Septic systems: Soil studies, septic designs, and code-related issues

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Soil studies, septic designs, and code-related issues all come into play when builders are working in areas that are not served by public sewers. We’ve already touched briefly on these issues, but it's now time to dig into the details.

As a builder, it's not your job to do soils studies. These studies are done by soils engineers or county officials. Drawing septic de signs isn’t part of your job description either. But, you are going to have to know how to interpret them. Code-related issues are the responsibility of the contractor you hire to install your septic systems. However, if you don’t have a cursory knowledge of code issues, you might find yourself feeling very foolish. So, what are we going to do about this? Well, I’m going to prepare you with enough background so that you can hold your own with any builder when it comes to talking turkey about septic systems.

Septic designs

I think we should start our discussion with septic designs. Chronologically, soil studies come first, but it might help you to understand the soil studies if you first have knowledge of design criteria. In order to be very accurate here, I’m going to use the actual septic design for my personal home as our example. This design was created by an engineering firm, and the data on it's consistent with design requirements in most areas.

As we run through the information required on a septic design, you are going to see that there are differences in the requirements for a new system and a replacement system. As a builder, you most often are going to be dealing with new systems. However, you might be come involved in the utilization of a building lot that once supported a house and septic system. If, for example, the home was destroyed by fire, the new house you are contracted to build might require a re placement septic system. For this reason, I am going to go over both areas of the design form.

The top section of the design form calls for information pertaining to the job location and the property owner’s name and address. This information is located at the top left corner of the form. In the right corner is an area for the septic permit to be attached once the design has been approved for a permit.

As we move into the main part of the design, there are many individual boxes that request pertinent information. For example, there is a box where you must indicate if the system is a new system, a re placement system, an expanded system, or an experimental system.

The next question box is one to be filled out by a code official. It asks if the system complies with rules, if it's connected to a sanitary sewer, if the system design is recorded and attached, and if system is installed.

If you are installing a replacement system, you must provide in formation that states when the failing system was installed. You must also indicate if the failing system is of a bed, trench, or chamber type. If it's another type, you must provide a description.

There is a question that asks for the size of the property. In my case, this was figured in acres, but some lot sizes would be given in actual dimensions. In addition to the lot size, it's necessary to identify the type of zoning in which the property is located. For example, my property is designated as shore-land zoning, since I have frontage on a river.

The next big box of questions deals with what the application re quires. The first question asks if a rule variance is required. Is a new system variance needed? Is a replacement variance needed? Sub-questions ask if a variance requires approval from a local plumbing inspector or from both a state and local plumbing inspector. The final question in this box asks if a minimum lot size variance is needed?

Moving along, the next box deals with the type of building being served. Is the building a single-family home? You must indicate if the dwelling is of a modular or mobile design. If the building is to be used as a multifamily dwelling, it must be reported. Buildings used for purposes other than those already discussed must be described.

As you shift to the right side of the design form, there is a box that asks if the system is engineered, non-engineered, or primitive (referring to the use of an alternative toilet). The next step is to determine the individually installed components of the system, such as a treatment tank, a holding tank, and alternative toilet, a non-engineered disposal area, or a separated laundry system. Just below this question box is another that requires you to describe the type of water supply used for the property.

Question boxes along the bottom of the first page of a design report start with questions about the type of treatment tank to be used. Is the tank aerobic or septic? What is its capacity? Is the tank a standard style, or is it a low-profile unit?

The next box asks about soil conditions, such as the soil profile and condition. Another question asks for the depth limiting factor. In my case, due to bedrock, the depth limiting factor at the septic area was 36 inches.

A third box in the bottom section asks about water conservation. Is there any special procedures used to conserve water? There is a box to check off if a house is equipped with low-volume toilets. There is a place to indicate that separate facilities are used for laundry waste. If alternative toilets are used, they have a box all their own to be checked.

When you enter the box pertaining to the size of a design, you are given five options. They are small, medium, medium-large, large, and extra-large.

The next box deals with septic systems that require pumping. A box must be checked to indicate if a pump is needed or not. Actually, there are three boxes to check, one if a pump is not required, another if a pump might be required, and a third for when a pump is definitely required.

The next to last box on the first page asks about the type and size of the disposal area. Is the system to be made with a bed layout? If so, what is the size of the bed in square feet? Are chambers needed? Are you planning a trench system? If so, how many linear feet are required? For other types of systems, you must specify your plans.

The last box on the first page wants to know the criteria used for design flow. Options include the number of bedrooms, seating capacity, employees, and water records, depending upon the type of building the system is to serve. In the case of a residence, the number of bedrooms is most often used as a guide to determining the num her of gallons that arc introduced into the system on any given day.

