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Atterberg Limits Soil Testing-Liquid Limit, Plastic Limit, and Shrinkage Limit
Complete Guide to Soil Consistency Testing
Atterberg limits define critical moisture content thresholds where fine-grained soils transition between solid, plastic, and liquid states. These standardized tests—liquid limit, plastic limit, and shrinkage limit—form the foundation of soil classification systems worldwide and directly influence foundation design, road construction, and engineering decisions. Understanding these limits is essential for managing Kenya’s challenging soils, particularly expansive black cotton clays.
Atterberg limits represent critical water content thresholds that determine when fine-grained soils transition between solid, semi-solid, plastic, and liquid states. For anyone involved in construction—whether you're a civil engineering student, a practicing geotechnical engineer, or working on building projects across Kenya—understanding these limits isn't just academic knowledge. It's essential practical information that directly influences whether your foundation will stand firm or your road will crack within years.
Think about it. Every building in Nairobi, every highway cutting through the Rift Valley, every dam holding back water—they all rest on soil. Yet soil isn't static. It changes. Add water to clay, and it becomes moldable. Add more, and it flows like thick soup. Remove water, and it shrinks, cracks, hardens. These transformations happen at specific moisture contents, and knowing exactly where these boundaries lie can mean the difference between a structure that lasts generations and one that fails catastrophically.
What Are Atterberg Limits?
Atterberg limits define the moisture content boundaries where cohesive soils change their physical state and behavior. Named after Swedish chemist Albert Atterberg who first proposed them in 1911, these limits provide a standardized way to classify and understand how clay and silt soils will perform under various conditions.
Here’s what makes them uniquely valuable: unlike granular soils like sand and gravel, fine-grained soils containing clay minerals exhibit plasticity—the ability to be molded and deformed without crumbling. This property comes from water molecules adhering to clay particles. The amount of water present determines whether soil behaves as a brittle solid, a moldable putty, or a flowing liquid.
The four distinct soil states are:
- Solid State: Soil behaves like a rigid material. Volume doesn’t change even when more moisture is removed.
- Semi-Solid State: Soil has gained some water but remains brittle. It will crack or break rather than deform.
- Plastic State: The sweet spot. Soil can be molded, shaped, and deformed without cracking or changing volume.
- Liquid State: Soil contains so much water it flows under its own weight.
Why does this matter for construction? Because the engineering properties of soil—its strength, compressibility, permeability, and volume stability—change dramatically at each state. A foundation designed for soil in a semi-solid state will perform very differently if that soil becomes saturated and enters a plastic or liquid state during heavy rains.
How Are Atterberg Limits Related to Soil Classification?
Atterberg limits form the backbone of modern soil classification systems. The Unified Soil Classification System (USCS) and AASHTO classification system both rely heavily on these values. By plotting a soil’s liquid limit and plasticity index on a plasticity chart, engineers can quickly determine whether they’re dealing with high-plasticity clay, low-plasticity silt, or something in between.
This classification isn’t just for paperwork. It directly informs critical decisions:
- What type of foundation should be used?
- How will the soil behave during compaction?
- Will the soil swell when wet or shrink when dry?
- What bearing capacity can be safely assumed?
Historical Development: From Agriculture to Engineering
Albert Atterberg wasn’t thinking about skyscrapers when he developed these tests. As an agronomist and chemist working in early 20th century Sweden, he wanted to understand soil behavior for agricultural purposes. He recognized that soil plasticity was unique to materials containing clay, and he proposed that soils with particles smaller than 0.002mm should be classified as clays—a definition we still use today.
But Atterberg’s work remained relatively obscure until two pioneering engineers recognized its potential.
Karl Terzaghi, often called the father of soil mechanics, saw how Atterberg’s simple tests could predict complex engineering behavior. His assistant at MIT, Arthur Casagrande, took it further. In the early 1930s, Casagrande refined and standardized the testing procedures, developing the equipment and methodologies that became industry standards.
