What is a Filter Pack?
A filter pack, also known as a gravel pack or filter sand, is a carefully graded layer of gravel placed in the annular space between the borehole wall and the well screen (or casing). Its primary purpose is to act as a physical filter that prevents formation sand, silt, and fine particles from entering the borehole while allowing water to flow freely from the aquifer into the well.
The filter pack is one of the most critical components of a borehole completion. A well-designed filter pack ensures that the borehole delivers clean water, protects the pump from abrasive wear, and maintains the structural integrity of the well over its lifetime. In Kenya's diverse geological conditions—from volcanic basalts to sedimentary sands—the proper design and installation of the filter pack is essential for long-term borehole performance.
The filter pack works on a simple principle: the gravel particles are sized so that they are large enough to allow water to pass through, but small enough to hold back the formation particles. This creates a stable, self-filtering zone around the well screen that prevents sand production and maintains water quality.
Why the Filter Pack is Critical
The filter pack is not an optional extra—it is an essential part of borehole construction. Here is why it matters:
- Prevents sand and silt intrusion – without a properly sized filter pack, fine particles from the formation will enter the borehole, causing turbidity, clogging the pump, and potentially damaging the screen.
- Protects the pump – sand and abrasive particles cause rapid wear on pump impellers, seals, and bearings, leading to premature pump failure.
- Maintains water quality – a well-designed filter pack prevents suspended solids from entering the water supply, reducing the need for extensive treatment.
- Extends screen life – the filter pack protects the well screen from direct contact with formation particles, reducing corrosion and physical damage.
- Enhances well efficiency – a properly sized filter pack ensures that water can flow freely into the borehole, minimising drawdown and maximising yield.
- Supports WRA compliance – proper filter pack design is part of the technical standards required by the Water Resources Authority (WRA) for borehole construction.
In short, the filter pack is the first line of defence against borehole failure. Getting it right is essential for the long-term reliability of your water supply.
Consequences of Poor Filter Pack Design
When the filter pack is not properly designed or installed, the consequences can be severe and costly:
- Sand intrusion – if the gravel is too coarse, formation sand will pass through and enter the borehole, causing turbidity and pump wear.
- Reduced yield – if the gravel is too fine, it can restrict water flow and reduce the borehole's yield.
- Screen clogging – if the filter pack is not properly graded, it can become clogged with fine particles, reducing water flow and increasing drawdown.
- Premature pump failure – abrasive particles in the water will damage the pump, leading to costly repairs or replacement.
- Inconsistent water quality – turbidity and sediment can make the water unsuitable for domestic or agricultural use without extensive treatment.
- Borehole abandonment – in severe cases, a poorly designed filter pack can lead to complete borehole failure, requiring expensive rehabilitation or new drilling.
The cost of correcting a poorly designed filter pack can be many times the cost of getting it right the first time. That is why professional design and installation are essential.
Sizing Gravel: The Sieve Analysis Method
The cornerstone of filter pack design is sieve analysis, a laboratory procedure that determines the grain size distribution of the formation material. Here is how it works and how the results are used.
The Process
- Collect formation samples – samples of the formation material are collected during drilling, typically from the water-bearing zones.
- Sieve analysis – the samples are passed through a series of sieves with decreasing mesh sizes (e.g., 2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.125 mm, 0.063 mm). The weight retained on each sieve is measured.
- Calculate the D50 – the D50 is the median grain size—the size at which 50% of the particles are finer and 50% are coarser. This is the key parameter for filter pack design.
The Rule of Thumb
The standard guideline for filter pack gravel sizing is that the D50 of the gravel should be 5–6 times larger than the D50 of the formation. This ratio provides optimal filtration while maintaining good permeability.
- If the formation D50 is 0.2 mm, the gravel D50 should be approximately 1.0–1.2 mm.
- If the formation D50 is 0.5 mm, the gravel D50 should be approximately 2.5–3.0 mm.
The gravel should also be well-graded (a mixture of particle sizes) rather than uniform. This ensures a stable pack that resists settling and maintains its filtration properties over time.
In addition to the D50 ratio, the gravel should be:
- Hard – resistant to crushing and abrasion.
- Well-rounded – angular particles can pack too tightly, reducing permeability.
- Free from clay and organic matter – these can clog the filter pack and reduce performance.
Technical Note: International standards such as the American Water Works Association (AWWA) guidelines provide detailed specifications for filter pack gravel. WaterLink Limited follows these standards on all projects.
