Pit device: digging, slopes, fastening. Fastening the walls of trenches and pits Fastening the walls of trenches with inventory wooden shields

In cramped conditions, as well as in the presence ground water, quicksand and under other difficult hydrogeological conditions, when it is impossible to provide the required laying of slopes, it is necessary to fix pits and trenches.

The permissible excavation depth, i.e., the maximum (critical) depth at which the cohesive soil slope is held in a vertical position without fixing the walls, is determined by calculation. Approximate values ​​of the critical depths of excavations arranged with vertical walls: 1.0 m in bulk, sandy and gravelly soils natural humidity; 1.25 m - in sandy loam; 1.5 - in loams and clays; 2.0 - in especially dense non-rocky soils.

The need for fasteners is established by the project. Mounting device vertical walls ok pits and trenches require significant costs manual labor, therefore, fastening is carried out only when it is economically feasible or when it is not possible possible device slopes.

Depending on the type of soil, the width and depth of the excavations and the service life, different kinds fasteners. Temporary support can be made in the form of wooden or metal sheet piles, wooden boards with support posts, boards with spacer frames. The design of any fastening includes a fence made of boards, beams or shields that directly perceive the pressure of the soil. Runs, spacers and other elements are used to hold the pick-up in a vertical position. There are horizontal fastening, when the boards or beams of the pick-up are brought horizontally behind the racks, and vertical, when the boards of the pick-up are installed vertically and fastened with horizontal runs with spacers.

For narrow trenches with a depth of 2-4 m in dry soils, a horizontal-frame fastening is used, consisting of racks, horizontal boards or plank (solid and not solid) shields and struts that press the boards or shields to the walls of the trench. Spacers are installed along the length of the trench at a distance of 1.5-1.7 m from one another and in height at 0.6-0.7 m.

Vertical fastening is most often used if the fasteners are placed in one row. The pick-up is made continuous if the soils are unstable and have high humidity, or with gaps (gaps) if sufficiently stable cohesive soils of normal moisture are attached. In difficult hydrogeological conditions, when there are highly water-saturated spreading soils, a solid fence made of wooden or metal sheet piles is used.

There are three types of fasteners to hold the pick-up in a vertical position: spacer, cantilever and strut. The spacer type of fastening is the most common due to the ease of assembly. In this case, the racks are freely installed on the bottom of the excavation and pressed against the intake by horizontal struts installed at several levels according to the calculation. The width of the spacer recess is limited. The spacer fastening is installed in the following sequence: after a section of the trench has been cut, two frames are lowered into it and they are installed on the bottom 2 m apart, temporarily unfastened with braces, then horizontal boards or shields are inserted from above into the gap between the racks of the frames simultaneously along both walls, after which push the spacer frames to the stop.

In cases where the possibility of installing spacers is excluded (for example, when developing wide pits), anchor or strut fastenings are used. Strut mounts consist of plank boards installed along the slopes of the racks, which are held by the struts, and stops driven at the base of the struts. However, such fastening, for all its structural simplicity, suffers from some drawbacks: such fastening hampers work inside the pit, in addition, driving of persistent anchors leads to a violation of the soil structure at the bottom of the pit.

Console mounts are characterized by the fact that the racks ( wooden piles) are held by pinching the lower part in the ground. Racks, piles, rails, steel rolled profile, pipes, etc. are hammered into the bottom of the excavation to a depth of 2.2-3.3 m. The horizontal pick-up boards are placed either behind the racks, or are inserted between the shelves of the I-beams. Cantilever fastening is carried out in the following order: along the trench broken on the ground, racks are hammered with a calculated step to a depth below the bottom of the future pit. After that, the soil is developed. If the soil is unstable, the horizontal elements of the intake are installed simultaneously as the trench deepens. At the same time, each subsequent board is brought from below under the previously installed one - it is grown. In sufficiently stable soils capable of holding a vertical slope at least for a short time, trenches are torn off in sections 3-4 m long to the design depth, and the pick-up boards are installed by lowering from above - building up. Fastening from a wooden or steel tongue is widely used; with a non-spacer fastening, the racks are located at a certain step, and in a grooved one they are hammered without an interval.

Cantilever non-spacer fastening is used for pits and wide trenches up to 4.7 m deep. If it is necessary to tear off deeper pits, additional fastening of the upper part of the posts with anchors is arranged. The anchor consists of one or two hammered anchors and braces. Anchors should be driven to a depth of about 3 m and at a considerable distance from the crest (equal to approximately one and a half depth of the excavation) in order to position them outside the destruction prism. The distance between anchors is determined by calculation. The disadvantage of this method is that the installation of anchors requires a significant free area along the excavation and, in addition, the braces interfere with the work in this area, so sometimes braces are arranged in trenches 0.5 m deep that are open for this purpose.

When constructing deep pits with sheet piling, first a steel sheet is hammered along the perimeter of the future pit 4-5 m below the bottom, then anchors are installed, after which the soil is torn off. Hanging mounts most often used for fastening rectangular pits up to 2-5 m deep, depending on the purpose; they have horizontal elements that play the role of persistent runs, which are suspended from a support frame laid on the surface of the excavation.

