

placed

in

the

slurry

starting

from

the

bottom

after

lowering

reinforcement

cages. Joint between the panels is a significant detail in water bearing soils

and steel pipe, H-beam or water stops are used.

1.1.5 Reinforced Concrete Retaining Walls Excavation in Stages

It

is

a

common

type

of

staged

excavation

wall

usually

supported

by

ground anchors. Soils with some cohesion are suitable because each stage is

first excavated before formwork and concrete placement. No water table or

appreciable amount of water should be present. Sometimes micropile support

is given if required due to expected cave-ins.

1.1.6 Soil Nail Walls

Similar to the method above excavation is made step by step (1.5 to 2 m

high). Shotcrete is common for facing and wiremesh is used. Soft facing is

also

possible

making

use

of

geotextiles.

Hole

is

drilled,

ordinary

steel

bars

are

lowered,

and

grout

is

placed

without

any

pressure.

Soil

should

be

somewhat

cohesive

and

no

water

table

or

significant

water

flow

should

be

present.

1.1.7 Cofferdams

Cofferdam

is

a

temporary

earth

retaining

structure

to

be

able

to

make

excavation for construction activities. It is usually preferred in the coastal and

sea environment like bridge piers and abutments in rivers, lakes etc., wharves,

quay

walls,

docks,

break

waters

and

other

structures

for

shore

protection,

large waterfront structures such as pump houses, subjected to heavy vertical

and horizontal loads. Sheet piling is commonly used in various forms other

than

conventional

walls

like

circular

cellular

bodies

or

double

walls

connected

inside

and

filled

with

sand.

Stability

is

maintained

by

sheeting

driven

deeper

than

base,

sand

body

between

sheeting

and

inside

tie

rods.

Earth embankments and concrete bodies are also used. Contiguous, tangent,

secant

piles

or

diaphragm

walls

are

constructed

in

circular

shapes,

and

no

internal

bracing

or

anchoring

is

used

to

form

a

cofferdam.

Reinforced

concrete

waling

beams

support

by

arching.

Shafts

are

also

made

with

this

method.

Large excavations or project details may require additional lateral

support.

1.1.8 Jet Grout and Deep Mixed Walls

Retaining walls are made by single to triple row of jet grout columns or

deep mixed columns. There is a soil mixed wall(SMW) technique specially

developed for wall construction where H sections are used for reinforcement.

Single

reinforcing

bar

is

placed

in

the

central

hole

opened

for

jet

grout

columns. Anchors, nails or struts may be used for support.

1.1.9 Top Down Construction

Retaining

structure

(generally

diaphragm

wall)

is

designed

and

constructed

as

permanent

load

bearing

walls

of

basement.

Piles

or

barettes

are similarly placed to complete the structural frame. Top slab is cast at the

ground surface level, and excavation is made under the slab by smaller sized

excavators and continued down forming basement slabs at each level. There

are

special

connection

details.

Top

down

method

is

preferred

in

highly

populated city centers where horizontal and vertical

displacements

are very

critical,

and

anchors

and

struts

are

very

difficult

to

use

due

to

complex

underground facilities and lifeline structures and site operations are difficult

to perform.

1.1.10 Partial Excavation or Island Method

It is possible to give strut support to retaining walls at a later stage after

constructing central sections of a building in large size excavations. Core of

the

structure

is

built

at

the

central

part

making

sloped

excavations

at

peripheral

areas

and

then

the

core

frame

is

used

to

give

support

to

walls

(Figure

1.9).

It

may

be

more

practical

and

construction

time

may

be

less

compared to conventional braced system. This method may not be suitable in

soft and weak soils due to stability and deformation problems during sloped

excavations.

2. EARTH PRESSURES ON IN- SITU RETAINING WALLS

2.1. Introduction

Earth pressures on in-situ retaining walls are rather different than those

on ordinary retaining walls due to the supporting elements. Free displacement

of

walls

are

not

allowed.

Type

of

support

affects

the

distribution

of

earth

pressure. Strut loads were measured in strutted excavations in many countries

in the past, and recommendations were given. Ground anchor technology is

relatively

new,

and

data

on

instrumented

anchored

walls

for

total

lateral

pressure

and

for

water

pressure

are

being

accumulated.

