alphonse, what and where are vents??...sorry, I'm clueless to this stuff...but the internet was totally my idea.

As Alphonse already stated, the water closets (toilets) are not the problem. In fact, Water closets are the exception that proves the rule prohibiting S-Traps.

The trapway that is built into a W.C. is an S-trap. When we flush a WC the water quickly fills the trap and as it spills over the top of the built in S-trap it starts a siphoning action that assists in lifting the remaining water & solid particulate matter up and over the top of the trapway. The siphoning action literally sucks the water from the bowl, leaving the bowl relatively empty. This is permitted by code only because all WC have a built in Trap primer, which is a small line from the water fill valve to the top of the standpipe in the tank. In this manner as the tank refills a small portion of the incoming water is diverted down through the standpipe and into the bowl to refill the trap water level, hence the term "Trap Primer". In a power assisted flush system water is forced into the bowl under pressure to insure the trapway fills quicker than it would by the normal gravitational fill from a conventional tank system, however once the water passes through the trapway and into the drain line under the W.C, it then flows from there under gravity flow regardless of what type of W,C, you have.

Having already ruled out collapsed or broken pipe and tree roots I suspect one or perhaps more of four potential problems, two of which can easily be corrected, while the other two could be very expensive.

1. As Alphons already meantioned, you could have an clogged roof vent. If the vents are clogged or even partially obstructed it could interfere with the flow of vent air into the pipes. Under extreme load conditions this could lead to a partial reduction in vent air pressure (slightly below standard atmospheric pressure). This reduction in vent air pressure results in a partial vacuum that retards the velocity of flow in the pipes. The solution: When you have your pipes snaked insist that they also check the vents and clean them if necessary.

2.You stated that you went outside and observed water gushing out of the cleanout. If water was gushing out of the cleanout obviously the cleanout is not properly capped with a threaded cap. Keep in mind that the plumbing codes require a house sewer line to be a minimum of 1"=' below grade, and in regions subject to frost it must be 6" below the average frost depth for your region. Water in a vertical column exerts 0.434psi per vertical foot, therefore if that cleanout was properly capped the vertical static head pressure required to lift that water to the top of the cleanout and force it to gush out would then be pushing the water through the pipe and on to the municipal sewer. This may account for why you have the problem and not your neighbors. Solution, Cap the cleanout with a threaded cap to trap the pressure in the line.

3. The house sewer line may be too large in diameter for the load on your structure. This may sound silly, but properly a drain line should be sized so that at full load the pipe is 1/2 full. In this manner the depth of the liquid in the line is sufficient to suspend any solid particulate matter while still keeping the upper half of the pipe open for the free transfer of vent air. Before the advent of water saver technologies W.C.'s discharged 3.5gpf or 5gpf and they required a 4" diameter waste line to maintain the proper 1/2 fill. When the 1.6gpf water saver type W.C. were first introduced they had many problems with clogged lines until it was realized that at 1.6gpf the 4" lines were barely 1/4 full, thus the depth of the water in the line was not sufficient to properly suspend larger chunks of fecal matter or other solid particulates that may get into the drain line. The result was that the solids then rubbed the bottom of the pipe, causing a reduction in the velocity of flow, and quite often the liquid flowed around the solids and on down the pipe, leaving solids laying on the bottom of the pipe where they dry in place once the liquid has passed. While the amount of solids that accumulate with any given flush may be quite small, none the less, with each successive flush more and more solids accumulate until you have a clog.

In the code we have tables that list all the fixtures in a structure that might be connected to the DWV (Drain, waste & vent) system and each fixture is assigned a DFU (Drainage Fixture Unit) value. When determining the size of any drain line we must first list all the fixtures that will discharge into that line, then compute the total DFU value for that particular line. From the DFU load value we then consult another table that tells us the proper size of pipe for that portion of the load. In turn, we must then compute the total DFU load for the structure to determine the correct size for the house "Main Drain" & "House Sewer".

Although it is impossible to give and exact answer without having a list of all the fixtures in your house, none the less, as a rule of thumb, if you have 2-1/2 baths or less, in all probability the proper size sewer line for your house would be 3" not 4",

4. You may not have sufficient pitch in the drain lines. Under the IRC(International Residential Code) all lines 3" or less are required to be installed with a 1/4" per foot pitch, however under extreme circumstances, such as when you have a long house sewer line and where the municipal sewer line if fairly shallow the code will permit a 3" line to be run with a 1/8" per foot pitch, with the expressed written consent of the local plumbing inspector. On the other hand, 4" lines are only required a 1/8" per foot pitch. If we then combine the shallow pitch with an over sized line the end result would almost certainly be exactly the problems you are experiencing.