Rice. 6. :
a - console; b - anchor; in - cantilever-spacer; g - spacer; d - strut; e - suspended; 1 - shields (boards); 2 - racks (piles); 3 - anchors; 4 - spacers; 5 - struts; 6 - stops (anchors); 7 - support; 8 - ring

In loose and unstable soils, spacer or log fastenings from plates and beams are installed. In viscous soils and with a strong influx of water, enclosing sheet pile walls made of boards or beams, reinforced with spacers, are clogged. On the surface of the earth, according to the size of the well, a wooden block frame is laid, and then from the outer sides of the frame beams, close to them, boards 1.5-2 m long are hammered with a certain slope and a foundation pit is dug under the protection of the hammered boards. After deepening by 1-1.5 m, a second similar frame is installed at the bottom of the well and the second row of boards is hammered. In the same order, work continues until the required depth is reached (Fig. 6).

The development of trenches with vertical walls by rotary and trench excavators in cohesive soils (loam, clay) is allowed without fastening to a depth of not more than 3 m. engineering networks etc. in trenches with vertical walls without fastenings should be carried out immediately after excavation in order to avoid shedding or sliding.

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During the earthworks, a number of side works have to be carried out, without which development is impossible. These jobs are called auxiliary.

The most common ancillary activities in earthworks include:

• fixing trenches and pits;
• dewatering (removal of water from pits);
• arrangement of temporary roads, entrances to the face and exits from the face for the transportation of soil during its development.

We must always strive to ensure that all auxiliary work is carried out by special workers and that the performance of auxiliary work does not delay or interfere with the main work.

Pit fixing device

As already mentioned in, not every soil can hold vertical slopes when digging. The value of the required slope of the pit is equal to the value of the angle of repose of the soil. This slope is the most reliable.

However, digging pits and trenches at great depths with gentle slopes is not considered economical, as it causes a significant amount of unnecessary excavation. Even at shallow depths, natural slopes are sometimes impossible to implement, for example, if buildings are located nearby. In cases where the bottom of the pit or trench is under water, free slopes are completely unacceptable, since they are not protected from water soaking and destruction.

That's why, in most cases, when arranging pits and trenches, you have to arrange various kinds of temporary fasteners. In addition, as mentioned above, a special type of fastening (sheet piling) serves to reduce the inflow of groundwater into the pits.

Fastening trenches and pits with wooden struts

The simplest fastening of the walls of pits and trenches up to 2 m deep are arranged as follows.

Along the walls of the trenches, 4 boards 50 mm thick are laid with spacers between them, placed every 1.5-2 m along the length of the trenches (Fig. 38);

Spacers are made from short logs or pipes 10-12 cm thick. This type of fastening is used for dense dry soils that can hold a vertical slope for some time and are not washed away by rains (dense clay, dense loam). Slopes in this case can be both vertical and with a slight slope (1/10).

At greater depths (up to 4 m), for dry soils that give local creep after a short period of time after the passage, the so-called horizontal fastening is arranged. It is arranged as follows: for the entire depth of the pit, a series of thrust posts are installed from boards up to 6 cm thick or plates at a distance of 2 to 3 m, depending on the depth of the pit (Fig. 39). For these racks, a fence is laid from horizontal rows of boards 4-5 cm thick, staggered or completely, depending on the soil. Wooden or steel spacers are used to hold the posts in place. The spacers should be slightly longer than the distance between opposite walls. When setting the strut, this circumstance makes it possible to “start” the struts with blows of a sledgehammer or hammer, and thereby tightly press the posts and the fence to the walls of the pit or trench.

In order to prevent the spacers from falling (Fig. 40), shorties (bosses) are placed under their ends from scraps of boards 4-5 cm thick. The shorties are nailed to the racks with 125-mm nails.

The distance between the spacers in height depends on the depth of the trench. With an increase in depth, the pressure of the soil on the fasteners increases, so spacers are placed below more often than at the top, namely: at the top - after 1.2 m and at the bottom - after 0.9 m in height. The upper horizontal board is placed slightly above the edge of the trench so that the soil from the edge does not crumble into the trench. To transfer the soil, shelves from boards are laid on spacers.

With loose and wet soils, as well as in crumbling soils, a vertical fastening is used, which differs from the horizontal one in that the horizontal boards in it are replaced by vertical ones, and the racks are replaced by horizontal clamping bars. The clamping bars are bursting with knurled spacers, forming spacer or clamping frames (Fig. 41).

Clamping frames with vertical fastening to a depth of up to 3 m are made of semi-edged boards 6 cm thick, and spacers are made of knurled or plates. At a depth of up to 6 m, the thickness of the pressure boards, as well as the spacer, must be increased to 10 cm.

The upper clamping frame must have, in addition to the inner board, an outer board 6 cm thick. This board cuts into the trench wall to its full thickness.

The height distance between individual pressure frames made of boards is 0.7 - 1.0 m, and with frames made of plates and beams - 1.0 - 1.4 m.

With a depth of up to 5.0 m, the number of spacers for each frame of boards 6.5 m long is set in 4 pieces, with a greater depth - 5 pieces.

Both with vertical and horizontal fastening, the walls of the trenches must be sheer. With inclined walls, the spacers under the pressure of the earth can jump up.

The lower clamping bars and struts of fastenings of water and sewer trenches should be located in such a way that there is a gap between them and the bottom of the trench, sufficient for unhindered laying of pipes.

Often there are cases (weak soil, the presence of water) when fastening is necessary before digging. In these cases, the fasteners are more complex.

These fasteners include:

Downhole fastening

In small in size, but deep pits and pits, the so-called downhole fastening is used (Fig. 42).

