土木工程专业外文翻译

DESIGN AND EXECUTION OF GROUND INVESTIGATION FOR

EARTHWORKS

ABSTRACT

The design and execution of ground investigation works for earthwork projects

has

become

increasingly

important

as

the

availability

of

suitable

disposal

areas

becomes

limited

and

costs

of

importing

engineering

fill

increase.

An

outline

of

ground

investigation

methods

which

can

augment

‘traditional

investigation

methods’

particularly

for

glacial

till

/

boulder

clay

soils

is

presented.

The

issue

of

‘geotechnical

certification’

is

raised

an

d

recommendations

outlined

on

its

merits

for

incorporation

with

ground

investigations and earthworks.

1. INTRODUCTION

The

investigation

and

re-use

evaluation

of

many

Irish

boulder

clay

soils

presents

difficulties

for

both

the

geotechnical

engineer

and

the

road

design

engineer. These glacial till or boulder clay soils are mainly of low plasticity and

have

particle

sizes

ranging

from

clay

to

boulders.

Most

of

our

boulder

clay

soils

contain

varying

proportions

of

sand,

gravel,

cobbles

and

boulders

in

a

clay

or

silt

matrix.

The

amount

of

fines

governs

their

behaviour

and

the

silt

content makes it very weather susceptible.

Moisture

contents

can

be

highly

variable

ranging

from as

low

as

7% for the

hard

grey

black

Dublin

boulder

clay

up

to

20-25%

for

Midland,

South-West

and

North-West

light

grey

boulder

clay

deposits.

The

ability

of

boulder

clay

soils to take-in free water is well established and poor planning of earthworks

often amplifies this.

The

fine

soil

constituents

are

generally

sensitive

to

small

increases

in

moisture

content

which

often

lead

to

loss

in

strength

and

render

the

soils

unsuitable

for

re-use

as

engineering

fill.

Many

of

our

boulder

clay

soils

(especially those with intermediate type silts and fine sand matrix) have been

rejected at the selection stage, but good planning shows that they can in fact

fulfil specification requirements in terms of compaction and strength.

The

selection

process

should

aim

to

maximise

the

use

of

locally

available

soils

and

with

careful evaluation

it

is possible

to

use

o

r

incorporate

‘poor

or

marginal

soils’

within

fill

areas

and

embankments.

Fill

material

needs

to

be

placed at a moisture content such that it is neither too wet to be stable and

trafficable or too dry to be properly compacted.

High moisture content / low strength boulder clay soils can be suitable for use

as fill in low height embankments (i.e. 2 to 2.5m) but not suitable for trafficking

by

earthwork

plant

without

using

a

geotextile

separator

and

granular

fill

capping

layer.

Hence,

it

is

vital

that

the

earthworks

contractor

fully

understands the handling properties of the soils, as for many projects this is

effectively governed by the trafficability of earthmoving equipment.

2. TRADITIONAL GROUND INVESTIGATION METHODS

For road projects, a principal aim of the ground investigation is to classify the

suitability

of

the

soils

in

accordance

with

Table

6.1

from

Series

600

of

the

NRA

Specification

for

Road

Works

(SRW),

March

2000.

The

majority

of

current

ground

investigations

for

road

works

includes

a

combination

of

the

following to give the required geotechnical data:

?

Trial pits

?

Cable percussion boreholes

?

Dynamic probing

?

Rotary core drilling

?

In-situ testing (SPT, variable head permeability tests, geophysical etc.)

?

Laboratory testing

The importance of ‘

phasing

’ the

fieldwork operations cannot be overstressed,

particularly

when

assessing

soil

suitability

from

deep

cut

areas.

Cable

percussion

boreholes

are

normally

sunk

to

a

desired

depth

or

‘refusal’

with

disturbed and undisturbed samples recovered at 1.00m intervals or change of

strata.

In

many

instances,

cable

percussion

boring

is

unable

to

penetrate

through

very stiff, hard boulder clay soils due to cobble, boulder obstructions. Sample

disturbance

in

boreholes

should

be

prevented

and

loss

of

fines

is

common,

invariably this leads to inaccurate classification.

