Misan University - College of Engineering Civil Engineering Department

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1 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). 4

2 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 5

3 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 7

4 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 8

5 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. 9

6 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 01

7 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. 00

8 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. 01

9 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 02

10 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, highslump 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. 03

11 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 middistance 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. 04

12 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. 05

13 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 07

14 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. 08

15 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 09

16 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. 11