Paving
Geotextile separation and stabilization fabric for pavement and site base
How a layer of fabric between the subgrade and the stone keeps a base from pumping into soft soil: the four functions, woven vs nonwoven, geotextile vs geogrid, AASHTO M288 classes, overlap, and placing stone without tearing it.
Direct answer
A geotextile is an engineered fabric placed between a soft subgrade and the aggregate base, and its main job is separation: keeping the clean stone from punching down and the fines from pumping up, which contaminates and weakens the base. It also filters and drains. The geotechnical engineer, the project spec, and the manufacturer set the class.
Key takeaways
- Separation is the geotextile's main job: a fabric between soft subgrade and aggregate keeps stone from punching down and fines from pumping up.
- Recommend separation where the subgrade carries more than roughly 10 to 15 percent fines or has a low CBR under heavy or repeated load.
- AASHTO M288 separation applies to unsaturated subgrades around CBR 3 and up; softer or saturated soils move to the stabilization application.
- M288 grades survivability into Class 1 (toughest, most severe), Class 2, and Class 3 (lightest, roughly 50 to 60 percent of Class 1 strength).
- Never drive on bare fabric: end-dump aggregate onto placed stone and push it forward so equipment always rides on stone, not cloth.
Geotextile fabric, and why a layer of cloth saves the base
A geotextile is an engineered fabric placed between the subgrade and the aggregate base so the two layers stay separate instead of mixing into each other. The subgrade is the natural soil at the bottom of the section. The base is the clean crushed stone above it. Put clean stone directly on a soft, wet, fine-grained soil and under traffic the two start to trade places. The stone punches down into the mud, the fines pump up into the voids of the stone, and a few months later the base that was 12 inches of clean rock is half its strength and full of clay.
Separation is the main job. The fabric is a barrier that lets water through but holds the soil particles back, so the stone stays clean and the soil stays put. Along with separation it filters, it drains water in some installations, and it adds a little tensile reinforcement, though reinforcement is the geogrid's job and the geotextile is the weaker player there.
Getting it right comes down to a few decisions. Choose the fabric for the function the engineer is asking for, lap it correctly so the seams do not open over soft soil, and do not drive equipment on the bare fabric before the stone is on it. This guide covers the separation geotextile. Chemical treatment with lime or cement and mechanical reinforcement with geogrid are in the subgrade stabilization guide, and building and compacting the base above the fabric is in the base and subgrade compaction guide, both cross-linked below.
The four functions, and which one you actually need
A geotextile does four jobs, and the one the engineer is after drives the whole selection. Name the function first, then pick the fabric, never the other way around.
Separation keeps two dissimilar layers from mixing. Filtration lets water pass while holding soil particles in place, the same logic as a sand filter. Drainage moves water along the plane of the fabric, inside its own thickness, to carry it out to daylight or an edge drain. Reinforcement adds tensile strength and lateral restraint to the soil-aggregate system, which a geotextile does modestly and a geogrid does far better.
Most pavement and site-base work is asking for separation first, with filtration riding along because the same fabric that separates also has to let pore water escape upward without letting the soil follow. Drainage as a primary function shows up in edge drains and blanket drains. Reinforcement as a primary function usually points you at a geogrid, or a high-strength woven, on the engineer's call. The mistake is buying one fabric and assuming it does all four well. A fabric tuned for high water flow is rarely the strongest one for survivability, and the strongest woven often has the least flow. Match the property sheet to the function the spec names.
Separation: the number one job
Separation is the function that pays for the fabric on most jobs, and it is the simplest to picture. Clean aggregate and soft subgrade want to mix under load. The fabric is the barrier that stops them.
Two things happen at once on an unseparated soft subgrade. The angular stone, under each wheel pass, presses down into the soft soil and embeds, so the bottom of your base disappears into the subgrade. At the same time the fines, especially low-plasticity silts and fine sands saturated with water, pump up into the open voids between the stones. The result is a contaminated zone at the bottom of the base where the clean drainage rock is now choked with soil. That zone holds water, loses strength, and the rutting starts from there.
