Sump pumps, Draingage, and Ground water

Factual Questions
1

Prompted by this thread - Downspout drainage - grading properly - Factual Questions - Straight Dope Message Board -

We live in a relatively new house (2006 construction) which is built into a hill, i.e., the basement is full exposed in the rear and completely underground in the front. Our neighbors recently pointed out to us that our sump pump seems to be flushing/ejecting water every fifteen to twenty minutes. I assume there is a bit of hyperbole in their time estimate, but they insist it is “way more than normal”. (We never noticed it because we’re usually not near the ejection pipe and the windows are usually closed.)

My first question is why we even have a sump pump? The house is built into a fairly steep grade – about 10 degrees – and I assumed the water would just follow gravity and run away from the house. (Apparently we do need one as the pump is active).

Second question, is it normal for a sump pump to operate as frequently as my neighbors insist it does?

Third question, the rear of our property has a 4’ tall rock retaining wall in the back of our property – there is a public walking trail running alongside the wall. The wall has PVC pipes jutting out of the bottom for what I assume to be drainage of water from behind the wall. But the inside of the pipes are always dry, actually contain debris. Is this normal – the ground around here is all clay, water here seems to evaporate rather than soak into the ground.

The ground is basically clay

2

I can only touch the second question, and even then only with anecdotal information.

My two (yes, two) sump pumps (34’ x 24’ full basement) each cycle every 7 to 10 minutes from roughly February to April or May. If I lose power for 45 minutes, I have one to two inches of water across my basement floor. Each sump hole has a primary pump and a 12 volt battery backup pump, and I have a generator (manually started, though). The hydraulic pressure outside is enough that I will get little fountains about one to two inches high of bubbling water around the edge of the foundation wall after a couple days of heavy rain. The fountains coincide with the two foot deep moat that develops in the back yard.

My neighbors all have similar water problems. I live in a wet area, though. Think of it as a bathtub made out of ledge about 2 miles long, a half mile wide, and about 80 feet deep in the middle. You can even see where the ledge comes above ground on the edges of the “tub.” The tub fills with water through the winter, and it evaporates below the level of our basement floor by June. In fact, looking at the water levels in the pump holes (which I do often), you’ll see water in the holes one day, and dry soil two days later. The level drops that fast.

Everyone’s leaching fields are also raised above ground because of the high water table - a great way to ruin a good yard.

I concede that my water problems are not “normal” by any stretch, but rest assured, you’re not the only one out there with a wet cellar.

3

Normal run times depend on circumstances. Find the pump and be sure it’s sensor works properly. In wet conditions it may come on every few minutes.

4

I’ll take a very rough stab at this, and we can refine it if I misunderstood.

Sump pumps, as I’m sure you know, take care of groundwater - that is to say, the water that is below the ground surface. Groundwater is funny stuff; I’ve worked with geotechnical engineers and hydrogeologists long enough to know that nothing below grade is completely predictable. In general, the “topography” of groundwater (it can be modeled and contoured) will follow the surface topography, but there are plenty of exceptions to that, so we should just start out with that as only a preliminary assumption. And the groundwater surface is not static like the ground surface. It can fluctuate based on weather conditions, or whether people are drawing it down with wells. Your neighbor up the street could put an addition on his house, and you might see results months later. So there is a great deal of “maybe” in this response without having groundwater monitoring wells or test pits.

Your sump pump’s frequent on/off behavior may be from two common occurences, although I am doubting it from your description. Nevertheless, let’s get them out of the way.

-Sometimes pumps “cycle” because of a very small difference between the float switches. If your pump “on” float turns the pump on, and it ejects the water in the sump, and quickly reduces the water in the sump, the pump will then shut off. If there is no valve to keep the water in the pipe from returning, this water will come back into the sump, triggering the on switch, ad nauseum. If so, you may be able to adjust the float switches or make the sump larger to avoid this.

-Or, the outfall for your sump pump could be discharging quite close to your sump. The water quickly finds its way back down below grade and fills the sump. In this case, extend the pipe discharge away from the sump.

Let’s say that neither of these is applicable.

