Note: This is part 3 of a 3 part series covering the design and install of conventional septic system for our cabin. Part 1 goes over my unsuccessful attempt to get a variance for a smaller system. Part 2 covers the considerations and process for designing a conventional septic system.
I rented a 1 ½ – 2 ton mini-excavator from our local Home Depot. It came with an 18 inch wide bucket. The excavator plus trailer was a too heavy to pull with our half ton truck so I had it delivered. As renting it for two weeks cost about the same as for a month, I rented it for a month. This was my first time operating an excavator so to practice I spent a couple of days digging a huge hole for the swimming pond we have in the works.
Site Preparation and Layout
A couple of days before the excavator arrived I staked out and painted the trench and septic tank locations. Several years ago Jean and made a detailed survey of our property including the location of the shed in the middle of your lot. I used the corners of our shed for reference points to locate the edge of the drain field. Then I used the 3-4-5 method to create a perfectly squared rectangle. Then I subdivided the rectangle into three 3 foot wide rectangles each separated by 4 feet. I worked backwards from there lining up the septic tank with drainfield.
The first step was digging the trench for the pipe run from the sewer stub out to the septic tank. The space was tight but the mini-excavator was narrow enough to get into the five foot wide space between the deck and the garden bed.
Digging the trench went quickly. Large limestone rocks that would have taken some effort to unearth by hand were dislodged easily by the excavator.
The septic tank required a hole approximately 8 feet long by six wide by 5 feet deep. That excavation took a good portion of the day. The most difficult part of the process was dealing with the spoils. My mobility was also limited by a couple of large trees. Once my spoils pile got to about six feet high I would drive to the opposite side and move the pile further away.
Once the hole for the tank was dug and cleaned up I put down a four inch layer of sandy loam soil in the bottom to provide the tank protection from any rocks.
The septic tank was bulky but not too heavy. Jean and I maneuvered it into the hole using several two by fours as rails and a ramp.
Digging the first drain field trench was little trickier as it needed to be level over the its span of 25 feet. The bottom of the trench also had to be a minimum of 18 inches below ground level. Once I began chewing up the terrain determining existing ground level was a little difficult.
The other consideration was the trench needed to be 36 inches wide. The bucket was 18 inches so I would dig out about a 4 foot long by 18 inch section of trench and then shift the excavator over to dig out the other half. In retrospect it would have been much easier if I had rented the slightly larger excavator with a 24 inch wide bucket and just installed 24 inch leaching chambers.
The rotary laser level was invaluable for keeping the trenches level. By the second trench I was getting a pretty good feel for keep the trench bottom elevation consistent. Dealing with the spoils continued to be a logistical issue. The first inspection required the full trench be open without the leaching chambers installed so I couldn’t back fill the first trench with the spoils from the second.
I cleaned up the trenches with a shovel, test fit the leach chambers and use the rotary laser level to set some grade stakes to confirm the trenches were level.
I called county development services Wednesday morning requesting an inspection. They called back that afternoon and we set up an appointment for Friday morning. For the inspection the septic tank had to be filled with water to demonstrate it was water tight. Unfortunately, I hadn’t understood the tank install instruction requiring the center column of the tank be back filled before filling the tank. Friday morning I noticed that the column had buckled. The inspector hadn’t shown up by noon so I pumped out the tank and pounded column back into shape back filling it with sandy loam and refilled the tank.
Over the course of the next week I left several messages with development services requesting an inspection. I finally got in contact with another inspector and set up an inspection for the following Monday. Fortunately, I had some other projects for the mini-excavator so that time didn’t go to waste. More on those other projects in future videos.
The first inspection went smoothly. This inspector showed up on time and checked the various excavations against the plan I had submitted. I had altered the septic tanks orientation to the drain field but she didn’t comment on that. I set up my laser level and demonstrated at a few locations in each trench that it was level. She checked that I had a two way sewer stub out at the cabin and there was adequate slope from the stub out to the septic tank. She observed that the tank was plumbed with tee fittings, full of water, had an effluent filter and that there was a least 1 foot of drop between the tank outlet and bottom and the first trench. The whole process took less than half an hour.
Next I connected the sewer pipe from the house to septic tank, put the leaching chambers in the trenches, connected the ends with 4 inch pipe. I used two elbows at the end of a run so that when the chamber fills up it will overflow into the next run of chambers below.
I wasn’t able to schedule the second inspection until Friday morning. She confirmed that the chambers were in the trench and snapped together and plumbed at the end of each run and to the septic tank. She also observed that the septic tank was plumbed to the sewer pipe running from the cabin. The only comment she had was that I needed to install splash pad where the pipe fed into the chamber. A flattish rock or piece of plastic would work to I guess prevent incoming water from eroding trench bottom. I told her I would install them and passed. This inspection took about 15 minutes.
I spent the rest of that day hunting day and installing a replacement battery for the one with melted terminal to get our house battery bank back on line. More on that here if you interested.
Over the weekend I back filled the drain field and septic tank excavations. Initially, I was really concerned about driving over the back filled drain field with the excavator. The leaching chamber specifications say that with at least 12 inches of soil covering them they will support up to 16000 lbs (per axle). The mini-excavator is about a quarter that weight. The treads also serve to better disperse the weight then a vehicle with wheels.
The mini-excavator went home Monday. The next week I rented a mini-skid steer to do some grading. I also had a total of 15 yards of top soil delivered to give my cover of clover, wheat grass and wild flowers a good start.
The final inspection took about five minutes. She observed that the trenches had been back filled and that access ports to the septic tank were securely fastened down. So, with the final inspection passed the cabin is now legally habitable.
Permits and Fees
The engineering fees partially covered attempting to get a variance and advising me on the conventional design. Per Texas administrative code chapter 285, you don’t need a professional engineer involved to design a conventional system.
Sand or Sandy loam is required for tank bed and for back fill (~5 yards in my case). The top soil (~10 yards) was more of an aesthetic choice.
The skid steer was technically not required but made moving the 15 yards of soil I had delivered easier. Renting the mini-excavator for two weeks instead of an entire month would have saved a few hundred dollars.
For time, site layout and prep took half a day. Trenching from the cabin to the tank and digging the hole for the septic tank took a day. Digging and cleaning out the drain field took two days. Installing and plumbing all the components totaled to about a day and back filling the excavation took a day. Grading with a mini-skid steer was perhaps half a day. Taking into account inspections I think it would be reasonable to install a similar size system in a two week time frame.
Here are more pictures if you still haven’t had you’re on-site sewage facility fix: