EXCAVATION FOR FOUNDATION - Methods & Temporary Earth Retaining Structures
- 1. EXCAVATION FOR FOUNDATION METHODS & TEMPORARY EARTH RETAINING STRUCTURE SHIVANANDA ROY B.TECH, M.TECH, AMIE
- 2. EXCAVATION FOR FOUNDATIONS AND BASEMENT FLOORS • Generally excavation means to loosen and take out materials leaving space above or below ground. Sometimes in civil engineering term earthwork is used which include backfilling with new or original materials to voids, spreading and levelling over an area. • A basement is part of a building that is either partially or completely below ground level. • Basements can be constructed using brick or concrete block, poured concrete, pre-cast concrete, or even timber. • Basements generally become more expensive as the depth increases. However, in prime locations, the cost of land may justify multi-story basements or even below ground parking garages.
- 3. • Excavation in most situations nowadays is done by mechanical means. However, the exact method to be adopted still depends upon a number of factors: • 1. Nature of subsoil – affect type of machine used and the necessity of soil protection. • 2. Size of excavation – affect type of machine used and method to excavate. • 3. Scale of work – large volume of excavation may involve complicated phasing arrangement and work planning • 4. Ground water condition – affect degree of protection (watertight sheet piling or dewatering may required.) • 5. Surrounding condition – impose certain restrictions and precautions (eg. diversion of a government drain, or underpinning work to the nearby building foundation)
- 4. METHODS • Full Open Cut Method • Bracing Excavation Method • Anchored Excavation Methods • Island Excavation Methods • Zoned Excavation Methods • Top Down Excavation Methods
- 5. FULL OPEN CUT METHOD • It divided into two major types: Sloped full open cut and Cantilever full open cut. • The former is assumed to be economical since the side of excavation would be sloped and does not need any support to held foundation wall. • However, if the slope is considerably gentle or the excavation is largely deep, it will costly. • The latter needs retaining wall to support foundation wall soil and prevent collapse of foundation The economical method may be distinguished based on analysis, design, and evaluation results.
- 6. BRACING EXCAVATION METHOD • Bracing excavation as shown in figure is the placement of horizontal struts in front of retaining wall to held excavation wall material pressure. Bracing system consist of wale, strut, center posts, end braces, and corner braces. • Earth pressure transfer to horizontal struts through wale, and the purpose of corner and end braces is to reduce wale span without increasing strut number. Center posts prevent the failure of struts due to their own weight.
- 7. ANCHORED EXCAVATION METHODS • In this technique, anchors are installed to counter act against earth pressure. • Bonded portion of the anchor provides anchoring force that works against earth pressure whereas unbonded part of the anchored transfer pressure to the anchor head. Anchor head transfer loads to the retaining wall. • The anchoring force is greatly based on the soil strength. The higher the soil strength the greater the anchoring forces. This technique is not suitable for clay and granular soil with high ground water table. • Lastly, it require short time to complete excavation with great efficiency and suitable for large areas and shallow depth.
- 8. ISLAND EXCAVATION METHODS • In this method, the center of excavation area is dug and excavated material placed close to the retaining wall to create a slope. After that, the major part of the structure would be constructed at the center of the excavation. Then, the sloped soil will be excavated and struts will be placed between retaining wall and the main structure. • Finally, the struts will be removed and remaining parts of the structure will be constructed. Sometimes, it might be required to use anchored or braced technique to removed slopes soil material, specifically when the excavation is too deep.
- 9. ZONED EXCAVATION METHODS… • Diaphragm walls are used as a retaining wall in the zoned excavation method. Deformation of the longer span wall would greater than short span wall as explained first figure. • So, the deflections of longer span walls are declined by dividing excavation area into small area to decrease wall deformation and settlement as shown in second figure.
- 10. …ZONED EXCAVATION METHODS • The excavation will begin in area B while area A would be left to support the wall of area B. then struts in area B would be installed and excavation starts in area A. This process will continue in stages till the whole excavation is completed. • It can be clearly observed that the load on diaphragm wall would be considerably large and hence deflection would great if the area had not been divided into smaller area.
- 11. TOP DOWN EXCAVATION METHODS… • In this method, construction begins from the top to the bottom of excavation and superstructure construction starts after the construction of the first slab is completed. • So, slabs are constructed after each stage of excavation is finished. The slabs play the same role as struts in holding earth pressure.
- 12. …TOP DOWN EXCAVATION METHODS • Construction process order include retaining wall construction, pile construction under column of superstructure, placing columns on piles, and installing formwork for the first slab at the top then other slabs of the would be constructed after each excavation. • This technique would need short construction time, but the cost is higher compare to other methods. Another advantage is that, construction area safer since slabs are stronger than struts.
