lecture earthworks

Lecture

Earthworks

1. Types of earthworks

2. Excavation & compaction materials

3. Design of cuttings & embankments

4. Specification of embankment fill materials

5. Example: Vajont Dam disaster

Types of earthworks

1. Cuttings

Road and railway cuts

Foundation excavations

2. Embankments

Constructed of engineered fill: road and rail

embankments & rock and earth fill dams

Non-engineered fills: loose tipped waste

dumps; e.g. landfill

When these structures fail?

Failing road cuttings; debris/rock falls

Failing road cuttings within road foundation material

CARSINGTON DAM an earth embankment in Derbyshire. This failed in 1984 during construction long before the reservoir was even filled.

(1) A slip surface developed through both the boot-shaped weak clay core and

a layer of periglacial head left on the shale bedrock beneath the placed and

compacted fill.

(2) The head was wrongly interpreted as in situ weathered shale, and the

design assumed an undisturbed angle of friction of = 20. Due to its origin the material contained shear surfaces with a residual angle of friction r = 12. This mistake and the rebuild cost 20 million at 1985 prices and the subsequent

litigation led to the company responsible being taken over.

(3) Incident could have been avoided: Periglacial head with shear surfaces at

residual strength is widespread on the shale of Derbyshire, therefore decent

applied geoscience investigations would have identified this as a potential

problem

(1) At 11:57 A.M. on June 1, 1976, in the Teton Canyon of Fremont

County, the collapse of an earth dam sent a wall of water toward

Idaho Falls.

(2) The subsequent flood killed 14 people and caused at least $1

billion in property damage. Ripping though Wilford, Sugar City,

Salem, Hibbard, and Rexburg.

(3) It destroyed 13,000 head of livestock, 3,500 farm buildings, and

4,000 homes. The federal government paid more than $300 million

to settle more than 7,500 claims.

Concrete Dam: St Francis

Dam, California 1928;

500 people killed.

Court of Inquiry blamed a

palaeomegaslide under the east abutment was

undetected

Scarp

Excavation/cutting design Removal of material without hazard requires

characterisation of rocks and soils involved;

Stability of cut slopes is critical factor in cutting

design, affected by:

1. Strength, frequency & orientation of rock

discontinuities such as joints and fracture sets

2. Water table level (affects effective strength of slope

material)

3. Changes in subsurface stress regime in response to

removal of overburden act to reduce cutting stability

Excavation/cutting design Different materials more able to resist shearing forces

than others. Ideal slope material has:

High shear strength, few discontinuities & low pore

water pressures (support steeper slope)

Results in a range of slope angles:

1. Massive igneous rocks and some metamorphic rocks

can support near vertical cuttings

2. Vertical slopes in horizontally bedded limestones

common (80-90)

3. Weak & fissile shales support slopes 45-70

4. SOILS: 10-40 (text book, Bell)

Cuttings: groundwater Short-term and long-term cutting stability

Pore

pressure, P

Factor of

Safety, F

1. Excavation causes

pore pressures to drop

considerably

- Response to fall in total

stress

- Materials expand

- FoS lowers sharply

2. Pore pressures

redistribute in

response to

overburden removal

3. Groundwater returns

to steady state

seepage flow

- Long term reduction in

FoS

1. 2.

3.

Excavation Considerations

1. Short & long term

slope stability

2. Reactivation of relict

structures

3. Construction methods

(next few slides)

4. Groundwater regime

during and after

construction

5. Heave (uplift) at base

of excavation

6. Risk of collapse

(tunnels)

(Pettifer and Fookes 1994)

NB point load index is a portable field test of

rock strength and is

approximately equal to

UCS/20

Which Excavation Method?

Rock strength increase

Fra

ctu

re s

pa

cin

g in

cre

ase

DIGGING

RIPPING

BLASTING

Point load machine

Soils and soft rocks: excavator and scraper

JCB.CO.UK

(Bell, 2007)

Digging method as a function of material

seismic velocity

Dig

gin

g m

eth

ods

Can dig through materials with low seismic velocities

JCB JS 360 Excavator

Soils and soft rocks: excavator and scraper

Cutting surface

in weak rocks

Soft to medium strong rocks: ripper plus scraper

Objective: break up the rock just enough

to allow its loading and transport

Rock rippability: depends on:

Intact strength, fracture index and

abrasiveness of rocks

Rippability a function of seismic velocity

(Bell 2007)

CAT D-9 Tractor with

ripper attachment

Rock cut blasting

Medium strong to strong rocks require blasting

Diggers remove the blast debris

Blast stem lines



To obtain a stable angle

in a rock face use

pre-split blasting

Delay time between blasts a function of burden, B

Subdrilling to depth B/3


How much dynamite to use?

