The source of the City of St. Helens drinking water is supplied from Ranney collector wells that are mechanically dug down 80 feet into the West bank of the Columbia River. The bottom of the collector well penetrates the zone of water saturated rock, sand and gravel, that lays far below the river bed of the Columbia river called the aquifer.
The Ranney collector well is a very large, vertical, steel reinforced concrete cylinder called a caisson (case-on) that has an outside diameter of 16 feet. The caissons installed in St. Helens, have 7 steel reinforced sections, called “lifts”. Each “lift” is 12 feet high, with a 13 foot inside diameter, (the walls of the caisson are 18” thick). Each “lift” or section of the caisson, has a tongue and groove joint on the top and bottom of the cylinder that allows each lift to be stacked, aligned, joined and tightly locked together.
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Construction of the Ranney collector well Caisson
The first section of the lift is a beveled cutting edge as seen in the picture above. Each of the sections or lifts, will be built on site. The inner wall of the cutting foot has been built,and here you see the rebar has been fabricated, then the outer concrete form will be built around the rebar and concrete will fill the form completing this section of the caisson. The caisson is carefully positioned on the river bank where the well will be dug. An overhead crane with a clam shell attachment is used to scoop out the mud, sand, rocks and gravel from the inside of the caisson. (pictured below) As the mud, gravel and water is removed, the “lift” or section of the caisson steadily lowers into the river bank. When each caisson section has lowered about 6 feet into the riverbank, another lift is stacked on top, locked into place and the digging resumes.
When the caisson reaches the designed well depth, a grid-work of steel reinforcing rods, rebar, is constructed creating a floor inside the caisson, as seen in the picture below. The beveled part of the cutting foot of the caisson is 12′ below the workman’s feet. They are standing on the water saturated gravel of the aquifer which is 40′ below the Columbia river riverbed. A pump is running constantly as the workman are building the floor because the surface of the Columbia river is 50′ to 60′ above their heads, on the outside of the caisson, and by the nature of gravity and natural hydraulics, the water is trying to get into the caisson by coming up through the gravel floor. The floor they are building is being attached to the ported section of the caisson where the lateral infiltration gallery of screened pipes will be installed out into the aquifer. When the reinforcing grid work of steel is installed, concrete is then poured over the rebar to a depth of 42”. The concrete creates a permanent plug that seals the inside of the caisson from the river, and the surrounding aquifer, and makes a solid concrete floor at the bottom of the collector well.
The Infiltration Gallery (laterals)
The second section of the caisson that was stacked on top of the beveled cutting foot, is where the infiltration gallery lateral ports are located. Through these ports the lateral screens are going to be pushed out into the aquifer using the green hydraulic tool seen in the pictures below. The hydraulic jacking tool is bolted down to the concrete floor, and walls, of the caisson, then a 10′ foot section of 10 inch, slotted ductile iron steel pipe, called laterals, are pushed out through the port from the caisson, into the sand and gravel aquifer. (As you can see in the cutaway illustration at the top of the page, the infiltration laterals are arranged out from the caisson like the spokes of a wheel). The first section of the screened, or slotted, lateral has a beveled, or pointed, end plug to close off the open end of the intake pipe as well as assisting the screen to be pushed out into the gravel aquifer. The screened infiltration gallery laterals are 40 feet below the riverbed of the Columbia river and 60 to 70 feet below the surface of the Columbia river.
A point of interest: The layers of mud, silt, sand and gravel that the Columbia riverbank filtration process supplies to the infiltration gallery laterals, provides an abundant, consistent, dependable, supply of high-quality source water, with a constant year round temperature, low turbidity (not muddy or murky looking) as well as low levels of undesirable constituents and contaminants, such as viruses, bacteria, pesticides, petroleum and pharmaceuticals. The riverbank filtration action also provides an additional barrier to reduce precursors (organics like algae, peat, tannin, leaves, submerged decaying trees, petroleum from boats etc.,) that can form disinfection byproducts during treatment.
Intake screens on the infiltration gallery
There are several infiltration gallery radial well intake screen configurations, some are machine slotted perforations, another is a continuous wedge wire wrapped around a framework of stainless steel ribs, (picture below). Generally the spacing between the wire wrapped screens is 1/8″ and the machine slotted perforations are 1/4″ to 3/8″ wide. (The spacing between the wire wound screen is given perspective when compared to the size of a penny. You can also see behind the penny, the framework of ribs the wire is attached to).
NOTE: There are more pictures of various screens in the slideshow
A short Ranney collector well history
The well is named after its inventor, a petroleum engineer, Leo Ranney. In the early 1920′s, he built his first oil collector well in a Texas oilfield. His idea was that he could gather larger quantities of oil by digging a single, large borehole (caisson) and then horizontally punching out from the caisson, with hydraulic jacks, slotted collector pipes into the oil rich bearing strata. The dozens of screened collection pipes would let the oil drain from the surrounding area and flow, by gravity, into the caisson. His concept was that he was going to be able to gather more oil from a larger underground oil field with his central caisson, than drilling many individual wells. When the bottom fell out of oil prices in the early 1930′s, Mr. Ranney’s method of an oil collector well technology was applied to tap into another vital and extremely valuable natural resource, a fresh, rich, clean, underground water supply to meet the demand for a pure drinking water source. The first Ranney collector wells to recover groundwater was dug in London, England in 1933. In 1936 the first Ranney collector well in America was installed in Canton, Ohio. Since then, hundreds of these wells have been used all over the world, including St. Helens, Oregon. return to top
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