The very bottom of the first page provides a statement regarding an on-site inspection. This section must be dated and completed by the individual who designed the system. Then, you move onto page two.

Page two of a septic design has the top half of the page created with grid boxes. This grid system is supplied so that the design professional can draw a site plan to scale. Part of the site plan shows the location of the building being served, its well (if one is to be in stalled), and the septic system. Other information might be included in the drawing, such as roads, rivers, ponds, property boundaries, and so forth.

The bottom section of the second page deals with soil descriptions and classifications as they were determined at the observation holes. The first question asks if the hole was a test pit or if it was created by boring. Subjects covered for the soil include texture, consistency, color, and mottling.

The box provided for soil data is ruled to allow for depths ranging from zero to 50 inches. In the case of my tests, the soil texture was sandy loam to a depth of 36 inches, where bedrock was en countered. The consistency was friable throughout. Color ran from brown in the first 6 inches to reddish in the 6- to 16-inch depths, to light brown in the final depths.

Additional information in the bottom section states the soil pro file, which in my case was a two. The soil classification was condition “A.” My slope was 10 to 15 percent, and my limiting factor was 36 inches, due to bedrock.

Page three of my septic design consists mostly of a detailed drawing of the septic layout. It mandates a low-profile, 1000-gallon septic tank, and no pump. All of the details of the septic system, including the septic tank, the distribution box, and the bed are drawn to scale.

Directly below the drawing of my septic system are some fill-in- the-blank spaces. The first one indicates the depth of fill required on the upslope. In my case, this was 12 inches. For depth of fill on the down-slope, I was required to have between 30 and 60 inches.

Construction elevations are also given in this section of the re port. My reference elevation was set at zero and marked with a nail and red flag on a tree. The bottom of my disposal area was set at 72 inches below the benchmark (the nail and red flag). For the top of my distribution lines, a calculation was made for 60 inches below the benchmark.

Two cross-sectional drawings were attached to my design. These drawings showed all the details of the installation, For example, the drawing started at the bedrock and showed the original soil surface. It then showed a 12—inch layer of crushed stone. It indicated a layer of hay and 4-inch perforated pipe. This was covered by a 12-inch layer of sandy type fill. Further details showed the rest of the fill needed to accommodate the slope of my system.

Anyone with a reasonable understanding of construction terms could look at my septic design and see exactly what was going to have to happen to make a satisfactory installation. Even if you are a builder who is not familiar with septic systems, reviewing a septic de sign brings you up to speed quickly.

Design criteria

Design criteria for septic systems can vary from one jurisdiction to the next. You should always consult local authorities to determine the requirements that are in effect within your region. However, I can give you a broad-brush understanding of how the criteria is often set.

Trench and bed systems

Trench and bed systems are two types of drain fields that don't re quire the use of chambers. Bed systems are the most common of the two. The design criteria for either of these systems is different from that used to layout a chamber system.

The landscape position normally considered suitable for a trench system should not have a slope of more than 25 percent. A slope greater than this can impair the use of equipment needed to install a system. Bed systems are to be limited to a slope of no more than 5 percent. Keep in mind, the numbers I’m giving you are only suggestions. They don't represent the requirements in your area.

These systems can be installed on land that's level and well- drained and also on the crests of slopes. Convex slopes are considered the best location. Trench and bed systems should not be installed in depressions or at the bases of slopes where suitable surface drainage is not available.

In terms of texture, a sandy or loamy soil is best suited to trench and bed systems. Gravelly and cobbley soils are not as desirable. Clay soil is the least desirable.

When it comes to structure, a strong, granular, blocky, or prismatic structure is best. Platy or unstructured massive soils are the least desirable.

When you are looking at the color of soil with the intent of in stalling a trench or bed system, look for bright, uniform colors. Such colors indicate a well-drained, well-aerated soil. Ground that gives a dull, gray, or mottled appearance is usually a sign of seasonal saturation. This makes soil unstable for a trench or bed system.

Be careful if you find soil that's layered with distinct textural or structural differences. This might indicate that water movement is going to be hindered, and this is not good.

Ideally, there should be between two and four feet of unsaturated soil between the bottom of a drain system and the top of a seasonally high water table or bedrock.

