All the details of digging a foundation pit: calculations, drawings and average cost

One of the first and most important steps in any construction project is digging a foundation pit. A well-built foundation sets the tone for the entire structure, no matter how big or small the building is. Stability and longevity are guaranteed by a well-executed foundation, so it is critical to comprehend every aspect of this process.

It’s crucial to have precise calculations and thorough drawings before starting construction. In order to guarantee that the pit is the right size and depth for your project, these will direct the excavation. Advance planning can avert expensive errors and postponements. We’ll guide you through the calculations and how to make accurate drawings for your foundation pit in this article.

Knowing how much it costs to dig a foundation pit will enable you to more efficiently manage your project’s budget. To help you prepare, we’ll break down the typical costs for supplies, labor, and equipment rental. Making educated decisions will be facilitated by having a clear understanding of the costs, whether you’re planning a do-it-yourself project or hiring experts.

Come along as we go over every stage of digging a foundation pit, from the preliminary design to the finishing touches. You can make sure that your construction project gets off to a strong start by being well-prepared and knowledgeable.

What is a pit pit??

A pit is a predetermined-sized hole dug out of the ground that is designated in the excavation plan and used to lay the foundation for a new building. It comes in various shapes and sizes.

Its measurements—height, breadth, area, and volume—are directly influenced by:

• geographical features of the region;
• soil level;
• operating conditions of the future building;
• ability to soil freezing.

The ground depressions must be deep and sturdy because they support the weight of the building as it is being built.

Here, the following factors are crucial:

• period of work;
• size and shape;
• planned order of actions;
• choice of the necessary equipment.

The earthworks plan-drawing could resemble this:

Digging the earth should be done correctly if groundwater level indicators are to be considered and soil samples should be collected for analysis. The quality and density of the soil material are ascertained through laboratory-based chemical and geological experiments.

These studies support the admissibility of constructing a home in a specific location, which serves as the foundation for the engineer’s drawing design.

The pit is being developed in compliance with all regulations. Reports must be prepared at the start and finish of the project (handover of the completed item), along with an estimate of the costs associated with the machinery, equipment, and labor force.

The type of foundation, which is a customer choice, always determines the shape and size of the future planned foundation pit.

Types of foundation pits and requirements for them

The features of the intended building must be considered when choosing construction recesses.

Excavated pits are among the foundation types available.

• for slab structures;
• for columnar foundations.

Additionally, there are sloped, inclined, step-equipped, vertical, round, trapezoidal, and rectangular pits that need to be reinforced, in addition to glass and strip pits.

For monolithic slab structures

A pit like this is designed for multi-story structures. Concrete forms a pit for monolithic slabs. It has a trapezoid or rectangle appearance. The walls’ vertical depth will always be greater than 50 centimeters.

Step technology excavation is appropriate for this kind of foundation. Layer by layer, the soil is removed uniformly, adhering to a specified layer thickness of 50 cm. The level that follows the first is excavated in a way that produces steps that are 25 cm wide each.

The following procedure can be followed by using the example of excavating a hole to a depth of 100 cm:

1. A rectangle is marked along the earthen plane, the dimensions of which exceed the indicators of the future building on all sides by 0.5 m.
2. According to the given markings, the earth is removed to a depth of 50 cm, with the simultaneous formation of vertical walls.
3. At the bottom of the pit, make a rectangular marking, with a distance of 25 cm from the sides.
4. Then remove the soil mixture to a depth of 50 cm.

The idea behind searching for extremely deep holes is the same.

Using a very straightforward formula, the engineer calculates the surface dimensions of these kinds of construction pits:

• L = l + 25 N / 50, where:
• L – specified length or width; l – length of the building (width); H – depth.

For base with pillars

Large holes are made using this principle for columnar foundations:

1. Carrying out preparatory work (cleaning, leveling and marking the area).
2. Digging a trench for the foundation of the house according to the specified markings, determining the location of the future installation of pillars, along the joints of the internal walls and the entire perimeter of the building, with steps of 1.6 – 2.2 m.
3. Formation of trench depressions with a depth of up to 40 cm and a width of up to 100 cm.
4. In places allocated for the installation of pillars, squares are dug, up to 70 cm in size, with a depth corresponding to the plan.
5. If necessary, the holes are secured from the inside with shields and rods made of wood or metal.

Using a shovel, pits for the pillar foundations are dug manually. When digging in extremely hard soil, a pickaxe or crowbar are used.

