1. Geocells for Affordable Low Volume Pavements
Oliver Whalley –transport analyst – World bank

2. Summary Rural Access and Poverty Geocell technology Engineering theory
Design
Construction
Materials
Performance
Maintenance
Case study
Conclusions

3. Rural Access and Poverty
4.6 billion people live in rural areas
1 billion have no access to an all-weather road
Low rural accessibility linked to
Poverty
Maternal mortality
Gender inequality
Low traffic volumes
Difficult to economically justify paving of roads
Despite rapid urbanisation, 4.6 billion people still live in rural areas
One billion of these don’t have access to an all weather road (refer to photo)
Work by World Bank on rural accessibility index has revealed that low rural accessibility is strongly linked to poverty, maternal mortality and gender inequality (example of each)
While the need is clear, it is often difficult to justify paving rural road on economic grounds. This is due to the high capital and maintenance costs of a sealed road, in addition to the low traffic which makes for less reduction in user costs

4. Geocell technology Traditionally used for soil stabilisation
Developed for concrete fill
In situ cast interlocking concrete blocks
Flexible HDPE formwork
75 to 150mm thick
150 to 300mm square
Geocells have traditionally been used with aggregate or soil fills for soil or pavement subgrade stabilisation
Sensing some promise, Visser and Hall conducted research on thin (0.2mm) HDPE cells filled with high slump concrete
Once poured into the formwork a flexible, interlocking concrete block pavement is formed
Load transfer occurs between adjacent blocks to distribute the traffic loads over a wider area, protect subgrade and provide impermeable surface

5. Engineering Theory Theory established by Visser and Hall (1999 & 2003)
Mechanistic design
Critical: Vertical subgrade compressive strain
Stiffness of geocells depends on subgrade stiffness
𝐴𝑙𝑙𝑜𝑤𝑎𝑏𝑙𝑒 𝑣𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑠𝑡𝑟𝑎𝑖𝑛 𝜇𝜖 = −𝑙𝑜𝑔10 # 𝐸𝑆𝐴 𝑟𝑒𝑝𝑒𝑡𝑖𝑡𝑖𝑜𝑛𝑠 (1)
Strength from lock-up (especially large deflections)
Impermeable layer prevents subgrade saturation
Like any pavement, the purpose of geocell surfacing is is to protect weaker underlying layers
Visser and Hall developed their theory with empirical testing and measurement using the mechanistic approach as a basis
This means that the vertical compressive strain is the critical consideration
Equation 1 can be used to predict the allowable strain as a function of the number of standard axle repetitions
Their research found that most of the geocell layer strength came from lockup of cells, especially at large deflections (thicker layers)
Cells form an impermeable layer which helps prevent subgrade saturation

6. Design C) Use A & B to find thickness
A) Calculate pavement loading (ESAs) then compressive strain (µƐ) (Eqn. 1)
B) Determine in-situ subgrade stiffness (MPa)
CBR from Dynamic Cone Penetrometer
Stiffness from Shell relationship: CBR x Z
C) Use A & B to find thickness
Example
A) ESAs = 1960 therefore Eqn. 1 gives
VCS = 6000 µƐ
B) CBR = 7 Stiffness = 7 x 7 ≈ 50MPa
C) Thickness to use = 75mm C
It’s as easy as A, B C!
Step A: calculate the pavement loading (including different traffic types ) using traditional approaches over the life of the pavement. From this the allowable compressive strain can be calculated using equation 1.
Step B: calculate the stiffness of the support. A simple way to do this is using CBR from a DCP, convert to stiffness with the Shell relationship.
Step C: With these two inputs, the design chart can guide practitioners to the correct thickness to use
Work through example (if needing a conservative design, could use 100mm) A B

7. Construction Methodology
1) Subgrade preparation 2) Geocell installation ) Concrete placing & curing
Pouring of concrete
Curing
Like design, the construction is quite straightforward and well suited to labour based methods.
Firstly, the preparation of the subgrade is required, including filling, shaping, excavation and formwork for edge beams and finally compaction (plate compactors or rollers)
Next, the geocell formwork can be installed. Anchor one end and expand the plastic form to the width and length of the road. Ensure anchored at regular intervals to prevent floatation. Tuck edges into edge beam.
Finally, high slump concrete can be placed, spread and broom finished. Curing requires no traffic till at least 15MPa strength is reached.

8. Material & Equipment Requirements
Materials
Geocells
Concrete
Water, aggregate, cement
No basecourse/subbase required
Equipment
Concrete mixers
Compaction equipment
Hand tools
Labour based
Geocell form required. Limitation in that there appears to be only a single supplier of this thin flexible formwork
Concrete requires 30MPa and mm slump (admixtures may be required)
Less use of aggregate as can do without basecourse and subbase layers
Compaction equipment can be hand plate compactors or rollers
Small concrete mixers (200L) to large agitator trucks
Hand tools for spreading, and finishing, installing wooden edge beam forms
Well suited to labour based approaches which addresses poverty reduction goals

9. Performance Best suited to light traffic low volume roads
Subgrade less protected
Thicker cells/basecourse for heavy loads
Integrated drainage for steep terrain
Long term study of performance required
Geocell pavements are best suited to light traffic and low volume roads.
As pavement trafficked, cells flex and cracks form. Also shrinkage cracking. Still impermeable.
For example, a 100mm thick geocell pavement over CBR 10 subgrade will form a 20mm rut after 2000 passes by a typical truck, or 124 million passes for a small vehicle
That said, thicker geocells (up to 250mm thick) have been used for port facilities with good results
Subgrade is less protected than with a normal flexible pavement, although the concrete surface provides a good impermeable barrier
Speaking of water, longitudinal drainage armouring can be integrated into a road cross section which makes for a robust approach in hilly terrain
Widely used in Africa, and now being considered as an alternative in the Pacific where traffic volumes are very light

10. Maintenance Limited edge break Straightforward
Pothole repairs
Service crossings
Upcycle at end of useful life
Limited edge break with edge beams
Pothole repairs – resolve drainage issue, remove cells, replace subgrade, compact and replace cells
Service crossings – remove strip of cells, install service and replace geocells
Many potential uses of blocks at end of pavement life (domestic, fill etc)

11. Case Study - Kiribati
Very poor, low lying Pacific Island nation of Kiribati.
USD60M World Bank, Australia and ADB funded road rehabilitation project
60,000 residents in a very small area, so not strictly rural roads
Very narrow and low traffic volumes. Limited working room.

12. Case Study - Kiribati
Hampered by poor accessibility, large potholes, dust and ponding
Geocells used for feeder roads. US$140k/km, 72% of chipseal, 52% of asphalt
Especially well suited due to limited supply of aggregate

13. Conclusions Promising solution for low volume roads Lower costs
Not suited to heavy traffic
Labour intensive construction
Easier to justify providing access
Potentially transformational impact
Design conservatism is a barrier
Geocells are a promising solution for low volume roads, with a significantly lower capital and maintenance cost due to no granular pavement layers being required
They are not suited to heavy traffic, but are ideal for enhancing rural access, or tight urban roads like we’ve seen in Kiribati
Labour intensive construction, low equipment requirements are ideally suited to development projects
Lower costs make providing all weather access more easy to justify, transformational impact!
Design conservatism is a barrier. More research required on long term performance, but Kiribati success is not alone in the low volume context.

14. Questions