Check with your local authorities to confirm the information I’ve given you here. You can also ask the authorities to give you accept able ratings for percolation tests. If you want to do your own perk tests for reference purposes, you simply dig a hole and fill it with water. Then you watch and wait to see how long it takes the soil to absorb the water. You should measure and note the number of inches of water in the hole when you begin the test and the number of minutes it takes for the water to be absorbed. This type of test has much to do with determining the type of drain field that must be installed. I am going to tell you more about perk tests in a moment.

Mound and chamber systems

Mound and chamber systems can have different design criteria than that used for bed and trench systems. I chose bed and trench systems to use as our example since they are the types of systems most often used. Your local code office or county extension office can provide you with more detailed criteria for all types of systems.

Perk tests

To do them properly, perk tests can’t be done quickly. The procedures used in the test tend to be consistent among authorities, but you might run into some variations. I know I have. At any rate, I am going to give you a rundown on what I consider to be the right way to conduct a professional perk test. My feelings are based on past re search in professional journals and books.

It is common practice to create at least three test holes for any site. You are looking for an average of the results from each hole. By doing this, you rule out the likelihood of having a false reading from just one hole. The holes should be dug or bored at random locations, not too close together, throughout the septic site.

Each test hole should have a diameter of about 6 inches. The depth of the holes should be about the same as the depth of the drain pipes. If you really want to do the test right, scrape the sides of the holes with something sharp and remove the fallen debris from the bottom of the hole. Next, place between one-half and three-quarters of an inch of crushed stone in the bottom of the hole. You are now ready to do the test.

Fill each hole with at least 12 inches of water. This depth of water should be maintained for a minimum of four hours, but overnight is even better. Realistically though, who is going to run out and pour water in a hole on a constant basis throughout the night? Professionals have special fillers and float kits to maintain their water levels. Do they use them? I’ve never seen it happen.

The reason for keeping the test holes full of water for so long is to saturate the ground. You are attempting to simulate conditions that are going to exist once a drain field is installed. On occasions when you are dealing with sandy soil that contains no clay, you can skip the soaking stage of the test procedure.

You should take your perk test within 15 hours of saturating the soil. Don’t wait more than 30 hours to take your tests. Clean out the bottom of the hole if debris has fallen in on the crushed stone, but leave the stone in place. Fill the hole with 6 inches of water. You can use a wooden yardstick as a measuring tool. It is necessary to leave the yardstick in the hole throughout the test, so if you want to test all three holes at the same time, you need three yardsticks.

A notepad and writing instrument are needed to keep records of the water absorption. You should make entries every time the water level drops by of an inch. During this test, you must monitor and record both the time and the loss of water. For example, if you see the first of water drop after four minutes, note the length of time it has taken to have that quantity of water become absorbed by the soil. You can note drops in the water level on a basis of 30-minute increments, if you like.

Testing should continue until two successive water level drops don't vary by more than 6 of an inch and at least three measurements have been made. When dealing with sandy soil, your water might seep out of the hole in less than 30 minutes. Under these conditions, use a standard measurement increment of 10 minutes to record your notes for a period of one hour. The last water level drop is used to calculate a perk rate. I should note that after each measurement of water drop is made, the hole should be replenished with water to a static level of six inches.

What you are after with a perk test is the amount of time it takes for one inch of water to be absorbed. This is done by dividing the time interval used between measurements by the amount of the last water drop. Once you have a rate from each hole, you average the results to come up with your test result. Your figure could be something like a one-inch drop in water every 40 minutes. This data allows you, or someone, to determine what types of septic systems are suitable for a specific isolation.

Soil types

What soil types are suitable for an absorption-based septic system? A great many types of soils can accommodate a standard septic system. Naturally, some are better than others. Let’s take a few moments to discuss briefly what you should look for in terms of soil types.

The best

What is the best type of soil to have when you want to install a normal septic system? There is not necessarily one particular type of soil that's best. However, several types of soil fall into a category of being very desirable. Gravels and gravel-sand mixtures are some of the best soils. Sandy soil is also very good. Soil that's made up of silty gravel or a combination of gravel, sand, and silt can be considered good. Even silty sand and sand-silt combinations rate a good report card. In all of these soil types, it's best to avoid what is known as fines.

Pretty good

Just as there are a number of good soils, there are several soil types that qualify as pretty good for septic tank installations. Gravel that has clay mixed with it's fairly good, and so is a gravel-sand-clay mixture. The same can be said for sand-clay mixtures. Moving down the list of acceptable soil types, you can find inorganic silts, fine sands, silty or clayey fine sands.

Not so good

Inorganic clay, fat clay, and inorganic silt are not so good when it comes to drainage values. This is also true of micaceous or diatomaceous fine sandy or silty soils. These soils can be used in conjunction with absorption-based septic systems, but the systems must be de signed to make up for the poor drainage characteristics of the soils.

