Building a house on a slope in Batumi

On a slope, a house relies on what lies beneath it and behind it: piles, retaining walls and drainage. Once the house is handed over, you hardly see any of this, but on a steep plot it takes a noticeable share of the budget, and it has to be costed before the room layout. The general order of a build is described on the page about house construction in Batumi.

What to find out about a sloping plot before you pay a deposit

A level patch on a slope is easy to mistake for a ready-made spot for a house. Often it is made ground: a previous owner cut away the soil higher up and dumped it lower down, and grass has since grown over the top. Fill won't carry a foundation, so the piles have to go through it down to a firm layer. On a site visit we look for traces of this kind of work and for signs that the slope is moving:

• tree trunks bent like a sabre near the ground: this is how trees grow on soil that is slowly creeping;
• cracks in the road, in neighbours' retaining walls and in the paved aprons around their houses;
• wet patches, springs and waterlogged ground after rain;
• a fresh cut or fill left by a previous owner;
• where water comes from off the plots above, and where it can be drained to.

Only drilling gives a final answer. The survey team sinks boreholes within the footprint of the house and along the line of the future retaining walls, and shows how the layers lie and where the water stands. The ground survey costs from 3,000 to 4,200 ₾, and before you buy the plot this is the most useful money you can spend: the report shows how many piles and how many metres of wall you will need, and sometimes that the plot is better left alone.

Access for machinery and getting materials down the slope

The design of a house on a slope begins with the question of how concrete, reinforcement and the piling rig will reach the plot. If a mixer truck can't get along the street and the rig can't be set up next to the house footprint, the structural engineer changes the foundation itself: for example, by specifying more piles of a smaller diameter that a compact rig can bore.

The usual solutions are these. We pump concrete with a boom pump from the road above, or bring it in small mixer trucks over several trips. Across the plot we build a temporary crushed-stone ramp, which later becomes the driveway. Heavy materials are lowered with a crane truck, and where it can't reach, we haul them with a winch or carry them by hand, which shows straight away in the delivery line of the estimate.

If the plot lies below the road, the entrance is often on the upper floor, straight from the street via a footbridge or a short flight of steps, and the parking space sits level with the road on its own retaining wall.

Piles, a stepped foundation or a lower floor set into the slope

The structural engineer chooses the foundation from the ground survey, and the general difference between piles and a slab is covered in the pile vs slab foundation comparison. On a slope, one of three schemes is usually used, or a combination of them.

The schemes are often combined: on the uphill side the house rests on a strip footing or the wall of the lower floor, and on the downhill side on piles. There is a subtlety here that the structural engineer calculates separately. The supports on the uphill side are shorter and stiffer than the long ones, and in an earthquake they take a larger share of the load, so their reinforcement and joints are heavier than on the same house on a level plot.

A light house also means a lighter foundation. On a steep plot, a timber frame house often needs simpler piles and walls than a masonry one.

Retaining walls and terraces

A retaining wall is needed wherever the plot changes level: behind the house, where the soil has been cut away; in front of it, where a platform has been built up for a terrace; along the drive; and along the boundary with a neighbour. The height, thickness and reinforcement are set by the structural engineer. We don't build these walls by eye, even low ones.

Next to the house, the road and neighbouring buildings, we make the walls of cast-in-place reinforced concrete on their own foundations. On the lower garden terraces, where the load is smaller, gabions or a bank reinforced with geocells and planted over are more appropriate. How the garden terraces, steps and planting will look is decided by the landscape design, and we prepare the base for it.

Behind every wall we give the water a way out: a layer of crushed stone wrapped in geotextile, a drainage pipe at the foot and weep holes through the wall. We photograph all of this before backfilling, because afterwards the only way to see it is with an excavator. If there is to be a pool on a slope on the lower terrace, its shell and the retaining wall below it are calculated together.

Water on a slope: three routes and one discharge point

Water reaches the plot by three routes, and each has its own solution. Rainwater running off the plots above is caught by a ditch or channel along the upper boundary. Groundwater seeps along the soil layers, and drainage around the foundations and behind the walls collects it. The rest falls on the roof, paths and paved areas, and the stormwater drains take it away.

All three systems meet at one discharge point: the street stormwater drain, or a soakaway on the plot if the ground will take the water. We find it before the design. Where the pipe runs change direction we install inspection chambers, so the pipes can be flushed without digging up the garden. After the first rainy season it is worth opening the chambers and checking them for silt: this shows whether the drainage is working.

Order of works and the season

On a slope, a lot depends on when the ground is opened up. A cut for the house left open over autumn slumps after downpours, and part of the earthworks has to be done again. So we work in this order:

1. a temporary ramp and a laydown area for materials;
2. cutting the soil in sections rather than along the whole length at once, with temporary support for the cut faces;
3. retaining walls and drainage while the plot is open;
4. piles or foundation steps, then the ground beams;
5. backfilling, compacted in layers, only once the drainage has been photographed.

We try to do the earthworks between late spring and late summer. If the permit comes through in autumn, piling can still go ahead, since rain gets in its way less, and the big cut moves to the dry months. We write the timing of each stage into the contract, with allowance for rainy weeks.

How much it costs to build a house on a slope in Batumi

A mid-range turnkey house on a slope costs from 2,280 to 2,760 ₾ per m² of total floor area. This price includes reinforced pile or stepped foundations, earthworks, getting materials down the slope, and finishing. Retaining walls, drainage and slope stabilisation are separate lines, because their scope depends on the plot rather than on the size of the house: a retaining wall from 360 ₾ per m² of wall, drainage from 87 ₾ per linear metre, geocells on banks from 69 ₾ per m². The ground survey and the design are not included in the price of the house and are costed separately.

Compared with the same house on a level plot, a slope adds 36–48% to the price. Fill and a deep firm layer, a plot below the road with no access, a lower floor set into the slope and high walls next to the neighbours all push the cost up. It comes out cheaper when firm ground is close to the surface, the house runs along the slope, and a light frame is chosen.

We send a preliminary estimate from photos of the plot and a plan of the house within 24 hours on business days, and until there is a ground survey, the below-ground works will be priced in two versions. The final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.