CHAPTER 2 Soil and Excavations - University of Misan · Excavation support using contiguous bored...

Dr. Abdulkhaliq Abdulyimah Jaafer

Misan University - College of Engineering Civil Engineering Department

4 Building Construction Lectures -2nd stage

CHAPTER 2

Soil and Excavations

Soil investigation including two phases: surface investigation and

subsurface investigation

Surface investigation involves making a preliminary judgment about the site’s

suitability for the proposed building. The first part of surface investigation is

a visual assessment of the site. The second part is the land survey provides

physical measurements of the site.

Subsurface investigation deals with conditions below the ground surface to

determine the requirements for the foundations and excavations. Subsurface

conditions have a significant influence on the building design,

construction materials, structural system, construction cost, and schedule.

For example, it is more expensive and time-consuming to excavate in a rocky

stratum or a stratum with a high water table.

Soil classification (Gravels, Sands, Silts, and Clays)

There are a number of characteristics that must be considered in

determining the ability of a soil to support building loads. One important

characteristic is soil classification based on the size of soil particles. The size

of soil particles is measured by passing a dried soil sample through a series of

sieves, each with a standardized opening size (see Figure).




Uniqueness of clay (swelling and shrinking) -the expansive

soils

Gravel and sand particles are approximately spherical or ellipsoidal in shape.

This is because gravels and sands are the result of mechanical weathering.

Clay particles are having flat, platelike shapes. Because of their flat particle

shape, the surface-area-to-volume ratio of clays is several hundred or

thousand times greater than the corresponding ratio for gravels and sands.

In the presence of water, the electrostatic forces that developed between

platelike surfaces are repulsive, which increases the space between plates.

Therefore, in the presence of water, clayey soils swell, and as water

decreases (i.e., when they dry), they shrink. Soils that are predominantly

clayey are unstable because they expand and contract, depending on the

amount of water present in them, and are referred to as expansive soils.

Cohesive and noncohesive soils

Fine-grained soil particles adhere to each other in the presence of water and

are, therefore, called cohesive s o i l s . Coarse-grained soils are typically




single-grained, lacking cohesiveness, and are referred to as noncohesive soils.

Geotechnical investigations—soil sampling and testing

The objectives of this exploration and sampling are to determine the:

• Engineering properties of the soil at various depths.

• Particle-size distribution of the soil.

• Plasticity index of the soil.

• Nature of the excavation that will suit the soil.

• Depth of the water.

• Compressibility of the soil.

Two methods are generally used for field exploration: (a) the test pit method

and (b) the test boring method.

Bearing capacity of soil

The bearing capacity of a soil is its strength to bear loads imposed on it by

the structure. In other words, the bearing capacity of a soil determines the

maximum load that can be placed on each square foot of the soil before it




fails structurally or has an unacceptable amount of settlement.

The bearing capacity of soil generally increases with increasing depth

below ground because the deeper strata of native soil are generally more

densely compacted and have a smaller amount of decomposed plant

matter. Therefore, increasing the depth below ground for the base of the

footing generally reduces the footing area but increases the depth of

excavation, (see Figure).

Presumptive bearing capacity of soil

The allowable bearing capacity of a soil should be obtained from

geotechnical investigations of the site. However, its approximate value,

based on the particle size of the soil at the location (without geotechnical

investigation) is allowed to be used in situations where

• The building is small;

• Adequate information about the soil from adjoining areas is available;

• The site does not contain fill of an unknown origin; and

• The soil is known to be stable (non expansive).

Excavation

Excavation is the first step of construction. It refers to the process of




removing soil or rock from its original location. Excavated material

required for backfill or grading fill is stockpiled on the site for

subsequent use. Unneeded material is removed from the site for appropriate

disposal. Excavations are generally classified as Open excavations, Trenches

and Pits.

Open excavations refer to large (and often deep) excavations, such as for a

basement. Trenches generally refer to long, narrow excavations, such as for

footings under a wall or utility pipes. Pits are excavations for the footing of an

individual column, elevator shaft, and so on. The depth of excavation

depends on the type of soil and the type of foundation. Excavation require

various types of power equipment, such as excavators, compactors, and

heavy earth-moving equipment (front-end loaders and backhoes), some of

which are shown in Figure.




Supports for open excavations

Excavations in the soil generally require some type of support to prevent

cave-ins while the foundation system or basement walls are constructed. The

simplest excavation support system consists of providing adequate slope in

the excavated (cut) face so that it is able to support itself, (see Figure). This is

feasible only if the site is large enough to accommodate sloped excavations.

