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*1. General aspects; Soil arrangementTo study the digging process, the adequate methods and means, the acting modalities on the soil to dislocate it from a place and put to another is necessary to know the nature and the crowd modalities of the earth. The soil is made up of rocks that can be classified after their making modalities in three main groups: Volcanic rocks; Sedimentary rocks; Metamorphic rocks.

The volcanic rocks are made of solidified magma that gets on earth surface after volcanic eruptions. The main volcanic rocks are granites and basalt. These kinds of rocks can be easily recognized after their surface texture. They have crystalline shining aspects and mosaic variegated colours.

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The main characteristic of these kinds of rocks is the primary fragmentation because of the fissure area apparition in the cooling process of the magma. These micro fissures create low resistance zones in the massif, zones that permit through mechanical actions, such as hammer knocks, the block fragmentation. As example, granite fragments are vertical such as columns and basalt fragments are horizontal like plates. Granite & BasaltThe volcanic rocks have great compression resistance (1000 daN/cm2).

*Sedimentary rocks are made of accumulated substance deposits during a period of time on the surface of the soil or at the bottom of the waters due to corrosion of pre existent rocks. The main sedimentary rocks can be classified such as: Detritus rocks (grit stone, clay, marl : Gresii, Argile & Marne) which are made by desegregation;

Fire destroyed rocks (tuff, volcanic tuff); Piroclastice, tufuri, tufuri vulcanice.

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Precipitation rocks (limestone, gypsum) de precipitaie, calcare, gipsuri.

Sedimentary rocks can be easily recognized after their stratified aspect, after the animal and vegetal fossils contain or after their colours, which are relatively uniform.Distinct categories of sedimentary rocks are the residual ones, which are made by the erosion of pre existent rocks without their movement. These soils can be found at the upper part of the earth and they are usually named vegetal soils. The width of these soils is 40 70 cm. They have low resistance, which implies that they cannot be used for building foundations.Metamorphic rocks can result from the transformation of pre existent rocks (volcanic and sedimentary) under different climatic actions such as temperature or pressure of the superior structures.Quartz (ardezia) from sand transformation or marble from limestone metamorphose are some examples of these kind of rocks.

*2. Mechanized digging of the soilDigging procedure particularitiesThe digging of the soil is mainly influenced by the type of the cutting device, by its position and by the properties and the nature of the earth.The soil digging equipment can be provided with specific working devices, on the acting level of the earth. Usually, this kind of device is called cutter. Its shape is presented in the figure :The shape and the characteristic angles of the cutting device

*The cutting device is characterized by its constructive shape and by its placing position regarding the cutting plane which takes into consideration the following angles: - The cutting angle; reprezint unghiul de tiere - The placing angle; unghi de aezare- The sharpening angle. unghi de ascuire

The digging process of the soil takes place in several stages and it is influenced also by the nature of the earth and its humidity degree.In the first stage the penetration and the advancing of the cutting device gives a soil particles tamping (the space between particle decreases). The compression efforts appear now.The next stage is characterized by the making of some slipping soil planes; it appears the cutting shear given by the advancing plane of the cutter.In the third stage takes place the soil portions displacement in front or to the upper part of the cutter.Taking into consideration the soil nature and its humidity degree the cut pieces of the earth can be disposed in different ways.

*To cohesionless soils such as sand, gravel, and ballast after the furrow cutting, a deformation of the terrain and an agglomeration in an unorganized shape in front of the cutting device take place. The prism that is made in the digging process becomes in time bigger and bigger.The digging process particularities of the cohesion less soilsIn the case of cohesive soils with an inferior humidity degree appears a fragmentation of the cutted furrow in pieces by making of several slipping planes at equal intervals.In time these soil parts have the tendency of moving in the upper part of the cutter.

*Plastic cohesive soils have the most favourable behaviour at digging. The prisms that appear stay relatively intact and can be directed to be organized in optimum conditions during the digging process.The particularities of the digging process for cohesive soilsThe digging process of plastic cohesive soils

*Taking into consideration the device characteristics, the cut furrow can be disposed in different shapes that lead to minimum resistance during the digging process.The digging process particularities of the hard cohesive soilsFor hard cohesive soils the soil dislocation appears in boulders. They have an unorganized shape in front of the cutter.

*General notions concerning the soil digging resistance The reactions that appear during the soil furrow dislocationBy the action of the digging device on the soil it appears a reaction R. The reactions that appear are: Rt The cutting resistance;Rn Normal resistance.The last reaction (Rn) gives during the cutting process a friction force between the cutter and the soil:

*This friction force is function of , the friction coefficient between the cutter and the soil.To realize the digging process is necessary to overcome the digging reaction Rs which is usually made by the cutting resistance Rt, the friction resistance Rf and the resistance of the soil furrow displacement Rd.To realize the digging process is necessary to overcome the digging reaction Rs which is usually made by the cutting resistance Rt, the friction resistance Rf and the resistance of the soil furrow displacement Rd.Rs = Rt + Rf + RdThe reactions that appear during the digging processIt becomes obvious that the digging force A has to be greater than the resistance force Rs.

*For different types of cutting devices the general relation will have particular values.In a simplified form, taking into consideration experimental data by introducing the concept of specific digging resistance k, the formula becomes:Rs = k S [daN] Where: k is the digging resistance [daN/cm2];S Is the furrow section surface perpendicular to the digging direction [cm2].

By taking into consideration the soil type and the working device there are presented in table the values of specific digging resistance.*From this table it results that the equipment with blade devices can work only for the first two categories. Blade cannot dig hard soils.

Soil typeSoil categoryk, daN/cm2BucketBladeSand, ooze, vegetal soilI0.16 0.700.28 1.20Argillaceous sand, argillaceous silt, loessII0.60 1.301.00 1.90Silty clay, clay, ballast, cobbleIII1.15 1.95-Tough clay, marl clay, cobbleIV2.00 3.00-Frozen smashed soilsV2.80 3.25-Frozen dislocated soilsVI3.80 4.70-

Equipments for soil digging and transportation*A general classification of the digging equipments can be made by taking into consideration the shape of the digging device: Bucket devices. Blade devices.

Excavators are equipments that have bucket digging devices.They can dig and place the soil in a transportation mean or in a storehouse.The excavators can have periodic action if they are provided with a single bucket or continuous action if they have several buckets.The classification of single bucket excavators can be made by taking into consideration the following points of view:a) Taking into consideration the universality point of view they can be:Universal excavators equipped with different types of working devices such as front acting buckets, back acting buckets, grab buckets, dragline buckets.Semi universal excavators equipped only with one, two or three working devices such as front acting, back acting and dragline buckets.Special excavators are specific for big soil volumes, in quarries or earth mines equipped with only one working device.

*b) Taking into consideration the acting system they can have:Mechanical action.Electromechanical action.Hydrostatic action.

c) Taking into consideration the rolling system they can be:

Caterpillar excavators & Rubber wheel excavators

Railway excavators & Floating excavators

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Walking excavator

All these kind of classifications are important, but taking into consideration the civil engineers point of view the main particularity is regarding the bucket of the equipment, so the universal excavators can be:Front acting excavators;Back acting excavators;Dragline excavators;Grab excavator.

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