Hello,
due to poor workmanship, we now have a 60 cm (24 inches) deep hole in the garden for a 6x3x2.5 m (20x10x8 feet) SwimSpa. The original plan called for 20 cm (8 inches) of gravel plus a 20 cm (8 inches) concrete base slab. Unfortunately, we now have a major problem. The pit can no longer be accessed by heavy machinery because of the actions of the previous landscaping contractor. This makes it difficult to deposit 8 cubic meters of gravel into the pit. Since the concrete base must be flush with the ground, 0.4 m (16 inches) of gravel would need to be installed. This would have to be done by wheelbarrow over a 30-meter (98 feet) access path with a slope.
For this reason, the current landscaping contractor has suggested pouring a 60 cm (24 inches) concrete base slab. They say the pipe from the pump to the pit can simply be laid that way. Attached is a photo of the pit. The garden soil consists entirely of limestone with soil classification 5. For this reason, the garden area was filled with 30 cm (12 inches) of topsoil. Along the long edge of the pit there is also a drainage pipe just above it. According to the construction survey, the soil has low permeability, kf ≤ 10^-4 m/s. The soil there is approximately class 3 to 5 — medium dense, dry to damp.
In the lower right corner, a layer of clay has appeared. I believe it is clay because on about 1 square meter, digging with a shovel is almost impossible — the soil sticks to the shovel and cannot simply be scraped off. However, my suspicion is that due to heavy rainfall, water collected in that corner over the last two weeks. Since the surrounding ground is mostly rock, it likely pooled there. I don’t know what this means in terms of construction.
The only access is on the right (visible in the photo where the fence panel is missing). For any other access, a crane would be necessary to lift materials over the house, which is about 50 meters (164 feet) for the crane’s reach, and there is no suitable crane setup area.
Of course, everything is somewhat possible... but my question is: is it really feasible to have a concrete base slab without gravel underneath in this situation? What would be a compromise—would 20 cm (8 inches) of gravel and 40 cm (16 inches) of concrete slab be acceptable? If so, should the gravel be compacted in layers or just the base once and then the 20 cm placed all at once? [Then the gravel could be delivered, and we would spread it ourselves by wheelbarrow over three days.]
We are really desperate. The reasons for this will be settled by our lawyer with the former landscaping contractor if needed, but the lawyer can only settle financial disputes. They can’t handle the practical work.
Best regards and Merry Christmas 🙂
due to poor workmanship, we now have a 60 cm (24 inches) deep hole in the garden for a 6x3x2.5 m (20x10x8 feet) SwimSpa. The original plan called for 20 cm (8 inches) of gravel plus a 20 cm (8 inches) concrete base slab. Unfortunately, we now have a major problem. The pit can no longer be accessed by heavy machinery because of the actions of the previous landscaping contractor. This makes it difficult to deposit 8 cubic meters of gravel into the pit. Since the concrete base must be flush with the ground, 0.4 m (16 inches) of gravel would need to be installed. This would have to be done by wheelbarrow over a 30-meter (98 feet) access path with a slope.
For this reason, the current landscaping contractor has suggested pouring a 60 cm (24 inches) concrete base slab. They say the pipe from the pump to the pit can simply be laid that way. Attached is a photo of the pit. The garden soil consists entirely of limestone with soil classification 5. For this reason, the garden area was filled with 30 cm (12 inches) of topsoil. Along the long edge of the pit there is also a drainage pipe just above it. According to the construction survey, the soil has low permeability, kf ≤ 10^-4 m/s. The soil there is approximately class 3 to 5 — medium dense, dry to damp.
In the lower right corner, a layer of clay has appeared. I believe it is clay because on about 1 square meter, digging with a shovel is almost impossible — the soil sticks to the shovel and cannot simply be scraped off. However, my suspicion is that due to heavy rainfall, water collected in that corner over the last two weeks. Since the surrounding ground is mostly rock, it likely pooled there. I don’t know what this means in terms of construction.
The only access is on the right (visible in the photo where the fence panel is missing). For any other access, a crane would be necessary to lift materials over the house, which is about 50 meters (164 feet) for the crane’s reach, and there is no suitable crane setup area.
Of course, everything is somewhat possible... but my question is: is it really feasible to have a concrete base slab without gravel underneath in this situation? What would be a compromise—would 20 cm (8 inches) of gravel and 40 cm (16 inches) of concrete slab be acceptable? If so, should the gravel be compacted in layers or just the base once and then the 20 cm placed all at once? [Then the gravel could be delivered, and we would spread it ourselves by wheelbarrow over three days.]
We are really desperate. The reasons for this will be settled by our lawyer with the former landscaping contractor if needed, but the lawyer can only settle financial disputes. They can’t handle the practical work.
Best regards and Merry Christmas 🙂
This is definitely doable by hand. Two people, one day of work. I’ve done this several times at home as well: having gravel delivered in a skip (to keep the ground clean) and shoveling it directly into a wheelbarrow from there. It’s great for muscle soreness and really a tough job—if a landscaping contractor made their workers do this, they would probably call in sick the next day. But as the homeowner, you would likely approach a task like this with a different motivation.
No one needs a 60 cm (24 inches) concrete slab for this. Simply install a well-compacted frost protection layer using 0/63 aggregate and pour the concrete on top of it.
In theory, a 60 cm (24 inches) concrete slab is sufficient if it is founded below the frost line, the appropriate concrete mix is chosen, and any potential groundwater or slope water around it is drained using a perimeter drainage system, especially if, as described, you are building on rock. However, if groundwater or slope water is not an issue, the concrete should perform equally well at 60 cm (24 inches) or 30 cm (12 inches) thickness, provided it is placed at frost-free depth. The subsoil must be properly prepared, and the slab reinforced accordingly if it contacts uneven soil layers. Therefore, it is advisable to construct a load-distributing frost protection layer from below, with the concrete slab placed on top.
In theory, a 60 cm (24 inches) concrete slab is sufficient if it is founded below the frost line, the appropriate concrete mix is chosen, and any potential groundwater or slope water around it is drained using a perimeter drainage system, especially if, as described, you are building on rock. However, if groundwater or slope water is not an issue, the concrete should perform equally well at 60 cm (24 inches) or 30 cm (12 inches) thickness, provided it is placed at frost-free depth. The subsoil must be properly prepared, and the slab reinforced accordingly if it contacts uneven soil layers. Therefore, it is advisable to construct a load-distributing frost protection layer from below, with the concrete slab placed on top.
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