Backward erosion piping (BEP) is an internal erosion mechanism that can occur in the granular foundations of dams and dikes consisting of uniform silts and sands covered by a cohesive layer . When a BEP incident occurs, timely emergency measures are critical—if not addressed promptly, it may lead to dike collapse .
Laying filtration materials at the water outlet of BEP is the most effective emergency response method, preventing continuous erosion of sand particles. Geotextiles offer advantages over traditional sand and gravel filters due to their low cost, energy efficiency, easy construction, and quick installation .
The Research Context
A study published in Geosynthetics International investigated the filtration characteristics of woven geotextiles for BEP prevention under the extreme soil-retention state . Laboratory experiments were conducted using woven geotextiles with different pore sizes to analyze extreme soil-retention characteristics and explore the optimal relationship between filter pore size and particle size of protected soil.
Key Findings
1. Optimal O95/d85 Ratios Identified
The results indicated that, for the medium sand and silty sand tested, the filter still met soil retention requirements when the ratio of the geotextile's equivalent pore size (O95) to the protected soil's characteristic particle size (d85) was 5.2 and 4.3, respectively .
When woven geotextiles are used for preventing BEP, to ensure optimal filtration performance of the filter, the recommended values for O95/d85 are :
Medium sand: 5.2
Silty sand: 3.3
2. Optimal Matching Relationship
With the increase of filter pore size, the clogging degree of the filter showed a trend of rapid decrease followed by an increase, indicating that there is an optimal matching relationship between the filter aperture and the particle size of protected soil . This is a critical finding—both too small and too large pores can compromise performance, but for different reasons.
3. Impact of Fine Particle Content
The study also examined the impact of fine particle content of the protected soil on the filter's filtration performance . Soils with higher fines content require different filter specifications than clean sands.
Additional Research on Clogging Mechanisms
A complementary study published in the Journal of Industrial Textiles examined both nonwoven and woven geotextiles for BEP emergency rescue . Key findings included:
Nonwoven Performance: Nonwoven geotextile filters with an equivalent pore size of 0.103 mm had a gradient ratio value of less than 3, and achieved the highest flow rate of 260 mL/min
Thickness Effects: Increasing the thickness and pore size of nonwoven geotextiles within a certain range helped enhance their anti-clogging ability
Clogging Mechanism: The main mechanism of clogging in nonwoven geotextiles was the deposition of fine sand particles on their surface, forming a layer of low permeability soil
Woven Limitations: Plain woven geotextiles experienced severe clogging with a 42% reduction in flow rate, and were deemed not suitable for rescuing BEP. The clogging mechanism involved the blocking of horizontal water passages by fine sand particles
Practical Implications for Emergency Response
For engineers and emergency responders, these findings provide actionable guidance:
1. Geotextile Selection Matters
Not all geotextiles are suitable for BEP rescue. The research suggests woven geotextiles may be problematic, while appropriately specified nonwovens can perform well .
2. Pore Size Must Match Soil
The O95/d85 ratio is a critical design parameter. For medium sands, ratios around 5.2 are recommended; for silty sands, more conservative ratios around 3.3 are appropriate .
3. Thickness Provides Benefits
Thicker nonwoven geotextiles offer enhanced anti-clogging ability, making them preferable for applications where clogging is a concern .
4. Testing Under Site Conditions
While these guidelines provide a starting point, site-specific soil testing is recommended for critical applications.
Conclusion
Geotextiles offer significant potential for rapid, effective response to dike piping emergencies—but only when properly selected for site-specific soil conditions. The new research providing recommended O95/d85 ratios for different soil types enables more informed selection and better emergency outcomes .
At HZ Geotextile, we offer a range of nonwoven geotextiles suitable for filtration applications, with documented pore size distributions and hydraulic performance data. Contact our team for assistance selecting the right filter fabric for your flood protection infrastructure.