Excavation in coarse-grained soils requires a shallower slope than excavation

in fine-grained soils (see Figure). Self-supporting sloped excavations cannot

be provided where the site area is restricted or adjoining structures are

present. In these cases, the excavation must consist of vertical cuts. In

cohesive soils, shallow vertical cuts (generally 5 ft. or less in depth) may be

possible without any support system. Deeper vertical cuts must be provided

with a support system. Some of the commonly used methods of supporting

deep vertical cuts in the soil are

1. Sheet piles

2. Cantilevered soldier piles

3. Anchored soldier piles

4. Contiguous bored concrete piles

5. Secant piles

6. Soil nailing

7. Bentonite slurry walls




Excavation support using sheet piles

For depths of up to about 15 ft., vertical sheets of steel, referred to as sheet

piles, can be driven into the ground before commencing excavations.

Sheet piles consist of individual steel sections that interlock with each other

on both sides. The interlocks form a continuous barrier to retain the

earth. Sheet piles are available in many cross-sectional profiles. The most

commonly used profile is a Z-section, (see Figure). The sections are driven

into the ground one by one using either hydraulic hammers or vibrators,

Figure (see Figure). For deeper excavations (generally greater than 15 ft.),

sheet piles are braced with horizontal or inclined braces or anchored with

tiebacks, (see Figure). Sheet piles are removed after they are no longer

required or can be left in place if needed.




Excavation support using cantilevered soldier piles

One of the disadvantages of sheet piles is the noise and vibration created in

driving them, particularly in stiff soils where the vibratory method is

ineffective and hydraulic hammers must be employed. An alternative to

sheet pile excavation support is the soldier pile system. In this support

system, H-shaped steel columns (called soldier piles or H-piles) are placed in

the ground. The piles are placed in predrilled circular holes approximately

6 to 8 ft. on center. After the piles are placed, the holes are filled with

lean concrete (see Figure). Excavation of the ground abutting the piles is

commenced after the concrete around the piles has gained sufficient

strength.




Excavation support using anchored soldier piles

The use of a cantilevered soldier pile system is uneconomical beyond a

depth of approximately 15 ft. because of the increase in pile cross section. For

deeper excavations, an anchored soldier pile system is employed, which is

similar to the cantilevered pile system except that the piles are tied back

(anchored) into the ground. The commonly used vertical support members

for this system consist of two steel channels with a space between them. The




channels are connected together with steel plates welded at intervals in this

space, (see Figure).

Drilling for tieback anchors is done through the space between the twin C-

sections of piles, (see Figure). After a tieback hole has been drilled, steel

bars or high-strength steel tendons are placed in the hole, and the hole is

grouted, (see Figure).

Excavation support using contiguous bored concrete piles

In situations where the (deep) excavation is close to an adjacent building or the

property line, tiebacks cannot be used. In this situation, closely spaced

reinforced concrete piles, called contiguous bored piles (CBPs), are often used, (see

Figure). Each pile is made by screwing an auger into the ground. The auger has

a hollow stem in the middle of a continuous spiral drill.

Once the drill has reached the required depth below the ground, high-

slump concrete is pumped down the hollow stem of the auger to the

bottom of the bore. Once the pumping starts, the auger is progressively

withdrawn. Immediately after the entire bore has been concreted, a

reinforcement cage is lowered in the concrete-filled bore.




Excavation support using secant piles

A major shortcoming of CBPs is the gaps between piles and the consequent

lack of water resistance of the excavation support. This problem is

overcome by the use of the modified version of CBPs called secant piles.

Secant piles essentially consist of two sets of interlocking contiguous piles.

The first set, called the primary piles, is bored and concreted in the same way

as the CBPs. The center-to- center distance between the primary piles is

slightly smaller than twice their diameter.

After the primary piles are constructed, the secondary piles are bored at mid-

distance between the primary piles, which also bores through part of the

primary piles, (see Figure). The secondary piles are concreted and reinforced

in the same way as the CBPs.




Excavation support using soil nailing

Soil nailing is a means of strengthening the soil with closely spaced, inclined

steel bars that increase the cohesiveness of the soil and prevent the soil from

shearing along an inclined plane. The inclined bars are almost

perpendicular to the possible shearing plane. In other words, the steel bars

connect imaginary inclined layers of the earth into a thick block that

behaves as a gravity-retaining wall when excavated, (see Figure).

