Creating an Infiltration Surface

Information for contractors to properly build an infiltration surface; the soil surface where stormwater or wastewater is expected to enter the soil profile.
Creating an Infiltration Surface - Articles

Updated: August 14, 2017

Creating an Infiltration Surface

Infiltration is the process by which water ponded or flowing over a soil surface is absorbed into the soil. There are many situations where a contractor is faced with the task of creating an infiltration surface. Probably the two most common situations are (1) creating an infiltration surface during the construction of a stormwater infiltration system, and (2) creating an infiltration surface during the construction of an on-lot wastewater absorption system. The first is usually on the soil surface and easier to get to and manipulate. The second is usually under the soil surface, hidden from sight and covered with either sand or aggregate. In either case the issues faced by the contractor are similar. Let's take a look at and discuss some of the major issues.

Natural soil has a long-term infiltration rate or permeability, which is a function of the soil's texture, structure and consistency. The goal of a contractor is to create the infiltration surface in such a way that the rate at which stormwater runoff or wastewater enters the soil is the same as the rate determined by the soil scientist's assessment. This almost never occurs. So, we must ask "what happens to the soil during construction that causes the soil's intake rate after construction to be less than the intake rate before construction?" The answer is compaction!

Wastewater Absorption Area

When we design or size an absorption area using the Perc Test results, unknown to most everyone is that the design loading equations from Chapter 73, have built into them a large factor of safety that has served the on-lot industry well. I have shown this reduction in effluent intake rate in Table 1. The first column shows selected site average Perc Rates. The second column shows the average measured water level drop from the Perc Rate expresses in in/hr. The third column shows the design loading or intake rate obtained by applying the appropriate loading equation from Chapter 73:15 Table A and the fourth column shows the ratio of the Column 2 results divided by the Column 3 results; the factor of safety built into the on-lot system designs when Perc Rate is used as the sizing criteria. In my opinion, this reduction in loading rate accounts for many of the difficulties encountered when constructing the infiltration surface.

Table 1. Evaluation of Design Loading Rates based on Perc Test Results.
Perc Rate
Drop in Hole Rate
Design Intake Rate
6 to 1560/10 = 6.00.055110
16 to 3060/23 = 2.60.05671
31 to 4560/38 = 1.60.03350
46 to 6060/54 = 1.10.02742
61 to 9060/75 = 0.80.02236

Infiltrating Stormwater

When a designer selects a soil's permeability from a Soil Survey or pays a professional soil scientist to measure the infiltration rate, the result is expected to be the true, long-term, sustainable steady-state infiltration rate of the soil profile in question. The resulting design is developed based on these intake values. Rarely is a factor of safety applied to the design.

Soil Compaction

Soil compaction is the physical consolidation of the soil by an applied force that destroys structure, reduces porosity, and limits water and air movement. The result is a dense block of soil with few large pores that has poor internal drainage and limited aeration. Compaction preferentially compresses large pores, which are very important for water and air movement in the soil and infiltration is greatly reduced. If our goal is to create and maintain a sustainable infiltration rate under an absorption area or under a stormwater infiltration surface, we must reduce, better yet eliminate, compaction of the infiltration surface. Soil compaction occurs when a physical force is applied to the soil. The physical force that may cause soil compaction may come from either falling rain or the application of a weight or force to the soil.

Do Not Let It Rain on an Exposed Infiltration Surface

Rarely would we consider rain to be damaging to the soil's infiltration rate. Jarrett's graduate students showed that when raindrops impacted a bare, tilled soil, the rate at which water entered the soil was reduced by two orders of magnitude. What does this mean in the practical sense? This means that if you excavate an infiltration surface, and it rains on the exposed soil, the intake rate of the soil will be much slower after the rain than it was just after you did the excavation. In hard numbers, if the intake rate just after excavation was 0.5 in/hr, it can be expected to be in the order of 0.005 in/hr after the rain. The reduction in intake rate occurs because the raindrops dissipate their energy on the bare soil and break down the soil aggregates leaving the soil surface covered with many individual soil particles. When these particles are largely clay and silt particles a thin, slowly permeable crust forms on the soil surface thus reducing the intake rate of the soil. Fix: Do not leave an infiltration surface open and uncovered during a rain event.