It is arranged as follows: on the surface of the earth at the location of the pit or pit, a horizontal block frame is laid according to the size of the pit. this frame is buried flush into the ground, after the frame a row of boards is hammered slightly obliquely. Then they start digging a pit under the protection of the walls formed by the clogged boards. When the excavation approaches the lower ends of the forgotten boards, a second frame is laid between them. In order for the upper frame not to fall down as the soil is worked out, short pieces of bars are placed under it, gradually lengthened. When the second frame is installed, between it and top frame install bars that support the upper frame. After that, another row of slightly inclined boards is hammered along the outer edge of the lower frame. Between the upper and lower rows of the fence, wedges of greater stability of the upper fence are hammered.

Fixing pits with piles with a wooden fence between them

The fastening of pits with piles with a wooden fence is used for weak soils that do not allow digging a pit to the full depth. In addition, often the device of transverse struts when fixing the pit is undesirable, since it makes it difficult to work in the pit. With a large width of the pit or its complex shape, spacers cannot be installed at all. Therefore, in all such cases, they resort to a device for fastening with piles with wooden driving between them. This type of fastening is as follows: before digging, wooden, and sometimes steel (iron) piles, the so-called lighthouse piles, are driven into the ground at a distance of 1.5-2 m from each other, depending on the depth of the pit (Fig. 43) ; between these piles, as the excavation deepens from the side of the slope, separate fastening boards are laid. Piles are driven to a depth slightly greater than the depth of the pit, so that the pile remains sufficiently stable until the digging of the pit is completed. To enhance the stability of lighthouse piles, their upper ends are anchored in a slope or supported by struts, resting the latter against the piles hammered at the bottom of the pit.

Pit fastenings with piles with a fence can also be arranged in pre-dug pits, if it is undesirable to have spacers in the pit, and the soil allows digging without pre-installed fasteners.

Sheet piling fastening

For fixing pits in soils saturated with water (slurry and quicksand), the so-called sheet piling is used. Sheet piling consists of a continuous row of vertically installed sheet piling pipes or boards (in which, a groove-groove is made in one edge, and a ridge on the other), pressed against the walls of the trench or pit by horizontal frames with spacers (Fig. 44). Everything that has been said about spacers in vertical fastening applies entirely to sheet piling, in that with sheet piling, the sheet pile is first driven in, and then the trench is digged with the gradual installation of spacer frames; in a vertical mount, a trench or pit is first dug, and then a mount is installed, gradually lowering down as the soil is further developed. The sheet pile boards are driven to a depth somewhat greater (by 0.2-0.5 m) than the depth of the trench or pit, so that after digging is completed, their lower ends cannot be moved by the pressure of the soil.

A wooden sheet pile is made from boards 6-7 cm thick or from bars 10x20 cm (Fig. 45). In each sheet pile (pile), a ridge and a groove are arranged. When driving piles, the crest of one enters the groove of the other. The cutting of the lower end of the pile is done in the form of a wedge with acute angle from the groove side. With such a fit, the piles fit snugly against each other during driving, which is very important in wet soils, when water seeps under pressure into the cracks of loose sheet piles. Sheet piles should be made from raw, freshly cut wood. If they are made from wood that has already lain in the air for some time, then before driving them they must be put in water for 10-15 days so that they have time to swell. This is done later that the sheet piling row, hammered from dried piles, swells in wet soil and, due to the increase in the volume of piles, the row is bent; individual piles turn out, forming cracks, and the row becomes unusable. pile driving begins with the installation exactly along the line of the future row of so-called lighthouse piles 2 m apart (Fig. 43).

These piles are driven first, and frame bars are attached to them on both sides. In the intervals between the lighthouse piles and the frame beams serving as guides, the rest of the sheet piles are driven. Each subsequent pile must be adjacent to the already clogged groove, and the crest must remain free, otherwise the grooves are heavily clogged with earth, and it will be difficult to achieve a dense row. Driving is done with a mechanical pile driver, and at shallow depths and soft ground, it can also be done manually with wooden broads.

Dismantling of fastenings of the sheet piling of the excavation

Dismantling of fasteners should be carried out, starting from the bottom, as the trenches are backfilled.

Horizontal mounts are disassembled one by one in case of weak soils, and in case of very dense soils - no more than 3-4 boards. Wherein vertical racks filed at the bottom desired height. Before sawing the posts, the spacers must be moved above the sawing point. The spacer is rearranged as follows: first, a new spacer is installed on top of the notch, and then the lower one is knocked out.

With vertical fastening and sheet piles, spacers and clamping bars are removed gradually as the backfill is completed, starting from the bottom: sheet piles and vertical boards are pulled out after backfilling is completed using a lever (Fig. 46). In this case, the engagement of piles is done according to one of the methods shown in Fig. 47.

Dismantling of fastenings with piles with a wooden fence is carried out by gradual sawing as the fence boards are backfilled, starting from the bottom; it is necessary to remove the fence one by one. The piles are removed after the entire backfilling is completed in the same way as when disassembling the sheet piles.

AT this moment time use steel railings: Larsen sheet pile, steel pipes bu with a diameter: from 159 to 426 mm.

The device of pits and trenches with fastening

For pits and trenches up to 3 m deep, as a rule, inventory fasteners are used, made according to standard designs. The choice of the type of fastening (Table 5.3) with an excavation depth of up to 3 m depends on the type of soil and its moisture content.

