Soil classification

Habari Mhandisi Mtarajiwa! Let's Talk Dirty: An Introduction to Soil Classification

Have you ever looked at the deep cracks in the ground during the dry season in places like Athi River and wondered why that happens? Or why the foundations for a tall building in Upper Hill, Nairobi, have to be dug so deep, while a small house in Malindi seems to sit right on the sand? The secret, my friend, is not just in the building, but in what the building stands on. The secret is in the soil!

Welcome to the fascinating world of soil classification. Think of yourself as a soil detective. Your job is to identify the soil, understand its character, and predict how it will behave. Getting it right means a safe, strong building. Getting it wrong? Well, that can lead to costly problems. So, let's grab our virtual shovels and dig in!

Why Do We Bother Classifying Soil?

Imagine you're a chef. You wouldn't use the same ingredients for ugali as you would for chapati, right? In the same way, engineers can't use the same design for a road on the black cotton soils of Mwea as they would for a road on the red, firm murram of Kiambu.

Soil classification is the language we use to describe soil properties consistently. It helps engineers and builders all over Kenya (and the world!) to:

• Predict soil behaviour: Will it swell when wet and shrink when dry? Will it be able to support a heavy load?
• Select the right foundation: Do we need a simple strip foundation or deep, expensive piles?
• Determine suitability for construction: Is this soil good for building a dam, a road base, or should it be removed from the site?
• Communicate effectively: An engineer in Kisumu can understand the soil report from a site in Mombasa without ever having seen the soil.

Real-World Scenario: The Tale of Two Foundations
An engineer, let's call her Akinyi, is designing foundations for two new homes. The first is in Karen, Nairobi, where the ground is the infamous "black cotton soil." The second is in Watamu, on sandy coastal soil. For the Karen house, she knows the soil will swell and shrink dramatically, so she designs a reinforced concrete slab on deep piles to anchor the house to stable ground. For the Watamu house, the sandy soil won't swell, but it has a lower bearing capacity, so she designs a wide, shallow "raft" foundation to spread the load. Same house, different soil, completely different engineering solution!

The First Step: Particle Size Distribution (Sieve Analysis)

The first thing a soil detective wants to know is the size of the particles. We separate soils into two main families:

• Coarse-Grained Soils: Particles you can see with your eyes. Think of gravel (like kokoto) and sand. These soils are often gritty.
• Fine-Grained Soils: Tiny particles you can't see individually. Think of silt and clay. These soils feel smooth or sticky.

In the lab, we use a stack of sieves with different mesh sizes to separate the particles. This is called a Sieve Analysis.

    --- ASCII Art: A Sieve Stack ---

      +-----------------+  <-- Coarse Sieve (e.g., for Gravel)
      | .   .   .   .   |
      +-----------------+
      | . . . . . . . . |  <-- Medium Sieve (e.g., for Sand)
      +-----------------+
      |.................|  <-- Fine Sieve (e.g., for Silt/Clay)
      +-----------------+
      |                 |  <-- Pan (Catches the finest particles)
      +-----------------+

Image Suggestion: A vibrant, well-lit photo of a soil mechanics laboratory. In the foreground, a lab technician in a dust coat is carefully pouring a sample of dry, reddish-brown Kenyan murram soil into the top of a brass sieve stack, which is clamped into a noisy, vibrating sieve shaker machine.

If more than 50% of the soil by weight is retained on the No. 200 sieve (0.075 mm opening), it's a coarse-grained soil (Gravel or Sand). If more than 50% passes through, it's a fine-grained soil (Silt or Clay).

The Fickle Fines: Atterberg Limits and Plasticity

Fine-grained soils are moody. Their behaviour changes dramatically depending on how much water they contain. This property of being mouldable or "plastic" when wet is what we need to measure. We use the Atterberg Limits for this.

• Liquid Limit (LL): The water content at which the soil starts to behave like a liquid. Imagine a thick porridge.
• Plastic Limit (PL): The water content at which the soil becomes too dry to be moulded and crumbles when you try to roll it into a thread.

The difference between these two gives us the most important value: the Plasticity Index (PI).

    --- Formula: Plasticity Index ---

    PI = LL - PL

A high PI means the soil is very plastic and will swell and shrink a lot (like black cotton soil). A low PI means it's not very sensitive to water changes (like silt).

Putting It All Together: The Unified Soil Classification System (USCS)

Now we combine our findings to give the soil an official two-letter name using the USCS. This is the ID card we talked about!

The First Letter (The "Family Name"):

• G - Gravel
• S - Sand
• C - Clay
• M - Silt (from the Swedish word 'mo' for silt)
• O - Organic (like the dark soils in a swamp)

The Second Letter (The "First Name"):

• W - Well-graded (a good mix of particle sizes, very stable - like good murram)
• P - Poorly-graded (most particles are the same size, less stable)
• H - High Plasticity (for fines, PI > 7 and on or above the A-line)
• L - Low Plasticity (for fines, PI < 4 or below the A-line)

To classify fine-grained soils, we plot their Liquid Limit and Plasticity Index on a special chart called the Plasticity Chart.

    --- Diagram: Simplified Plasticity Chart ---

       |
    60 +                 |               / U-Line (Upper Limit)
       |                 |              /
       |        CH       |             /
    P  |_________________|____________/ A-Line: PI = 0.73(LL-20)
    I  |                 .
       |        CL       .  (CL-ML)
       |................. 
     7 +-------/---------+------------
     4 +------/----------+------------
       |  ML or OL       |
     0 +----------------------------------> LL
                         20             50

So, a soil with LL = 65 and PI = 40 would plot in the CH zone. What does that mean? C for Clay, H for High Plasticity. This is our classic black cotton soil! An engineer sees "CH" on a report and immediately knows to be careful.

Image Suggestion: A close-up, dramatic shot of deeply cracked, dark grey earth under a hot Kenyan sun. The cracks are wide and form a polygonal pattern. In the background, a house with a visible crack running up its foundation wall, illustrating the destructive power of expansive clay (CH soil).

Let's Classify a Soil Sample!

Imagine you get this lab report for a soil sample from a site in Thika:

• Percentage passing No. 200 Sieve = 65% (So it's fine-grained!)
• Liquid Limit (LL) = 45%
• Plastic Limit (PL) = 25%

Step 1: Calculate the Plasticity Index (PI)

    PI = LL - PL
    PI = 45 - 25
    PI = 20

Step 2: Plot on the Plasticity Chart

We have the coordinates (LL=45, PI=20). Let's check the A-Line equation for that LL:

    A-Line PI = 0.73 * (LL - 20)
    A-Line PI = 0.73 * (45 - 20)
    A-Line PI = 0.73 * 25
    A-Line PI = 18.25

Our soil's PI (20) is above the A-Line's PI (18.25), and our LL (45) is less than 50. Looking at the chart, this puts us squarely in the CL zone.

Conclusion: The soil is a CL - Clay of Low Plasticity. This soil is still clay, but it's much more stable and easier to build on than the notorious CH black cotton soil. Hongera, you've just classified your first soil!

Tafakari (Reflection)

You see now that the ground beneath our feet is not just "dirt." It's a complex engineering material with a unique personality. By learning the language of soil classification, you are gaining a powerful tool to build a safer and more resilient Kenya.

Next time you pass a construction site, don't just see the building going up. Look at the earth being excavated. Is it red and gravelly like murram (maybe a GW or GC)? Is it dark and sticky like black cotton (CH)? Or is it sandy like at the coast (SP)? You are no longer just a student; you are a soil detective. The ground holds many secrets, and now you have the key to understanding them.