Typical Gravel Sizes for Different Formations
The grain size of the formation determines the appropriate gravel size. The table below provides general guidance based on formation type:
| Formation Type | Typical D50 (mm) | Recommended Gravel Size (mm) |
|---|---|---|
| Fine sand | 0.10 – 0.20 | 0.50 – 1.20 |
| Medium sand | 0.20 – 0.50 | 1.20 – 3.00 |
| Coarse sand | 0.50 – 1.00 | 3.00 – 6.00 |
| Fine gravel | 1.00 – 2.00 | 6.00 – 12.00 |
| Medium gravel | 2.00 – 5.00 | 12.00 – 30.00 |
| Volcanic/basalt | Variable (fractured) | 4.00 – 12.00 (based on fracture openings) |
These are general guidelines. The actual gravel size for a specific borehole should always be determined by a sieve analysis of the formation material.
Filter Pack Installation Methods
Even the best-designed filter pack will fail if it is not installed correctly. The two primary installation methods are:
1. Tremie Method
The tremie method involves pouring the gravel through a pipe (tremie pipe) that is lowered to the bottom of the annulus. The pipe is slowly raised as the gravel is placed, ensuring that the gravel is deposited evenly without trapping air or creating voids. This method is widely used and provides good control over the placement.
2. Reverse Circulation Method
In the reverse circulation method, water is circulated down the annulus and up through the screen, while gravel is added at the surface. The water flow carries the gravel into place, ensuring a dense, well-compacted pack. This method is often used in larger-diameter boreholes.
Key Installation Best Practices
- Maintain a uniform thickness – the filter pack should be of consistent thickness around the screen (typically 75–150 mm).
- Avoid bridging – gravel can form bridges or voids if not placed carefully. The tremie method minimises this risk.
- Use clean gravel – gravel should be washed and free from dust or fines before placement.
- Gradual placement – gravel should be placed gradually to avoid displacing the screen or casing.
- Monitor placement – the amount of gravel placed should be measured and compared to the calculated volume of the annulus to ensure completeness.
After installation, the borehole is typically developed to remove any fines and ensure the filter pack is functioning correctly.
Quality Control and Testing
Quality control is essential to ensure the filter pack meets design specifications. Key steps include:
- Sieve analysis certification – the gravel supplier should provide a sieve analysis certificate confirming the grain size distribution.
- Visual inspection – the gravel should be inspected for cleanliness, uniformity, and absence of clay or organic matter.
- Placement monitoring – the volume of gravel placed should be tracked and compared to the calculated requirement.
- Post-development testing – after the borehole is developed, a pumping test should confirm that the water is free of sand and that the yield meets expectations.
- Camera inspection – a downhole camera can be used to visually inspect the filter pack and screen, confirming proper placement and integrity.
At WaterLink Limited, we conduct rigorous quality control on every project, ensuring that the filter pack meets our high standards and the client's expectations.
Common Mistakes and How to Avoid Them
Despite its importance, filter pack design and installation are often overlooked or poorly executed. Here are the most common mistakes and how to avoid them:
- Using the wrong gravel size – always conduct a sieve analysis of the formation material. Never rely on "standard" gravel sizes without testing.
- Using angular or soft gravel – use hard, well-rounded gravel. Angular particles can pack too tightly, reducing permeability.
- Poor placement technique – use the tremie method or reverse circulation to avoid bridging and ensure even placement.
- Insufficient gravel volume – measure the annulus volume and place the correct amount of gravel. Under-filling leaves voids that allow sand intrusion.
- Not developing the borehole after installation – development is essential to remove fines and ensure the filter pack is functioning correctly. Skip this step at your peril.
- Ignoring the upper seal – the filter pack should be sealed at the top with a bentonite seal or grout to prevent surface water contamination and gravel migration.
By following best practices and working with an experienced contractor, these mistakes can be easily avoided.
How WaterLink Limited Ensures Proper Filter Pack Design
At WaterLink Limited, we treat filter pack design with the seriousness it deserves. Our approach includes:
- Detailed hydrogeological surveys – we collect formation samples during drilling and conduct sieve analysis to determine the D50 and other grain size parameters.
- Customised gravel selection – we source gravel specifically for each project, ensuring it matches the formation characteristics and meets international standards.
- Expert installation – our experienced crew uses the tremie method to place the gravel evenly and avoid bridging or voids.
- Quality control testing – we test the gravel before placement and conduct post-development tests to confirm the filter pack is performing as designed.
- Comprehensive documentation – we provide clients with a detailed report on the filter pack design, installation, and test results, supporting WRA compliance.
- Warranty and aftercare – we stand behind our work and offer ongoing support to ensure your borehole continues to perform.
When you choose WaterLink Limited, you can be confident that your borehole's filter pack is designed and installed to the highest standards, ensuring clean, reliable water for decades.