The fabric breaks the cycle. The stone bears on the fabric and the fabric spreads the load instead of letting each stone punch through, and the woven or fine-pored nonwoven holds the fines on the subgrade side. Separation is recommended where the subgrade carries more than roughly 10 to 15 percent fines, because that is the soil that loses strength and migrates under dynamic load. The exact trigger, the fabric, and the class belong to the geotechnical engineer and the project specification, but the principle does not move: on a soft, fine, wet subgrade, separation keeps the base out of the soil and the soil out of the base.
Filtration: pass the water, hold the soil
Filtration is the balance the fabric strikes between letting water through and holding soil particles back. Too open and the fines wash through and you have not separated anything. Too tight and the fabric blinds or clogs, water backs up behind it, and the pore pressure it was supposed to relieve builds against it.
The property that governs retention is the apparent opening size, the AOS, sometimes written O95. It is the size of the largest soil particle that effectively passes through the fabric, measured by the glass-bead method, and a smaller AOS holds finer soil. The property that governs flow across the fabric is permittivity, the cross-plane flow rate per unit thickness. The selection is a retention-versus-permeability trade run against the actual subgrade gradation, which is why the soil report matters: the fabric that filters a silty clay is not the fabric that filters a clean sand.
AASHTO M288 ties the AOS and permittivity requirements to the percent fines in the subgrade for exactly this reason. Do not pick the AOS off a habit or a shelf. The engineer sets the retention and flow criteria from the gradation, and the manufacturer's tested values on the property sheet are what you verify against. For most soils the AOS lands at or below about 0.43 mm, but treat that as a common ceiling, not your number, and confirm the spec value.
Drainage: moving water in the plane of the fabric
Drainage is the fabric carrying water along its own thickness, sideways, out to a daylight or an edge drain. It is a different property from filtration. Filtration is flow across the fabric. Drainage is flow within it, measured as transmissivity.
This is where a thick needle-punched nonwoven earns its place. The lofty, three-dimensional structure of a nonwoven has enough open thickness to move water in the plane, so it can act as a thin drainage layer as well as a filter. A thin woven has almost no in-plane thickness, so its transmissivity is low and it does little for drainage. That difference is the reason a separation-and-drainage application leans nonwoven while a high-strength separation-only or reinforcement application can go woven.
On a soft subgrade with a high water table, the drainage function matters more than crews expect. The base only stays strong if water can leave it. A nonwoven that filters at the subgrade and also relieves pore water sideways keeps the bottom of the section from staying saturated. Whether transmissivity is a design requirement on a given job is the engineer's call, and a fabric is rarely a substitute for a proper edge drain. When drainage is the goal, confirm the required transmissivity against the spec and the manufacturer's tested value, and pair the fabric with real drainage, not in place of it.
Reinforcement: real, but the smaller part
A geotextile does add some tensile reinforcement. Under load the fabric stretches slightly, picks up tension, and provides a small membrane effect and lateral restraint to the aggregate above it. A high-strength woven can carry meaningful tension and is sometimes specified where the engineer wants both separation and a reinforcement contribution from one product.
Be honest about the size of the effect. For confining and stiffening an aggregate layer, a geogrid does the job better, because its open apertures let the stone strike through and interlock, which locks the aggregate against lateral spreading in a way a closed fabric does not. The geotextile reinforces mostly by friction and membrane tension, the geogrid by mechanical interlock. When the design problem is reinforcement, the answer usually involves a geogrid, often with a separation geotextile under it on a fine subgrade.
That two-layer combination is common and worth understanding: the geogrid for interlock and confinement, the geotextile for separation and filtration, because a geogrid alone does not separate. The geogrid and chemical reinforcement are covered in the subgrade stabilization guide cross-linked below. For the fabric, treat reinforcement as a bonus on a separation product, not the reason you bought it, unless the spec specifically calls a high-strength woven for the tensile contribution.
What is the difference between geotextile and geogrid?
A geotextile is a continuous fabric that separates and filters. A geogrid is an open grid of ribs and apertures that reinforces by interlocking the aggregate. They do different jobs, and confusing them is one of the most expensive mistakes on a soft-subgrade section.