For strip

For this kind, a soil depression is excavated using the same methodology as for slabs:

1. First, preparation is done: the area is cleaned and the fertile layer is removed soil up to 20 cm.
2. Then, according to the project plan, markings are made for the walls of the excavation, as well as for the trenches, the depth of which should be 50 cm.

Experts advise that the building’s future load-bearing walls be 80 cm. The earthen pits that have been dug out shouldn’t have vertical walls. Their external resemblance to slopes and their placement at a specific angle, or slope, whose dimensions are specified in the project, are the primary requirements.

Stakes work best for marking the base. At the end of the building’s perimeter, at the wall joints, they must be hammered into the corners. Next, draw ropes to indicate the pit’s minimal boundaries and mark the trench’s desired width.

Soil excavation and sampling commence at the highest point on the angle. Panels with spacers on the sides are inserted when the depth reaches 70 cm; these spacers will come in handy later on when the formwork is being installed.

Use a metal ruler and a tape measure to mark the pit for strip bases. It is possible to use a building level for control.

How to determine what kind of hole is needed and its dimensions?

The foundation upon which the construction will occur is taken into consideration in the design of the excavation pit.

The project’s actual location is determined by a few natural variables:

• type of land and the level of its possible freezing;
• dimensions and weight of the future structure;
• type of selected base;
• seismicity;
• level approach of water in the ground.

Only after geological exploration and laboratory research on climate conditions can the work process start on the site. SNiP 3 standards are always considered in this situation. 02.01. The excavation’s shape—tape, round, pile, or rectangular—must also be considered.

The following formula is used to determine the excavation’s volumes:

• N – specified depth;
• a, b – width + length of the bottom;
• c, d – width + length of top.

The absence of a mark up to the design indicator of 0.1 m is considered when developing a pit by a mechanized method. In order to prevent disturbances in the natural soil, the shortage is required. Thus, the only labor employed here is manual labor. The volume of the base is deducted from the final volume of the entire development during backfilling procedures.

Examples of volume calculations

Let’s look at a few different recess options and how to calculate the right amount in each unique situation.

Excavation with vertical walls

1. Width (L1), m – 4.
2. Length (L2), m – 6.
3. Height (H), m – 2.
4. Volume (V) = 48 m3 – V = L1 * L2 * H = 4 * 6 * 2 = 48 m3.
5. Area (F) = 24 m2 – F = L1 * L2 = 4 * 6 = 24 m2.

Recess with vertical walls, with different vertex sizes

1. Beginning of the form.
2. Width (L1), m – 4.
3. Length (L2), m – 6.
4. Height (m):
5. (H1) – 2.
6. (H2) – 2.
7. (H3) – 2.
8. (H4) – 2.
9. Volume (V) = 48 m3, V = L1 * L2 * (H1 + H2 + H3 + H4) / 4 = 4 * 6 * (2 + 2 + 2 + 2) / 4 = 48 m3.
10. Plan area (F) = 24 m2, F = L1 * L2 = 4 * 6 = 24 m2.

With slopes

1. Width(L1), m – 4.
2. Length (L2), m – 6.
3. Height (H), m – 2.
4. Selected soil: loam> coefficient. m = 0.5.
5. Recess volume (V) = 70.667 m3.
6. Top width (L3) = 6 m2.
7. Top length (L4) = 8 m2.
8. V = (H / 6 * (2 * L1 + L3) * L2 + (2 * L3 + L1) * L4) = (2 / 6 * (2 * 4 + 6) * 6 + (2 * 6 + 4) *8) = 70, 667 m3.
9. L3 = H * m + L1 + H * m = 2 * 0.5 + 4 + 2 * 0.5 = 6 m.
10. L4 = H * m + L2 + H * m = 2 * 0.5 + 6 + 2 * 0.5 = 8 m.

The foundation of the building being built should be no closer than 50 cm from the ground’s water, according to the primary construction rule. The better, the higher this value is.

You must always keep in mind that the perimeter’s indicators—height, width, and length—are simply multiplied together when performing calculations for earthworks. – This is the loudness.

An engineering degree is required to perform engineering computations. For instance, determine the slope angle—which varies depending on the type of soil—while approving a plan. It can only be accurately calculated by a specialist.

The soil depths that are taken into account when calculating the volume of work based on the loosening coefficients—which are calculated using the tables in these guidelines—are listed in table SNiP 12-03-99.

Geodesy methods are used to determine the composition of the soil. The following formula determines soil resistance, as shown in the picture:

Example: Determine the resistance of clay at depth d = 2.5 m, foundation width b = 2, specific gravity above the base γ´ = 17 kN/m3, K porosity e = 0.85, fluidity IL = 0.45.

Examine the soil indicators in the table, then enter the corresponding values.

In this instance, the primary formula for determining soil resistance is as follows:

With width b1 = 1 m and depth d1 = 2 m, R0 refers to foundations. As a result:

GOST 24847-2017 is used to determine the level of soil location, seismicity, groundwater, and ability to freeze. On the website, you can see them.

This document states that certain conditions must be met when choosing measurement techniques and groundwork projects. Using specialized drilling techniques, the Ratomsky permafrost meter (MP) measures seasonal freezing.

Prior to choosing to drill holes, the soil’s bearing capacity is also determined. Use the following formula to accomplish this:

The area of the lower portion of the foundation is always divided by the weight of the house. The approximate indicators are divided by the soil value and the allowable load indicator to get the area of the reinforced concrete base.

The design and development of a space-planning solution for the building under construction need to be approved by experts from the local authorized authorities’ architecture department.

The document in the picture illustrates how a pit plan drawing is created for a particular foundation.

The density of the stale soil layers is disturbed during excavation work if special transport is employed. Thus, it is always considered in calculations that the K value (20–30%) is applied when developing land. This coefficient is regarded as a factor for correction.

With soil removal

1. Length – 70 m
2. Depth – 5 m
3. Width – 30 m
4. Total volume = 70*5*30 =10,500 m3

The overall volume will always be higher in the case of land removal. To accomplish this, you’ll need:

12,600 m3 (10,500 m3*20%). The answer is 12,600 m3.

Accurate calculations and consideration of all relevant technological, laboratory, and natural factors are necessary to ensure the correctness of earthworks and the longevity of the structure.

The walls of foundation trenches are reinforced, inclined, and vertical.

Process and technology of dimple formation

The process of digging starts with development, planning, and geodetic work. It also involves calculating the size and depth of the hole. Excavation starts as soon as the technology map is complete.

Specialized equipment is used for this:

• machines that dig pits (single- and multi-bucket excavators);
• transport and earthmoving (bulldozers, graders, ditch diggers).

Additionally, they employ manual labor: experts form slopes, fill excavation pits, oversee soil sampling, and compact the earth.

Builders need to be precise about:

• the expected load that the foundation must withstand;
• type of building foundation;
• soil features;
• end point for possible soil freezing;
• amount of expected annual precipitation and ground moisture level.

Experts with years of experience advise doing work between early May and late October. Because of the pliable and soft soil, now is a better time to dig a hole than it is during the colder months, when labor costs increase significantly.

It is necessary to consider the digging depth while accounting for soil freezing. The pit’s bottom needs to be lowered to 40 cm if this happens. Groundwater location is also important. There should always be a 50 cm higher recess.

The following moments (in centimeters) are considered when calculating the depth after identifying the type of soil that permits digging:

• sandstone – 1,100;
• sandstone – 1,300;
• clay and loams – 1,400;
• dense soil – 1,800.

It is not permitted to perform any work if the numbers exceed or fall short of the specified indicators. But a foundation that is too deep is covered by this rule. 40 to 70 cm should be the depth if the foundation is shallow.

It is necessary to carry out the following tasks prior to digging:

1. Check the place for the absence of underground utilities. This can be clarified with the local administration, when submitting and registering the drawing (during the registration of documents for construction).
2. Cut off the fertile structure (up to 40 cm) and leave it in the place from which it will be disposed of, when tidying up the adjacent territory and landscaping, at the end of the stand.
3. Pump out or drain surface water using pumps (if necessary).
4. Mark the place. For this, use a theodolite or level, fix the corners (use stakes, pull marking cords).
5. Start digging from the bottom side.
6. Strengthens at great depths.
7. Remove excess soil.

For all three major types of foundations—strip, monolithic, and columnar—there are general guidelines for development, excavation, and preparatory work. The subtleties are limited to specific measurements and computations.

Excavation is done manually for the necessary small volumes of low depth pits (strip and column foundations). Loaders and excavators with direct and reverse buckets are used for massive and slab foundations.

The walls of the foundation pit are fastened with:

• cementing (for dense buildings);
• tongue and groove (suitable for sandy, weakened and waterlogged soil);
• piles (permissible for deep holes).

The pit’s bottom needs to be leveled. Hence, you must complete the task by hand when there are 10 cm left. Filling the sinuses with a draining soil mixture is preferable.

Development should take place beneath the foundations of any nearby buildings if there are any.

Main difficulties and mistakes

When excavating recesses, it is imperative to follow the developed technological map, which includes all necessary calculations, to avoid mistakes.

