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Excavation support or earth retaining structures are required where excavation is deep and required slope cannot be provided. Types of excavation supports are discussed.

Soil type	Unconfined compressive strength (psf)	Cohesion(psf)	Safe height (ft)
Very soft	Less than 500	<250	<5
Soft	500-1000	250-500	5-10
Medium	1000-2000	500-1000	10-20
Stiff	2000-4000	1000-2000	20-40
Very stiff	4000-8000	2000-4000	40-80
Hard	>8000	>4000	>80

Types of Excavation Supports

Soldier Pile and Lagging

Soil Nailing Method of Excavation Support

Groundwater causes extreme geotechnical problems in excavations such as sand running for most of construction projects such as tunneling. So, issues caused by ground water would increase construction budget and extend construction time unless the ground water is properly controlled which is the root cause of the problems.

There are two major methods for controlling ground water including exclusion technique (Figure 1a) and pumping technique (Figure 1b).

Figure 1a: Ground water control in excavations by exclusion

Figure 1b: Ground water control in excavations by pumping

Why to use Exclusion Methods to Control Ground Water?

There are cases in which the application of pumping techniques to control ground water is not recommended, for example, in water bearing rock formation and high permeability ground.

This is because utilization of large capacity pumps, which are required for high permeability ground and formation of well points in rock formation would be considerably costly.

So, it would be economical to consider exclusion methods in the aforementioned situations and alike cases.

There are number of techniques by which ground water exclusion are obtained:

• Forming impervious barriers by grouting with cement, clay suspension
• Chemical consolidation for controlling ground water in excavation
• Ground water control by compressed air
• Freezing ground water control

Forming Impervious Barriers by Grouting with Cement, Clay Suspension or Bitumen

This strategy is considered in water bearing rock formation or high permeability ground where the use of high pump capacity or digging well point is expensive.

In this technique, the permeability is reduced by creating an impervious barrier by injecting suspension material or fluids into the fissures of rocks or pore spaces. Fineness of fissures in rocks or soil particle size distribution would control types of materials used for grouting.

This means that the grout material particle size should be considerably smaller than the pore spaces. Figure 3 illustrates limiting particles sizes of materials which may be grouted by different types of grout.

Figure 3: Various Types of Soils Which can be Grouted with Different Types of Grouting Material

Additionally, groutability ratio, which is the ration between D.15 size of soil to the D.85 size of grouting material particle, is also used to determine suitability of suspension grouts.

So, suspension grouts would not be appropriate choice for the soil under consideration unless the groutability ratio is higher than 5:15 for clay grouts and 11:25 for cement grouts.

Furthermore, it is necessary to pay attention to the quantity of materials used for grouting since it could be costly if the excessive amount is employed. This concern might be dealt with by considering chemical grouts even though its cost is higher than clay and cement for the same quantity.

As far as fluid gout is concerned, it is more effective than suspension grout since it fills all pores and spaces in soil whereas small size pores would be left empty in the case of suspension grout.

When grouting technique is considered, it is required to practice great care regarding structures and facilities such as sewer sanitary system around the grouting area. This is because grouting is conducted under great pressure, so it might impair considerable damage to these facilities.

Figure 4: Distribution of Grouting Pipes Around Excavation Area

Finally, there are three major methods for injecting grouts which are provided in Table-1 along their application conditions and procedures.

Figure 5: Tube-a-Manchette Used for Grouting in Soils

Sleeve grouting Suitable for grouting soils It makes use of Tube-a-Manchette as shown in Figure-5. After a hole is drilled to a determined depth and cased a sleeve tube is inserted and surrounded by partially plastic grout. Then, the case is pulled up and perforated injecting pipe is inserted into the sleeve pipe. finally, the grouting material is injected and plastic grout would be broken and the grout material will spread through the ground.

Chemical Consolidation for Controlling Ground Water in Excavation

Figure 6: Chemical Grout Formation prior to Injection

Chemical consolidation method is suitable for sandy gravels and fine grading sands. The most usual chemical material used for chemical consolidation is the sodium silicate. If the sodium silicate is mixed with other chemicals, moderately strong and insoluble silica gel can be produced.