The geotextile works because it is closed. It puts a continuous barrier between the stone and the soil, so the two cannot mix, and its pores filter water while holding fines. The geogrid works because it is open. The aggregate pushes down into the apertures and strikes through, and the ribs grip the stone and stop it from spreading sideways under load, which stiffens the layer and spreads the wheel load wider. The interlock depends on matching the aperture size to the stone, so rounded or poorly graded aggregate confines poorly.
Here is the part crews miss. A geogrid does not separate. Field studies that dug up both have found the aggregate over a geogrid alone contaminated with the subgrade soil, while the section over a separation geotextile stayed clean. So on a soft, fine subgrade you often want both: a separation geotextile against the soil and a geogrid above it for reinforcement, or a single product the engineer has chosen for the combined duty. Do not swap one for the other. If the spec says separation, a geogrid alone will not deliver it, and if the spec says reinforcement, a light fabric will not deliver that. The geogrid and stabilization choices live in the subgrade stabilization guide cross-linked below.
Woven vs nonwoven: match the weave to the function
Woven and nonwoven geotextiles are built differently and do different things, and picking the wrong one for the function is a common field error. The short version: woven for strength and separation with low flow, nonwoven for flow, filtration, and drainage.
A woven geotextile is made of flat tapes or monofilaments woven like cloth. It has high tensile and grab strength at low elongation, which makes it good for separation and for the reinforcement contribution, but its tight weave and thin section give it low water flow and almost no in-plane drainage. A nonwoven is a felted mat of fibers, usually needle-punched, with high elongation, high flow across the fabric, and enough thickness to move water in the plane. It filters and drains well and conforms to a rough subgrade, but at the same weight it carries less tensile strength than a woven.
So the function points you to the type. Separation with reinforcement and not much water to move tends toward a woven. Separation with filtration and drainage on a wet, fine subgrade tends toward a needle-punched nonwoven. Plenty of jobs could run either with the right class, and some specs name a specific construction. The wrong-type failure is real: a tight woven where the subgrade needed the high flow and drainage of a nonwoven traps water, and a light nonwoven where the job needed woven strength tears under the stone. Match the construction to the function and confirm the choice against the geotechnical engineer and the project spec.
| Property | Woven | Nonwoven (needle-punched) |
|---|---|---|
| Strength at low strain | High | Lower at equal weight |
| Water flow across fabric | Low | High |
| In-plane drainage | Minimal | Meaningful |
| Best primary function | Separation, reinforcement | Filtration, drainage, separation |
| Conforms to rough subgrade | Less | More |
The soft, wet, fine-grained subgrade
The fabric earns its money on a soft, wet, fine-grained subgrade, and barely matters on a firm, dry, granular one. Knowing which you have is the difference between a smart line item and a wasted roll.
The problem soils are clays and silts with a low CBR, the California Bearing Ratio that the geotech reports as a measure of subgrade stiffness. A low-CBR soil deflects under each wheel pass, and when it is wet and fine it pumps and migrates. AASHTO M288 frames the separation application for unsaturated subgrades at a CBR of about 3 or higher, with shear strength on the order of 90 kPa, and points softer or saturated subgrades toward the stabilization application with a heavier fabric and more aggregate. That CBR-3 line is the practical break between separation duty and stabilization duty, but the geotechnical engineer sets it for the site.
On a firm, dry, well-draining granular subgrade the fabric does little, because the stone is not punching anywhere and there are no fines to pump. Spending on a separation fabric there is mostly insurance. The judgment call is reading the subgrade, the moisture, and the gradation, and that call belongs to the geotech and the proof roll, which is covered in the base and subgrade compaction guide cross-linked below.
Pumping and contamination: the failure the fabric prevents
Pumping is the mechanism that destroys a base from the bottom, and it is the specific failure a separation geotextile is there to stop. Understand it and the fabric stops being optional on the wrong soil.