Casagrande’s most significant contribution was the liquid limit device—commonly called the Casagrande apparatus—which uses a mechanical cam system to drop a brass cup containing soil onto a hard rubber base. This standardization meant that an engineer in Nairobi would get the same result as one in New York or Mumbai, provided they followed the same procedure.
Today, these standardized methods appear in specifications worldwide: ASTM D4318, BS 1377-2, AASHTO T89 and T90. Whether you’re conducting tests at certified materials testing laboratories in Kenya or anywhere else globally, you’re using essentially the same procedures Casagrande established nearly a century ago.
The Three Primary Atterberg Limits
Liquid Limit (LL): The Upper Boundary
The liquid limit marks the moisture content where soil transitions from a plastic state to a liquid state. At this point, soil still has measurable shear strength—it hasn’t become pure liquid—but it flows readily under small applied forces.
How is it determined? Two standardized methods exist:
1. Casagrande Cup Method A soil paste is placed in a brass cup and a groove is cut down the center with a standardized tool. The cup is then dropped repeatedly from a height of 10mm onto a hard rubber base at a rate of 2 drops per second. The groove gradually closes due to the impact. The liquid limit is defined as the moisture content at which it takes exactly 25 blows for the groove to close over a distance of 12.7mm (half an inch).
Procedure:
- Take about 200g of material passing through 0.425mm sieve.
- Place sample on the glass plate and mix it thoroughly with distilled
water using spatula (soaking for 24 hrs is required for clayey soils). - Place a portion of the mixed sample in Casagrande apparatus and
level it parallel with the base of apparatus. - Cut a groove along the diameter through the center of the hinge.
- Turn the handle at a rate of 2 revolutions per second until the two
sides of the groove come into contact (along a length of 13mm)
and record the number of blows that closed the groove. - Take a small portion of the material for moisture content.
- Remove the soil from apparatus, add little water, mix thoroughly
and repeat step 3) to 6) at least four times. - Plot the moisture content on y-axis and number of blows on x-axis
on a semi-logarithmic chart and join the points by a straight line
(best-fit line). - Interpolate the moisture content corresponding with 25 blows
report it as the Liquid limit.
In practice, multiple tests are run at different moisture contents, and the results are plotted on semi-logarithmic paper. The moisture content corresponding to 25 blows is interpolated from this “flow curve.”
2. Fall Cone (Penetrometer) Method A standardized stainless steel cone of specific weight and angle is allowed to penetrate the soil under its own weight for 5 seconds. The liquid limit is the moisture content at which the cone penetrates exactly 20mm. This method is preferred in many countries including the UK (BS 1377-2) because it’s less dependent on operator technique.
Procedure:
- Sieve air-dried soil on 0.425mm sieve and take about 200g of the
material passing the sieve. - Place sample on a glass plate and mix it thoroughly with distilled
water using spatula (soaking for 24 hrs is required for clayey soils). - Place a portion of the mixed sample in the soil cup and level it off
with the edge of the cup. - Place the cup under cone and lower the cone until it just touches
the surface of the soil; then record the dial gauge reading. - Release the cone for 5 seconds and then lock it in position.
- Lower dial gauge to touch the cone and record the reading.
- Lift the cone up, remove some soil from the cup and add more soil
from the mixing glass. - Place the cup under the cone, penetrate it again (repeat this step
three times) and record the average of the three readings.
50
Linear Shrinkage Mould
A Digital Balance - Take a small portion of the material from the cup for moisture
content. - Lift the cone up, remove the soil, add little water, mix thoroughly
and repeat step 3) to 8) at least four times by adding little water in
the same sample. - Plot cone penetration on y-axis and moisture content on x-axis on
a linear scale and join the points by straight line. - Interpolate the moisture content corresponding with the
penetration of 20mm
and report it as the Liquid limit.
What does LL tell us? Soils with high liquid limits (above 50) contain significant clay minerals and will exhibit high compressibility. The liquid limit strongly correlates with:
- Compressibility and settlement potential
- Permeability characteristics
- Organic matter content (high organic soils have elevated LL values)
Plastic Limit (PL): The Lower Boundary
The plastic limit represents the moisture content where soil changes from a plastic to a semi-solid state. Below this water content, soil can no longer be molded without cracking and crumbling.
