Excavations on site
This article describes the Excavations on site, works that give rise to the esplanade below the initial grade of the land where to locate the foundation level. The problems derived from the exacation, the factors that intervene in the stability of the land, the slopes, the entivacones and the solutions to the respective problems will be analyzed.
Prior to site excavations the following tasks will be performed:
- Expropriations and easements.
- Clearing and stripping.
- Identification and diversion of affected services.
- Planning and execution of accesses, landfills and storage areas.
- Stake the expropriation or designated area.
- Materialization of the stations of the project axes every 20 m. by means of stakes, with an indication on each stake of the station and the level to be excavated.
- Obtain the topographical documentation of the land. Cross profiles.
- Materialization by means of stakes every 20 m., of the slope starts; with indication in each station stake, level to be excavated and inclination of the slopes.
Recommendations for the execution of on-site excavations
The slopes are left with their approximate profile and, if the characteristics allow it, already finished. Otherwise, they will be reprofiled with a motor grader.
The excavation on site must not reach the final grade level; The last 30 to 50 cm will then be re-profiled with a motor grader, avoiding deterioration due to decompression and the passage of heavy traffic.
Keep the area in optimal drainage conditions. For this, the work platforms will have slopes of the order of 4%, avoiding erosion on the slopes, diverting and directing the waters that may affect the slopes and outlining the ditches.
Throughout the execution of the tasks, control the stability of the slopes and the appearance of undesirable cracks or materials of lower quality than expected in order of their specific treatment.
The characteristics of the extracted material will be determined to establish its use in other parts of the work if appropriate.
The topsoil, which has not been removed in clearing, will be collected separately for later use, taking care that over time it does not spoil due to lack of aeration or drainage.
Before loading the material for immediate use, measure the humidity and correct it by bringing it to the required levels.
Before the start of the work, it is necessary to know a series of circumstances that may affect their safety and which, at a minimum, will be:
problems in the excavation
The following problems are commonly encountered in on-site excavation:
- Characteristics of the land in relation to the works to be carried out, such as: natural slope, bearing capacity, water table, moisture content, possibility of leaks, stratification, previous alterations of the land, etc.
- Proximity of buildings and characteristics of their foundations, as well as possible overloads in the vicinity of the excavation walls.
- Existence of vibration sources (roads, factories, etc.).
- Existence of or proximity to water, gas, electricity and sewage installations and pipes.
To know the terrain, it will be necessary to carry out a geotechnical study, which will give us information about the type of terrain that we are going to find and its behavior. In order to have in advance a series of means and calculations with which to undertake the work with a series of risks already controlled.
If the building is exempt, this problem does not exist, but in most cases, the site that is the object of the excavation on site is dividing with other buildings, so it will be necessary to carry out the necessary felling operations to avoid settlements and subsidence. of the adjoining foundations and party walls, as the load capacity of the party foundation is reduced as a result of the excavation.
It is necessary, before proceeding with the excavation on site, to know the exact situation of the public services that affect the site, with the data provided by the different organizations. Once these have been obtained, the place where they are located will be marked on the ground, choosing a system that lasts until the excavation is carried out in that area. The exact depth at which they are found will be noted, protecting them against eventual overloads produced by the circulation of heavy vehicles.
The excavation will be carried out mechanically, up to 1 meter, before reaching the conduit and from then on, the excavation will be manual with pneumatic drills, picks, etc., up to 0.50 m. Using the manual shovel from this distance.
Buried cables generally only cause one type of accident and that is direct contact by perforation of the insulation and through the tool we use to dig (shovel, hammer drill, pick, etc.).
The perforation of an unknown sanitation or gallery, which we can find when excavating on site, can cause a typical accident, caused by the fact that there are toxic gas emissions, mainly CO. And that when the workers descend without the proper protections, they become intoxicated.
overhead power lines
The risks of overhead power lines are different depending on whether these lines cross the site or are more or less close to it.
In the first case, we must not start working until the supply company has eliminated said power line, or has raised it sufficiently, so that the minimum safety distances established in the Low Voltage Electrotechnical Regulation (REBT) are met and the Regulation of High Voltage Overhead Power Lines.
Factors that intervene in the stability of the land
When we start an excavation on site, we are breaking the balance that exists between a system, sometimes very complex, of forces or tensions. If we carry out the excavation in dry sand, the grains of the walls slide towards the bottom and this displacement stops when a certain angle of natural slope is achieved. This angle is independent of the height of the slope. Sand is a cohesionless soil.
If we do the same operation on clay, we can obtain a certain depth, with almost vertical walls. In this case, we could see that the natural slope angle varies with height since the clay has greater cohesion.
Between pure sand and plastic clay, there is a wide range of soils, with different coefficients of friction and cohesion.
Experience shows us that the ground always tends to restore this balance that we are breaking. In some cases, it does so immediately (in the case of sand), in others, it is slower and can last for hours, days, months, or even years.
Therefore, it is crucial to take into account in site excavations:
- – Friction angle
- – Granulometry
- - Consistency
- - Humidity
- – permeability
- – Statigraphy, dip and faults
- – Climatic factors (water, rain, ice, drought).
- – Vibrations
The critical depth of on-site excavations is the maximum depth that can be excavated in a stable vertical wall, without any type of fortification.
The factors that influence the stability of the terrain and that can affect the critical depth are:
- – Climatological.
- – Overloads.
In clay soils, water acts as a lubricant for the clay, causing displacement of more or less compact masses.
Another climatological factor is rainwater or water from pipe breaks, which can lead to the flooding of pits with the consequent danger of diluting the ground or undermining the walls of the excavation; if necessary, due to their importance, bilge pumps will be used.