Embankment Design Embankments built up of laying and compacting layers of soil

Engineering properties of embankment fill affected by amount

of compaction

1. Compaction; expulsion of air at ~constant MC increases water saturation

and dry density

Amount of compaction depends on the optimum design

performance of structure

Degree of compaction necessary dictates:

1. Compaction equipment used (rolls, tamps or vibrates)

2. Soil type (granular; natural MC, cohesive; optimum MC)

3. Quantity of material needed

4. Layer thickness geometry of proposed earthworks

Embankments: groundwater

Pore

pressure, P

Factor of

Safety, F

Short term and long term stability of embankments 1. Building embankment causes pore pressures

rise

- Response to increase in

total stress

- Materials contract

- FoS lowers as

overburden supported by

pore water

2. Pore pressures

redistribute until equal

with original regime

- FoS rise

3. Groundwater returns

to steady state

seepage flow

- FoS rises leading to

long-term stability

1. 2. 3.

Embankment Considerations 1. Interaction with existing

features (relict landslides,

cavities)

2. Influence of loading greater

than normal foundations

(piles, rafts)

3. Settlement and lateral

movements; phase project to

minimise

4. Locally sourced fill material

(project costs lower)

Engineered Fill Material Embankment built up of many 0.3 m lifts placed by scraper

(tight guidelines on construction procedure)

The material is then compacted after each lift

Proper placement and compaction ensures maximum

strength is obtained and settlement minimised

Embankment material is identified based on its compaction

qualities; related to: dry density/moisture content

relationship but also undrained shear strength,

consolidation characteristics

Embankments often contain layers of free-draining sandy

material to ensure settlement and pressure dissipation

occurs rapidly

Embankment design factor of safety: 1.5 (cover in prac.)

Laboratory Compaction Proctor test

Laboratory determination of

material compaction properties:

1. Build up three layers of sample

2. 25 blows per layer to compact

3. Measure dry density, MC and

air volume

4. Repeat

Laboratory Compaction Proctor test

Most desirable degree of compaction achieved at:

highest dry density which occurs at an optimum

moisture content and a high

shear strength

Too much compaction: 1. Samples crack/fissure

2. Moisture content rises

3. Reduces soil strength

In field: measure MC to

determine compaction

Increased

compaction

Field Compaction

(Bell, 2007)

Grid roller

The Vajont Dam disaster, Italy

1963

Deaths: 2043

Concrete dam completed in

1960

1963: 270M m3 of rock forming a

slab 200m thick moved 400 m at

20-30 m/s.

The sliding block landed in the reservoir creating a 100m high

flood wave that overtopped the

dam

Daves Landslide Blog: http://www.landslideblog.org/2008/12/vai

ont-vajont-landslide-of-1963.html

Worlds worst civil engineering disaster

Map of area (Waltham, 2009)

Sliding block

Back scarp Monte Toc

~1 km

Dam

~1 km

50 m Structure within slumped mass

Village of Casso

~1 km

Cross section through gorge before landslide Geology:

Downslope-dipping dolomitic limestones interbedded with thin plastic clay horizons 1963 main failure occurred along a reactivated clay horizon; ancient slip surface;

also minor 1960 rock slide

Sharp rise in groundwater level when reservoir filled

(Waltham, 2009)

Trigger Mechanisms 1. Minor landslide activated in Feb

1960 during first reservoir filling

event (impoundment)

2. October 1960: reservoir filled;

leading to high displacement rates

- Reservoir lowered; displacement

much reduced

3. November 1963: reservoir filled

and high rainfall lead to

catastrophic failure of rock slump

block

1.

2.

3. 4.

Trigger Mechanisms

4. Increasing the level of the

reservoir drove up pore water

pressures within the clay

interbeds, reducing shear

resistance (strength)

NB: importance of understanding

water within a landslide system!!

Key Outcome:

Understanding of ground

conditions imperative if

earthworks are to be

successfully constructed

Summary

Earthworks involves: design of stable slopes;

excavation of material; placement of fill

In the UK Earthworks construction is carried

out according to BS6031 and Eurocode 7

Minimise failure by understanding the effect of

earthworks on the site

Earth Dam during construction:

Earth Dam during construction: Dam

wall

lecture earthworks

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