- 13. TEMPORARY EARTH RETAINING STRUCTURES • Deep excavation, unlike a shallow one, often requires to protect the sides of cut using suitable support. Besides, the problem of ground water cannot be avoided. There are methods to overcome this, such as: Dumpling method Braced wall Diaphragm walling Soil Nail Wall Using cofferdams Soldier Pile and Lagging Walls Sheet Steel Piling Sheet Pile Walls Ground anchor Reinforced Concrete Wall Construction
- 14. DUMPLING METHOD • This is used where there are buildings or street in the proximity. The method is to construct a series of retaining wall in trench, section by section, around the site perimeter ,leaving a centre called "dumpling“ • When the perimeter walls are in place, excavation may start at the centre of the dumpling, until exposing a section of the wall. Then the wall may be side supported by struts, shoring or soil anchor etc., again section by section in short length, until the excavation is all completed. • This method does not require much heavy mechanical equipment and thus cost of work is relatively lower. It can excavate up to a maximum depth of about 3m. • Sometimes in very poor soil or in waterlogged ground, interlocking steel sheet pile may be driven to confine the area to be excavated .After that excavation can be done in section and properly supported similar to that mentioned above.
- 15. SHEET PILE WALLS … • Sheet piles can be driven into soil by stricking or static vibrating and have them interlocked or connected with one another. • There are several shapes of sheet pile section viz- U-section, Z-section and the line section. • If the interlocking is well done, sheet piles can be quit efficient in water sealing. If not, leaking may occur at the joints. • In clayey soils, having low permeability, sheet piles do not necessarily require perfect connection to prevent leaking. • In sandy soils, with high permeability, any breach in sheet piles may well cause leaking. If leak occur, sand in back of the retaining walls will possibly flow out, which may cause settlement in turn.
- 16. … SHEET PILE WALLS … • The construction method for sheet piles can be described as follows: 1. Drive sheet piles into soil by stricking or static vibrating. 2. Proceed to the first stage of excavation 3. Place wales in proper places and install horizontals struts. 4. Proceed to next stage of excavation. 5. Repeat 3 & 4 till the designed depth. 6. Complete excavation and begin to build the foundation of the building. 7. Build the inner walls of the basement. Dismantle the struts level and build the floor slabs accordingly. 8. Complete the basement. 9. Dismantle the sheet piles.
- 17. … SHEET PILE WALLS • The merits of steel pile method are: • It is highly watertight • It is reusable • Has higher stiffness than soldier piles. • The drawbacks of steel pile method are: • Lower stiffness than column piles or diaphragm walls. • Susceptible to settlement during stricking or dismantling in a sandy ground. • Not easy to strike piles into hard soils. • A lot of noise is caused during striking. • Leaks cannot be completely avoided and sealing and grouting are probably necessary if leak occurs.
- 18. DIAPHRAGM WALLING… • This method need to construct a R.C. retaining wall along the area of work. Because the wall is designed to reach very great depth, mechanical excavating method is employed. Typical sequence of work includes: • a) Construct a guide wall • b) Excavation for the diaphragm wall • c) Excavation support using bentonite slurry • d) Inert reinforcement and concreting
- 19. …DIAPHRAGM WALLING… • Construct a guide wall – guide wall is two parallel concrete beams running as a guide to the clamshell which is used for the excavation of the diaphragm wall. • Excavation for the diaphragm wall – In normal soil conditions excavation is done using a clamshell or grab suspended by cables to a crane. The grab can easily chisel boulder in soil due to its weight. • Excavation support – Excavation for the diaphragm wall produces a vertical strip in soil which can collapse easily. Bentonite slurry is used to protect the sides of soi1. • Reinforcement – Reinforcement is inserted in form of a steel cage, but may require to lap and extend to the required length.
- 20. …DIAPHRAGM WALLING • Concreting – As Concrete being poured down, bentonite will be displaced due to its density is lower than concrete. Bentonite is then collected and reuse. Usually compaction for concrete is not required for the weight of the bentonite will drive most of the air voids in concrete. • Joining design for the diaphragm wall – • Diaphragm walling cannot be constructed continually for a very long section due to tremendous soil pressure. The wall is usually constructed in alternative section. • Two stop end tubes will be placed at the ends of the excavated trench before concreting. The tubes are withdrawn at the same time of concreting so that a semi-circular end section is formed. • Wall sections of this type are built alternatively leaving an intermediate section in between .The interior sections are built similarly but without the end tube . • At the end a continual diaphragm wall is constructed with the sections tightly joined by the semi-circular groove.
