technical note

R&D TECHNICAL NOTE

SFGC CHARACTERISTICS NOVEMBER 2017

ICM

ICM TECH NOTE: SFGC CHARATERISTICS AUTHORS: ICM • • • •

Angus Jackson Simon Restall Bobbie Corbett Sam King

GSR •

Simon Restall

REVISIONS:

nil

Table of Contents 1.

INTRODUCTION ......................................................................................................................... 2

2.

AUSTRALIAN PERSPECTIVE - 2017 .............................................................................................. 2

3.

SFGC CHARACTERISTICS ............................................................................................................. 5

4.

SUMMARY ................................................................................................................................. 6

ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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ICM TECH NOTE: SFGC CHARATERISTICS

1. INTRODUCTION Filter fabrics were initially used and developed to provide a robust filter layer behind coastal structures such as rock walls. In this location, the geotextile needs the following characteristics: • •

High permeability while retaining fines High tensile strength and deformability (puncture resistance) to cope with initial rock placement and longer-term settlement and rock movements

These high strength filter fabrics, later named geotextiles and geosynthetics in the 1970s, were adapted with improved UV resistance for the fabrication of forms (sandbags and tubes) that could be easily hydraulically filled (due to their permeability) with sand and are now wellaccepted construction modules for construction of coastal engineering structures such as seawalls, groynes and breakwaters. The available products have evolved considerably from the hessian and similar natural textile “sandbags” that have a very long history, and are still used, as a successful emergency protection measure against coastal erosion and flooding as they can be filled quickly and inexpensively by unskilled labour with local sand sources. As an example, standard military issue hessian sand bags weighing only about 20- 30kg, when filled with dry sand, were very effective, when well stacked and interlocked, as temporary erosion scarp armouring during the severe storm surge and erosion events on the Gold Coast of Australia in 1967 (Tomlinson et al 2007). For SFGC, the geotextile effectively acts as the armour layer and the selected material must be puncture resistant. The geotextile materials used for fabrication are either: • • •

Woven Non-woven Composite

The type of material used is important as it affects the durability, tensile strength and friction between modules of SFGC.

2. AUSTRALIAN PERSPECTIVE - 2017 Australia was an early adopter of the use of synthetic geotextiles (geosynthetics) for filter layers and fabricated into sand filled geotextile containers (tubes and bags) for coastal engineering works. There is now over 30 years of experience with geotextiles in the coastal zone and with the design, modelling, construction and maintenance of coastal structures constructed using sand filled geotextile containers (SFGC). There were 4 milestone prototype projects In Australia between 1985 and 2001 (Figure 1 and Table 1) that developed the core design, modelling and construction methods for small and large SGC structures that are still used in Australia (and elsewhere, particularly the UAE and South Pacific where Australian design and construction expertise have been used).

ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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ICM TECH NOTE: SFGC CHARATERISTICS

Vandal resistant composite geotextile

1967

Narrowneck Reef Maroochy Groynes

1985

1996

2000

Narrowneck Reef top up

2.5m3 / 4.5t bags

Stockton Beach wall

0.75m3 /1.5t bags

Nth Kirra Groyne

Emergency sandbagging along GC beaches

STATE OF THE TECHNOLOGY

20-30kg sandbags

Mega- containers

Narrowneck Reef review Noosa Trial groynes Maroochy Groynes review

Tubes

2017

TIMELINE Figure 1

The implementation and monitoring of these milestone projects has provided considerable real-world data to provide ongoing improvements in materials, design, construction and repair (Table 1).

ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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ICM TECH NOTE: SFGC CHARATERISTICS Table 1 Project Date Location Description Wave conditions Construction methods

North Kirra Groyne 1985 Gold Coast, Australia 5m high x 120m long groyne Exposed ocean coast. Up to 14m Hmax. Offshore. Depth limited ~5m at head. Bundled 1.2m dia x 120m long tubes (Non-woven with impermeable liner) filled in-situ in the dry with dredges (3 wks)

Innovative features

First large SFGC coastal structure in Australia. Largest in world at time. Proof of concept for SFGC structures in exposed ocean conditions in Australia. $350,000aud (1985) Approx. 60% cost of rock. User friendly - safe for swimmers. Physical modelling showed need to have tubes tightly filled or sand would migrate causing failure of loose material & displacement. Damage to head occurred after vandal damage and leakage. Practical repair methods were developed. Groynne was covered after about 5 years (design life). Proof of concept re viability of temporary large scale SFGC coastal structures. Jackson 1985, Jackson 1987

Cost Comments

References Project Date Location Description

Stockton Beach seawall 1996 Stockton, Australia

Project Date Location Description Construction methods Wave conditions Innovative features

Narrowneck submerged reef breakwater 1999/2000 Gold Coast, Australia Artifical reef to provide coastal protection and improved surfing. First pre-fabricated mega-containers filled in hopper dredge and dropped accurately into place Up to 14m Hmax. First pre-fabricated mega-containers filled in hopper dredge and dropped accurately into place development of composite “vandal resistant” geotextiles $2.5M aud (2000) approx. 50% cost of rock Jackson et al 2013

48m long x 4.5m high seawall constructed of 0.75m3 SFGC Wave conditions Exposed ocean coast Construction methods bags filled with excavator in filling frame, sewn closed and stacked in place Innovative features First use of 0.75m SFGC "bags" for seawalls in Australia. Cost ~$40,000 aud (1996) Comments Constructed as quick and inexpensive temporary emergency structure - still in good condition after 11 years. Provided prototype for emergency seawalls. References Restall et al 2002

Cost Comments References Project Date Location Description Wave conditions Construction methods Innovative features Cost Comments References

Maroochydore groynes (4) 2001-03 Sunshine Coast, Australia Groynes approx 100m long constructed of 2,5m3 Hmax = 10m offshore. Depth limited to ~3m at heads. bags filled with excavator in filling frame, lashed closed and stacked in place First use of 2.5m3 containers filled in filling frame and placed. Development of failure modes and stability curves (Jackson et al 2006) $210,000 aud (2001) User friendly - safe for swimmers. Restall et al 2002

ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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ICM TECH NOTE: SFGC CHARATERISTICS Present conditions: • •

•

•

Kirra groyne provided the temporary protection required for 5 years and is now buried under large scale beach nourishment. Stockton Beach seawall is exposed during storms and continues to provide protection with ad hoc maintenance. Was originally designed for a 6 month life expectancy and was fabricated from durable, but standard (entry level) geotextile that has survived for 21 years, and has been extended. Narrowneck reef continues to provide its primary function of coastal protection but has a lower crest height due to long term settlement and loss of crest megs-containers due to anchor damage. A top up to restore the crest height and surfing amenity is underway. Maroochy groynes continue to provide its primary function of coastal protection, but the groynes are due for maintenance to replace damaged and displaced bags. No decision has been made yet on whether to use rock or SFGC but there is solid campaign “Don’t Rock Maroochy” for retention of the user-friendly bags.

This need for maintenance of Narrowneck and Maroochy groynes led an in-depth review of the present state of the art of SFGC characteristics and ongoing maintenance requirements to evaluate the options of repairing these structures using present day SFGC or replacing with rock. SFGC have been found to have very different characteristics to “conventional” hard construction materials such as rock and concrete. An important element of the reviews was understanding the different characteristics of rock and SFGC.

3. SFGC CHARACTERISTICS From the long-term monitoring and observation of SFGC structures, the key characteristics of SFGC compared to rock are as per Table 2:

ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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ICM TECH NOTE: SFGC CHARATERISTICS Table 2 Charateristics

SGGC c/f rock

Modular

Wide range of uniform sizes 40kg -300t provides for higher stability than rock and construction ranging from hand to large equipment to suit the site.. Cost per unit volume generally decreases with size wheras rock costs increase with size such that unit rates using large modules can be much lower rock Able to accommodate overtopping so better than rock for low crested overtopped structures. Fill ratio and type of fill affects shape and stability due to interlocking friction and flexibility. Boulder supply now limited to about typically 3-6t grading requiring flatter slopes for larger wave conditions or overtopping.

Stackable

Individual SFGC modules can be stacked at steeper slope than rock while maintaining stability through friction and interlocking. .

Flexible / deformable Able to accommodate settlement. Rock structures can also accomodate settlement.

Interlocking

Flexibility and friction of the geotextiles provides higher level of interlocking and stability than rock.

Permeable

Modules absorb some energy internally but sand fill will migrate if not filled tightlly. Poorly filled SFGC may fail due to fatigue of loose material or loss of weght and reduced stability.

Filled onsite using local sand.

Only empty forms need to be imported to site reducing transport impacts. Rock often needs to be transported hundreds of km by road causing traffic hazards and damage to roads.

“Soft”

More user friendly for recreational use and good substrate for marine growth but less durable than rock.

4. SUMMARY SFGC have very different characteristics to “conventional” hard construction materials such as rock and concrete. They also need different design and construction methods and equipment. SFGC have advantages and disadvantages when compared to “conventional” hard construction materials such as rock and concrete. Drivers to the use of SFGC have included: • • • • • •

Increasing shortage of good quality rock and increasing rock costs making sand filled structures lower capital cost. Sand filled geotextile structures can be constructed in developed areas with restricted access without the need to transport rock or concrete units to the site. SFGC provide soft, user friendly structures in high use areas SFGC can be used for low crested overtopped structures. SFGC provide good emergency and temporary structures that can be easily removed or modified, if required. Provide a good media for energy absorption often resulting in accredtion near the structure rather than erosion often observed with rock or concrete structures.

There have been significant advances in material properties and fabrication methods. Continued analysis of extracted geosynthetic materials from existing structures indicates excellent relative durability, likely better than originally predicted, however SFGC ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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ICM TECH NOTE: SFGC CHARATERISTICS structures are still not as durable as well constructed and maintained conventional rock structures. As a result, SFGC are most suited to locations where safety and user friendliness are important, and maintenance can be carried out as required. SFGC also have a strong relevance in remote locations such as island communities where the availability of suitable armour is non-existent or prohibitively expensive. There are a number of successful projects now completed.

ICM: Angus Jackson, Bobbie Corbett & Sam King GSR: Simon Restall

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