Institution of Civil Engineers

Geocomposite Drainage in Highway Engineering and Earthworks

Mike Stephen Retired/Consultant, formerly Technical Manager, ABG Ltd.

What is a geocomposite drain?

• Essentially, a plastic drainage core with geotextiles (usually non-woven) bonded on one or both sides

• The function of the core is to provide a flow path

• The geotextile acts as a filter, may include a pipe sleeve

• A number of different core types exist (nets, ribbed sheets, cuspated sheets) – all perform differently

• A range of thicknesses, strengths and flow capacities are available

What is a geocomposite drain?

Single textile, single cuspated core Double textile, double cuspated core

2D geonet 3D geonet Fibre core Ribbed core Cuspated core

What is a geocomposite drain?

Double textile, double cuspated core with pipe sleeve (Type 6 Fin Drain)

Why use geocomposite drainage?

• It has been around for a long time (25+ years in MCDHW)

• It works (or it would not still be in MCDHW)

• It can save time and money

• It helps conserve aggregate resources

• It can greatly reduce vehicle movements during construction

• Compared to aggregates it is light and easily moved

• It is more efficient at drainage than aggregates

So why NOT use it more often?

More efficient?

Drainage aggregate Drainage geocomposite

Endoscope view, with water flow and under load

Why not use geocomposite drainage?

• Only common specification is MCDHW Clause 514 - Fin Drains

• Little guidance or specifications for other applications

• Not trusted, not well understood?

• Past bad experiences?

• May not offer savings where aggregate is locally available and abundant

This talk will attempt to outline other possible applications, give guidance on specification and explain

differences between types available.

Well known applications

MCDHW Volume 3: Standard Highway Details

Well known applications

Type 6 (A1 Dishforth) Type 10 (A556 Knutsford - Bowden)

Vertical and steeply sloping applications 1

Drainage behind abutments and retaining walls:

• Easy to put in place and fix

• No issues of chemical compatibility with concrete

• Range of strengths available to suit different wall heights or high pressures from integral bridges

• Range of flow capacities available

• Range of textiles available to ensure filtration against various soils and fill materials

Vertical and steeply sloping applications 1

Back of abutment or retaining wall drainage

Vertical and steeply sloping applications 1

Behind retaining wall Behind bridge abutment (M6 – Heysham link)

Vertical and steeply sloping applications 2

Drainage behind reinforced soil walls and slopes:

• Often required for pressure relief, not quantity of water, so discrete strips may be enough

• Easier to construct than aggregate drains on steep faces

Drainage behind tunnel linings, especially in rock

Drainage behind facings to bored pile and diaphragm walls

Runoff from footpaths, cycleways and verges

Vertical and steeply sloping applications 2

Surface water/verge/ cut-off drainage

Geocomposite drainage strips behind reinforced slope repair

Vertical and steeply sloping applications 3

Drainage within soil slopes:

• Replace aggregate in counterfort and stabilisation drains or ‘herring-bone’ drains – narrower trenches possible

• Applicable to new works or remediation of slips

Drainage under embankments:

• May be used to aid consolidation of a relatively thin, weak layer instead of gravel-filled trenches or removal and replacement

Vertical and steeply sloping applications 3

Slope drainage Drainage of weak soil layers

Moderately sloping applications (in which the geocomposite is laid on a sloping surface,

rather than in a trench)

Interception of seepages from cutting slopes:

• Can be placed beneath topsoil with no additional excavation required

• Can be a continuous blanket, or individual strips only where required

• Replace gravel blankets or ‘herring-bone’ drains

Behind face of slopes subject to occasional inundation:

• To prevent slumping after rapid drawdown

Horizontal applications 1

• To replace starter layers or drainage blankets beneath embankments and development platforms

• Beneath pavement foundation layers in wet cuttings

• Drainage under reinforced soil

• Relief of uplift pressures

As these tend to involve large areas; it is worth investigating:

• Range of strengths and flow capacities available

• Range of textiles available to ensure filtration against various soils and fill materials

Horizontal applications 2

• Use of geocomposite drains as ‘blankets’ to promote drainage and accelerate consolidation of marginal fills – Ashton Moss, UK*

• Geocomposite drains can also be used to enhance stability of slopes in embankments, slip repairs and landscaping bunds:

Flow capacity required is usually small

Drainage may be more effective than reinforcement, especially in high-plasticity clays

Alternative to treatment of wet fills with lime

*Robinson, N, Bamforth, A and Erak, G: 2016: Advanced engineered solutions using geocomposite drainage for fast consolidation in compacted fill. Session 8, Proc EuroGeo 6, Ljubljana.

Horizontal applications: Starter layer

Drainage/starter layer replacement under embankment

Horizontal applications: Starter layer

The Highways Agency (as it was) has taken a positive approach to replacing drainage/starter layers with geocomposites

Starter layers - things to consider

• What determines the required thickness of a starter layer? (Drainage capacity, strength, precedent?)

• An open-graded aggregate such as Class 6C will usually need filtration/separation textiles above and below. If a geocomposite is used, the core replaces the aggregate and the whole thing is placed in one operation rather than 3!

• How will a starter layer be imported, placed and compacted over soft ground? Geocomposites are light and easily moved by comparison!