Given that the future fate and functionality of the structure depend on precisely laying the foundation in the excavated pit, the development stage is regarded as a crucial procedure. Inadequate excavation could cause the foundation to sink.

Among the primary danger spots are:

1. Perekok, due to sampling of land below a given level. Here it is worth paying attention to the functions performed by the excavator, which should be monitored by specialists. If excavation work is carried out below a given level, and more soil is taken than necessary, then it will be very difficult to compact it, since the overall density will suffer. If a mistake is made, you will have to fill the hole with medium-grain gully sand, followed by compacting it with a vibrating compactor.
2. Incorrectly made slopes. It is worth paying special attention to this point, since if the slopes are made too steep, they may collapse. And in order to carry out restoration, separate installation work will be required, which will lead to additional financial costs. In this case, you will have to use manual labor.

For this reason, it’s essential to create a workflow in the work to prevent errors:

1. Conduct soil reconnaissance and laboratory testing.
2. Calculate groundwater distance level.
3. Properly prepare the site by removing debris, green spaces and topsoil.
4. Make markings according to the approved work plan.
5. Use equipment and labor of experienced builders (crews).
6. Comply with required standards and technologies.
7. Monitor occupational safety and health regulations.
8. Remove soil layers gradually and evenly.
9. Upon completion of work, dispose of unnecessary soil and clear the construction site.

Any nuances in this kind of work are significant, and a trained specialist should develop the plan. Waste soil, sand, or ASG are used, all of which are compacted as much as possible, to fill the pit’s sinuses.

Peat can be taken out, walls reinforced, and water pumped out of the excavations if needed.

Digging price

In figuring out the cost of digging a pit and setting up a hole for laying a foundation, you have to factor in the type of soil, the amount of time, the means of production, and the number of workers involved in the project (manual, mechanical, transport).

Digging one cube costs, on average, between 180 and 250 rubles. Excavation work will always be far more expensive in areas with rocky, heavy soils.

When the soil mixture is removed by a dump truck (10 m3), the cost of transportation and disposal will be between 400 and 700 rubles.

The cost of bringing heavy machinery (excavator, tractor, loader, Kama, or dump truck) to the job site is billed separately and is directly correlated with the machine’s power type and the vehicle’s distance traveled. Prices for heavy equipment services in the Russian Federation’s central regions range from 10 to 30,000 rubles per day.

An engineer’s creation of the project is taken into account at an additional cost. In order to give their customers good deals, many construction companies that offer these services combine excavation development with the primary work and disposal of excavated earth.

The average cost of the project, including setup, excavation, and soil removal, is 500 rubles per m3.

Paying for the excavator’s digging time rather than the earth’s cubic capacity is more profitable. In this instance, an hourly wage of 600 rubles and a daily wage of 5,000 rubles will be paid.

Any construction project must start with the excavation of a foundation pit, and by knowing how to do it right, you can save time and money. Everything you need to know, from producing precise drawings and accurate calculations to estimating the average cost, will be covered in this article. Whether you’re a do-it-yourselfer or intend to hire experts, we’ll offer simple, clear instructions to guarantee your foundation pit is completed correctly the first time.

Useful video

Watch the video to see an example of how an excavation pit is actually dug:

Topic	Details
Calculations	Determine the size of the pit based on the building plan, considering soil type and load-bearing capacity. Calculate the volume of soil to be excavated.
Drawings	Create detailed drawings showing the dimensions and shape of the pit, including depth and width. These help guide the excavation process.
Average Cost	The cost varies based on factors like pit size, soil type, and labor rates. On average, expect to pay between $2,000 and $5,000 for a standard residential foundation pit.

A foundation pit must be dug as a first step in any construction project. Correct computations guarantee the longevity and stability of your structure. You can steer clear of common pitfalls and guarantee a smooth excavation process by carefully planning and preparing.

Making thorough drawings is crucial. They give workers a clear blueprint for the excavation, making it easier for them to understand the precise measurements and depths needed. By acting as a guide, these drawings lower the possibility of mistakes and guarantee that the foundation pit satisfies your requirements.

There are various factors to consider when it comes to costs. The location, kind of soil, and pit size all affect the total cost. You should budget a sizeable sum, but in the long run, you can save costs by investing in careful planning and expert guidance.

In conclusion, the secret to excavating a foundation pit successfully is meticulous planning. For your construction project, precise calculations, thorough drawings, and a clear understanding of the associated costs will lay a strong foundation. You can create a sturdy and safe building by taking your time and making thoughtful plans.

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