Picture a saturated fine subgrade under a clean stone base, no fabric between them. A wheel rolls over. The load presses the stone down and squeezes the wet soil, and because the soil is saturated the water has to go somewhere, so it shoots up into the voids of the stone carrying fines with it. The wheel passes, the pressure releases, and some water draws back, but the fines stay behind in the stone. Repeat that a few thousand times and the bottom inches of the base are no longer clean rock. They are a soft, fine-choked slurry that holds water and carries no load.
The base does not fail all at once. It thins, because the bottom is now mud, and it weakens, because the contaminated zone cannot drain or carry load. The surface starts to rut and the cracks follow. By the time you see it on top, the damage has been growing at the bottom for a season. The fabric stops it at the start by keeping the water and fines from ever crossing into the stone. That is the entire argument for separation, and it is why skipping the fabric on a soft wet subgrade is a decision you pay for later, not a saving.
When do you need separation fabric?
You need a separation geotextile when the subgrade is soft, wet, and fine-grained and the section will carry real load, and you do not need it when the subgrade is firm, dry, and granular. It is a targeted fix, not a default line item on every job.
The triggers stack up. A soft or wet subgrade with a low CBR. A subgrade with more than roughly 10 to 15 percent fines, the silts and clays that pump. A high water table or poor drainage keeping the soil saturated. Heavy or repeated loads, like truck routes, equipment yards, and haul roads, that drive the pumping action. A thin pavement section that puts the base close to the soft soil with little structure to spread the load. Any one of these can put a fabric in the section, and several together make it a clear call.
What it is not is automatic. A firm granular subgrade that proof-rolls tight, drains well, and carries light traffic rarely needs separation, and adding it there buys little. The decision is the geotechnical engineer's, made from the soil report, the proof roll, the water table, and the design loads. When the geotech calls for it, the fabric is cheap relative to rebuilding a pumped base, and when the geotech does not, do not add it on a hunch. Confirm against the geotechnical report and the project spec.
AASHTO M288 and the geotextile classes
AASHTO M 288 is the specification most US highway and site work points to for geotextiles, and it is the document the spec usually references by name. It sets property requirements by application, so the first thing it makes you do is name the function.
M288 covers several standard applications, including subsurface drainage, separation, stabilization, permanent erosion control, sediment control, and what it calls enhancement. Separation and stabilization are the two that matter under a base. The separation application is written for firmer, unsaturated subgrades, around CBR 3 and up. The stabilization application is written for softer and saturated subgrades, with heavier survivability and the expectation of more aggregate to bridge the soft soil. The application you fall under is set by the subgrade conditions and the engineer, not by the roll on the truck.
Inside each application, M288 sets minimum values for the strength and hydraulic properties and ties the hydraulic requirements, the AOS and permittivity, to the percent fines in the subgrade. The edition matters, because the values and tables are revised over the cycles. Cite M288 by name and application, confirm the adopted edition and the project spec, and verify the specific class and property values with the engineer and the manufacturer's certified sheet rather than from memory.
Survivability: the fabric has to live through installation
The hardest moment in a geotextile's life is the day it gets installed. Stone gets dumped on it, dozers track over the fresh lift, and a sharp aggregate over a soft subgrade tries to punch through. Survivability is the property set that says the fabric will live through that without tearing, and M288 grades it in classes.
M288 sorts survivability into Class 1, Class 2, and Class 3, ranked by the harshness of the installation. Class 1 is the toughest, for the most severe conditions where there is the greatest chance of damage, such as a soft subgrade, angular stone, and a thin first lift. Class 3 is the lightest, for mild conditions. Class 2 sits between them. As a rough guide a Class 3 fabric runs roughly 50 to 60 percent of the companion Class 1 strength, so the class is a real step in toughness, not a label.
The harder and softer the conditions, the higher the survivability class you want, because a fabric that fails during placement never gets to do its job. A torn separation layer separates nothing. The required class for the conditions is the engineer's and the spec's call under M288, and the manufacturer's certified properties are what you check it against. Do not down-class to save money on the exact job where the installation is roughest, which is the soft wet subgrade where you needed the fabric most.