Two approaches have been practiced to conduct chemical consolidation, namely, two shot process and one-shot process. By and large, the latter process which is the most common one has replaced the former process.

In two shot process, two pipes with spacing of 50cm are forced into the ground, then sodium silicate are driven to one pipe and calcium silicate injected into the other while they are pulled up gradually.

Alternatively, one chemical is injected while the pipe is driven into the ground, the other chemical material is driven though the pipe as it is withdrawn.

Figure 7: Acrylic Polymers

Therefore, it is desired to have low viscosity grout during injection and the increase in grout viscosity occurred after the completion of injection process.

Finally, several attempts have been made to achieve gouts with such favored property, for example, resins and lignins and acrylic polymers.

Control Ground Water in Excavation by Compressed Air Method

There are several factors that motivate the application of compressed air to control ground water in excavations. For example, the use of other ground water control methods is not possible due to hydrological conditions.

The use of compressed air is advised in the case where environmental concerns are encountered specifically when ground water employed as a reservoir for drinkable water, consequently the use of solid materials like cement is prevented.

Figure 8: Preparation for Compressed Air Technique to Control Groundwater, Diaphragm Wall Construction

Compressed air technique is commonly employed for controlling ground water in excavations of tunnels and shafts.

Figure 9: Removing Waste Materials from Excavations

Controlling ground water by compressed air cannot be carried out unless certain conditions are met. Firstly, the side walls and lid of the structure in which air is kept should be nearly impermeable. Secondly, compressed air static pressure shall be equal to the hydrostatic pressure of ground water at the lowest point of the to be maintained dry. Thirdly, air static pressure throughout the entire dry hollow space of the structure is should be constant.

In order to achieve the above conditions, the following construction procedures should be considered.

• Construct diaphragm wall as shown in Figure-8 along the side of the planned structure such as tunnel. The depth of the wall should extend below the final bottom slab of the structure.
• After that, construct a lid for the tunnel and the joint between the lid and diaphragm wall should be compressed airtight. In this stage, preliminary drainage may be employed provided that the conditions do not pose obstacles. It should be bore in mind that both diaphragm wall and the lid should be airtight as well.
• Construct a dividing wall or bulkhead with locks for workers and materials at one portal of the tunnel to avoid the escape of air toward the other end of the tunnel. Complementary momentary diaphragm wall might be placed at specified spacing along longitudinal axis of the tunnel and at its far end portal.
• Configuring mechanical and electrical machineries for waste material disposal as shown in Figure-9, compressed air provision and supporting plant.
• Finally, the excavation processes and compressed air utilization will be started under the lid at the tunnel portal.

In this technique, substantial care should be practiced to prevent undesired events since compressed air techniques involves high level of risk that could lead to human loss.

Figure 10: Excavation and compressed air application, PL: air pressure height in the tunnel, Dtt: distance between artesian ground water and invert WK: artesian Tithonian water pressure

Control of Ground Water in Excavations by Freezing

Controlling excavation ground water by freezing is not recommended to use unless all other methods fails to provide desired result or inappropriate to choose due to certain factors. This is because the cost of controlling ground water by freezing is significantly high due to large number of boreholes required to be drilled around the excavation area.

However, there cases in which freezing is the only practical method to control ground water for example in extremely deep shaft excavation where the pressure of ground water is seriously high.

Figure 11: Controlling Ground Water in Excavation by Freezing Method

To prevent the formation of unfrozen spaces in the frozen area, boreholes shall be exactly vertical and errors must be kept as minimum as possible in addition to provide small spacing between boreholes.

Regarding disadvantages of freezing method, considerable time needed for the completion of drilling boreholes, installing plants, freezing grounds and certain types of soils might experience heaving.

Added to that, compressed air operation is possible to hinder due to low temperature of excavation and construction activities such as concreting will face difficulties.

Nonetheless, it should be known that the most outstanding benefits of freezing technique is the effective controlling of ground water which other methods are lacking.

Freezing procedure involves drilling boreholes around excavation area, then inserting an outer plastic or steel tube with diameter of 10-15cm and an inner tube of 3.8-7.5cm into the boreholes, the outer tube end is closed whereas the inner tube end is opened.