How is it determined?
The test is beautifully simple yet requires skill. A small ellipsoidal mass of moist soil is repeatedly rolled by hand (or using a plastic limit roller device) on a glass plate or non-porous surface. The operator rolls the soil into a thread approximately 3mm in diameter (about 1/8 inch).
The soil is at its plastic limit when the thread crumbles and breaks apart just as it reaches 3mm diameter. If it crumbles before reaching 3mm, the soil is too dry—add water and try again. If it can be rolled thinner than 3mm without crumbling, it’s too wet—continue rolling to evaporate more moisture.
Once the thread crumbles at the correct diameter, collect the pieces in a moisture content container and determine the water content. This value is the plastic limit.
Test Procedure:
- Sieve the air-dried soil on 0.425mm sieve to obtain about 300g,
and take about 20g of the material for the test (reserve the
remaining amount of sieved soil for liquid limit test). - Place the sample (20g) on a glass plate and mix it thoroughly with
distilled water using spatula (soaking for 24 hrs is required for
clayey soils). - Mould a ball between the fingers, and then roll it between the
palms of the hands until slight cracks appear on the surface. - Form a thread of about 6mm between the first finger and the
thumb. - Roll the thread between tips of the fingers and the glass plate until
it cracks (ensure the cracks appear when the thread is about 3mm) - Collect broken pieces of the thread into two containers, and
determine the moisture content. - Report the average moisture content (in whole number) as the
Plastic limit.
What does PL tell us? The plastic limit indicates the minimum water content for plastic behavior. It correlates with:
- Minimum moisture needed for compaction
- Toughness and cohesion of soil
- Clay mineral type and activity
Shrinkage Limit (SL): The Volume Change Boundary
The shrinkage limit is the moisture content below which further drying causes no additional volume reduction. Below this limit, air has entered the soil pores and additional moisture loss only replaces water with air—the soil skeleton doesn’t shrink further.
How is it determined?
A soil paste at its liquid limit is placed in a porcelain shrinkage dish (44.5mm diameter, 12.5mm high). The dish and soil are weighed, then oven-dried at 105°C. After drying, the volume of the dried soil pat is determined by mercury displacement or by coating with wax and measuring volume by water displacement.
The shrinkage limit is calculated from the initial and final weights and volumes.
Test Procedure:
- Clean the mould, measure the length (h1) and apply a thin film of
grease at the inside walls. - Take about 150g of the soil paste at Liquid limit, fill it fully in the
mould and tap it on a hard surface to remove air pockets. - Level the mould and remove surplus soil around it.
- Allow specimen to dry in air for 24 hours, and then dry it in oven.
- Allow specimen to cool and measure its length (h2).
Why is SL less commonly used? While theoretically important, the shrinkage limit test is more complex, requires mercury (a hazardous substance), and provides information that can often be inferred from the liquid and plastic limits. However, it remains crucial for evaluating soils with high shrink-swell potential—particularly relevant in Kenya where black cotton soils (expansive clays) are common in many regions.
What Is the Difference Between Liquid Limit and Plastic Limit?
The liquid limit and plastic limit mark opposite ends of the plastic range—that moisture content window where soil can be molded and worked with effectively.
Liquid limit is the upper boundary (higher moisture) where soil becomes too wet to maintain shape and begins to flow. Plastic limit is the lower boundary (lower moisture) where soil becomes too dry and brittle to mold without cracking.
The gap between them—the plasticity index—tells us how wide that workable window is. Large PI means a wide window; soil remains workable across a broad moisture range. Small PI means you have little margin for error.
Why Are Atterberg Limits Important in Construction?
Every construction project starts with a fundamental question: Will this soil support the loads we’re planning to place on it? Atterberg limits provide critical insights to answer this question.
Foundation Design: Soils with high plasticity indices undergo significant volume changes with moisture fluctuations. In Kenya’s climate, with distinct wet and dry seasons, this becomes critically important. A foundation designed without considering these limits might experience differential settlement, cracking, or even structural failure as soil expands during rains and shrinks during dry periods.