The key properties on the spec sheet
A geotextile spec sheet lists a handful of properties, and each maps to either surviving installation or doing a function. Read them against the M288 class and the project spec, and verify the manufacturer's values are certified, not nominal.
The strength and survivability properties are grab tensile strength, the force to pull the fabric apart in a clamp; trapezoidal tear strength, the force to propagate a tear; CBR puncture strength, the force to push a plunger through, which models a stone trying to punch the fabric; and UV stability, the retained strength after sun exposure for fabric left uncovered. The hydraulic properties are the AOS for retention and permittivity for cross-plane flow, plus transmissivity where drainage is a function. M288 ties the AOS and permittivity to the subgrade percent fines.
Match the property to the duty. On a rough subgrade with sharp stone, puncture and tear strength carry the survivability. On a wet fine subgrade, the AOS and permittivity carry the filtration. A high-strength woven for separation and reinforcement will show high grab and tear with low permittivity, while a needle-punched nonwoven for filtration and drainage shows high permittivity and transmissivity with lower grab strength. The values that govern your job are in the project spec and the M288 table for the application, confirmed against the manufacturer's certified sheet.
| Property | What it tells you | Tied to |
|---|---|---|
| Grab tensile strength | Pull-apart resistance | Survivability class |
| Trapezoidal tear strength | Tear propagation resistance | Survivability class |
| CBR puncture strength | Stone punch-through resistance | Survivability class |
| UV stability | Retained strength after sun | Exposure time |
| AOS (O95) | Soil retention, opening size | Subgrade percent fines |
| Permittivity | Cross-plane water flow | Subgrade percent fines |
| Transmissivity | In-plane drainage flow | Drainage applications |
Rolling out the fabric
Installation starts on a prepared subgrade, the same surface you would accept for the base if there were no fabric. Grade it, remove the soft pockets the proof roll finds, and knock down the sharp objects and ruts that would tent or puncture the fabric. The fabric conforms to the surface under it, so a rough, rutted subgrade gives you a rough, stressed fabric.
Roll it out flat, in the direction the spec calls, with the long axis of the rolls usually running the direction the aggregate will be placed so the trucks back over the overlaps the right way. Pull the wrinkles out as you go. A wrinkle is a fold with no soil grip, and it walks and bunches under the first lift, opening gaps and creating weak lines. On a windy day or a soft subgrade, pin or sandbag the edges so the panels do not lift or slide before the stone holds them.
Keep the fabric clean and covered reasonably soon. UV exposure degrades it, so the spec limits how long it can sit uncovered before the aggregate goes on, often a small number of days. The roll direction, the laps, and the exposure limit are on the manufacturer's installation instructions and in the project spec, and those govern over any habit. Follow the printed sheet.
Overlap: more lap on softer soil
The seams are where a separation layer fails if you let them, because a gap at an overlap is a gap with no fabric, and the soil and stone mix right through it. The rule that holds across jobs: the softer the subgrade, the more overlap you need, because soft soil lets the panels shift and the laps creep open under the first lift.
On firm soil a modest lap holds. On soft soil it has to grow, and on very soft soil the panels should be sewn rather than lapped. As a feel for the range, a common spec runs the lap on the order of 12 inches on firm ground up to 24 to 36 inches on a soft subgrade below about CBR 2, and on very soft soils below about CBR 1 the guidance moves to sewn seams and starting the layout from firm ground at the edge of the site to anchor the fabric before rolling onto the soft area. Those numbers vary by spec, so treat them as the shape of the rule, not your dimension.
Lap in the direction of placement so the trucks back over the upstream panel onto the downstream one and never catch and peel a leading edge. Do not leave gaps, do not let a lap shrink below the spec where the soil is worst, and do not assume a firm-ground lap survives on mud. The exact overlap width and whether seams must be sewn are set by the project spec and the manufacturer for the subgrade conditions. Confirm them before the first roll goes down.
Placing the stone without driving on the fabric
How you place the first aggregate over the fabric decides whether the fabric survives to do its job. The rule is blunt: do not drive on bare fabric. End-dump the stone onto already-placed aggregate and push it forward over the fabric, so the equipment always rides on stone, never on the cloth.