Table 5.3

Rice. 5.2. Ground fixing design solutions:

a) fastening with spacers (1 - stand, 2 - bosses 3 - spacer), b) anchor fastening (1 - stand, 2 - fixing boards, 3 - coupler, 4 - anchor) c) fastenings (1 - stand; 2 - fastening boards 3 - bosses 4 - struts) d) tongue and groove fastening (1 - wooden sheet pile 2 - girders) e) device for fastening trenches (1 - stand, 2 - guide 3 - sliding struts 4 - extensions, 5 - steel shields) g ) a device for fastening the walls of trenches (1 - fencing shields, 2 - spacers 3 - thrust 4 - spread the pads 5 - loops;) h) fastening the walls of trenches when laying pipelines (1 - wooden shield, 2 - spacer frame 3 - sector support, 4 - a pipe that is lowered, 5, 6 - hinged struts) e) temporary fastening when laying pipelines (1 - anchor elements 2 - soil is removed 3 - shields 4 - racks, 5 - bracket, 6 - hook)

The advantages of inventory fastenings: the assembly of elements, the possibility of installing them from above without descending into the trench, the possibility of mechanizing installation and disassembly, calculating all elements for strength and stability.

Consider the basic requirements for labor safety in the preparation of pits and trenches. It is necessary to install fasteners down as the excavation is developed to a depth of not more than 5 m (in unstable soils by 0.2-0.25 m). When installing fasteners top part they should protrude above the edge of the notch by at least 15 cm. wooden fasteners trenches up to 3 m deep must meet the following requirements: for fixing soils of natural moisture, except for sandy ones, boards with a thickness of at least 4 cm are used, and for sandy soils and soils high humidity- at least 5 cm, which are laid behind the vertical racks close to the ground with the reinforcement of the strut. Racks of fasteners are installed at least every 1.5 m, spacers of fasteners are placed at a distance of no more than 1 m, at the end of the spacers clog the bosses. When excavating soil of excavations with a depth of more than 1.8 m, it is necessary to arrange flooring shelves on spacers, which must be protected by side boards with a width of at least 15 cm. mechanized way. Dumping it into trenches or pits is prohibited. The condition of the fasteners must be systematically monitored. Mounts installed in winter are especially carefully examined and, if necessary, reinforced. Fill up the recesses in parts. In this case, the fastening is disassembled from the bottom up, removing no more than three boards at a time in stable soils, and no more than one in unstable soils. When removing boards, you need to rearrange the spacers accordingly. Disassembly of fasteners is carried out under the supervision of a foreman or foreman. When erecting underground structures in loose and water-saturated soils, it is not necessary to disassemble the fasteners, as this can lead to an accident. Usually, the recesses fall asleep without disassembling the fastening, about which an appropriate act is drawn up.

Arrange in dry and low-moisture stable soils.

If the height of the pit h to ≤5 m, then the slope (ratio h to /b) is determined from the tables depending on the type of soil.

If the height h to >5 m, then the calculation of the steepness of the slope is necessary.

Such pits are the simplest, but at the same time, the volume of earthworks increases sharply, especially with deep pits. Besides, in vivo cities excerpt excavation with a natural slope is not always possible (closely located buildings)

2.2.B Pit pits with vertical walls

can be: - with fastening

Without mount

Without fastening it is allowed only in dry and low-moisture stable soils for a short period. The depth of such pits should not exceed:

in sands up to 0.5 m

in sandy loam up to 1.0 m

in loams and clays up to 3 x m

The design of the pit fasteners is selected depending on the following conditions:

pit depth;

soil properties;

fixture service life.

Depending on these conditions, the following fastening designs are selected:

embedded fasteners;

anchor or strut mounts;

sheet piling.

2.2.B. Embedded mounts

Arranged at a pit depth of up to 2 ... 4 m in dry and low-moisture soils (Fig. 14.2 a, b). Mortgage fastening consists of racks, struts and horizontal boards (pickup), which lead behind the racks from below as the pit or trench deepens, and the racks are gradually replaced with longer ones, carefully unfastening them with struts.

Rice. 14.2. Fastening of the vertical walls of the recesses:

a, b - mortgage; c - anchor; g - strut; 1 - rack; 2 - boards; 3 - spacer; 4 - pile; 5 - coupler; 6 - brace

A more convenient fastening that does not require replacement of racks as the excavation deepens, consists of I-beams steel beams previously hammered into the ground, behind the shelves of which boards are gradually laid.

2.2.D. Anchor and strut fastenings

Arrange in those cases when the possibility of installing struts is excluded (a wide pit, also if the struts interfere with the construction of the foundation).

For device anchor(Fig. 14.2 c) the fasteners along the wall of the pit are hammered with inclined piles, which are connected with anchor rods to the fastening posts.

In a strut mount (Fig. 14.2 d), the walls are held by struts that transmit shear forces to the stop, which is clogged at their bases.

2.2.E. sheet piling

Serve for fastening the vertical walls of the pit at a depth of more than 4 meters, as well as at any depth, but at a groundwater level above the bottom of the pit.

Sheet piles are made up of individual elements(shuntin), which are immersed in the ground even before the excavation of the pit and form a solid wall that prevents the soil from sliding and the penetration of water into the pit.