• If additional strength is needed, geogrids can be used instead of a granular layer and are comparatively cheap

• A geocomposite-soil interface is usually stronger than the soil itself, so does not introduce a weakness

Horizontal applications: Upward seepage

Drainage layer under pavement with upward seepage from bedrock

Horizontal applications – Pressure relief Artesian groundwater pressure at depth

DIAPHRAGM WALL DIAPHRAGM WALL Alluvial clay

Terrace Gravel

A331 BLACKWATER VALLEY ROUTE IN 6m DEEP

RETAINED CUTTING 600 m LONG x 30 m WIDE

PROP SLAB

Gravel drainage layer (200 mm thick)

Sand filter layer (300 mm thick)

London Clay

London Clay Basal Beds

Lambeth Group

The drawing shows the original design. The Contractor proposed a geocomposite drain to replace the gravel drainage layer. This was accepted. Saving in excavation was 600 x 30 x 0.19 m = 3 420 m3 (for off-site disposal). Saving of imported gravel was 600 x 29 x 0.2 = 3 480 m3. Apart from the costs, this also saved over 600 round-trip lorry movements.

Drainage layer beneath underpass prop slab

Transportation aspects - Example

Replacement of 300 mm starter layer with geocomposite:

• 7 mm double cuspate, 220 m2/roll

• 27 rolls per load = 5 940 m2 (limited by bulk)

• Volume of 5 940 m2 starter layer 300 mm thick = 1 780 m3

• At 1.6 t/m3 and 22 t/load = 2 850 t = 130 loads – replaced by 1 load of geocomposite

• Replaces 1 780 m3 of aggregate with 1 740 m3 of general fill

• Potential to save 1 740 m3 of disposal if general fill was surplus to requirements

Carbon footprint

• Carbon footprint of 1 780 m3 of aggregate quarried 10 km from site = 17.8 tonnes (material), 3.8 tonnes (transport)

• Carbon footprint of 6 000 kg of geocomposite supplied over 100 km = 15.2 tonnes (material), 0.1 tonne (transport)

• Saving on one load of geocomposite = 6.3 tonnes CO2e

(Source: EA Carbon Calculator version 3.1.2)

Life cycle analysis Comparison of 500 mm aggregate drainage layer (including separators) with geocomposite drainage (EAGM study*):

*Stucki M, Büsser S, Itten R, Frischknecht R. and Wallbaum H: 2011: Comparative Life Cycle Assessment of Geosynthetics versus Conventional Construction Materials. ESU-services Ltd. Uster, ETH Zürich, Switzerland.

Life cycle analysis: Conclusions

• The use of a geosynthetic drainage composite leads to lower environmental impacts for all indicators investigated, except land competition

• The geosynthetic drainage composite results in savings of approximately 73 tonnes CO2 - equivalent for an area of 10 000 m2

Value engineering?

Approximate costs for the replacement of a 300 mm thick starter layer – will vary with location and availability of aggregates:

• Aggregate 300 mm thick = 0.5 tonne/m2 = £10/m2

• Geotextile separators above & below = £0.30/m2 each

• Total materials cost = £10.60/m2 delivered.

• Drainage geocomposite of similar flow capacity, permeable both sides, double textile = £5 to £6/m2

• Total materials cost = £ 5.60/m2, say, delivered.

• Materials cost saving is approximately £5/m2

This is materials cost only, placement costs not considered

Specification of geocomposite drainage 1

• Must have a clear idea of what it is required to do!

• Clause 514 requirements a starting point

• Drainage is all about FLOW – what capacity would an aggregate layer realistically have when placed on site? (NOT Hazen permeability based on lab D10 of a perfect sample! Consider placement, fines intrusion, clogging)

• For applications in or against soils, must judge in-plane flow capacity by soft platen performance at pressure in service

• Through-plane capacity of most non-woven geotextiles is high enough not to be a constraint – 50 l/m2.s is ample

Specification of geocomposite drainage 2

• Pore size (O90) needs to meet filtration criteria, e.g. Heibaum, 2014*. Most critical for uniform fine grained materials such as PFA

• Tensile strength is not an issue – this is NOT reinforcement! (Minimum of 10-20 kN/m should be adequate)

• Static (CBR) penetration resistance is a good indicator of general robustness

• Dynamic (cone drop) penetration resistance is an indicator of resistance to damage by sharp objects – a low value is good! Replaces trapezoidal tear (a deprecated test)

*Heibaum, M: 2014: Rethinking geotextile filter design. Proc 10th ICG, Berlin.

Specification of geocomposite drainage 3

• Creep performance – should be able to demonstrate no collapse and <25% thickness/capacity reduction at 115 yrs at the required pressure in service.

• UV stability – generally sufficient if covered within 14 days but best to lay and cover same day (wind, contamination)

• Long term durability is generally accepted as satisfactory according to EN 13252 for polyethylene and polypropylene

• Look for CE marking and BBA/HAPAS certification as evidence of proven quality

Core type: 2D geonet 3D geonet Fibre Corrugated/

ribbed Cuspated

Property:

MD flow

(soft platens) * *** * *** ***

CMD flow * * * * ***

Permeable

(one side)

*** (with

membrane)

*** (with

membrane)

*** (with

membrane)

Depends – core may

be permeable or not *** (single)

Permeable

(both sides) *** *** *** ***

* (single)

** (perforated)

*** (double)

Resistance to

pressure ** *** * ** or *** ***

* = poor ** = fair *** = good

Which type of geocomposite drain?

Conclusion: Why not consider drainage geocomposites?

(shamelessly copied from a presentation by Ian Fraser, TCS)

Institution of Civil Engineers

Geocomposite Drainage in Highway Engineering and Earthworks

Thank you for listening!