Trucks back to the edge of the placed stone and dump there, and a dozer or loader spreads the pile forward onto the fabric, advancing the working face. The fabric never sees a tire or a track directly. Drive a loaded truck across exposed fabric over a soft subgrade and the tires shear it, fold it, and punch it, and the separation layer is torn before the base is even built. This is the single most common way a good fabric gets ruined on site, and it happens because somebody took a shortcut across the open area.
Spread to the design lift thickness in one pass over soft soil rather than working it thin, and on a soft subgrade use static rolling rather than heavy vibratory compaction on that first lift, because vibration can displace the fabric and pump the subgrade. Compact the aggregate, not the fabric. The placement method, the lift, and the compaction approach for the first lift are on the manufacturer's instructions and the spec for the conditions.
The first lift over soft soil
The first aggregate lift over the fabric on a soft subgrade has to be thick enough to carry the equipment without rutting through to the fabric. Too thin and the dozer ruts the lift, dishes the fabric, and stresses it past what it can take. The first lift is the one that protects everything below it.
There is no single number, because the thickness depends on the subgrade strength, the equipment weight, and the stone, and it is a design value the geotech sets. The principle is that the softer the subgrade, the thicker the first lift has to be to bridge it, and the M288 stabilization application assumes more aggregate over softer soil for exactly this reason. Watch the working face as you advance: if the lift is rutting and the fabric is dishing under the dozer, the lift is too thin for that subgrade and the fix is more stone, not more passes.
Track the dozer carefully and keep it on the stone. A first lift that ruts is a first lift that is working the fabric, and on a soft subgrade that is where a fabric tears or a seam opens. The first-lift thickness over soft soil is a design call from the geotechnical engineer and the project spec, tied to the subgrade and the equipment. Confirm it rather than eyeballing it.
The fabric is not a substitute for the section
A separation geotextile helps a soft subgrade, but it does not replace drainage or a structural section, and selling it as a cure-all is how it gets blamed for failures it was never meant to prevent.
The fabric separates and filters. It does not make a soft subgrade into a firm one, it does not carry the structural load the pavement section is designed to carry, and it does not drain a site that has no edge drains or daylight for the water to reach. A section that is too thin for the traffic will still fail with a fabric in it, just a little slower. A site with a high water table and no drainage will still hold water against the bottom of the base. The fabric is one tool in the design, working alongside the aggregate thickness, the compaction, and the drainage, not in place of them.
Set the expectation early on the job. The geotech designs the section, the drainage, and the treatment, and the fabric does the separation job inside that design. If the section is undersized or the water has nowhere to go, that is a design conversation, not a heavier fabric. Manage the expectation so the fabric is judged on the job it actually does.
Separation plus drainage: the water has to leave
Separation keeps the base clean, but the base still has to drain, and the fabric and the drainage work together rather than one covering for the other. On a wet subgrade the smartest sections pair a filtering geotextile with real drainage so the water the fabric relieves actually has somewhere to go.
A needle-punched nonwoven that filters at the subgrade can also move pore water sideways in its own thickness, which helps relieve the pressure that drives pumping. That in-plane drainage is a help, not a drainage system. The water still needs an edge drain, a daylighted base, or an underdrain to carry it off the site. Filter the subgrade with the fabric, then give the water a path out, and the bottom of the section stays workable.
Where the spec wants the fabric itself to drain, it will call a transmissivity, and that is a property to verify against the manufacturer's tested value. Where the spec wants the base to drain, that is an edge-drain and grading decision that lives with the design. The combination of separation and drainage is the engineer's to detail; the field job is to install both as drawn and not let the fabric become an excuse to skip the drains.
Other site uses for the same fabric
The same separation and filtration logic shows up across the site, not just under pavement, and recognizing it helps you spec the right fabric in the other places it appears. The functions are the same; the application names change.
Under riprap and rock slope protection, a geotextile filters the soil behind the rock so the embankment fines do not wash out through the voids while water passes through. In a French drain or an edge drain, the fabric wraps the trench and the aggregate around the perforated pipe so the surrounding soil cannot migrate in and clog the stone and the pipe, the same filtration job with the flow reversed. In erosion control and on slopes, geotextiles and erosion blankets hold soil while vegetation establishes.
