We need strong soils and aggregates to create safe, long-lasting roads and runways. R-value testing measures soil and aggregate strength to help roadbuilders choose the best design for a given project. In this article, we’ll discuss:
The California Division of Highways (now the California Department of Transportation) developed the r-value test in the late 1940s to improve road design, mainly to prevent ruts.
The r in r-value stands for a material’s resistance to applied vertical force. Simply put, the r-value test tells us how much a material will move laterally if we press down on it. That ratio of vertical force and lateral movement is the r-value of soil or aggregate. Soils with a low r-value are weaker, while soils with a high r-value are stronger.
Originally, its inventors hoped the r-value test would improve on the California bearing ratio test (CBR). While the r-value is more extensive, it has yet to gain the same widespread use as the CBR. It’s most common with state highway agencies in the western U.S., as well as a few airports.
A road is only as strong as the materials you build it with. If the native soil under the road is soft, expansive, or unstable, it can cause structural failures. Likewise, the wrong aggregate subbase can lead to shifting pavement and other safety problems.
The r-value test helps engineers understand how a material will perform under pressure from vehicle traffic. With this knowledge, they can determine the proper design for the road, such as subgrade and pavement thickness, to ensure it lasts.
Fun fact: Did you know there’s another type of r-value that’s not for soil? The other r-value measures how warm insulation is, so it keeps your house and sleeping bag cozy!
American roadbuilders typically use one of two standard r-value tests: the AASHTO T-190 or the ASTM D-2844. Since both tests differ slightly and you can use them for soil or aggregate, we’ll keep things simple with a high-level overview of an r-value test for soil.
The steps for an r-value test are:
First, engineers break up any clumps and remove debris so the soil can pass through the #4 sieve. They separate the sample by particle size, weigh each segment to meet testing standards—usually around 1,200 grams of material—and calculate its mass.
They then combine the varying particle sizes into standard, proportional soil samples for the r-value test. Typically, engineers use five soil samples. They take an initial moisture content reading from one of these, which we’ll revisit in step five.
Engineers soak at least four samples to reach around half to two-thirds of the soil’s highest possible saturation level. The soil should no longer be loose; it should stick to itself and clump together when someone squeezes it in their hand. These partly saturated samples need to cure for at least 16 hours.
After curing, engineers place the soil into a compaction mold and add more moisture. Surprisingly, there's no rule for how much water to add. They use the ol’ eye-crometer and give it their best guess! Their goal is to add enough water for the soil to exude moisture under a standard amount of pressure.
That’s why having four samples is helpful: engineers can test one “pilot” sample and then adjust the moisture content of the other samples based on its performance. Then, they compact each sample.
The r-value test measures three pressure components: exudation, expansion, and horizontal pressures.
Engineers place a phosphor bronze disc and filter paper on top of each soil mold. They flip the sample upside down and set it on a detection plate in a moisture-exudation indicating device. This device applies pressure to the soil and wrings water out of it.
Next, engineers measure the sample and recalculate its mass. This tells them how much moisture the soil emitted due to the pressure, helping them better understand how the soil will perform under traffic when it’s wet from rain or groundwater.
Some soils with very low r-values, such as heavy clays, may ooze out of the mold. If that happens, engineers discard the sample and mark it as less than five r-value. This weak soil won’t perform well as an unpaved road or paved road subgrade.
Expansion pressure testing indicates how much force the soil will exert when it expands due to moisture. Since expansive soils can seriously damage roadways, this vital information helps roadbuilders know how thick to make each layer of the road and how to stabilize the soil so it won’t shift.
Engineers place the soil samples in an expansion pressure device. They add more water to the samples and let them sit for at least 16 to 24 hours. During this time, the soil absorbs the water and expands. The device records the amount of pressure the expanding soil exerts.
Generally, silt and clay soils have the highest expansion pressures, meaning they expand the most forcefully when they absorb moisture and are most likely to cause structural problems for the road.
Immediately after measuring the soil’s expansion pressure, engineers measure its horizontal pressure and displacement using a device called the Hveem stabilometer. Francis Hveem invented this machine to test soil’s stability and resistance to lateral spread by placing a vertical load onto the soil—just like the load a road bears when vehicles drive over its surface.
Engineers place the soil into the Hveem stabilometer, which has a neoprene diaphragm fixed in position and filled with hydraulic fluid. The stabilometer applies vertical pressure to the soil, causing the hydraulic fluid to move a gauge that measures the soil’s resistance to the pressure. But, the gauge doesn’t just spit out the r-value. The engineers have to do a little math to get there.
The r-value of soil falls on a scale from zero to 100, with zero being totally unstable like water and 100 being as strong as steel. To calculate the soil’s r-value, engineers take the horizontal pressure, vertical pressure, and displacement readings from the gauge. Then, they plug them into this equation to find the r-value:
Different soil types tend to fit within a consistent range of r-value results. In the table below, you’ll find the typical r-value for several different soil types. It includes both the United Soil Classification System (USCS) and AASHTO soil categories, so you can compare how people classify soil using both methods.
You may have noticed that some soils in the graph have wide r-value ranges. That’s because those soils are loams, or mixtures of multiple soil types. The percentage of each soil type within the mixture determines how high or low the r-value is. Generally, soils with more coarse particles and lower plasticity tend to have higher r-values and are therefore more likely to hold up to traffic than highly plastic, fine-grained soils.
Engineers discard the samples they used to calculate the soil’s r-value, but they typically have one or two soil samples remaining. They use those “leftovers” to calculate the soil’s optimum moisture content and maximum dry density.
First, they calculate the soil’s moisture content at the time of compaction based on the amount of water they added to the soil during the r-value test and the initial moisture reading they took when preparing the samples. Since soil compacts best at optimum moisture and since the sample successfully compacted during the test, the moisture content at the time of compaction is at or close to the soil’s optimum moisture.
Using the soil’s measurements for mass, weight, and optimum moisture content, engineers calculate the soil’s maximum dry density—or how much roadbuilders must compact the soil so it will support the traffic weight without shifting.
R-value testing is an important measure of soil and aggregate strength. Engineers use r-value test results to make recommendations about soil stabilization and road design. Each soil type tends to have its own specific range of r-values, meaning that if you know your soil type, you’re one step closer to building a safe, long-lasting road.