Comparison of OSA With Concrete Options

8/10/2019 Comparison of OSA With Concrete Options

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www.hesselberg-hydro.com

Information for Chelmsford FAS

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Comparison of alternatives :

Specification

Grasscrete GC2 Dycel 155 Open Stone Asphalt

Thickness (mm) 150 150 150 200

Unit Weight

(kg/m2)

~220? 230 300 400

Geotextile filter As per literature As per literature Propex 6081 or

similar

Propex 6081 or

similar

Max Flow Rate

(m/s)

8 m/s 8 m/s 9.8 m/s* 9.8 m/s*

Surface voids 40%? - uniform 30% - uniform 20-25% - Irregular 20-25% - Irregular

*Average during overtopping tests

Installation

Grasscrete GC2 Dycel 150 Open Stone AsphaltPreparation Earthworks to correct line

& level

Earthworks to correct line

& level

Earthworks to correct line &

level (by others)

Underlayer Very regular profile (sand

blinding advised to ensure

formers sit correctly)

Very regular profile (sand

blinding advised to ensure

blocks sit correctly)

Regular profile (in-situ

material follows minor

irregularities)

Geotextile Yes Yes Yes

Erosion

Protection

Rigid layer Individual blocks Bound flexible layer

Edge Details As per literature Beams 7 anchors Thickening at edge

Expansion

Joints

Every 10m x 10m Every 10m x 10m Not required

Topsoil Placed in holes plus layer

over surface

Placed in holes plus layer

over surface

Layer over surface

Installation :

• The installation time for an Open Stone Asphalt (OSA) revetment is usually significantly less than

for a concrete revetment. (approx. 2 weeks for 3,300m2, actually 1 week for 2,200m

2 at Forres)

• The installation is a simple process with easily-managed quality control steps.

• No sand blinding is required to form a perfectly regular formation. OSA will follow minor

undulations in the formation.

• No expansion joints required.

• No separate edge details are required. Against structures and along the edges of the constructionthe OSA may be thickened by ~100mm.




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Technical Information :

An OSA layer of 150mm thick results in 300 kg/m2

plus topsoil. This is approx. 30% heavier than

the Dycel 150 layer which is approximately 230 kg/m2 plus topsoil.

OSA is permeable and homogeneous with a void content of 20-25%. Used as a protection layer it

acts as one large bound plate which relies on its permeability and monolithic nature to withstand the

hydraulic loads associated with overtopping spillways.

The in-situ OSA can follow all contours and changes in gradient with ease.

Asphalt is a bound, flexible material and so can withstand settlements expected with new

earthworks without any detrimental effects. Settlement below a concrete block revetment may

cause the blocks to move leading to protruding edges and gaps, or voids may be ‘bridged’ beneath a

rigid grasscrete layer

If damage were to occur, the damage is limited in a bound material. With concrete blocks theremoval of one block can lead to rapid progressive failure of a large area.

A gravel blinding is often specified with block systems as the gravel makes a significant

contribution to the friction between the blocks, on which resistance to pull-out depends. This

unbound gravel is vulnerable in very high water flows.

Brochures and examples of the use of OSA are included.

Design Flow Velocities :

In April 2008 Rijkswaterstaat executed tests with a dike overtopping simulator on an Open Stone Asphalt

revetment. Each test, with increasing water volume/flow rate was carried out for 6 continuous hours. The

highest flow rate achievable with the apparatus was 1.22 m3 /s/m, resulting in a maximum water velocity of

9.8 m/s and this test was also carried out for 6 hours. Open Stone Asphalt showed no sign of damage after

the tests.

In the 1980’s the stability of OSA in high water velocities has been tested at the Delft Hydraulics

Laboratory, Holland, and also in a test canal. The maximum velocities tested were 6 m/s and 7m/s

respectively. In the test canal the OSA was placed on geotextile over a subsoil of fine sand (D50 = 110µm),

without any damage to the asphalt or migration of the subsoil.

OSA mattresses are used for scour protection in ports and so the material has to withstand very high

currents generated by ships’ propellers. The maximum water velocities generated are of the order of 10m/s, but the velocity over the mattresses is considered to be 6-7 m/s.

As with concrete blocks, OSA has not been tested much beyond ~7/8 m/s because the test flumes are not

capable of achieving higher flows. Flows of 10 m/s may have been achieved in the field, and OSA has

performed very well in areas of high water velocities.




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Durability

• OSA is a very robust material against impact and abrasive loads.

• Asphalt is a flexible material and so can withstand and follow settlements expected with newearthworks

• In areas prone to vandalism, OSA has been used successfully.

• Plants growing through the asphalt will not damage it as the material can withstand deformations

over time (thermoplastic properties).

• The oldest OSA revetments (coastal) are in excess of 40 years old and are still performing well.

Aesthetics

• The most common treatment for the OSA in spillway applications is the addition of a layer oftopsoil over its surface plus application of seed if required. The thickness of topsoil will depend on

the types of plants/grasses desired, location (water), etc.. A thin layer (~50mm) of soil will allow

the roots to penetrate through the OSA which will help retain the vegetation during overtopping

events. In some cases a full 150mm layer of soil may be used to provide a totally ‘soft’ appearance

with the erosion protection layer beneath.

• Once the grasses grow over the revetment, the roots will penetrate through the open structure of thematerial and will then be able to survive longer through dry periods. The OSA forms a complete

layer over the embankment, helping to retain moisture, whereas some concrete blocks have holes

through the whole block which allows the underlayer to dry out more quickly.

• In the short term the grasses/plants will be dictated by the types of seed applied, but in the long

term, the area will 'naturalise' with wind blown seeds of local plants. Surveys in Holland and

Norfolk have shown that in the long term a wider variety of plants are found over OSA revetments

than over concrete block revetments. It is thought that the regular voids in the blocks suit certain

plants, which over time become dominant. The voids on the OSA surface are smaller and irregular.




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Environmental

• The aesthetics are discussed above.

• OSA is compatible with the environment – it is used in drinking water reservoirs, SSSI’s, etc.Asphalt is manufactured with bitumen refined from petroleum which is inert and will not harm the

environment. Tests investigating the leachability of PAHs, heavy metals and other chemicals from

bitumen show that concentrations in the test water was well within the surface water limits for EEC

countries, and were also more than an order of magnitude lower than the current EEC limits for

potable water.

• OSA can be produced at practically any asphalt mixing plant, so the material procurement willbenefit the local community.

• At the end of its design life, OSA can be recycled as a fill material. After a research project with

the Dutch authorities into manufacturing new asphaltic materials using reclaimed OSA, it is now

accepted as a sustainable material by the Dutch authorities.

.

Quality Control

• A typical specification for the material is enclosed.

• Prior to work on site the Open Stone Asphalt is designed using the raw materials available at themost convenient mixing plant to supply the project. This mix design is carried out in our laboratory

and is submitted for approval prior to works commencing on site.

• During the project our engineer supervises the mixing at the plant and the following checks are

made- mixing temperature

- placing temperature

- placing thickness

- sample analysis to check constituents (daily)

- raw materials checked (weekly/monthly)



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Comparison of OSA With Concrete Options