The upper end of inner tube is connected to refrigeration plant from which cooled brine is pushed into the inner tube and after that returns to the refrigerator plant. The time during which the ground is frozen ranges between 1 to 4 months.

Finally, it is recommended to use liquid nitrogen rather than brine because freezing time would be reduced considerably. One might argue that the liquid nitrogen is expensive but its low construction cost may offset that and it freezes the ground five times faster than case where brine is used.

Previous article: What is Grading of Soil?

Common types of equipment used during the grading operations are as follows:

1. Bulldozer (Fig. 9). The bulldozer is used to clear the land of debris and vegetation (clearing, brushing, and grubbing), excavate soil from the borrow area, cut haul roads, spread out dumped fill, rip rock, and compact the soil.

Figure 9: The bulldozer is in the process of spreading out a layer of fill for compaction.

2. Scraper (Fig. 10). The scraper is used to excavate (scrape up) soil from the borrow area, transport it to the site, dump it at the site, and the rubber-tires of the scraper can be used to compact the soil. Push-pull scrapers can be used in tandem in order to provide additional energy to excavate hard soil or soft rock.

Figure 10: The scraper is used to excavate material from the borrow area, transport it to the site, dump it at the site, and then compact the soil.

3. Loader (Fig. 11). Similar to the scraper, the loader can be used to excavate soil from the borrow area, transport the soil, and then compact it as structural fill.

Figure 11: The loader can be used to move soil about the job site and compact the soil as structural fill.

4. Excavator (Fig. 12). This type of equipment is ideally suited to excavating narrow trenches for the construction of utilities such as storm drain lines and sewer lines. This equipment is also

used to excavate footings and other foundation elements.

Figure 12: The excavator is ideally suited to excavating narrow trenches for the construction of utilities such as storm drain lines and sewer lines.

5. Dump Trucks and Water Trucks (Fig. 13). If the borrow area is quite a distance from the site, then dump trucks may be required to transport the borrow soil to the site. Dump trucks are also needed to transport soil on public roads or to import select material.

Figure 13: The water truck is adding water to fill that is in the process of beingcompacted.

Especially in the southwestern United States, the near surface soil can be in a dry and powdery state and water must be added to the soil in order to approach the optimum moisture content. A water truck, such as shown in Fig. 13, is often used to add water to the fill during the grading operation.

The Caterpillar Performance Handbook (1997), which is available at Caterpillar dealerships, is a valuable reference because it not only lists rippability versus types of equipment, but also indicate types and models of compaction equipment, equipment sizes and dimensions, and performance specifications.

Compaction equipment can generally be grouped into five main categories, as follows:

1. Static weight or pressure. This type of compaction equipment applies a static or relatively uniform pressure to the soil. Examples include the compaction by the rubber-tires of a scraper, from the tracks of a bulldozer, and by using smooth drum rollers.

2. Kneading action or manipulation. The sheepsfoot roller, which has round or rectangular shaped protrusions or feet, is ideally suited to applying a kneading action to the soil. This has proven to be effective in compacting silts and clays.

3. Impact or a sharp blow. There are compaction devices, such as the high-speed tamping foot and the Caterpillar tamping foot, that compact the soil by imparting impacts or sharp blows to the soil.

4. Vibration or shaking. Nonplastic sands and gravels can be effectively compacted by vibrations or shaking. An example is the smooth drum vibratory soil compactor.

5. Chopper wheels. This type of compaction equipment has been specially developed for the compaction of waste products at municipal landfills.

A common objective of the grading operations is to balance the volume of cut and fill. This means that just enough earth material is cut from the high areas to fill in the low areas. A balanced cut and fill operation means that no soil needs to be imported or exported from the site, leading to a reduced cost of the grading operation.

When developing a site so that the cut and fill is balanced, consideration must be given to the bulking or shrinkage factor associated with the compaction operation. Bulking is defined as an increase in volume of soil or rock caused by its excavation. For example, very dense soil will increase in volume upon excavation and when compacted, the compacted soil may have a dry unit weight that is less than existed at the borrow area. Conversely, when loose material is excavated from a borrow area and worked into a compacted state, the compacted soil usually has a dry unit weight that is greater than existed at the borrow area. The shrinkage factor is often defined as the ratio of the volume of compacted material to the volume of borrow material (based on dry unit weight).