Earth

pressure

diagrams

on

strutted

and

anchored

walls

are

expected

to

be

somewhat

different

due

to

stiffer

support

conditions

in

the

former.

Theoretical

approaches will also be discussed.

2.2. Earth Pressure Distributions on Walls

Terzaghi

and

Peck

(1967)

and

Peck

(1969)

based

on

load

measurements on struts recommend the pressure distribution shown in Figure

2.1 for cohesionless soils. It is a uniform pressure and given by Eq. 2.1;

p = 0.65 K

A

γ

t

H

2.1

where K

A

is the active earth pressure coefficient, H is the height of wall.

Unit weight (γt) is described as the bulk unit weight

in the original references.

Since

braced

excavations

were

generally

dewatered

in

the

past

projects

the

unit

weight

in

the

expression

was

described

as

wet

or

bulk.

If

wall

is

watertight

and

water

table

is

present,

buoyant

unit

weight

should

be

used

under water table and water pressure should be added.

The rectangular diagram proposed in the figure is not an actual pressure

distribution

but

an

envelope

obtained

by

plotting

the

measured

strut

loads

converted to

pressure distribution at

each stage

of excavation including

the

final

depth

covering

all

distributions.

It

is

also

called

apparent

pressure

distribution.

It

is

regarded

as

a

conservative

approach

because

strut

loads

calculated by such an envelope are generally greater than the measured loads.

Rectang

ular envelope with

p = 0.2 γt H

is also recommended by Twine

and Roscoe (1996) based on more recent field measurements. Similarly use

of submerged unit weight below water table and addition of water pressure is

recommended. Data on cohesive soils are classified for soft to medium stiff

clays and stiff clay.

Anchor or nail supported walls may show higher lateral displacements,

and

stress

increases

at

the

upper

levels

of

walls

may

be

somewhat

less

compared

to

the

distributions

on

strutted

walls.

However,

there

are

no

documented comparisons. In the solution of anchored walls by finite element,

boundary

element,

finite

difference

softwares

or

simpler

spring

models

the

analyses may be repeated without assigning pre-tensions initially like in case

of

nail

supported

walls

and

then

assign

the

calculated

reactions

as

pre-tensions.

There are also recommendations on selection of the type of distribution

in

relation

to

height

of

braced

walls.

Distributions

based

on

pressure

cell

records

are

recommended

for

all

heights

but

distributions

by

strut

load

measurements are not found suitable for walls higher than 15 m, they may be

used for walls of 10

–

15 m height depending on conditions of the ground and

construction and recommended for heights less than 10 m.

Another common case is an alluvial profile where clay, silt, sand layers

mixed in different proportions lie in different thicknesses. If a dominant layer

is

present

one

of

the

above

distributions

may

be

selected,

otherwise

a

theoretical

approach

like

Coulomb’s

earth

pressure

expression

may

be

followed

making

use

of

effective

parameters,

submerged

unit

weights

and

added water pressure.

Effect of different surcharge loads on walls may be calculated by stress

distributions in elastic medium (e.g. NA

VFAC 1982). For the upper limit of

very rigid walls the distributions are doubled. Wide surcharge loads may also

be converted to equivalent heights of soil layer.

3. SUPPORTING ELEMENTS

3.1 Ground Anchors

3.1.1 Introduction

Ground

anchor

is

a

common

type

of

supporting

element

used

in

the

design and construction of in-situ retaining walls. It is an installation that is

capable

of

transmitting

an

applied

tensile

load

to

a

load

bearing

stratum

which may be a soil or rock. A summary about ground anchors will be given

in this section. Types, capacity, design, construction and quality control will

be reviewed.

3.1.2 Types and Capacity of Anchors

Temporary anchor and permanent anchor are the main types and as the

names imply the former is used in temporary works and usually a period of

maximum

two

years

are

assigned

as

the

design

life.

Design

life

of

a

permanent

anchor

is

the

same

as

the

life

of

structure.

Corrosion

protection

details and factors of safety are the main differences between the two types.