Each of these has its own M288 application and its own property requirements, so the riprap filter fabric is not automatically the pavement separation fabric. The point is that separation and filtration are general functions, and once you understand them under a base you can see where the same fabric belongs elsewhere. Match the fabric to the application and confirm the class against the spec for that use.
Common mistakes
The failures cluster, and almost all of them trace back to either skipping the fabric where the subgrade needed it or mishandling it during placement. None of them are exotic, and every one of them is avoidable.
- Leaving the fabric out on a soft, wet, fine subgrade, so the base pumps and contaminates from the bottom.
- Using the wrong type, a tight woven where the wet subgrade needed nonwoven drainage and filtration, or a light nonwoven where the job needed woven strength.
- Confusing geotextile and geogrid, expecting a geogrid alone to separate, or a light fabric to reinforce.
- Inadequate overlap on soft soil, or a firm-ground lap that creeps open over mud, leaving gaps the soil migrates through.
- Driving loaded trucks or equipment on the bare fabric and shearing or puncturing it before the stone is on it.
- Expecting the fabric to replace a proper structural section or site drainage.
- Specifying the wrong M288 survivability class for the conditions, so the fabric tears during the first lift.
What to document
A separation layer is buried the day after it goes in, so the record is the only proof of what was installed and why. When a base shows distress later, the record answers whether the right fabric was specified, laid, and protected.
Capture the function the fabric is serving, the product and its M288 application and class, the manufacturer's certified property values, the overlap width and whether seams were sewn, the first-lift thickness and placement method, and who inspected the subgrade and the fabric before cover. Tie it to the geotechnical report and the spec section that called the fabric, so the decision has an owner.
| Function | Fabric | Note |
|---|---|---|
| Separation | Woven or nonwoven per spec | M288 application and class, AOS for fines |
| Filtration | Nonwoven or fine woven | AOS and permittivity vs subgrade gradation |
| Drainage | Needle-punched nonwoven | Transmissivity verified, paired with edge drain |
| Reinforcement | High-strength woven or geogrid | Per stabilization guide and engineer |
| Overlap and seams | Per spec for subgrade | Width, sewn vs lapped, direction of placement |
| First lift | Aggregate over fabric | Thickness, end-dumped, static roll on soft soil |
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
AASHTO M 288, the standard specification for geosynthetics in highway applications, is the document most US pavement and site specs point to for geotextiles. It sets property requirements by application, including separation and stabilization for the work under a base, and it grades survivability into Class 1, 2, and 3 by the harshness of installation. The application, the class, and the AOS and permittivity requirements are tied to the subgrade conditions and percent fines, and the values are revised across editions, so confirm the adopted edition.
The property test methods sit under ASTM: grab tensile, trapezoidal tear, CBR puncture, permittivity by the water-flow method, and AOS by the glass-bead method. Cite them by what they measure rather than by a guessed designation, and read the actual values off the manufacturer's certified property sheet. State DOT specifications often adopt or amend M288 with their own tables, so the project spec and the DOT special provisions can override the base standard.
The decisions that carry risk, the fabric type, the survivability class, the AOS and permittivity, and the overlap width, belong to the geotechnical or civil engineer, the project specification, and the manufacturer's instructions, in that order of authority on a given job. Three things hold across all of them. Separation keeps the base out of a soft subgrade and the subgrade out of the base. Woven versus nonwoven follows the function, strength versus flow. And you lap it for the soil and never drive on bare fabric. The geogrid and chemical treatment decisions are in the subgrade stabilization guide, and building and compacting the base above the fabric is in the base and subgrade compaction guide, both cross-linked here.
Units and terms
Geotextile work carries its own vocabulary, and the same idea reads differently across a soil report, a spec, and a manufacturer sheet. These are the terms that matter under a base.
- Geotextile
- An engineered permeable fabric placed in soil, here between subgrade and aggregate, to separate, filter, drain, or reinforce.