Fill placement should proceed in thin lifts, i.e., 6 to 8 in. (15 to 20 cm) loose thickness. Each lift should be moisture conditioned and thoroughly compacted. The desired moisture condition should be maintained or reestablished, where necessary, during the period between successive lifts. Selected lifts should be tested to ascertain that the desired compaction is being achieved.

There are many excellent publications on field compaction equipment. For example, Moving the Earth (Nichols and Day, 1999) presents an in-depth discussion of the practical aspects of earth moving equipment and earthwork operations.

Since most building sites start out as raw land, the first step in site construction work usually involves the grading of the site. Grading basically consists of the cutting or filling of the ground in order to create a level building pad upon which the foundation and structure can be built. The three types of level building pads that are created by the grading operations are cut lots, cut-fill transition lots, and fill lots as illustrated in Fig. 1.

Figure 1 Three types of building pads created during the grading operation.

The typical steps in a grading operation are as follows:

1. Easements. The first step in the grading operation is to determine the location of any on-site utilities and easements. The on-site utilities and easements often need protection so that they are not damaged during the grading operation.

2. Clearing, brushing, and grubbing. Clearing, brushing, and grubbing are defined as the removal of vegetation (grass, brush, trees, and similar plant types) by mechanical means. This debris is often stockpiled at the site and it is important that this debris be removed from the site and not accidentally placed within the structural fill mass. Figure 2 shows one method of dealing with vegetation, where a large mechanical grinder has been brought to the site and the trees and brush are being ground-up into wood chips. The wood chips will be removed from the site and then recycled.

3. Cleanouts. Once the site has been cleared of undesirable material, the next step is the removal of unsuitable bearing material at the site, such as loose or porous alluvium, colluvium, and uncompacted fill.

4. Benching (hillside areas). Benching is defined as the excavation of relatively level steps into earth material on which fill is to be placed. The benches provide favorable (i.e., not out-of-slope) frictional contact between the structural fill mass and the horizontal portion of the bench.

5. Canyon subdrain. A subdrain is defined as a pipe and gravel or similar drainage system placed in the alignment of canyons or former drainage channels. The purpose of a canyon subdrain is to intercept groundwater and to not allow it to build up within the fill mass.

6. Scarifying and recompaction. In flat areas that have not been benched, scarifying and recompaction of the ground surface is performed by compaction equipment in order to get a good bond between the in-place material and compacted fill.

7. Cut and fill rough grading operations. Rough grading operations involve the cutting or excavation of earth materials and the compaction of this material as fill in conformance with the grading plans. The location of the excavated earth material is often referred to as the borrow area. During the rough grading operation, fill is placed in horizontal lifts and then each lift of fill is compacted to create a uniformly compacted material such as shown in Fig. 3.

Other activities that could be performed during rough grading operations are as follows:

a. Ripping or blasting of rock. Large rock fragments can be removed from the site or disposed of in windrows. Seismic wave velocity can be used to determine if rock is rippable or nonrippable. Figure 4 shows a Caterpillar D10 tractor/ripper that can be used to excavate rock.

b. Removal of rock fragments. Large rock size fragments interfere with the compaction process and are usually an undesirable material in structural fill. The large rock size fragments may become nested, creating open voids within the fill mass. Figures 5 and 6 show one method used to remove large rock size fragments. A screen is set up as shown in Fig. 5 and then a loader is used to dump the material on top of the screen. As shown in Fig. 6, the large rock size fragments roll off of the screen while the material that passes through the screen is used as structural fill.

FIGURE 5 A screen has been set up in order to remove large-size rock fragments from the soil.

FIGURE 6 A loader is in the process of depositing material on top of the screen in order to

separate the large size rock fragments.

Sand replacement test method is used to determine the field density or in-place density of earth embankments, road fills, sub-grade, sub-base or any of compacted material. This method serves as base upon which one can accept the density of a compacted material to a specified magnitude or to a percentage of maximum unit dry density determined as proctor.