Rice. 14.3. Wooden sheet piling:

a - from the boards; b - from bars; c - the lower end of the wooden sheet pile

Sheet piles can be made from:

→ Wooden sheet piling is used for fixing shallow pits (3 ... 5 m) (Fig. 14.3) can be:

Plank (thickness up to 8 ... 10 cm)

Paving stones (t from 10 to 24 cm)

Rice. 14.4. Rolled steel sheet pile profiles:

a - flat; b - trough; in -Z-shaped

The length of sheet piles is determined by the depth of their immersion, but, as a rule, does not exceed 8 m, since the longer one is not expensive and in short supply.

To completely close the sheet piles, they are provided with a ridge or a groove, and the lower end is made with one-sided sharpening, due to which the submerged sheet pile is pressed against the already submerged one, which makes the wall more dense.

The gradual swelling of wood in water also contributes to additional compaction of the tongue joint.

Wooden sheet piling is distinguished by ease of manufacture, but there are limitations to its use:

The impossibility of driving sheet piles into dense soils;

Small length of sheet piles (6 ... 8 m);

And relatively low strength.

→ Metal sheet pile is used at a depth of more than 5 ... 6 m. Due to its design (Fig. 14.4), it has great strength and rigidity.

It consists of a rolling profile l=8…24 m.

Korytnaya; ) at large bending moments

Z shape

The connection between the sheet piles along the vertical is carried out using " castles". The design of the locks provides a tight and durable connection between the sheet piles. The remaining gaps in the locks are quickly filled and the metal sheet pile wall becomes practically watertight.

reinforced concrete sheet pile is used in the construction of embankments, moorings and hydraulic structures, or in cases where the sheet pile is subsequently used as part of the structure.

Reinforced concrete pile

Solid reinforced concrete row of piles (driven or bored)

Permitted row of piles in clayey soils.

Sheet pile wall structures:

Without mounts (console);

With spacer fastening;

With ground anchors.

Rice. 14.5. Sheet piling schemes:

a - console; b - with spacer fastening; c - with anchor fastening; 1 - sheet pile wall; 2 - spacer; 3 - strapping; 4 - anchor pile; 5 - anchor rod.

The use of expansion and anchor type fasteners increases the stability of the sheet pile wall, reduces the resulting bending moments and its horizontal displacements, which makes it possible to make the walls lighter.

2.8.1 The development of trenches and pits with vertical walls in soils of natural moisture without fastening can be carried out at a depth of:

no more than 1 m - in bulk, sandy and gravelly soils;

no more than 1.25 m - in sandy and loamy soils;

no more than 1.5 m - in clay soils;

no more than 2 m - in especially dense soils. At the same time, work should be carried out immediately after excavation of trenches and pits.

2.8.2 If the specified depths are exceeded, digging trenches and pits is allowed only if vertical walls are fastened or slopes of acceptable steepness are arranged (Figure 2.7).

Figure 2.7 - Determining the steepness of the slope

The maximum allowable steepness of slopes of trenches and pits in soils of natural moisture should be determined according to Table 2.4.

2.8.3 Digging trenches and pits in frozen ground all rocks, with the exception of dry sandy, can be carried out with vertical walls without fastenings to the entire depth of their freezing. When deepening below the freezing level, fastening should be carried out.

2.8.4 Trenches and pits in dry (loose) sandy soils, regardless of the degree of their freezing, should be developed to ensure the established steepness of slopes or with a wall fastening device.

2.8.5 Digging trenches and pits in warmed (thawed) soils should be carried out with the necessary steepness of the slopes or with the device for fastening walls in those cases (or places) when the depth of the warmed area exceeds the dimensions indicated in Table 2.4.

Table 2.4 - Maximum allowable steepness of slopes of trenches and pits

2.8.6 At intersections with railway or tram tracks, it is necessary to develop trenches and pits with the obligatory fastening of their walls. Tracks should be fastened with rail packages only in cases provided for by the project agreed with the maintenance service for these tracks.

2.8.7 Types of fastening of pits and trenches with vertical walls are shown in Figure 2.8 and Table 2.5.

a) horizontal-frame mounting;
b) fastening horizontally solid;
c) horizontal fastening with gaps;
d) mixed fastening: horizontal, solid and tongues;
e) vertical-frame fastening;
f) vertical-solid fastening

Figure 2.8 - Ways of fastening the walls of trenches and pits

Table 2.5 - Types of fastening of pits and trenches with vertical walls

2.8.8 As a rule, trenches and pits up to 5 m deep should be fixed with inventory devices. Inventory metal screw struts (Figure 2.9) are used to reduce the consumption of forest materials.

Figure 2.9 - Screw struts for fixing trenches

At a depth of more than 3 m, fastenings must be made according to separate projects approved by the management of the construction organization

2.8.9 In the absence of inventory devices, the details of fastening trenches and pits must be made on site in compliance with the following requirements:

a) for fixing soils of natural moisture (except sandy ones), boards with a thickness of at least 40 mm should be used, and for soils of high humidity - at least 50 mm. Boards should be laid behind vertical posts close to the ground with reinforcement by spacers;

b) fastening posts should be installed at least every 1.5 m;

c) the vertical distance between the struts should not exceed 1 m. The struts are fixed with a stop;

d) the upper boards should protrude at least 15 cm above the brows;

e) the attachment points on which the shelves for transferring the soil rest must be made reinforced. Shelves are protected by side boards with a height of at least 15 cm.