Road Construction: The Proctor compaction test determines optimum moisture for soil compaction, but Atterberg limits tell us about the soil’s fundamental plasticity characteristics. Subgrade soils with high plasticity can create problems with pavement performance, requiring special treatment or replacement.
Soil Classification: Before any engineering analysis, soil must be properly classified. Atterberg limits combined with grain size distribution allow engineers to classify soil according to USCS or AASHTO systems. This classification then connects to decades of empirical correlations and engineering experience.
Predicting Behavior: Soils with similar Atterberg limits tend to exhibit similar engineering behavior. If strength, compressibility, or permeability relationships are known for one soil, they can be reasonably estimated for another soil with similar limits. This makes preliminary design possible before extensive testing.
Construction Scheduling: Understanding the plastic range helps contractors plan work around weather. Soil near its plastic limit can be efficiently compacted and worked. Soil too wet (near liquid limit) or too dry (near plastic limit) creates problems. Knowing these boundaries helps schedule earthwork during appropriate conditions.
Derived Indices: Beyond the Basic Limits
While the three basic limits provide fundamental information, engineers calculate several derived indices that offer additional insights:
Plasticity Index (PI)
The plasticity index is perhaps the most important derived value. It’s calculated simply as:
PI = LL – PL
This represents the range of water content over which soil remains plastic and workable.
What PI tells us:
- PI < 7: Low plasticity (lean clay or silt)
- PI 7-17: Medium plasticity
- PI > 17: High plasticity (fat clay)
- PI = 0: Non-plastic (sand, gravel, or non-plastic silt)
High PI values indicate high clay content and predict several behaviors:
- Greater volume change potential (swelling and shrinkage)
- Higher compressibility
- Lower permeability
- Potential problems for construction unless properly managed
In Kenya, many areas have soils with high PI values, particularly the notorious black cotton soils found across parts of the Rift Valley and Central Kenya. These soils can have PI values exceeding 30, requiring special foundation treatments like deep foundations, soil replacement, or chemical stabilization.
Can Atterberg Limits Predict Soil Swelling?
Yes, with important limitations. The activity number specifically addresses swelling potential:
Activity = PI / (% clay fraction)
Where clay fraction is the percentage of particles smaller than 0.002mm.
- Activity < 0.75: Inactive clay (low swelling potential)
- Activity 0.75-1.25: Normal activity
- Activity > 1.25: Active clay (high swelling potential)
Active clays expand significantly when wetted and shrink when dried—exactly the problematic behavior seen in many Kenyan soils. However, Atterberg limits alone don’t tell the full story. For critical projects, specific swelling tests (free swell test, consolidation-swell test) should be conducted alongside Atterberg limits determination.
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Test Procedures and Laboratory Equipment
Sample Preparation: The Foundation of Accurate Results
Everything starts with proper sample preparation. You can’t just grab soil from the site and start testing. The material must be carefully prepared to ensure consistent, repeatable results.
Standard specifications require sieving soil through a No. 40 sieve (0.425mm opening) to remove larger particles. This makes sense. Atterberg limits measure the behavior of fine-grained soil—the clay and silt fractions. Larger particles like sand and gravel don’t exhibit plasticity, so they’re excluded from the test.
Two preparation methods exist:
Wet Preparation Method: Take the as-received field sample and add distilled water to create a soil-water mixture. Mix thoroughly with a spatula on a glass plate. Allow the soil to cure in a humid environment (covered container) for 16-24 hours. This curing period, especially important for clayey soils, allows water molecules to fully hydrate the clay particles. The moisture distributes uniformly throughout the sample.
Dry Preparation Method: Air-dry the soil sample first, break up clumps, sieve it, then add water. This method works better for samples that arrive very wet or for laboratory storage purposes. However, it can alter test results for certain soils, particularly those with organic content.
Which Preparation Method Should You Use?
The requesting authority specifies which method to use; if not specified, wet preparation is the default. In Kenya, most geotechnical testing follows British Standards (BS 1377-2) which typically employs wet preparation for natural moisture content testing.