Do Not Drive or Walk On an Exposed Infiltration Surface

Most of the damage caused by soil compaction is the result of applying a force or load to the soil during excavation of the infiltration surface. There are two very important factors that control the amount of soil compaction that may occur to an exposed infiltrating surface; (1) the moisture content of the soil, and (2) the size of the compacting load.

Soil Moisture.

Dry soil is very hard and difficult to dig or work. As the water content in the soil increases, the soil slowly becomes friable or plastic and can be shaped. The soil's plastic limit is the point where the soil takes on plastic qualities. When farmers till the soil to prepare a seed bed, they want the soil to be friable. As more water is added to the soil, the soil slowly is transformed from a plastic body to a liquid and the soil will begin to flow. The soil reaches a point where it becomes liquid (the liquid limit).

Soil moisture can also be evaluated based on a soil-science model where moisture levels in the soil was related to when the gravitational water was all gone (field capacity) and when the plants could no longer extract water from the soil (wilting point).

Another soil-water related aspect is that the topsoil (A horizon) can be at a wide range of moisture contents. When it rains, the top soil gets wet, very wet. A day later it is probably at field capacity and a week or two later this topsoil may be at the wilting point. If we consider the subsoil (B horizon) where an in-ground on-lot absorption area is located, the moisture content is almost always wetter than the topsoil. In fact, as you move downward through the soil profile, the moisture content almost always increases and the soil becomes more friable and approaches the plastic limit.

When it comes to construction and related earth moving activities, "When is it too wet for creating an infiltration surface?" What do you think? If preserving the soil's intake rate is your goal, you want the soil to be at or near the wilting point when the actual surface is cut/created/graded. If you take a small quantity of soil (from the depth of the desired infiltration surface) and place it between your thumb and 2nd finger and squeeze it as hard as you can and you can make a "ribbon" it is too wet for the creation of the infiltration surface.

Compacting Load.

It is easy to visualize that when you drive over soil you apply a compacting force to the soil. This compacting force, or ground pressure, reduces the volume of the soil, thus decreasing the pore space, especially the larger pores that transport air and water. This force also increases the density of the soil and destroys the soil's structure. The end result is a hard, nearly impermeable block of mud and soil that is no longer capable of transporting water. The construction industry refers to "Ground Pressure" as an indicator of how much compaction to expect from equipment operating on the site. Ground pressure is usually reported in pounds per square in (psi) and can be estimated for each piece of equipment. Have you ever considered how large the ground pressure is when you drive your equipment over the soil? Consider Example 1.

Example 1: You have a 20,000 pound wheeled excavator. The excavator has four wheels, each with a footprint 3 ft. long and 1 ft. wide. Compute the average ground pressure for this excavator?

Solution: The area where the excavator's weight contacts the soil is 3 x 1 = 3 ft2 for each wheel. There are four wheels, so the total area contacting the soil is 4(3) = 12 ft2. Since we want the ground pressure in units of psi, we need to convert the contact area to square inches, or 12 ft2 x 144 in2/ft2 = 1,728 in2.

By dividing the excavator weight by the contact area, we get the ground pressure, GP as

GP = 200lb/1728in2 = 11.6 psi
It is easy to see that driving over soil compacts and destroys the soil's ability to transmit water and air (see Table 2). But there are other actions an installer does that also compacts the soil. Any procedure that drives a blade, a shovel, or a tooth of any kind into the soil also compresses or compacts the soil. We think of excavation as being a process that opens and loosens the soil; decreasing its density. But this is only true for a portion of the soil that is moved. Before any movement occurs, there must first be compaction to lift and start the movement. When an installer leaves tooth marks in the trench bottom, he is compacting the trench bottom. Actually it is very difficult to excavate a hole, trench, bed or surface and not compact the bottom of the excavation. Use of a straight-blade bucket, without teeth moved perfectly horizontal creating a perfectly level trench bottom still creates some compaction of the trench bottom. The only way to avoid compacting the bottom of an excavation is to do the work when the soil is so dry that it has sufficient strength to not compact and for this the soil must be at or near the wilting point.
Table 2. Average ground pressures for various applications.
Force ApplierGround Pressure (psi)
Adult horse25
Passenger car30
Human standing8
Human walking16
Track Hoe5.6
Mini excavator4.0
Dump truck20

Note that even when holes are dug for a Perc Test, the holes are to be scarified with a knife blade or pointed instrument to remove any smeared soil surfaces (Chapter 73.14). When the Perc hole is dug, the sides are compacted by the digging tool. Surface compaction can be attenuated by similarly scarifying to remove the smeared surface. Compaction that affects the soil below the exposed surface is much harder to attenuate. Typically freezing and thawing does the best job, but this is hardly practical.

