Well construction: how to make a caisson

Recommendation points

• How to dig a caisson
• Wall and floor arrangement
• Overlapping and blind area
• Caisson technical equipment

If there is not enough free space in the yard, a caisson is the best way to equip the head of the well and place a pumping station. As a result, you will get a discreet hatch, under which an arbitrarily complex set of equipment will be located. Let’s talk about the correct arrangement of the well and how to make a caisson with your own hands.

How to dig a caisson

Upon completion of construction, the well looks like a piece of pipe sticking out of the ground. The soil in the surrounding area is usually very eroded due to previous flushing of the well, so it is best to let the site dry out for several weeks. Close the headband almost immediately after drilling..

Up to the installation of the caisson cover, the casing pipe will remain in its original form, including during excavations. First, you need to mark the area by defining a round or rectangular section corresponding to the outer dimensions of the caisson.

There are no strict requirements to which part of the chamber the well casing should protrude. It can be right in the center, it can be at the very edge – it depends on the layout of the equipment inside the chamber, its shape and size. The only wish is to position the manhole in the plan so that the well is directly below it. In this case, it will be much easier to reach the suction pipe with a pump, especially if it is rigid and long..

There are two options for carrying out land works. In the first case, the soil is removed below the freezing depth, forming a rectangular pit with straight walls approximately 1×1.2 m in size.In the second case, the sod is first removed to obtain a round pit, into which a concrete ring with a quarter is installed. Further, the digging is carried out by a sequential method.

The choice of method depends on the characteristics and stability of the soil, the availability of suitable materials in a particular area. The best type of device cannot be named, however, with a low GWL, reinforced concrete rings look like the least costly method of device. If the caisson is potentially susceptible to flooding, it is better to think of nothing better than a monolithic concrete box.

Wall and floor arrangement

In fact, there are much more options for the execution of the caisson. It can be a plastic, steel container, or a masonry structure made of small-sized blocks. And again, it will not be possible to name a contender for the title of the best device method. And all because, in addition to material costs, there are also labor costs, besides, we repeat, not every decision will be optimal for a specific combination of indicators:

• groundwater level;
• density and hardness of the soil;
• the action of the forces of frost heaving.

However, it is safe to say that in terms of the floor, it is a monolithic reinforced concrete slab that will be the best solution among others. It will evenly distribute the load on the ground and, with properly performed waterproofing, will not allow groundwater to seep inside. The floor should be cast over a layer of incompressible bedding: 10–12 cm of compacted gravel and 5–7 cm of sand compacted with water. A wide canvas of polyethylene film with a thickness of at least 150-200 microns should be spread over the bedding as an external hydro-barrier. The edges should be tucked on the sides of the pit for about a meter for subsequent joining with waterproofing walls.

The floor and walls are cast in concrete of class B25 or higher. A reinforcement mesh is laid on top of the film on 50 mm distance plugs, the same protective layer will be poured over the reinforcement. With a small area, the floor of the caisson can be reinforced with a self-knitted mesh made of 12 mm reinforcement with a periodic profile with a cell of about 100–120 mm. However, the caisson is often combined, for example, with a compressor station of a car wash or a pool water treatment complex. In this case, it is better to use factory-welded gratings with a rod thickness of 14 mm or more. On each side of the mesh, an anchor bent at right angles is tied to every second rod of the mesh as a foundation element for connecting with wall reinforcement.

The walls are also cast quite simply, but you need to correctly orient the overlap of their waterproofing with the floor. It is optimal to use polyethylene film supplied in closed sleeves for these purposes. The total width of the curtain is up to 6–8 meters, so this is quite enough for a small caisson. The sleeve is inserted into the pit and straightened, while the protruding edges of the underfloor insulation are bent down and overlaid on the walls from the inside. The upper joint along the entire length is glued with tape.

The casting is carried out on the OSB panel formwork. Inside, spacers are installed from a bar in the form of two or three square grids 2×2. The walls are reinforced with vertical rods of 10–12 mm every 25–30 mm of the wall, as well as horizontal rods of the same dimensions in the amount of three pieces per wall. In the corners, the reinforcement is connected with a bent anchor. The thickness of the casting is very small – about 60 mm, therefore it is recommended to fix the reinforcement on the spacer rings.

Overlapping and blind area

Further work is carried out a week after the casting of the walls. By the way, they need to be brought up to a level 30–40 cm below the planned yard cover. After the concrete has set, the rest of the pit can be excavated 35–50 cm in each direction, bringing the bottom to the height of the walls. A small groove with a depth of about 10 cm is also dug around the perimeter of the expanded pit.After casting, it forms a side of the concrete cover, which prevents the fluid from flowing under the forces of surface tension.

It is also necessary to build a panel formwork. For this, U-shaped supports are made from a bar in the amount of 3-4 pieces. Their legs are connected at the sides with longitudinal slats for greater stability. The task of the props is to support 2–3 parts of the OSB (solid slabs are difficult to dismantle) that form the floor formwork. Accordingly, the height of the U-shaped racks should be 30-40 mm less than the height of the caisson walls from the floor.

For casting the neck, it is better to use a 4 mm thick fiberboard impregnated with water, rolled into a pipe and stitched with self-tapping screws to a thin rail from the inside. The sheet is rolled up according to the installation diameter of the plastic hatch (about 60 cm), the formwork can even be pulled to the seat ring with small screws from the inside. The external formwork of the neck can be of any shape, the wall thickness is about 70 mm.

In the slab formwork, it is also necessary to cut a round hole with a jigsaw, inside which the fiberboard is attached to the end of the slab formwork. Such a scheme can be easily disassembled from the inside:

1. Removing the side rails holding the legs of the props.
2. We knock out legs from under one plate, cut it and give it up.
3. Similarly, dismantle the second plate..
4. We twist the round neck formwork.

The slab is reinforced with exactly the same mesh as the floor was reinforced. For the neck, four longitudinal rods, surrounded by rings of structural reinforcement, will be enough. It is also allowed to use large plaster nets with a rod from 4 mm.

As for the waterproofing of the floor, it is carried out by coating with waterproof mastics for the internal processing of concrete containers. Both the flat plate and the neck must be covered with at least two layers. Also note that from the inside, the walls, floor and ceiling of the caisson also do not hurt to cover with a hydrophobic coating. In such operating conditions, the protective layer of concrete should be at least 60 mm, and casting walls of 120 mm for such a light and simple structure would be a clear excess. Therefore, moisture should be prevented and, if possible, frost resistance modifiers should be used when mixing concrete..

Caisson technical equipment

Before the floor is poured, consider all possible additions to the structure in accordance with the technical purpose of the building. Here’s the minimum required:

1. A welded staircase made of reinforcement, permanently fixed to the neck wall and resting on the floor. To save space, only cut-in brackets can be placed in the neck, and a ladder or scaffold can be installed in the extended part of the chamber.
2. Electricity input pipe. Should have a “gander” in the part protruding above the surface, while the inner side is brought close to the input distribution device.
3. Item arising from the previous one: at least one pylon for 3-5 sockets, a small lamp and a switch.
4. Also, a small fan riser should be installed 10–20 cm from the hatch so that high humidity does not harm the electrical equipment.
5. It’s time to cut the casing at a height of 30–35 cm from the caisson floor.
6. Also consider how to suspend and secure the pump along with the suction line.