2.8.10 The development of excavations in soils saturated with water (quicksand) should be carried out according to individual projects, providing safe ways works - artificial dewatering, tongue and groove fastening and etc.

2.8.11 Fixings of pits and trenches should be dismantled from the bottom up, as backfill soil and simultaneously remove no more than two or three boards in normal soil, no more than one board in quicksand. Before removing the boards of the lower part of the fastening, temporary oblique struts must be installed above, and the old struts may only be removed after the installation of new ones; fasteners must be dismantled in the presence of a responsible contractor. In places where the dismantling of fasteners can cause damage to structures under construction, as well as in quicksand soils, it is possible to partially or completely leave fasteners in the ground.

2.8.12 The walls of pits and trenches developed by earth-moving machines must be fastened with ready-made shields, which are lowered and burst apart from above (workers are prohibited from descending into an unsecured trench). The development of trenches by earth-moving machines without fasteners must be carried out with slopes.

After excerpts, trenches and pits are prone to rapid destruction. Therefore, their walls must be further strengthened. This, firstly, keeps the shape of the notch for a while further work, and secondly, it protects workers from accidents due to landslides. Most often, the walls of the trench are fastened with inventory boards, boards or tongues. In what cases are such measures necessary and how exactly is soil strengthening carried out?

When is it necessary to fix the walls of trenches and pits?

In preparation for the construction or laying of communications, recesses without slopes, with vertical walls, are usually preferred. Such trenches have a number of advantages:

• they are more economical in execution, because attachments excavators are mainly designed to create vertical walls;
• pits and trenches without slopes occupy a smaller area, which is very important when digging in conditions of dense buildings or natural landscape, which is undesirable to destroy;
• the presence of a slope can complicate further construction work on a dug trench or in a pit.

But recesses with vertical walls are prone to collapse and shedding. Therefore, without additional reinforcement, you can only dig pits and trenches of small depth:

• on bulk, sandy and coarse soils - up to 1 m;
• on sandy loam - up to 1.25 m;
• on loams, clays, loess-like soils - up to 1.5 m;
• on especially dense soils, for the development of which it is necessary to use crowbars, picks and wedges - up to 2 m.

When digging at a greater depth, it is necessary to fasten the walls of the trench with special devices. In addition, it is necessary to strengthen the walls even at shallow depths if the soil is oversaturated with moisture, and the excavation can “float”.

Attention! The need to strengthen the walls of pits and trenches is spelled out in normative documents. This requirement cannot be ignored. Soil collapse can lead to the destruction of construction sites, landslides and accidents. The presence of workers in excavations without slopes or reinforced vertical walls is strictly prohibited!

Ways to strengthen the walls of the trench

Most often, the slopes of the recesses are strengthened in the following ways:

• inventory boards and spacers;
• sheet piles;
• boards.

The method and design parameters of the reinforcement are selected depending on the type and condition of the soil, the height of the groundwater, the depth and purpose of the excavation. At the same time, the fastening of the slopes of the trench is performed small areas, as it digs. As a rule, this process follows the excavator, at a safe distance from the excavation site. With a large digging depth, the structures are installed from top to bottom, after excavating the excavation to a depth of no more than 0.5 m.

The upper part of all fasteners should protrude at least 15 cm above the edge of the trench. During backfilling, the wall reinforcements are usually dismantled. The exception is cases when the dismantling of structures is technically impossible or may cause deformation (destruction) of the construction object. The fasteners should be disassembled from the bottom up, as the excavation is backfilled.

Strengthening trenches with inventory shields

This is the most popular way of fastening the walls today:

• it is convenient in execution and safe;
• requires less labor and materials than the use of board and sheet piling (for example, use inventory shields for fastening trenches is 3-4 times cheaper than building reinforcing fences from boards).

Also, inventory fasteners are indispensable when extracting recesses with a ditcher. Indeed, in this case, the width of the excavation is so small that the installation of reinforcing structures is possible only from above.

Inventory fastening for trenches consists of:

• metal screw spacer frames;
• wall fencing shields.

The spacer frame is a simple fixture of two stops and a screw that connects them. With the help of a screw, the stops are moved apart to the desired width and press the elements of the trench fence against its walls.

Inventory shields for fixing trenches are made from different materials. It could be:

• waterproof plywood;
• bituminized cardboard;
• wavy sheet metal and etc.

The types of shields are selected based on working conditions and economic feasibility.

Installing inventory mounts is pretty easy. First, two already assembled spacer frames are lowered into the trench. Then, shields are placed in the gaps between their posts and the walls of the recess. After that, it remains only to push the stops to fix the fence.

Take off inventory fences in the process of backfilling the trench. Spacers are removed as the backfilled soil reaches their lower ends. The shields are removed after removing the uppermost struts. Since by this time they are already covered with soil, crane equipment is required to lift them.

Plank and sheet piling wall reinforcements

Fastening the trench with boards is done in different ways. There are four main types:

• vertical solid;
• horizontal solid;
• horizontal with a gap;
• horizontal frame.

AT different occasions boards are located on the walls of the trench vertically or horizontally, solid or with gaps through one board. For their fixation, spacers, tongues and other additional elements are used.

Fastening the trench with a tongue is used in difficult cases:

1. In areas with a strong influx of groundwater, when soil particles can be carried out by water and the walls of the excavation are washed out. For example, a solid fence made of Larsen's trough-shaped sheet piles makes it possible to hold even swampy, spreading, water-saturated soils and withstands a sharp rise in groundwater.
2. With very deep development.
3. If the trench runs close to the foundation of the building.