Your house has a high ground surface in the front, and a low ground surface to the back. Let’s start with the assumption that the groundwater is following the existing grade. Therefore, the groundwater will be higher along your basement wall to the front, and will slope down to where it is some distance below your basement slab in the rear. By the time it reaches your retaining wall (I am assuming here that the high side of the wall is on your property and the base of the wall is the walking path), the groundwater is far enough below grade that the weepholes (relief drains in the wall) are not seeing any water.

In a perfect world, your builder should have installed a subdrain (sometimes called a french drain) around the footings of your foundation. This is typically a PVC perforated pipe surrounded by crushed stone, and that whole package surrounded by an envelope of filter fabric. The idea is to intercept groundwater at an elevation lower than your basement slab so that your basement won’t see groundwater.

It is possible that you have this, but instead of discharging to the rear of your property, the pipes enter through your foundation walls and into your sump. Can you see pipes coming into the sump hole? If so, you may be able to connect to this line (at the rear of your house if it extends all around the footings) and take the drainage away from your house. If not, then you may not have a subdrain.

And that’s too bad, because the cost and effort to install this is laughably small compared to the cost of the foundation work - at least before the foundation and footings are backfilled. If you want to do this now, you will be in for some backbreaking work at the front of your house. If you are up for the work, or know someone who has a backhoe, you could install it now.

But I wouldn’t have it discharge to your sump. The sump should be the emergency backup for any water the subdrain didn’t intercept. The ideal solution would be to bring the discharge pipe to the rear of your property and have it “daylight” or discharge at grade elevation. With a 10% grade, it shouldn’t be too long to reach grade.

5

Thanks to everyone for your responses.

Part of the problem may be the discharge too near the sump. The discharge pipe comes right out of the wall and used to hit a splash block. My wife reports the splash block seems to have been moved – large washed out area underneath the discharge pipe.

I’m going to attach one of those flexible accordion-like plastic pipes to discharge further from the house and hope that works.

Short of asking the builder – who I wouldn’t trust to give me the time of day – how can I tell if a french drain was installed? I haven’t looked into the hole yet but it doesn’t seem to make sense to run what a french drain captures into the sump – just making the sump do work that gravity and pipes could take care of. Of course with my builder anything is possible.

6

While you are checking things, also check where the discharge for your gutters are. In my last house, you could always tell when the downspouts got disconnected from the extenders at the bottom and dumped their water right next to the house. Suddenly the sump pump would be running every couple of minutes, when it rained.

I hope that’s your problem, because that’s an easy fix.

7

If a house were to be constructed on level ground, where the ground surface is very high up on a basement wall, and if there is an anticipated high groundwater table, some home builders will install this subdrain around the building footings, and have it tie into a sump inside the basement.* As the groundwater rises and enters the pipe, it will fill the sump and be ejected by the pump. If your builder is used to putting these drains in, and if he really wasn’t thinking when he built your house, he may have just installed a perimeter drain without considering bringing the discharge ends to daylight. If you can’t see pipes entering your sump, then it is unlikely you have a subdrain. But you are absolutely correct - any system that works by gravity is far superior to one that relies on pumping. Every so often I work on a project where someone suggests pumping storm drainage and I grit my teeth. When rain comes down, it can come down hard and you have to size the system for a worst case scenario. This means big pumps, and now you have the possiblity that these pumps may fail when you need them most (power shortages during a storm) which means backup emergency generators. The same applies to your sump pump. Gravity may only be a theory :), but in this case it is your friend.

Tastes of Chocolate brings up an excellent point. All stormwater (whether from downspouts or from your sump pump) should be discharged as far from your house as practicable.

*You may have seen ads for basement refinishing companies who promise to get rid of water in your basement. One of their techniques is to install a subdrain inside the foundation wall by jackhammering out your basement slab where it meets the wall. Same thing, they tie it into a sump and have the pump discharge to the outside.

8

Start with city hall.
Depending on where you live, the city or county may have plans which were filed when the building permit was issued. Or not…depends on local requirements, procedures, and if the record archive got flooded or burned down.
Oh, of interest: You can get backup sump pumps that are driven by city water pressure. Eductors I’d call them, but not sure what the trade term is. They waste water, but only run when the power is out.