Laboratory Equipment Requirements
Walk into any properly equipped soil testing laboratory in Kenya, and you’ll find these essential tools:
For Liquid Limit Testing:
- Casagrande liquid limit device (brass cup mounted on cam mechanism)
- Standardized grooving tool (either flat or curved ASTM tool)
- Spatula for mixing and placing soil
- Glass or rubber base for cup impact
- Moisture content containers (numbered aluminum cans)
- Precision balance (0.01g accuracy)
- Drying oven (105-110°C)
Alternative equipment:
- Fall cone penetrometer apparatus with 80g, 30° cone
- Dial gauge or digital readout for penetration measurement
- Sample cups for cone test
For Plastic Limit Testing:
- Glass plate or ground glass mixing surface (minimum 30cm x 30cm)
- 3mm diameter comparison rod
- Moisture content containers
- Some laboratories use plastic limit rolling devices for consistency
For Shrinkage Limit Testing:
- Porcelain shrinkage dish (evaporating dish)
- Mercury displacement apparatus or wax coating materials
- Volumetric measurement equipment
How Long Does Atterberg Limits Testing Take?
Time varies depending on the laboratory’s workload and testing method:
Typical turnaround in Kenya:
- Sample preparation and curing: 16-24 hours
- Liquid limit testing: 2-4 hours (including multiple moisture contents)
- Plastic limit testing: 1-2 hours
- Moisture content determination: 16-24 hours (oven drying overnight)
- Total time: 2-3 working days for complete results
Cost in Kenya ranges from Ksh 1,500 to 5,000 depending on the laboratory and whether additional tests are included. Certified laboratories like Spectralab and GeoIssa typically charge on the higher end but provide ISO 17025:2017 accredited results.
Testing Standards: Ensuring Quality and Consistency
International Standards
ASTM D4318 is the primary American standard covering liquid limit, plastic limit, and plasticity index determination. This standard provides two methods for liquid limit (multipoint and one-point) and two procedures for plastic limit (hand rolling and rolling device).
British Standards BS 1377-2 remains widely used in Kenya due to historical ties to British engineering practices. This standard includes both Casagrande cup and cone penetrometer methods for liquid limit determination.
AASHTO T89 and T90 cover similar procedures for road and highway applications. Many KeRRA projects specify AASHTO standards for subgrade evaluation.
Quality Control in Kenyan Laboratories
KENAS (Kenya Accreditation Service) serves as the sole accreditation body for testing laboratories in Kenya. Laboratories must meet ISO/IEC 17025:2017 requirements, which mandate:
- Documented procedures for all tests
- Regular equipment calibration
- Proficiency testing participation
- Qualified technical signatories
- Traceability of results
Several KENAS-accredited laboratories operate throughout Kenya, concentrated primarily in Nairobi with branches in Nakuru, Mombasa, Kisumu, and Eldoret.
Atterberg Limits and Kenyan Soils
Soil Types Across Kenya
Kenya’s geology creates remarkable soil diversity. Understanding regional variations helps engineers anticipate challenges before site investigation.
Black Cotton Soils (Expansive Clays)
More than 60% of Nairobi area is covered by black cotton soils, particularly in the Robi valley lacustrine sedimentary deposits. These problematic soils dominate satellite towns like Kitengela, Syokimau, Athi River, and Kamulu.
Black cotton soils in Kenya experience volume changes of 120-300% with swelling pressures reaching 8-10 kg/cm². These soils typically show:
- Liquid limits: 60-100%
- Plastic limits: 30-50%
- Plasticity indices: 30-60 (high plasticity)
- Activity numbers > 1.25 (indicating active clay minerals)
The high clay content, particularly montmorillonite minerals from the smectite group, causes significant swelling when exposed to water and dramatic shrinkage during dry periods. This creates the characteristic surface cracking visible during Kenya’s dry season.