Sheet piling can be solid or with boards. Depending on the depth and width of the trench or pit, wooden, steel or reinforced concrete sheet piles of various profiles (flat, trough-shaped, tubular) are used. They are hammered in before the start of excavation and, if necessary, additionally fixed with anchor braces.

Standard inventory fasteners for the walls of trenches and pits are used at a development depth of up to 3 m. If the excavation needs to be dug deeper, the reinforcing structures are developed individually and approved by the project.

Facing pits and trenches can be done as follows:
a) vertical facings, consisting of a system of vertical bearing posts, the space between which is sheathed with horizontally arranged elements. The fastening is installed after excavation of the soil from the pit or trench without first fixing the walls;
b) vertical retaining structures. The fastening consists of a vertically placed sheet pile, which is installed before or parallel to the excavation and secured with horizontal braces or special pile nozzles;
c) load-bearing sheet pile walls. They are constructed from vertical load-bearing sheet piles, which are driven into the ground until it is excavated, and then unfastened by horizontal struts. Pole anchoring is possible;
d) fastening with special plates. The lining of the walls of the pits is carried out with specially made large-sized slabs, installed immediately after excavation. Their fastening is carried out by vertical or horizontal bearing elements, which can be additionally secured with spacers.
When arranging deep pits with vertical walls in residential, industrial and agricultural construction before installation load-bearing structures the walls must be lined with protective elements coated with various film materials.
In accordance with the above classification, their areas of application are distributed as follows:
a) for small and medium-sized pits;
b) limited width of the construction site;
c) the proximity of the building soil;
d) in conditions that exclude the possibility of concussions. The types of claddings described in this chapter are only applicable
in conditions of limited inflow of groundwater. The groundwater level should be below the bottom of the pit. If necessary, dewatering should be carried out in a closed way.

3.1. FASTENING PIT WALLS WITH HORIZONTAL ELEMENTS

The method of fastening the walls of pits appeared a very long time ago and is still widely used in the construction of small and complex objects. It is used when, according to soil conditions, the height of the loose part of the wall should not exceed 0.5 m, and the elevation of the bottom of the pit is above the groundwater level. Facing the vertical walls of the pits with a horizontal sheet pile is used in cases where two parallel walls of the pit are slightly removed from each other. The design of such walls consists of:
horizontal - wooden, metal or reinforced concrete elements;
vertical posts made of round wooden posts or steel beams;
horizontal or inclined braces wooden round timber or beams, steel beams or screw struts for narrow trenches;
elements providing local rigidity of the structure, consisting of additional racks and struts.
The following advantages of fastening racks with horizontal elements can be called:
the possibility of arranging pits of complex configuration;
small mass of individual building elements;
possibility of multiple use of fastening structures.
However, this solution has several disadvantages: restrictions on the use of machines for laying pipelines and performing other types of work due to the presence a large number cross struts;
the need to re-stability during disassembly and reassembly of struts;
the possibility of loss of stability of the walls when removing the struts in the process of execution construction works.

Rice. 3.6. Fastening the vertical walls of a wide pit
a - driving a guide trench; b - the device of the bearing element with its anchoring; c - support of the walls of the pit on load-bearing element; d - top view of the anchor fastening; 1 - anchor made of round steel; 2 - spacers; 3 - metal bearing racks; 4 - racks; 5 - sheathing; 6 - traverse; 7- cross beam; 8 - wedges

3.1.1. Trench linings.
When implementing a structure with horizontal elements to protect the walls of trenches when laying pipelines (Fig. 3.1), the following design solutions are used:
the thickness of the boards used for sheathing must be at least 50 mm;
wooden posts with a section of at least 100x140 mm must support at least four horizontal elements or boards along the length;
when using metal racks, their cross section must be at least 10;
wooden stud diameter round section must be at least 100 mm and have a chamfer at the ends.
For sheathing the walls of trenches, boards are usually used with a length of 4.0 to 4.5 m, a width of 200 to 300 mm and a thickness of 50 to 70 mm. For everybody a separate section sheathing, it is allowed to use boards of only the same length, since their extension along the length is not allowed. Can be used instead of wood metal cladding from profiled elements. Under normal conditions, the length of the posts and braces is from 1.5 to 2.5 m. The posts should be located at a distance of no more than 200 mm from the end of the horizontal board. Boards 2.5 long; 4.5 m are attached to three posts. The length of the vertical posts is at least 1 m; this allows at least two spacers to be placed along their length. On fig. 3.2 shows the design of the fastening of the walls of the trench for laying the pipeline, made of boards. Structural solutions for the construction of a manhole at the end of a narrow trench are shown in fig. 3.3.
When laying pipelines, the distance between the lower tier of spacers and the bottom of the trench is often insufficient. This leads to the need to install stronger and longer racks that can absorb large bending moments. On fig. 3.4 shows a solution that eliminates the need for a bottom brace. In this case, in addition to the usual short posts, long vertical posts with greater strength are additionally installed almost to the entire height of the walls, and the lower struts are attached at a greater height from the bottom of the trench.
On fig. 3.5 shown constructive solution fixing the walls of the trench for laying a steel pipeline with a diameter of 800 mm. Racks for fixing the horizontal cladding are made of metal profiles. Metal tubular spacers with a diameter of 60 mm and a wall thickness of 4 mm are laid at the base (Fig. 3.5, a) and two wooden spacers are installed in the upper part of the trench. This provides the space needed for the pipeline installation work. For the installation of individual sections of the pipeline, sections 6 m long were provided, on which spacers were not installed. Pipes were laid in a trench in these places and then stretched to the installation site.
On fig. 3.5,c shows a plan of the site on which the lowering of individual pipe sections is carried out. After excavation, special metal frames are installed in the trench, eliminating the possibility of wall collapse.
Served as the bottom brace concrete base trenches into which the lower ends of the racks were brought.
If the edge of the trench provides for the movement of vehicles or construction mechanisms, the vertical fastening of the walls of the trenches must be strengthened.
In this case, the upper struts should be located at a depth of not more than 0.5 m from the ground surface, and if the road for transport access is located at a distance of less than 1 m from the edge of the trench, their number should be doubled.