On-Lot Absorption Area

So, finally the area has been excavated with the desired level infiltration surface. What is the next step? Place the aggregate. How do you place the aggregate in a 20- by 40-foot bed? You dump some gravel in the bed from a skid loader, truck, or excavator and then you push/move the stone with the machine. To do this you must drive onto and over the gravel. If a skid loader has a ground pressure of 20psi, what is the ground pressure exerted on the infiltration surface located a foot below the top of the gravel layer? About 20 psi. Gravel transfers any weight loaded onto the top of the gravel straight downward to the layer (the infiltration surface you worked so hard to get right) below. It has been destroyed.

Figure 1. Angle of repose for sand

When it comes to sand, the ground pressure transferred to the surface below the sand is different. Because sand has a tendency to compress and spread when it is loaded from above, the loading weight is transferred differently than for gravel. Sand has an angle of repose of about 32°, see Figure 1 (angle of repose for sand).

This means that the force pushing down on the top of the sand is spread over a larger area by the time the force reaches the bottom of the sand. With an angle of repose of 32°, the downward force is applied over an Area widened by 1.6 ft. for a sand depth of one foot (see Fig. 2. Influence of angle of repose on force area). Example 1 is repeated as Example 2 to show the influence of driving over sand with a wheel excavator. As you can see the force applied by the excavator is distributed over a much larger area, thus reducing the compactive force on the infiltration surface to about 1.3 psi.

Figure 2. Influence of angle of repose on force area

Example 2: You have a 20,000 pound wheeled excavator. The excavator has four wheels, each with a footprint 3 ft. long and 1 ft. wide. Compute the average ground pressure for this excavator?

Solution: The area where the excavator's weight contacts the soil is 6.2 x 4.2 = 26 ft2 for each wheel. There are four wheels, so the total area contacting the soil is 4(26) = 104 ft2. Since we want the ground pressure in units of psi, we need to convert the contact area to square inches, or 104 ft2 x 144 in2/ft2 = 14,980 in2.
By dividing the excavator weight by the contact area, we get the ground pressure as

GP = 20000lb/14980 in2 = 1.3 psi

Compaction Summary

Creation of an infiltration surface is an extremely difficult activity. The soil being excavated must be at or about a moisture content equal to the wilting point. This very dry moisture content rarely, if ever, occurs in the soil located a foot or two below the soil surface. Once excavated, never drive or walk on this surface, even if covered by a layer of sand or aggregate. All excavator marks or local compacted soil areas should be scarified with a knife or blade to expose the undisturbed soil below the compacted surface soil. Never let it rain on or introduce water onto an exposed infiltration surface until the surface has been covered (with aggregate, sand or surface mulch). I am aware that I have just created an excavation protocol that is impossible to follow without a great amount of hand labor.

Practical Summary

From a practical perspective, where can we compromise the above restraints to make it possible for a contractor to create an uncompacted infiltration surface in a reasonable amount of time and with a reasonable amount of labor? I would start with moisture content. I would excavate the proposed infiltration surface to about 3 inches above the final grade. Let the excavation open to the atmosphere and let it exposed to evaporation for a few days. Cover the surface if it rains. When the exposed surface has dried so you cannot create a "ribbon" between your fingers, finish excavating to grade with a straight-blade bucket, but do not drive on the area. If the site is sloped, do not drive downslope of the area either. Scarify all smeared and locally compacted areas with a blade and remove the loosened soil.

On-Lot Absorption Areas.

Introduce and level the aggregate (no equipment on the infiltration surface). Choose the smallest person with the biggest feet (wearing snow shoes would be better) to make final adjustments and place the distribution pipes. Add additional aggregate, cover with a spunbonded geotextile and replace the excavated soil to form the cover and berm; do not drive on the infiltration surface.

Stormwater Infiltration.

Cover the infiltration surface with a rolled erosion control product (RECP) or straw mulch. Get grass growing as soon as possible. Never drive or walk on the infiltration surface.