Volcanic Soils
Found extensively in Central Kenya, Rift Valley, and parts of Western Kenya. These soils derived from volcanic ash and lava weathering typically exhibit:
- Lower plasticity than black cotton soils
- Better drainage characteristics
- Moderate bearing capacity
- PI values: 10-25 (low to medium plasticity)
Coastal Sandy Soils
Mombasa and coastal regions feature predominantly granular soils with minimal clay content. These soils often classify as non-plastic (NP)—they cannot be rolled into threads at any moisture content. Their LL and PL cannot be determined because insufficient cohesion exists.
Why Are Atterberg Limits Important for Black Cotton Soils?
The plasticity index directly correlates with swelling potential. Higher PI means greater clay content and more dramatic volume changes. For black cotton soils with PI > 30, engineers must implement special foundation designs.
Common solutions include excavating to 1.5-2 meters depth and replacing with murram or gravel, using raft foundations to distribute loads evenly, or employing suspended ground floor systems with ground beams and pad foundations.
Atterberg limits testing allows engineers to quantify the problem. Rather than guessing whether soil is “bad,” they get numerical values that connect to decades of engineering experience. A soil with PI = 45 behaves predictably differently from one with PI = 20.
Testing Laboratories in Kenya
Major Testing Facilities
Kenya hosts numerous soil testing laboratories, though quality and accreditation status vary significantly. Here are established facilities:
Spectralab operates from Credit Bank House on Butere Road, Nairobi. The laboratory provides comprehensive geotechnical tests including Atterberg limits according to BS 1377 Part 3. They serve water, environmental, agricultural, and geotechnical sectors.
Cropnuts (Crop Nutrition Laboratory Services Ltd) maintains facilities in Limuru off Limuru Road. The company holds ISO/IEC 17025 accreditation from KENAS and specializes in agricultural soil testing but also conducts geotechnical analyses.
Kenya Bureau of Standards (KEBS) laboratories at Popo Road, off Mombasa Road, provide government-backed testing services. KEBS maintains ISO/IEC 17025:2017 accredited laboratories with approximately 45 qualified technical signatories covering civil engineering tests.
SGS Kenya operates internationally recognized testing facilities. Using state-of-the-art technology, SGS can test soil samples for a wide range of parameters with accredited procedures.
Other Notable Laboratories:
- GeoIssa (Nairobi)
- Rockridge Engineering
- Material Testing and Research Division (MTRD)
- AgriQ Quest (Plessey House, Nairobi)
- Various university laboratories (University of Nairobi, JKUAT, Technical University of Kenya)
Choosing a Testing Laboratory
When selecting a laboratory for your project, verify:
- KENAS accreditation for specific tests you require
- Turnaround time that fits your project schedule
- Experience with similar projects
- Quality of test reports (clear, comprehensive, professionally presented)
- Cost transparency
Practical Applications in Construction
Foundation Design Based on Atterberg Limits
Soil classification from Atterberg limits directly influences foundation selection:
Low Plasticity Soils (PI < 15):
- Suitable for standard shallow foundations
- Minimal volume change concerns
- Strip footings or pad footings work well
- Normal bearing capacity assumptions apply
Medium Plasticity Soils (PI 15-30):
- Require careful moisture management
- Consider raft foundations for even load distribution
- Implement proper site drainage
- Monitor construction moisture content
High Plasticity Soils (PI > 30):
- Mandate special foundation treatments
- Deep foundations to reach stable strata
- Soil replacement or improvement
- Chemical stabilization (lime or cement)
- Suspended ground floors
How Do Atterberg Limits Relate to Soil Strength?
The relationship isn’t direct, but strong correlations exist. Soils with high PI values generally exhibit:
- Lower undrained shear strength
- Higher compressibility
- Slower consolidation
- Lower permeability
Liquidity Index provides immediate strength indication:
- LI < 0: Soil is drier than plastic limit (stiff, brittle)
- LI = 0 to 1: Soil is within plastic range (moldable)
- LI > 1: Soil is wetter than liquid limit (very soft, may flow)
The liquidity index formula is: LI = (Natural Water Content – PL) / PI. This tells you where current field conditions sit relative to the plastic range.