3.1.2. Fixing the walls of pits.
When constructing wide pits with vertical walls in conditions that exclude the possibility of ground shaking, it is possible to use the structures described above with horizontal cladding elements. Fixing the walls of the pit is carried out by two methods:
1) a slit-like trench is constructed along the perimeter of the future pit, the walls of which are fixed by the method described above, after which the main soil of the pit is excavated (Fig. 3.6);
2) first, the main part of the soil is excavated in a pit with slopes, after which the soil is excavated, which lies at the base of the slope, its subsequent fixation with sheathing, which is unfastened to the existing structure (Fig. 3.7).
On fig. 3.8 shown phased implementation works according to the first option during the construction of a railway bridge in conditions of incessant traffic. Initially, two slit-like slots were developed with their fastening by horizontal skin. After completing this stage of work, numerous wooden struts were installed. Durable spacers and their heads were made of metal. In parallel with the excavation, the gradual replacement of wooden struts with metal ones was carried out and the lining of the trench walls was installed. On fig. 3.9 shows the structural solution of the bridge crossing support.

The geometric dimensions of the trenches are determined based on the depth of the pipelines, the required width of the trenches along the bottom (along the bottom) and the configuration of the walls.

The width of the trench along the bottom consists of the size of pipelines and technological gaps that ensure all construction work. The width of the trench along the bottom b (m) depending on the outer diameter of the pipeline D (m) is taken equal to:
b = D + (0.5...0.6) m with D ≤0.5 m;
b = D + (0.8...1.2) m for D > 0.5 m.

The width of the trench along the bottom can be specified in the project for the production of works, but should not be less than 0.7 m.

The trenches are torn off with inclined or vertical side walls. Vertical wall trenches are more economical. However, due to the danger of soil collapse, their greatest depth in dense soils without special calculations and wall fastening should not exceed 2 m. Therefore, trenches with slopes (with inclined walls) are mainly used, the greatest steepness of which in soils of natural moisture ranges from 1: 0 .25 to I: 1.25 (Table 5.3).

In waterlogged clayey and dry sandy soils, the steepness of the slopes should be taken as for bulk soils. In all cases, it is necessary to check the stability of the slopes in the project for the production of works, taking into account specific hydrogeological conditions and the presence of a temporary load on the collapse prism.

Sloped trenches, due to their large width at the top, can only be developed in undeveloped areas. In cramped conditions, trenches with vertical reinforced walls are mainly used. Depending on the depth of the trench, soil and hydrogeological conditions, the type (Table 5.4) and the design parameters of the fastening are selected.

The most widespread in the construction of gutters are embedded fasteners installed in the trench as the soil is developed. They consist (Fig. 5.9) of a pick-up, risers and spacers. A horizontal or vertical fence is made of boards 4-6 cm thick, installed close to one another or with gaps (with gaps) equal to the width of the board.

Rice. 5.9. Embedded trench fasteners
a - horizontal apart; b - vertical solid; in - inventory; 1 - pickup boards; 2 - risers; 3 - spacers; 4 - bosses; 5 - runs; 6 - wooden shields; 7 - tubular frames; 8 - screw struts

The horizontal boards of the pickup (Fig. 5.9, a) are supported by vertical risers pressed against the walls of the trench by spacers. Risers made of boards with a thickness of at least 5-6 cm or pipes are installed at a distance of 1.5-2 m along the length of the trench. Spacers from logs with a diameter of 12-18 cm or pipes are placed through 0.6-0.75 m 410 in the depth of the trench. In continuous vertical mounts(Fig. 5.9.6) the boards of the pick-up after 0.7-1.4 m are combined with horizontal belts (runs) pressed against the walls of the trench by struts.

Inventory mounts consist of standard, most often plank shields and metal tubular frames with screw struts (Fig. 5.9, c). These are collapsible mounts, so they are less labor-intensive and material-intensive compared to the considered wooden mounts.

The development of trenches during the construction of a drainage network is carried out, as a rule, by single-bucket excavators equipped with a backhoe or dragline with a bucket volume of 0.25-1 m 3. The backhoe excavator provides more high performance and digging accuracy, but has limitations on the size of the face. Therefore, with a large width and depth of trenches, a dragline is used. To develop trenches with vertical reinforced walls, excavators with clamshell equipment are used.

Excavators continuous action have the highest productivity, but can only develop relatively narrow trenches with vertical walls, so their use is limited to separate laying, mainly cable networks.