If I ever had any notion that the basement/dampness problem was forever solved by the work done and described in House vs. Water, Round One, it was effectively dispelled less than two months later (January 2019) when follow-up infrared scans revealed underground water contact that had not been detected before. This time it was along the front, rather than the back, of the basement walls and foundation. Scans repeated every couple of days thereafter for the next three weeks showed that the area of water contact was increasing at a rate of roughly one foot per every five days along the front basement wall, and about one foot per week from the outer edge of the footing toward the center of the basement. This was, to put it succinctly, Not Good.
The height of the subsurface water was constantly changing every few days as well, from a low point of 9 feet beneath the soil (existing grade) surface to a high point of less than 4 feet below. Because the footing (house foundation) is approximately six feet below grade – basement has a 7-foot ceiling – this meant that not only was the underground water exerting pressure on the walls and footing, but it was also creating erosion via movement: Think of how the incoming and outgoing action of ocean waves will pull sand from beneath your feet if you stand in one spot on the beach as the waves roll in. Substitute “my house foundation” for those feet and that’s the effect this underground water was producing.
Locating the Enemy
The first step was to determine where the water was coming from and the path it was taking to get from there to here. Again, infrared scans to the rescue.
The origin of the stream is a large pool of subsurface water located in the center of the nearby intersection of my street with the one crossing it (blue X on photo.) In terms of general topography, the intersection is downhill – although not dramatically so – from the northern part of the street my house is on. The cross street (going from left to right in my photo) is fairly level. The concrete slab is a section of my driveway. The outside edges of the underground stream are marked with yellow flags.
This photo was taken from the same corner but looking toward my house/driveway. The approximate width is between five and six feet.
The stream widens out a bit as it gets close to my driveway, but narrows again as it travels beneath it.
It narrows even more as it passes from the driveway area into the foundation-plantings area, and then swings to the right in a narrow curve to dead-end smackdab against my house foundation. (The frontmost part is the same area where the Killer Viburnum lived before being finally yanked out, roots and all, last year.)
In simpler circumstances the typical approach would be to create a barrier to prevent the water from making that right turn. Unfortunately this one wasn’t that simple.
Remember the Big Dig in July 2018 when a new cesspool had to be installed in the front yard? It is pretty much directly on the opposite (left) side of the walkway shown in the above photo. Any barrier across the front of the house – which wouldn’t be possible in any case because of the waste line extending directly outward from the house foundation, just to the left of the righthand white column and running under that walkway – would make the water “back up” directly into the cesspool and quickly fill it up with water, which would then start backing up into the house plumbing. A situation that would be Definitely Not Good.
The Battle Plan
Therefore, the stream had to be diverted from reaching either the house foundation OR the septic system. In other words, it had to be stopped at the ‘inside driveway edge.’
Another wrinkle is that although lots in my neighborhood are larger than average (at minimum ½ acre zoned) homeowners must be careful not to create a water problem – either surface or subsurface – for adjacent properties by making changes in theirs. (Things like that keep lawyers busy.) So, we had to change the path of the underground stream but without affecting either my neighbor’s property or the public street.
Eddie (you remember him from House v. Water Round One) designed an underground dam that would effectively divert the stream while not impacting anything else. Well, anything else but my budget/bank account, that is. We’ll quietly tiptoe away from that subject; suffice it to say that this sort of work does not come cheap. The dam shape and location were plotted out to run from the front of my house about 75% of the way toward the street. It would be of poured concrete with iron rebar for stability.
Unlike a traditional dam, though, it would not be a solid rectangular structure but instead rather like a hair comb: solid along the top section but with evenly spaced concrete “teeth” extending downward from that upper part, for a total depth of about ten feet. At its first contact with the barrier at the location shown in the fourth photo above, the underground stream’s momentum would be broken as it began to swirl as well as shift toward the left (away from the house.) The water’s flow would continue to swirl, shift, and increasingly diffuse until finally, by the time it reached the outermost end of the dam’s length, the flow would have been broken down completely. It was now time to mobilize the forces and dig.
The dam begins close to the house at the very edge of the existing (and to be replaced a few months from now) driveway and about 24” in from same. The root ball belongs to a mugo pine that gave its life in the line of duty; the weeping shrub is a dwarf redbud that seemed to be barely just out of harm’s way.
Looking from that point toward the far end of the dam, as the digging began. The redbud is just out of the photo, to the right. The cesspool area is on the other side of the (still small) pile of dirt.
The trench gets deeper..
…and deeper. This will be the top of the “comb”. The entire dam had to be excavated by hand.
Next, the holes are bored into the soil for the “teeth.”
As the digging took place, the soil composition turned up (no pun intended) some surprises. This general area is clay soil but at about six feet down they started hitting rocky sand, and then below that, a vein of very fine white “beach” sand that no one would expect to see in this area. Such sand would indeed be expected about 20 miles south, where the soil is glacial washout instead of glacial moraine as hereabouts … but not here. Was this fine sand a geological oddity or was it brought in for some reason when the house was built during the 1960s? (and if so, why?)
The typical subsoil here is rocky clay. There was also a deposit of bank run, another puzzlement unless the area has a long history of rivers and streams (a distinct possibility) in the past.
But even the normal and “beachy” sands were not rock-free.
Anyway, back to the war zone. Even with heavy clay soil, as the excavations went deeper the sides had to be shored up against any risk of collapse.
The cement mixer arriveth! Eddie ordered a very specific mix that is normally used by the Army Corps of Engineers (his alma mater) for dam construction. It is so dense and sets up so quickly that it can even be poured completely underwater.
Rebar was laid across the bottom of the trench before the pouring began. The concrete first filled up the “teeth” posts and then began forming the solid top section. The end result would be a monolithoic structure of solid concrete.
The concrete had to be smoothed and leveled so that the construction would be of the same density and strength throughout. When finished, the top of the dam ended up 24” below the soil surface. Clearly this will impact whatever might or might not be planted above or even close to it, and vice versa.
After rough grading, a few days’ settling showed the outline of the dam, so it was smoothed out again. Obviously this is all subsoil, the original topsoil having been another casualty of war.
Before they began filling it with concrete I made a sketch, with measurements, of the exact location of the dam so that I will know what can and can’t be planted in this area in the future.
They also spread the extra subsoil over much of the front yard, which admittedly had some “grading issues” already due to prior removal of trees and so forth. This was how the area looked for the next several months….
…and after the rebuilding of the front walkway in the spring. The next steps taken in this area are related in Tales From The Money Pit!
Unfortunately I forgot to take photos of an auxiliary part of the battle plan which was the excavation down to the house footing in the area directly affected, and that side of the foundation sealed against water penetration. In so doing it was discovered that a section of the footing had already been water-eroded to such an extent that the concrete holding the stone/rock matrix together was almost completely gone; the next phase would have been the settling of that part of the foundation and of course the house walls it supports. The area was repaired and strengthened before that excavation was backfilled.
So who emerged the victor in Round Two: House, or Water? Follow-up scans over the past four months have shown that the dam is working exactly as it was designed to: the stream now stops, swirls, shifts, and diffuses out to a complete non-problem at the far end. The basement walls and foundation are back to being within normal moisture limits on that side of the house. In idle moments I do wonder what a distant-future owner of this house will think if they ever take it into their heads to plant a tree over that “dam district” someday though…
In the two bouts thus far, I think it’s fair to say that it’s a draw – but only because the optimal anti-water campaign could not be waged in Round One, whereas it could and was in Round Two. My fervent hope is that there won’t ever be any need for a Round Three!