River Sand Quality During The Rainy Season: What Construction Buyers Need to Know

Understanding the Risk: Why Rainy Season Sand Requires Extra Caution

When ordering river sand for construction in Kenya, timing matters more than most builders realize. During the country’s rainy seasons—particularly the long rains from March to July and the short rains from October to December—river sand quality can be significantly compromised. Understanding this risk is essential for anyone involved in construction, from homebuilders to large-scale developers.

Kenya’s Rainy Seasons: A Critical Context

Kenya experiences two distinct rainy periods:

Long Rains (March to July): The more intense of the two seasons, with April typically being the wettest month. Rainfall during this period can range from 50-200mm per month, though some areas receive over 300mm monthly.

Short Rains (October to December): Generally lighter but still significant, these rains can be unpredictable in timing and intensity.

During these periods, rivers that remain dry or low for much of the year transform into flowing waterways carrying substantial volumes of water—and contaminants.

The Contamination Problem: What Gets Into River Sand

Silt and Clay Content

Research indicates that sand quality from Ukambani sources varies considerably by season, with dry season sand typically containing less silt while wet season extraction may include excessive clay content. This is not merely a quality preference—it’s a structural concern.

Silt refers to very fine soil particles (finer than sand) that can pass through a 75-micron sieve. Construction standards specify that sand should not contain more than 8% silt content, with many experts recommending levels between 3-5% for optimal concrete strength.

The Problem: During rainy seasons, runoff from agricultural lands, roads, and settlements carries fine silt and clay particles into rivers. These particles remain suspended in the water and settle onto the sand being extracted. Monsoon seasons can dramatically increase silt content in most sand sources.

The Impact: Silt weakens the bond between cement and sand particles. It creates a sort of film on sand grains that prevents proper adhesion, reduces concrete strength, increases drying shrinkage, and can retard cement setting. Concrete made with high-silt sand is more permeable, has higher erosion risk, and shows inferior binding properties—increasing the chances of cracks, splits, and structural failures.

Organic Matter Contamination

Perhaps even more concerning is organic matter contamination during rainy periods. Heavy rainfall washes debris, decayed vegetation, humus, leaves, roots, and other organic materials from surrounding lands into river systems.

The Impact: Organic impurities in sand affect concrete in multiple ways. They reduce strength, delay setting time, cause surface stains, and increase the risk of cracking. These materials can also have corrosive effects on reinforcement steel within concrete structures.

Research shows that organic matter can be detected through colorimetric testing—when sand is mixed with a 3% sodium hydroxide solution, excessive organic content causes the liquid to turn yellow or brown rather than remaining a light straw color.

Additional Contaminants

Rainy season runoff can introduce:

• Salts: Particularly problematic in coastal areas, salts can affect concrete durability
• Coal particles: These can corrode reinforcement
• Mica: Excessive amounts (over 2-3%) affect concrete performance
• Industrial and agricultural waste: Depending on the river’s watershed

Why Rainy Season Sand Harvesting Persists

Despite quality concerns, sand extraction continues—and often intensifies—during rainy seasons in Kenya. The reasons are primarily economic:

1. Increased Accessibility: Seasonal rivers that are dry most of the year become navigable, allowing harvesters access to new deposits
2. Higher Water Flow: Moving water makes extraction easier as sand is already in suspension or loosened from riverbeds
3. Economic Pressure: Kenya’s construction boom demands approximately 50 million metric tonnes of sand annually worth roughly $600 million, creating constant pressure for supply
4. Regulatory Gaps: While regulations exist, enforcement of seasonal extraction bans remains inconsistent

The Kenyan Sand Industry Context

River sand extraction in Kenya is primarily sourced from:

• Machakos County: Particularly from the Athi River watershed, with extraction concentrated in Kathiani, Matungulu, and Yatta sub-counties
• Makueni, Kitui, and Kajiado counties: Semi-arid regions with seasonal rivers
• Coastal areas: Though marine sand requires washing due to salt content

The National Environment Management Authority (NEMA) and the Mining Act of 2016 regulate sand extraction, requiring Environmental Impact Assessments for commercial operations. However, illegal sand harvesting remains widespread, with unlicensed operations avoiding quality standards and environmental protections.

Many traditional sources now operate under seasonal limitations, with dry season extraction meant to minimize riverbed disturbance and wet season bans allowing ecosystem recovery—though enforcement varies.

Testing Sand Quality: Practical On-Site Methods

The Kenya Bureau of Standards (KEBS) has developed standards for construction materials, including natural aggregates (KS 95). For construction projects, testing should be conducted for every 20 cubic meters of sand delivered, or at least once per 200 cubic meters for large projects.

Silt Content Test (Decantation Test)

This simple field test can be done on any construction site:

Procedure:

1. Fill a graduated glass cylinder (100ml capacity) halfway with sand
2. Pour in clean water until the cylinder is three-quarters full
3. Add one teaspoon of salt per pint of water to speed settlement
4. Shake vigorously and let settle for about one hour

Interpretation:

• Clean sand settles immediately
• Clay or silt will settle slowly on top of the sand layer
• The thickness of the silt layer should not exceed one-seventh (approximately 14%) of the sand layer below
• For construction purposes, silt content should not exceed 8%, with 3-5% being ideal

Hand Rubbing Test

A quick preliminary assessment:

1. Rub a sample of sand between damp hands
2. Clean materials will leave hands slightly stained
3. If hands stay dirty after throwing away the sand, it indicates excessive silt or clay content

Organic Matter Test

Procedure:

1. Place sand sample in a plain glass bottle
2. Add equal volume of 3% caustic soda solution (sodium hydroxide)
3. Shake well and let stand for 24 hours

Interpretation:

• Liquid should not be darker than light straw (pale yellow) color
• Marked yellow or brown color indicates excessive organic matter
• Such sand should be washed before use or rejected

Protecting Your Construction Investment

During Rainy Seasons:

1. Source Verification Verify your sand supplier’s extraction license and confirm they operate during permitted seasons. Ask for documentation showing when and where the sand was harvested.

2. Always Test Before Acceptance Never skip testing during rainy seasons. Test every delivery, as quality can vary significantly batch to batch when extracted during wet periods.

3. Consider Alternatives

• Rock Sand (Artificial River Sand): Manufactured by crushing ballast and washing through water pools, rock sand offers consistent quality year-round at approximately Ksh 3,000 per tonne
• Stockpiled Dry Season Sand: Some suppliers maintain properly stored dry season inventories—verify storage conditions to ensure no contamination

4. Specify Washed Sand If you must use river sand during rainy seasons, specify that it must be thoroughly washed. While this adds cost, it can reduce silt content from dangerous levels (above 8%) to acceptable ranges.

5. Proper Storage Matters Even quality sand can be compromised by poor storage. Sand should be:

• Stored on concrete pads or plastic sheeting (not directly on muddy ground)
• Covered to prevent rain from mixing surface dirt into piles
• Protected from dusty conditions
• Used first-in, first-out to prevent long-term contamination accumulation

Year-Round Best Practices:

1. Regular Testing Schedule

• Visual inspection of every delivery
• Silt content testing for every 20 cubic meters
• Organic matter testing weekly during wet seasons
• Comprehensive lab analysis (sieve analysis, specific gravity) periodically

2. Supplier Relationships Build relationships with licensed suppliers who prioritize quality. Reputable suppliers understand seasonal quality variations and will work with you to provide the best available materials.

3. Documentation Maintain records of:

• Sand source and extraction date
• Test results for each batch
• Supplier licenses and permits
• Delivery dates and quantities

This documentation is valuable for both quality control and potential legal requirements from the National Construction Authority.

The Broader Environmental Picture

The concerns around rainy season sand extraction extend beyond quality to environmental sustainability. Excessive sand harvesting in Kenya has led to:

• Riverbed degradation and reduced water retention capacity
• Drying of aquifers that communities depend on for water
• Bank erosion and increased flood risk
• Disruption of aquatic ecosystems
• Loss of sand dam effectiveness in water-scarce regions

In counties like Makueni, Kitui, and Kajiado, communities have constructed over 1,000 sand dams to capture and store water during rainy seasons. These structures depend on sand accumulation to create natural aquifers. Excessive commercial extraction undermines these water security initiatives.

Regulatory Landscape and Standards

Kenya has established regulatory frameworks, though implementation challenges persist:

Key Regulatory Bodies:

• Kenya Bureau of Standards (KEBS): Sets standards for construction materials including natural aggregates
• National Environment Management Authority (NEMA): Oversees environmental protection and requires EIAs for commercial mining
• National Construction Authority (NCA): Enforces construction compliance

Relevant Standards:

• KS 95: Natural Aggregates for Concrete
• KS EAS 18-1: Cement standards
• Various testing standards aligned with international protocols

Construction professionals should familiarize themselves with these standards and verify that suppliers comply.

The Path Forward

The message is clear: caution is essential when ordering river sand during Kenya’s rainy seasons. The combination of increased silt, clay, and organic matter contamination poses genuine structural risks that no builder should ignore.

Key Recommendations:

1. Plan ahead: Order and stockpile quality sand during dry seasons when possible
2. Test rigorously: Never skip quality testing, especially during or immediately after rains
3. Know your sources: Work with licensed, reputable suppliers who understand quality standards
4. Consider alternatives: Rock sand and other manufactured options offer consistent quality
5. Document everything: Maintain comprehensive records for quality control and compliance
6. Stay informed: Monitor weather patterns and adjust procurement schedules accordingly

Conclusion

River sand remains a crucial construction material in Kenya, but its quality is not consistent throughout the year. The rainy seasons—March through May and October through December—introduce significant contamination risks that can compromise structural integrity if ignored.

By understanding these risks, implementing rigorous testing protocols, and working with responsible suppliers, construction professionals can protect their projects from the hidden dangers of contaminated sand. The extra time and cost of proper quality control during rainy periods is minimal compared to the potential costs of structural failure, repairs, or building collapse.

In Kenya’s rapidly developing construction sector, quality cannot be compromised. Your building’s foundation—literally and figuratively—depends on it.

About Quality Standards: For detailed information on construction material standards in Kenya, contact the Kenya Bureau of Standards (KEBS) at their toll-free number 1545 or visit their website. For reporting substandard materials, use the “Wajibika Na KEBS” program by texting the S-Mark code to 20023.

Sources: This article draws on research from the Kenya Bureau of Standards, National Environment Management Authority, construction industry studies, and peer-reviewed research on sand quality and seasonal variations in East Africa.