Just won’t do

Some types of soils just won’t do when it comes to installing a standard septic system. Of these types of soils, organic silts and clays are included. So are peat and other soils that have a high organic rating.

I’ve read some pretty sophisticated books on the subject of soils. Believe me when I tell you that an entire book could be written on the subject of soils alone. Operating on the assumption that you are a builder and don’t wish to become a soils scientist, I’m going to spare you all the technical information that's available on various types of soils. As a builder, you must rely on someone else to design your septic systems, so I can see no need to go into great detail on the finer points of soil types.

Code-related issues

You and your septic installer must deal with code-related issues. For example, the code does not allow you to begin the installation of a septic system without first obtaining a permit. Don’t take a design and install it before obtaining a permit. The code officer might require changes in the proposed design. These changes are much easier to deal with on paper than in the dirt.

Local codes often control the types of materials that can be used in the construction of a septic system. The code officer, or some other official, has the requirements pertaining to the sizing of a system. This sizing procedure might be based on the number of bedrooms in a house or on an estimated daily use of plumbing facilities. In residential homes, the number of bedrooms is often the yardstick by which sizing requirements are made. Commercial and special-use buildings normally have their septic systems sized by using a table of daily water usage.

What else does the local code have to do with your installation? Inspections are a big part of the job. Some communities require multiple inspections. You might have to get the excavated site inspected before any further work is done, It’s almost a given that all the installations must be inspected and approved before they are covered up. A third inspection, if there is one, might be done to see if the system was covered and protected properly.

I talked about zoning in Section 4. The requirements set forth by zoning regulations must be observed when installing a septic tank. A code officer should inspect for set back, so you have to make sure that any system you install is in compliance with set-back rules and other zoning requirements.

Who is allowed to install a septic system? Does such a system have to be installed by a licensed plumber? I’m not aware of any jurisdiction that requires a licensed plumber to install septic systems. Some areas don’t regulate who can install a septic system. Other jurisdictions might require septic installers to be licensed, but the installers don’t necessarily have to be plumbers. In fact, I’ve only known one plumbing company that installed septic systems.

Septic systems are often installed by site contractors. The same people who clear building lots, dig trenches, and grade finished lots routinely install sc systems. Check with your local code office to see if there are any local requirements that dictate who can install septic systems. If you have access to a backhoe and dump truck, you might be able to install the systems yourself. The work is pretty simple, and the profit from installing a septic system is enough to get any co attention.

When it comes to code questions, it's always best to pursue answers on a local level. Rules that apply in Maine can be meaningless in Georgia. Changes in soil types, topography, and water tables can have a lot to do with the installation of septic systems. Talk to your local code office to get the facts as they pertain to your particular region.

As a home builder, you can take several approaches to the installation of septic systems. You can ignore the mechanics of septic systems and concentrate on subcontracting the work. Or, you can learn a lot about the various systems and consider installing them yourself for extra money. Then, there is the middle ground option, where you know enough to talk intelligently about septic systems with any installer and code officer, but you choose not to take a hands-on approach to the installation of the systems. Which option should you choose?

I have taken the middle ground. In the areas that I have built houses, it would have been acceptable for my building company to install the septic systems. I chose not to do the installations. Why? Mostly because of the cost of equipment. As a builder, I use a lot of subcontractors. My emphasis is on volume, and I use my time to sell houses, rather than build them stick by stick myself. It’s more profitable for me to sell a lot of houses and sub-contract the work out, instead of trying to grab every piece of work just because I have a work crew available.

Not all builders operate like I do. Many builders in Maine drive their own nails. Volume building is not a reality in Maine, so local builders make the most out of every house they sell. In Virginia, where I built homes in volume, builders did much more work in the office and in the sales arena than they ever did in the field. Only you know your personal circumstances.

It’s not difficult to understand the job of installing septic systems. Some of the labor can be rugged, but the technical end of the job is pretty simple. A good septic design tells you nearly everything you need to know. With the aid of a code officer or engineer, you can get any other questions answered on an individual basis. Unless local code requirements prohibit you from installing septic systems, you should be able to take on this phase of your building projects for extra money. The choice is yours.

When we get to Section 10, we are going to talk about pipe-and gravel systems. These systems are quite common and easy to install. Whether you want to learn how to install these systems on your own or just want a good understanding of your next septic job, you are going to enjoy Section 10. But, to get a break from septic systems, let’s turn to the next section and discuss drilled wells.

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Updated: Sunday, April 4, 2010 11:43