As we know that moisture content of the soil vary from time to time and hence the field density also, so we are required to report the test result in terms of dry density. In order to determine the dry density we must have to examine the moisture content in the soil by using general method.
Moisture content (%) = m = ((wt. of wet soil – wt of dry soil) / wt of dry soil)  x 100
Dry density = (bulk density ) / (1 + w)

• Sand pouring cylinder
• Calibrating
• Metal tray
• Excavating tool
• Balance
• Glass plate
• Metal tray
• Clean uniform sand (1mm pass 600 mirco retain)
• Water content determination apparatus

The dry density of the sample obtained as a result is divided by the proctor test result i.e. the maximum dry obtained of the sample that can be obtained in the laboratory by using standard AASHTO compaction test or Modified AASHTO compaction test and the result is reported as percentage.

The acceptance criteria for these percentages depend on the specification requirements and generally following rules is followed;

No less than 98% within 150 mm below formation level
No less than 95% between 150 mm and 1200 mm below formation
No less than 90% beyond 1200 mm below formation level

As we know density means weight per unit volume or in other words how much mass is being enclosed in a specific quantum of volume. We can easily determine the mass of soil by using the physical balance or digital balance, but the problem lies in finding the volume of the hole dug.
This problem is solved with the help of a calibrated sand whose unit weight or density is already being determined and thus if we could determine how much weight of calibrated sand is going to rest in the dug hole we can find the volume of the hole by using following formula;
Volume of dug hole = weight of soil in hole dug  / unit weight of calibrated soil

The standard procedure of this test is being divided in two parts in first part we will find the unit weight of the standard soil by calibration process described  as follows;

1. Determine the internal volume of the calibrating container by using the dimensions as follows;
                                                    V = πd²h/4
2. Now fill the sand pouring cylinder with the sand to be calibrated within about 10 mm of its top left vacant and then determine the mass of the sand pouring cylinder along with sand and note it as w1.

3. Now place the sand pouring cylinder on top of calibrating cylinder of known volume and open the shutter to allow the sand to fall in to the cylinder after no more sand is falling close the shutter and determine the mass of the calibrating cylinder filled with sand and note it as W2

4. Now as we also have the weight of the sand in the conical portion of the sand pouring cylinder, we must subtract the weight of sand that can accumulate within that conical portion. For that take a flat glass plate and place the sand pouring cylinder. Open the shutter till no more sand falls and determine the mass of sand in the conical portion and note it as W3.

5. Now the weight of the sand in the calibrating cylinder is determined as
                        Wa = W1 – W2 – W3

6. The bulk density of the sand is determined by dividing the mass of sand in the calibrating cylinder with the volume of the calibrating cylinder.

1. Prepare the area of the embankment subject to test, level the top of the soil using the scrapper tool.

2. Place the metal tray on the flat surface, if required insert the nails into the small holes of the metal tray.

3. Trace the circular hole of the tray on the ground and excavate the soil carefully without loosing any of
the soil fragment. Dig a hole of approximately 150 mm in the ground.

4. Collect all the excavated material in a metal container and clear the hole using a brush.

5. Determine the mass of this soil as weight of wet soil from hole Ww.

6. Fill the sand pouring cylinder with the calibrated sand and determine its mass as W1.

7. Place the cylinder directly over the excavated hole. Allow the sand to run out the cylinder by opening the shutter. Close the shutter when the hole is completely filled and no further movement of sand is observed.

8. Now weigh the remaining sand in the sand pouring cylinder and note it as w4.

9. Take a sample of the excavated soil in an air tight sampler for the determination of the water content or moisture content.

10. Volume of the hole is determined by using the unit weight of the calibrated sand already known;

Volume of calibrating container = V (cm³) = 1000 cm³
Weight of cylinder + sand (before pouring) , W1(g) = 7476 g
Mean weight of cylinder + sand (after pouring), W2 (g) = 5610 g
Mean weight of sand in cone (of pouring cylinder), W3 (g) = 436 g
Weight of sand to fill calibrating container Wa = w1 – w2 – w3 = 1430 g
Bulk density of sand = Gamma b = Wa / v (g/cm³) = 1.43

Weight of calibrated sand in hole wb = W1 – W4 – W2
Volume of hole = Vh = Wb  / Unit weight of sand
Dry density of soil = ww / vh
% compaction = (dry density / proctor density )  x 100