Within the second group of modifying factors of the critical depth of excavation, are the overloads, which in turn can be static and dynamic.
Static overloads can be caused by various circumstances such as:
Accumulated earth at the edge of ditches, which will be placed at a sufficient distance from the edge of the excavation, so that they do not suppose an overload that could give rise to landslides or landslides, having to adopt at least a distance equal to or greater than half of the depth of the trench, in general. In sandy terrain, this distance will be greater than or equal to the depth of the trench.
Dynamic overloads are produced by the circulation on highways, railways, streets, in the vicinity of the works, as well as the vibrations caused by pile drivers, etc., or the movement of the machinery itself on the work. For this reason, precautions will be taken for the movement of machinery on the edge of the excavation, especially in the case of recent rain. The itinerary of the machine will be checked, with no personnel under its path, since the heterogeneity of the terrain must be considered, since an overload can affect the partial stability of the slope.
A general classification of the land can be made according to its stability:
- – Stable, (rocky, limestone, marl).
- – Not very stable, (gravel, with clay, dragged terrain).
- – Shifty, (loose gravel and sand).
slopes of excavations on site
The stability limit of a piece of land is given by the angle of the natural slope of that piece of land. This angle is the maximum slope (angle with the horizontal) that the plane of an excavated wall of any height can maintain indefinitely, without the material tending to slide or crumble.
Next, we include a table of inclinations and slopes of the slopes that depend on the nature and water content of the land.
For depths less than 1.30 m. in coherent terrains and without solicitation of roads or foundations, vertical cuts may be made without shoring.
In loose terrain or that are requested, an adequate shoring must be carried out.
For greater depths, the adequate embankment of the excavation walls constitutes one of the most effective measures against the risk of landslides.
Maximum height required on slopes
The following table (TABLE 1) is used to determine the maximum admissible height in meters of slopes free of stress, depending on the type of terrain, the angle of inclination of the slope not greater than 60º and the simple compressive strength of the ground
The maximum admissible height H max in sloped sections of the ground, provisional, with an angle between 60º and 90º (vertical slope), without overload stress and without shoring, can be determined by means of TABLE 2 based on the simple compression resistance of the terrain and its apparent specific weight. As a safety measure at work against «venting» or small detachment, stepped berms with plateaus not less than 0.65 m and counter-plateaus not greater than 1.30 m will be used (fig. 1).
The cut of land will be considered requested by foundations, roads and equivalent stockpiles, when the horizontal separation «S» (fig. 2), between the crown of the cut and the edge of the request, is greater than or equal to the values «S» from TABLE 3
In excavations next to flush or deeper foundations, it should be checked if there is a danger of raising the bottom. In general, there is no danger as long as it is verified (fig. 3) that:
qs ≤ 0.9 (m.Rw + n)
qs= Test stress transmitted by the foundation to the ground in its support plane in Kg/cm2
Rw= Simple compressive strength of the ground in Kg/cm2.
m= Influence factor (table 4).
n= Overload due to the shoulder in Kg/cm2 (table 5).
For values of A < b, n= 0 should be taken in general.
* Being, (fig. 3):
b= Width of the foundation in the direction normal to the cut in m.
L= Length of the foundation in a direction parallel to the cut in m.
D= Unevenness between the foundation support plane and the bottom of the excavation in m.
Shoring is the set of wood or other materials, conveniently arranged, that constitute the underpinning of the excavations on site.
The need for shoring arises from the problem of ensuring the stability of the excavations. Added to this requirement in urban areas is the lack of space in many cases, as it is not possible to give the excavation the natural slope of the land, or economic constraints in ditch or well type excavations. Due to what has been said above, the most frequent use of shoring is in temporary excavations of the trench or well type, although more rarely, they are used in castings or excavations of a single face.
The type of shoring to be used will be determined by the type of land in question, whether or not there are stresses and the depth of the cut (see table).
The Technological Standard NTE-ADZ/1976, «Terrain conditioning.
Clearing: Ditches and wells”, establishes the criteria to determine if the cut in the land can be considered without requesting nearby foundations or roads, this circumstance occurring when it is verified that:
P ≤ (h + d/2) or P ≤ d/2 respectively, (next fig.)
P= Depth of cut
h= Depth of the support plane of the next foundation.
d= Horizontal distance from the crowning edge of the cut to the foundation or road.
Types of shoring for site excavations:
Shoring with horizontal boards.
It is used when the cut is carried out in a ground with sufficient cohesion that allows it to be self-supporting while the excavation is carried out. By alternating excavation (0.80 m. to 1.30 m.) and shoring, the total depth of the trench is reached. (Fig. below)
Shoring with vertical boards
When the ground does not have sufficient cohesion or there is no guarantee of it, it is more advisable to carry out the shoring with vertical boards. If the ground presents acceptable cohesion and resistance, it is excavated in successive sections of up to 1.50 – 1.80 m. of maximum depths, in variable longitudinal sections that in no case should exceed 4 m.; and if the ground has little or no cohesion, the vertical boards must be driven into the aforementioned sections before proceeding with the excavation of the land, reaching the planned depth in successive stages.
Regardless of whether the shoring is carried out with horizontal or vertical boards, these may fully cover the walls of the excavation (solid shoring), the 50% (semi-solid shoring), and even less than this proportion (light shoring).
The Technological Standard NTE-ADZ/1976, allows to determine its use depending on the depth of excavation, the type of terrain and whether there is a request for foundation or road, (previous table), through the tables that follow, it can be determined the separation and thicknesses of the different elements that constitute the shoring of the main cases.