Trial

pits

are

considered

more

appropriate

for

recovering

appropriate

size

samples

and

for

observing

the

proportion

of

clasts

to

matrix

and

sizes

of

cobbles, boulders. Detailed and accurate field descriptions are therefore vital

for

cut

areas

and

trial

pits

provide

an

opportunity

to

examine

the

soils

on

a

larger

scale

than

boreholes.

Trial

pits

also

provide

an

insight

on

trench

stability and to observe water ingress and its effects.

A suitably experienced geotechnical engineer or engineering geologist should

supervise the trial pitting works and recovery of samples. The characteristics

of

the

soils

during

trial

pit

excavation

should

be

closely

observed

as

this

provides information on soil sensitivity, especially if water from granular zones

migrates into the fine matrix material. Very often, the condition of soil on the

sides

of

an

excavation

provides

a

more

accurate

assessment

of

its

in-situ

condition.

3. ENGINEERING PERFORMANCE TESTING OF SOILS

Laboratory testing is very much dictated by the proposed end-use for the soils.

The engineering parameters set out in Table 6.1 pf the NRA SRW include a

combination of the following:

?

Moisture content

?

Particle size grading

?

Plastic Limit

?

CBR

?

Compaction (relating to optimum MC)

?

Remoulded undrained shear strength

A number of key factors should be borne in mind when scheduling laboratory

testing:

?

Compaction

/

CBR

/

MCV

tests

are

carried

out

on

<

20mm

size

material.

?

Moisture

content

values

should

relate

to

<

20mm

size

material

to

provide a valid comparison.

?

Pore pressures are not taken into account during compaction and may

vary considerably between laboratory and field.

?

Preparation

methods

for

soil

testing

must

be

clearly

stipulated

and

agreed with the designated laboratory.

Great care must be taken when determining moisture content of boulder clay

soils. Ideally, the moisture content should be related to the particle size and

have a corresponding grading analysis for direct comparison, although this is

not always practical.

In

the

majority

of

cases,

the

MCV

when

used

with

compaction

data

is

considered

to

offer

the

best

method

of

establishing

(and

checking)

the

suitability characteristics of a boulder clay soil. MCV testing during trial pitting

is

strongly

recommended

as

it

provides

a

rapid

assessment

of

the

soil

suitability directly after excavation. MCV calibration can then be carried out in

the

laboratory

at

various

moisture

content

increments.

Sample

disturbance

can

occur

during

transportation

to

the

laboratory

and

this

can

have

a

significant impact on the resultant MCV’s.

IGSL

has

found

large

discrepancies

when

performing

MCV’s

in

the

field

on

low plasticity boulder clays with those carried out later in the laboratory (2 to 7

days).

Many

of

the

aforementioned

low

plasticity

boulder

clay

soils

exhibit

time dependant behaviour

with significantly different MCV’s recorded at a later

date

–

increased values can be due to the drainage of the material following

sampling,

transportation

and

storage

while

dilatancy

and

migration

of

water

from granular lenses can lead to deterioration and lower values.

This

type

of

information

is

important

to

both

the

designer

and

earthworks

contractor

as

it

provides

an

opportunity

to

understand

the

properties

of

the

soils

when

tested as

outlined

above.

It

can

also

illustrate

the

advantages of

pre-draining

in

some

instances.

With

mixed

soils,

face

excavation

may

be

necessary to accelerate drainage works.

CBR testing of boulder clay soils also needs careful consideration, mainly with

the preparation method employed. Design engineers need to be aware of this,

as it can have an order of magnitude difference in results. Static compaction

of boulder clay soils is advised as compaction with the 2.5 or 4.5kg rammer

often leads to high excess pore pressures being generated

–

hence very low

CBR

values

can

result.

Also,

curing

of

compacted

boulder

clay

samples

is

important as this allows excess pore water pressures to dissipate.

4. ENGINEERING CLASSIFICATION OF SOILS

In

accordance

with

the

NRA

SRW,

general

cohesive

fill

is

categorised

in

Table 6.1 as follows:

?

2A Wet cohesive