Note: Under the UPC (Uniform Plumbing Code) all lines 4" or less are required a 1/4" per foot pitch but again, under extreme circumstances a 4" line may be run with a 1/8" per foot pitch with the approval of the local inspector.

the Mystery is solved! But i am sooo saving all this information because after what i discovered today added to this stuff, I'm learning a ton. The city came out, agreed to dig down to the main and found that our sewer pipe that connected with the main by a "saddle" fitting had slid right thru the fitting down into the main. I guess it did it gradually and had finally fallen all the way in because nothing was getting thru. Got a new saddle fitting and pipe, cemented it in so it wouldn't go anywhere this time, created another cleanout trap, filled in the swimming pool-sized hole and we are NOW FLUSHING!!! It really was a bigger deal than everyone thought it would be---12 feet down. Thank you city subcontractors!!! I made them cookies and margaritas for their hard work in 95 degree heat! And thanks lazypup and alphonse for your quick help. I'm starting to get it.
Lazypup(you must be a plumber or a professor, or both)I went thru your points and here's what i have--for anyone's curiosity:
(1) roof vents were in good shape.
(2) water really was gushing out of the cleanout, but it's threaded type that has this flappy valve on top that allows water to escape. we installed that instead of a solid one because the last time this block happened the entire contents of my son's shower overflowed the downstairs toilet...what a mess.
(3) sewer lines were code, they looked about 4".
(4) pitch was to code. at about 50 feet from house, it went from 22 degrees to 90 degrees and that's the pipe that fell in to the sewer (I think they called it a "stack")

gracias to all!

Here is a link that might be useful: {{gwi:2027061}}

trying again.
One of my heroes in a 12 foot hole in Houston's humid heat:
{{gwi:2027069}}
New Saddle Fitting that "rides" on the sewer main:
{{gwi:2027071}}
New pipes, "stack?", and new cleanout in progress:
{{gwi:2027074}}

No 'shoring' in that hole. He could have very
easily been a dead hero.

Umm. I would guess that OSHA would be interested in these pics.

People are injured or die all of the time when the sides of a hole cave in and trap the worker. They suffocate because the dirt around them prevents them from inhaling - even if their head is above ground. They also suffer oxygen deprivation to the limbs because blood flow is cut off.

To prevent that, various methods are used including a metal structure lowered into the hole to prevent it from collapsing while the worker is below grade. Trenches over 5 feet deep must be supported in such a manner. To be down 12 feet with no protection is major stupidity and the supervisor should be called out on this.

Really - make an issue of it... Send the photos to OSHA and the head of the department. The next guy in the hole may not be so lucky. Education and enforcement is the only way not to end up with dead workers.

Here is a link that might be useful: Trench OSHA Quick Card

Naw, that 'ol boy has poop pertecters on his kaboy boots, no wainhell mud gettin' him!

I'm sure you all mean well. But this was all done properly. Houston earth is like clay. Things are done differently in different parts of the country. I only posted the pictures to be helpful to someone else. I'll definitely be more judicious here next time.

You didn't make a mistake - you don't need to be judicious.
This is a plumbing forum, it's a teachable opportunity and other people are reading this.

You don't go into a hole that's 5 feet or more deep without shoring. I don't see any exceptions to that OSHA regulation for Huston, or clay soil, or different parts of the country, etc. Unfortunately, people do it and people get injured or killed. These were professionals and they should know better.

For DIY's the way to avoid this is to know the 5 foot rule. For professionals, there's additional information about sloping the sides, not having machinery near the trench, etc

Here is a link that might be useful: Examples of trench accedents

Wow, I'm sorry you pulled the pictures. Really.

What is done is done - but you aren't doing anyone any favors, very least of all the guy who is told to go down in the hole the next time.

The only way to not have guys buried is through education and enforcement.

Kacy27, I worked cross country pipeline through different parts of Texas. Everywhere you got ditch it was WIDE because of the sandy soil, not much danger of collapse when the banks are so far apart. When I saw your pic, I figured that was clay and probably wet to boot (all puns intended), maybe caliche. Local hands know conditions better than weblookers...but the law is the law.

The images are gone.
I can relate to a tranch accident. When they were replacing the sewer line from my house to the street the guy almost got burried. He was down in the 8 foot trench and as I was watching him I saw the side start to shift and yelled my $%&@( off to get out of the trench. He got out of the way quick enough or he would have been under 6 feet of clay.

Lazypup, Your explanation of the situation is an education for me. Does this mean that with millions of older houses that replaced old 3.5-7 gallon toilest with 1.6 or smaller toilets, we could expect more clogged waste pipes after some time? Has this been happening and showing up in different areas or national statistics?

I am just about to update the last of four toilets in our 60-year old house. Hope I do not now have to look forward to periodically getting the waste pipe snaked out to the main sewer line every couple of year or so?