For quick estimates: soils with LI approaching or exceeding 1.0 likely cannot support construction loads without significant improvement. Conversely, soils with LI < 0.2 behave much more favorably.
Road Construction Applications
Road construction in Kenya heavily relies on subgrade soil classification. Atterberg limits determine whether natural subgrade is acceptable or requires treatment.
AASHTO Soil Classification groups soils primarily by grain size distribution and Atterberg limits:
- A-1, A-2, A-3: Granular materials (excellent to good subgrade)
- A-4: Silty soils (fair to poor subgrade)
- A-5: Elastic silts and clays (poor subgrade)
- A-6: Clayey soils (fair to poor subgrade)
- A-7: Highly plastic clays (poor subgrade)
The classification directly determines pavement thickness, need for subgrade improvement, and appropriate construction methods. CBR testing then confirms the design, but Atterberg limits provide the initial classification.
Can Atterberg Limits Predict Settlement?
Indirectly, yes. The compression index (Cc), which quantifies soil compressibility, correlates strongly with liquid limit:
Empirical relationship: Cc ≈ 0.009 (LL – 10)
This means a soil with LL = 60% has Cc ≈ 0.45, while one with LL = 100% has Cc ≈ 0.81. Higher compression index means greater settlement under loads.
For preliminary design before detailed consolidation testing, engineers use this correlation to estimate potential settlement magnitudes. It’s not precise enough for final design but helps identify problem soils early.
Interpretation and Analysis
The Plasticity Chart: Visual Classification Tool
Plot liquid limit (x-axis) and plasticity index (y-axis) on the plasticity chart. A diagonal line called the A-line separates clays from silts.
A-line equation: PI = 0.73 (LL – 20)
Interpretation:
- Above A-line: Clay behavior (designated “C”)
- Below A-line: Silt behavior (designated “M”)
- LL < 50: Low plasticity (designated “L”)
- LL > 50: High plasticity (designated “H”)
Combined USCS classifications:
- CL: Low plasticity clay (most common)
- CH: High plasticity clay (black cotton soils often fall here)
- ML: Low plasticity silt
- MH: High plasticity silt (elastic silt)
What Is a Good Plasticity Index Value?
“Good” depends entirely on application:
For foundations: PI = 5-15 is ideal. Soil has enough cohesion for stability but limited volume change potential.
For clay liners (environmental applications): PI > 20 is preferred. High plasticity means low permeability—excellent for containing waste.
For pavement subgrade: PI < 10 is best. Limited moisture sensitivity and good compaction characteristics.
For agricultural use: PI 20-40 can be excellent (like black cotton soil for growing cotton). The same soil properties that create construction nightmares benefit certain crops.
There’s no universally “good” value—context matters.
Common Testing Challenges
Operator Variability: The plastic limit test especially depends on operator technique. ASTM D4318 specifies acceptable ranges for single-operator precision—for CH soils, two PL results should differ by no more than 1%. Different operators may produce slightly different results.
Sample Disturbance: Field sampling methods affect results. Disturbed samples often show slightly different Atterberg limits than undisturbed samples, particularly for sensitive clays.
Moisture Conditioning Time: Rushing the 16-24 hour curing period can yield lower liquid limits. Clay particles need time to fully hydrate.
Presence of Organics: Soils with substantial organic matter show dramatically reduced liquid limits after oven-drying compared to testing at natural moisture content. This phenomenon actually serves as a qualitative organic content indicator.
Soluble Salts: Marine soils or those with high soluble salt concentrations show affected LL and PL values. The degree of salt dilution during testing must be carefully considered.
Case Studies: Kenyan Projects
High-Rise Construction in Upperhill, Nairobi
A recent high-rise development in Upperhill encountered CH soils (high plasticity clay) at foundation level. Initial borings revealed:
- LL = 72%
- PL = 32%
- PI = 40
- Natural moisture content = 35%
The liquidity index calculated as: LI = (35 – 32) / 40 = 0.075
This indicated soil was relatively dry, near plastic limit—favorable for construction timing. However, the high PI flagged potential long-term settlement concerns.
Engineering solution: Deep pile foundations extending 12 meters to reach denser strata, combined with raft foundation at pile cap level to distribute loads. Foundation design explicitly accounted for consolidation settlement calculated from liquid limit correlations.
Highway Project, Kajiado County
A highway widening project encountered variable soils ranging from sandy silts to clay. Atterberg limits testing at multiple locations revealed problematic sections with PI > 25.
Project response: Sections with PI < 15 received standard pavement structure. Sections with PI 15-25 required lime stabilization (3-4% lime by weight). Sections with PI > 25 underwent complete subgrade replacement to 600mm depth with imported select material.
This differential treatment, guided by systematic Atterberg limits testing every 200 meters, optimized costs. Rather than over-designing the entire alignment, resources concentrated where soil conditions demanded.
Frequently Asked Questions
What is the difference between liquid limit and plastic limit?
Liquid limit marks the highest moisture content where soil maintains measurable shear strength before flowing. Plastic limit marks the lowest moisture content where soil can be molded without cracking. The gap between them defines the moisture range where soil behaves plastically.
Why is plasticity index important?
Plasticity index (PI = LL - PL) indicates the range of moisture content over which soil remains workable and moldable. It correlates with clay content, volume change potential, compressibility, and strength characteristics. Higher PI generally means more challenging construction conditions.
Which soils require Atterberg limits testing?
Only fine-grained soils containing clay or silt fractions require this testing. Purely granular soils (clean sands and gravels) don't exhibit plasticity and are classified as non-plastic (NP). Testing is mandatory for cohesive soils used in foundations, earthworks, and pavement subgrades.
How do temperature and humidity affect results?
Laboratory temperature and humidity affect evaporation rates during testing, particularly for the plastic limit test. Most standards specify testing at 20-27°C (68-80°F) with controlled humidity. Kenyan laboratories typically operate within this range, though those without climate control may show slight seasonal variability.
What are typical values for different soil types?
Illite clays show shrinkage limit 15-17%, plastic limit 24-52%, and liquid limit 30-110%. Kaolinite exhibits shrinkage limit 25-29% Legacyengineering. These minerals occur in various Kenyan soils, affecting their engineering behavior.
General ranges:
- Lean clay (CL): LL 20-50%, PI 7-25
- Fat clay (CH): LL > 50%, PI > 25
- Low plasticity silt (ML): LL < 50%, PI < 7
- High plasticity silt (MH): LL > 50%, PI < 7
Can Atterberg limits predict soil swelling?
They provide strong indicators but aren't definitive. High PI (>30) and high LL (>60) suggest swelling potential. The activity number (PI / % clay fraction) more specifically addresses this—values exceeding 1.25 indicate active, expansive clays. However, specific swell tests provide conclusive data for critical projects.
What is the cost of Atterberg limits testing in Kenya?
Costs range from Ksh 1,500 to 5,000 depending on laboratory accreditation, turnaround time, and whether testing is bundled with other geotechnical analyses. KENAS-accredited laboratories typically charge higher rates but provide internationally recognized results necessary for major projects.
How do Atterberg limits relate to compaction?
Proctor compaction testing determines optimum moisture content (OMC) for compaction. This OMC typically falls within or near the plastic range defined by Atterberg limits. Soils with high PI require more precise moisture control during compaction and show narrower acceptable moisture ranges.
Why do black cotton soils cause foundation problems?
Black cotton soils contain montmorillonite clay minerals that absorb water and expand significantly, then shrink dramatically when dried. This repeated expansion and contraction creates unstable conditions. With PI values often exceeding 30, these soils exhibit some of the most problematic behavior for construction.
What happens if foundations are built without knowing Atterberg limits?
Building without understanding soil plasticity characteristics is essentially gambling. You might encounter:
- Unexpected differential settlement
- Foundation heaving during rains
- Cracking in floors and walls
- Premature structural failure
- Costly remedial work
Geotechnical investigation including Atterberg limits testing isn't an optional luxury—it's fundamental due diligence for any construction project. The testing cost pales compared to foundation failure repair costs.
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