House foundation on a plot with a slope

Recommendation points

• Slope problems
• Classification of slopes
• Stepped MZLF
• Foundation on piles
• Recessed slab

The more actively new territories are developed for private development, the worse, on average, the geodetic and geomorphological conditions on the sites. Today we’ll talk about one of the most unpleasant situations – the need to build a house on a site with a medium and strong slope..

Slope problems

Building a house on a site with a slope requires not only a foundation of the appropriate type and design. Rather, it is a question of the rational choice of building materials and the use of certain technical solutions aimed at reducing the mass of a building with a limited supporting capacity of the soil. Indeed, in the end, the surface relief on the site does not play a decisive role, it is much more important how the layers of sedimentary rocks are located and how groundwater is distributed. And although the structure of the soil usually follows the shape of the surface, exceptions to this rule are not at all rare..

Before building a house on an inclined site, in addition to a geodetic survey, it is imperative to conduct a hydrogeological survey. It is performed two times during the year: the first – in the period from late July to early September, the second – two to three weeks after the positive air temperature is stable in the region. The purpose of such surveys is to determine the location of sedimentary layers using soil punctures to a depth of 3.5–5 meters, take samples from various layers, and also determine the seasonal fluctuation and maximum GWL.

Depending on the geomorphology, a decision is made on the choice of a certain type of foundation, in which layer-by-layer sliding, punching of the layer in the plane of the support due to its insufficient thickness, undermining of the foundation and the location of its various parts on soils with very different bearing capacity will be excluded. In addition, taking into account geodetic planning, the optimal method of drainage of waters flowing down the upper water and migrating in the upper soil layer.

Classification of slopes

When building a foundation, the steepness of the slope is determined not by absolute values, but relative to the depth of the effective deepening of the concrete base. It, in turn, is determined by the requirements for stiffness and strength, as well as for reasons of counteracting the forces of frost heaving.

Sites with a small steep slope are considered those on which the difference in surface heights along the entire length of the foundation strike does not exceed the effective depth of its foundation at the lowest point. As a rule, in such cases, the difference in height is in the thickness of the fertile layer and only occasionally affects the upper sedimentary layer. Therefore, on small slopes, it is usually possible to dispense with the device of a conventional MZLF with careful planning of the bottom of the trenches in a single plane and their correct preparation, as well as enclosing the house around a reliable insulated blind area. The adjacent territory in the lower part of the slope to a width of 3-4 meters from the house is leveled over the surface with fertile soil taken from the central part of the foundation. In the upper part of the slope, some of the soil can be removed to ensure sufficient elevation of the basement in accordance with the average depth of snow cover.

If the height difference between the extreme points of the foundation is equal to its effective depth or exceeds this value by up to 50%, the slope is considered average. In this case, placing the base on a single base plane is advisable only for small buildings. When the foundation extends along a slope of more than 10 meters, the overrun of the concrete mixture to ensure the standard deepening and elevation will be especially pronounced, therefore the most reasonable option would be to carry out large-scale planning of the adjacent territory, if the thickness of the layer of dense sedimentary rocks allows it. If the hydrogeological situation is unfavorable, preference is given to alternative types of foundation.

Those sections are considered to be steeply inclined, where the difference in heights during the construction of the strip foundation at its extreme points exceeds the effective deepening by 1.5 or more times. Planning a single support plane in such cases is completely unprofitable, this causes a colossal amount of earthwork and at least a two-fold overrun of the concrete mixture. On steep slopes, where terracing is impossible, pile-grillage or buried slab foundations with a basement superstructure are arranged for the construction of low-rise buildings.

Stepped MZLF

The cascade strip foundation works effectively in cases where the thickness of the formation on which the base rests is sufficient to accommodate all the support planes in its thickness with as few steps as possible. In general, the more cascading the tape is, the lower the concrete consumption, but the complexity of monolithic work increases..

Trenching begins at the lowest point of the construction site, focusing on the design depth of the foundation and the actual location of the soil layers. As you move towards the top of the slope, the bottom of the trench is planned at the same level, while the actual excavation depth gradually increases. When it becomes 0.3–0.4 meters higher than the calculated one, a new horizon is cut in the trench and then the bottom is planned along it. The last horizon is planned in such a way that at the highest point of the site, the deepening of the foundation is 15-20% more rationally necessary.

The operation of a stepped foundation is associated with a number of risk factors. There is no guarantee that sufficient thickness of the dense formation will remain in the support zone, plus everything, due to the difference in depth in local areas, the frost heaving forces differ in magnitude. This leads to an inhomogeneous action of the loads, which requires:

1. Casting with extended support.
2. To make a trench 25–30% wider than the supporting part and fill the CPB under the foundation.
3. Backfill sinuses with non-porous soil.

Foundation on piles

Pile-grillage technology is used in those areas where there are simply no sufficiently dense layers in the upper layers, or too much deepening of the foundation is required, which is economically unprofitable. Piles allow, at comparable costs, to lean on a stony layer of marl or even limestone, without taking into account the surface relief and slope of the site at all. If there are no stony or even dense soil layers at depths that are reasonable for development, the piles work according to a different principle, taking the load due to lateral friction forces on the compacted clay layer.

There are several types of pile foundations:

1. Bored piles are made by a mechanized method using heavy construction equipment on the site. Despite the relatively high cost of work, this is a truly reliable and versatile method that allows you to create a stable foundation for “heavy” buildings.

2. Concrete shallow piles are optimal for making a foundation on soils, where the upper layers are weakened by groundwater, but already at a depth of 2–2.5 meters dense clay occurs. Well drilling can be carried out with light crane-boring machines, this class of equipment is very common in many regions. It is allowed to fill piles on our own, which greatly reduces the cost of work.

3. TISE piles – a kind of concrete pile manufacturing technology designed for civilian use. Well drilling is performed using a special hand drill that forms an expansion in the bottom to increase the support area.

4. Screw piles – iron pipes with wide blades, which can be screwed into the ground manually or mechanically. The technology is distinguished by a high speed of work, however, in order to arrange a foundation even for a light frame house, a high density of piles is required.

Recessed slab

The third type of foundation, suitable for constructing a foundation for a house on an inclined area, consists of a monolithic slab resting on a dense layer of clay below the freezing depth, the estimated depth of the slab is determined by the lowest point of the slope. The production of monolithic work continues with the formation of concrete walls, thus, a sealed caisson with one or two half-open walls is cut into the ground.

Building a house on a recessed slab allows you to equip an exploited basement with natural light with relatively little effort. However, this type of foundation is most susceptible to layer-by-layer sliding, therefore, the slab should be cast with stiffening ribs located across the slope and increasing friction against the support layer. In some cases, it is reasonable to replace the ribs with short rectangular anchor piles..