- Separation
- Keeping two dissimilar layers, the clean aggregate and the fine subgrade, from mixing under load.
- Filtration
- Letting water pass across the fabric while holding soil particles in place, balancing flow against retention.
- AOS (apparent opening size)
- The opening size that controls which soil particles pass, written O95; a smaller AOS retains finer soil.
- Permittivity vs transmissivity
- Permittivity is water flow across the fabric; transmissivity is water flow in the plane of the fabric for drainage.
- Woven vs nonwoven
- Woven is high strength and low flow for separation and reinforcement; nonwoven is high flow for filtration and drainage.
- Geotextile vs geogrid
- A geotextile is a closed fabric that separates and filters; a geogrid is an open grid that reinforces by aggregate interlock.
- AASHTO M288
- The geosynthetic specification setting geotextile properties by application and survivability Class 1, 2, or 3.
- Subgrade pumping
- Saturated fines forced up into the aggregate voids under repeated load, contaminating and weakening the base.
FAQ
What does geotextile fabric do under a pavement base?
A geotextile placed between the subgrade and the aggregate base separates the two so the clean stone does not punch into soft soil and the fines do not pump up into the stone. It also filters water and, in a nonwoven, drains it. The geotech and the spec set the fabric and class.
What is the difference between geotextile and geogrid?
A geotextile is a closed fabric that separates and filters, keeping the aggregate and subgrade from mixing. A geogrid is an open grid that reinforces by letting stone strike through and interlock. A geogrid alone does not separate, so on a soft fine subgrade you often install both, the fabric below and the grid above.
Woven vs nonwoven geotextile: which do I use?
Woven geotextiles are high strength and low flow, good for separation and reinforcement. Nonwoven needle-punched geotextiles are high flow and filter and drain well, good for a wet fine subgrade. Match the type to the function the spec names. A tight woven where the soil needed nonwoven drainage traps water.
When do you need separation fabric?
You need separation fabric when the subgrade is soft, wet, and fine-grained, with a low CBR or more than about 10 to 15 percent fines, especially under heavy loads or a thin section. A firm dry granular subgrade rarely needs it. The geotechnical engineer makes the call from the soil report and proof roll.
Why does an aggregate base pump into the subgrade?
Under each wheel pass on a saturated fine subgrade with no fabric, the stone presses down and embeds while pore water shoots up into the stone voids carrying fines with it. Repeated, the bottom of the base becomes a soil-choked slurry that holds water and carries no load, so the base thins, weakens, and ruts.
What is AOS in a geotextile spec?
AOS is the apparent opening size, written O95, the opening size that controls which soil particles pass through the fabric. A smaller AOS retains finer soil. AASHTO M288 ties the AOS and permittivity to the subgrade percent fines, so the engineer sets it from the gradation. Verify the value on the manufacturer's certified sheet.
How much overlap does separation fabric need?
The softer the subgrade, the more overlap. A common spec runs roughly 12 inches on firm ground up to 24 to 36 inches on soft soil below about CBR 2, with sewn seams on very soft soil. Lap in the direction of placement. The actual width and sewn requirement are set by the project spec and manufacturer.
Can you drive equipment on geotextile fabric?
No. Do not drive on bare fabric, especially over soft soil, because tires and tracks shear, fold, and puncture it before the base protects it. End-dump the aggregate onto placed stone and push it forward so equipment always rides on stone. Use static rolling on the first lift over soft subgrade to avoid displacing the fabric.
What are the AASHTO M288 geotextile classes?
M288 grades survivability into Class 1, Class 2, and Class 3 by how harsh the installation is. Class 1 is the toughest for the most severe conditions, and Class 3 is the lightest, running roughly 50 to 60 percent of Class 1 strength. The required class for the conditions is the engineer's and the spec's call.
Does geotextile fabric replace a structural pavement section?
No. A separation geotextile keeps the base clean and helps a soft subgrade, but it does not carry the structural load, stiffen soft soil into firm soil, or drain a site with no edge drains. It works inside the design alongside the aggregate thickness, compaction, and drainage, not as a substitute for any of them.