The process of soil nailing consists of the following steps:

1) The soil is first excavated 5 to 7 ft. deep, depending on the ability

of the cut face to remain vertical without supports.

2) Holes are drilled along the cut face at 3 to 4 ft. on centers so that

one hole covers approximately 10 to 15 ft2 of the cut face, Figure

11.26(a).

3) Threaded steel bars (approximately 1 in. in diameter) are inserted

in the holes. The length of the bars is a function of the soil type but

is approximately half the final depth of excavation. The bars

protrude a few inches out of the holes.

4) The holes are grouted with concrete.




5) WWR is placed over the wall and tied to the protruding bars.

6) A layer of shotcrete is applied to the mesh.

7) Plates and washers are inserted in the protruding bars and locked in

position with a nut.

8) A second layer of shotcrete may be used if the soil-nailed wall is

the finished wall, or a cast-in-place concrete wall may be

constructed against it.

9) These steps are repeated with the next depth of cut.

Excavation support using bentonite slurry as trench support

Another excavation support system, commonly used in situations where the

underground water table is relatively high, is a reinforced concrete wall.

Construction of such walls is done by excavating 10-ft- to 15-ft-long




discontinuous trench sections down to bedrock, called primary panels. The

width of the trench sections is the required thickness of the concrete wall.

So that the soil does not collapse, the trench is continuously kept filled with

bentonite slurry as the excavation proceeds. (Bentonite slurry is a mixture

of water and bentonite clay, which pressurizes the walls of the trench

sufficiently to prevent their collapse during excavation.)

Special excavation equipment is used to extract soil through the slurry-filled

trench. After the excavation for the entire primary panel is complete, a

reinforcement cage is lowered into the trench. Concrete is then placed in the

trench panel using two or more tremie pipes, typically one at each end of the

panel. Concrete is placed from the bottom up, and the discharge end of the

tremie is always buried in concrete. A tremie pipe is generally an 8-in. - to

10-in.-diameter steel pipe with a hopper at the top, (see Figure).

As concreting proceeds, the slurry is pumped out from the top of the trench and

stored for later use. After the primary panels have been constructed,

excavation for secondary panels (between the primary panels) is undertaken

in the same way as for the primary panels. To provide shear key and water

resistance between primary and secondary panels, a steel pipe is embedded at

the end of each primary panel prior to its concreting. These pipes are removed

after the concrete in the primary panels has gained sufficient strength.

The tremie pipe method of concrete placement requires great care and

expertise, particularly the initial placement of concrete, which is generally

a richer mix. The concrete must also be placed slowly so that it does not get

too diluted by the slurry.




Keeping excavations dry

It is important to keep excavations free from groundwater. Groundwater

control in an excavation consists of two parts: (a) preventing surface water

from entering the excavation through runoff and (b) draining (dewatering)

the soil around the excavations so that the groundwater level falls below the

elevation of proposed excavation. Two commonly used methods of

dewatering the ground are sump pumps and well points

Dewatering through sumps

Sump dewatering consists of constructing pits (called sumps) within the

enclosure of the excavation. The bottom of sumps must be located below

the final elevation of the excavation. As the groundwater from

surrounding soil percolates into the sump, it is lifted by automatic pumps

and discharged away from the building site, (see Figure). The number of

required sumps is a function of the excavation area.

Dewatering through well points

Sump dewatering works well in cohesive soils, where the percolation rate is




slow and where the water table is not much higher than the final elevation

of the base of the excavation. A more effective dewatering method uses

forced suction to extract groundwater. This is done by sinking a number

of vertical pipes with a screened end at the bottom (called well points)

around the perimeter of the excavation. The well points reach below the floor

of the excavation and are connected to large-diameter horizontal header

pipes at the surface.

The header pipe is connected to a vacuum-assisted centrifugal pump that

sucks water from the ground for discharge to an appropriate point. For a

very deep excavation, two rings of well points may be required.

Whereas the sump method of dewatering does not greatly affect the existing

water table, dewatering by well points can lower the water table

considerably. The effect of this on the adjoining buildings must be

considered because it can cause consolidation and settling of the

foundations of existing buildings on some types of soils.

Dewatering of excavations can be fairly complicated and generally requires

an expert dewatering subcontractor for large and complicated operations.

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CHAPTER 2 Soil and Excavations - University of Misan · Excavation support using contiguous bored...

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