- 21. USING COFFERDAMS • A cofferdam may be defined as a temporary box structure constructed in earth or water to exclude soil or water from a construction area, such as for foundation or basement works. • Use of cofferdam suitable for excavation of larger scale can be of : a) Sheet pile cofferdam – Also known as single skin cofferdam. Interlocking type steel sheet pile is used and can use for excavation up to 15m. Sheet pile in this case acts as a cantilever member to support the soil therefore adequate depth of pile or suitable toe treatment may be required. In addition, cofferdams are need to be braced and strutted or anchored using tie rods or ground anchors.
- 22. USING COFFERDAMS • b) Double skin cofferdam – This works similarly like the sheet pile to form a diaphragm. However, the diaphragm is double-skinned using two parallel rows of sheet pile with a filling material placed in the void between. This creates somewhat a gravity retaining structure and increase the ability to counteract the soil behind. However, more working space is required.
- 23. BRACED WALL • This soil retaining arrangement is suitable for deep and less wide soil excavations. Initially, horizontal members are installed against the soil support. These horizontal members are called as waler. From one waler to the other, intermediate struts are installed across the excavation. • The excavation is performed step by step. • This retaining system is suitable for soils with cohesion and low or no water table level. • The method is often limited to small dimension shafts and trenches with and without penetration below base.
- 24. SOIL NAIL WALL … • Soil nail wall construction is a technique used to bring soil stability in areas where landslides might be a problem. • Soil nail can prevent landslides by inserting steel reinforcement bars into the soil and anchoring them to the soil strata. • It is called Soil Nail because it’s like having a nail being hammered into the soil, where the nails are the steel bars.
- 25. SOIL NAIL WALL INSTALLATION PROCESS • Soil nail wall provides a resisting force against slope failures. Its construction process is faster than other similar methods. • The construction procedure starts, drilling into the soil, where the nail, steel bar, is going to be placed. • Then, it must be grouted into the soil to create a structure similar to a gravity wall. After placing the nail, a shot-crete layer is usually placed as a facing material, to protect the exposed nail, and then other architectural options are placed over the shot-crete, creating an aesthetic finish to the project. • Drainage is a critical aspect of soil nail wall construction. Face drainage is virtually always used with permanent walls, and very commonly used with temporary walls. Face drainage usually consists of synthetic drainage elements placed between the shot-crete and the retained soil. • The grouted soil nail hole typically has a minimum diameter of 4 inches. Centralizers are placed around the soil nail to maintain an even thickness of grout around the bar. For permanent applications, nails may be epoxy-coated or provided with a protective sheath for corrosion protection.
- 26. SOIL NAIL WALL INSTALLATION PROCESS • For a soil nail wall the general construction procedure involves: 1. Excavate for the first nail (soil must be sufficiently self standing) 2. Install the 1st nail. 3. Construct 1st phase shotcrete on soil face (optional if shotcrete is constructed) with wire mesh or other reinforcement if required. 4. Install soil nail head plate (with or without other attachments) 5. Construct 2nd phase shotcrete (depending on staging specifications). 6. Excavate to next soil nail level, and install next soil nail, shotcrete etc. 7. Repeat steps 3 through 6 until the final excavation level is reached. 8. Construct additional permanent facing if required.
- 27. GROUND ANCHOR • Ground anchor is basically a pre-stressing tendon embedded and anchored into soil or rock to provide resistance to structural movements by a “tying back" principle. • Common applications are : 1. General slope stabilization 2. Tying back/stabilizing a retaining structure 3. Tying back/stabilizing for diaphragm walls, but for a temporary nature during excavation 4. Tying back the entire building from up possible uplifting • Ground anchor can be classified into: 1. Rock anchor – for anchorage in rock 2. Injection anchor – suitable for most cohesive and non-cohesive soils
- 28. GROUND ANCHOR • Method to form a ground anchor 1. A hole is predrilled on soil or rock in position carefully calculated. A dense high strength grout is injected over a required length to develop sufficient resistance to hold the bar when it is stressed. 2. For injection anchor, a hole should be bored usually with an expanded end to increase anchorage ability. The pre-stressing bar is placed into the bore hole and pressure grouted over the anchorage length. 3. Gravel placement ground anchor can also be used in clay soils for lighter loading. In this method irregular gravel is injected into the borehole over the anchorage length to form an end plug. The gravel p1ug is then force into soil using percussion method through casing, forming an enlarged end. A stressing bar is inserted into the casing and pressure grouted over the anchorage length as the casing is removed. 4. It should be noted that certain protection measure against corrosion or rusting is required for the stressing bar.
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