Sand bed for strip foundation - whether it is necessary?

One of the most popular branches on FORUMHOUSE is an section on foundationsBecause our experts are always ready to answer any question. Many novice developers interested: "Do I need to make a sand pillow under strip foundation?".

At first glance, the bedding sand is needed, because it is nearly always in the majority of foundation work. But is it right? Some builders believe that it is necessary in all cases, others - that only under certain conditions, and in some cases sand cushion is simply harmful.

So what to do - pour the sand or not? Let us turn to the practical experience of experts of our website.

The main purpose of the foundation - it is load balancing the weight of the house to the lower soil layers. Any negligence committed in the preparation of the base under the house can lead to unpleasant consequences - cracks in the walls due to the subsidence of the ground.

It is believed that the sand cushion under the base of the strip foundation - a guarantee of its strength and durability. Often, however, the sand in the excavated trench is filled, guided by the principle "the more, the better, because so do all." This does not include any portion geology or soil characteristics and its carrying capacity nor the groundwater level.

Denisminko (User FORUMHOUSE):

Many of my friends before pouring strip foundation made of sand cushion. To the question "why" answer - "saw our neighbors." I read that the strip foundations should be based on undefeated (intact) soil layer. Is it really?

To clarify, it is necessary to clearly define for themselves: what kind of a strip foundation in question. Namely:

• malozaglublonnom strip foundation - MZFL;
• tape foundation laid below the freezing depth;
• monolithic strip foundations;
• precast strip foundation blocks of PBS.

Each of these four types of foundation requires an individual approach in the preparation of the base

RNikonov (User FORUMHOUSE):

If it is a monolithic strip foundations, the sand bed under it, you can not do. But under the following conditions:

- Soil nepuchinisty.

- The soil under the base of the foundation of sand.

- Structural poured below the freezing depth, with a concrete foundation base under preparation.

Concerning precast foundation blocks of PBS, There is a sand cushion is simply necessary, because sand can only align all the irregularities that are left after the sample (i.e., removal of loosened layer) of soil. Because only sand can be compacted practically to the natural density.

If this is not done, the foundation block will fall on uneven ground (not the entire surface of the sole), and below it may be empty. This will lead to non-uniform pressure on the foundation soil. Accordingly, it is increasing the likelihood of uneven subsidence of the ground.

putnik 777 (User FORUMHOUSE):

I once asked the chief engineer of the construction firm: by executing the bedding sand? He said that the sand is filled to level ground under the "0" and for concrete savings.

If arranged monolithic strip foundations, then seek the "mirror" the base is no longer necessary, since due to the plasticity of poured concrete fills all irregularities. Accordingly, the foundation sole to rest on the ground over its entire surface and redistribute the load on the ground.

When it comes to malozaglublonnom strip foundationsThen sand cushion is necessary because with its help heaving soils is replaced by nepuchinisty. Moreover, malozaglublonny strip foundation requires a device drainage system, warmed the blind area and the insulation of the foundation. These measures minimize the forces of frost heaving, acting on a shallow foundation laying.

When the device of the sand cushion is important to understand what is under the ground to the future base. If the sand is poured into the soil with low permeability (which is primarily concerned with loam and clayey soils), the sand, being less dense soil, will be a place where you will constantly accumulate water. Result - eventually bearing capacity permanently waterlogged soil will decrease, which may lead to subsidence of the foundation.

Read articles aboutright cake warm otmostkiandfeatures of the drainage system and the foundation portionyou can, by clicking on the links.

The issue of cushion sand must be considered because of the particular characteristics of the soil, ground water table level, the weight of the building, the freezing depth. It can not be considered a universal means of sand bed, suitable for the preparation of any foundation, without its binding to the specific conditions of use of the building.

Practical recommendations on the production of sand cushion

• 20-30 cm - the minimum thickness of sand cushion under the foundation.
• To align sufficiently sand base thickness of 5-10 cm.
• Backfill best coarse sand.

• Sand necessary ramming in layers, for 1 time embankment no more than 15-20 cm - in the case of mechanical compaction, and 10-15 cm when using the hand tamper.

• Before rammer sand moisten. Because water, due to surface tension, "glues" particles of sand and does not allow them to "scatter" when compacting.
• Moisten the sand up to his best placing in the trench, and not in it. This will avoid waterlogging and "raskisaniya" ground under the base.
• For douplotneniya sand and obtaining better base can be used rubble coarse fraction (40-60). In this case, after tamping the sand, it poured gravel layer of 5-10 cm, and already on it again traversed rammer. Through point load (shock) is driven into the gravel and sand, due to wedging effect, the seal depth is increased from 15-20 to 30-40 cm.

• To avoid silting of sand cushion and mixing sand with the surrounding ground, dug in the trench can be pre-lay geotextile.

Algorithm design strip foundation for the "Dummies" is available is set out here. This theme - of Finnish version melkozaglublennogo strip foundation. Why should insulate the foundation foamYou will learn from this thread. Reinforcement of the foundation in the corners - do it right, read this topic. And in this video shows how to make a foundation on a swampy soil.