Design Requirement for Deep Footing - Lab Report Example

Design Requirement for deep footing Deep Foundation this is suitable when the soil top layer is soft and has good bearing soil strata. This means that it can withstand super-embossed stress. If the two mention conditions are made, then deep foundation is used. In designing a footing for the storey building various requirements need to be determined in order for the strong structural building. Among the requirements include a structure and loading capacity. Loading capacity is very important as it will allow the footing to withstand maximum weight. The soil shown from the well is soft at the top and has good bearing soil strata; it can as well withstand super-embossed stress. This soil provided requires soil is of high compressibility this calls for Punching shear failures and the initial will be as follows If the column is too tall it will be crushed under its own weight therefore the maximum height of the column is limited because of compressional pressure at the bottom cannot exceed compressional strength of the material. However, the maximum height at which a footing buckles is less than the height it will be crushed. In designing the footing, the sizing of the footing should be as shown below. Sizing the footing If the tensile or compressive stress exceeds the proportional limit of soil, the strain is no longer proportional to the stress. The footing will tilt when the strength as shown. If the stress exceeds the elastic limit, the material is permanently deformed. For still larger stresses, the solid fractures when the stress reaches the breaking point. The maximum stress that can be withstood without breaking is called is called the ultimate strength. The ultimate strength can be different for compression and tension; then we refer to the compressive strength or the tensile strength of the material. A ductile material continues to stretch beyond the ultimate strength without breaking it; the stress the decreases from the ultimate strength. Examples of ductile solids are the relatively soft materials such as gold, silver, copper and lead. These metals can be pulled like taffy, becoming thinner until finally reaching the breaking point. Calculation In calculating the loading capacity, the footing a number of factors are taken into account. These include; ultimate strength, durability of the footing, serviceability, stability and fatigue. Ultimate strength involves the determination of compressional capacity, tensional capacity, and lateral of piles. Compressional capacity involves the determination of geotechnical strength which is calculated by multiplying ultimate geotechnical with geotechnical strength reduction factor. The following table provides ultimate strength parameters for base bearing and skin friction. The table shows base bearing and skin bearing for foundation of different soils. Material Base bearing kpa) Skin friction (kpa) XW rock 1500 60 DW rock 4500 200 DW/SW rock 7500 235 SW rock 1200 300 Active lateral pressure on piles is earth pressure on footing cause slight movement from the backfill. In this case the foot rotates about the bottom to take a new position which is tilted to the point of plastic equilibrium. It is recommended that the deep foundation used on the project designed in accordance with standard Pilling- Design and installation. This code uses the limit state design method the following must be taken into account:- Ultimate strength- the design of a single pile or a pile group must be such that both the geotechnical strength Rg * and the structural strength Rs*are greater than or equal to the Design action effect S* that is. Rg* S* and Rg* S* Serviceability- single piles or pile groups shall be designed for serviceability by controlling or limiting pile movements. Durability – this is outlined in section AS 2159-1995 and will not be discussed any further. any other factors that need to be considered i.e stability, fatigue, cyclic loading or seismic actions – as we are not aware that any of this Lateral pressure - Lateral pressure on the footing of a building is calculated after considering the force that is likely to be transferred to the base of the footing without exceeding the laid down soil pressure of an area. This is necessary to avoid chances of a footing sinking. Lateral forces are associated with seismic reactions on the base of a footing. If the reaction exceeds the required footing pressure, there will be tilting of the footing which may cause destruction to the building. According to the laid down standards, allowable strength ultimate strength on the footing should not be less than the soil pressure. This has been provided in section 1629. This section gives standards how the footing should be seized and the proper combinations of loads that will not cause harm to the building. The following are the required formulas in determining the load pressure on the footing. Where D is dead load, L is live load and S is lateral or soil pressure. This formula can be restructured to be as follows: Where E/1.4 represents soil pressure. It can further be written as This is because; buildings have specific bearing strength which is expected to be exerted on the soil. Without the soil upward pressure being greater than allowable strength, the footing will sink. Therefore the allowable strength is calculated as shown above. However, the footing should also be in a position to resist earthquakes once they occur. The following formula helps in determining the load that will be resisted by the footing in case of earthquake. in of this case it is assumed that E=0 during design for ultimate strength of the load. This gives the following formula. In this case the strength deduction factor is assumed to be 0.45. another table provides the required allowable footing forces, lateral bearing pressure and friction coefficient of the sliding of the soil. The values provided in the standards are necessary if specific grounds are indentified after site investigation. Where is the ultimate pile capacity, is the shaft resistance and is ultimate force resistance. They are calculated as follows: Where is the area of the pile surface area which is pile perimeter length, is undrained shear strength of clay soil and is adhesion factor, is the area of the toe and is the pile adhesion or force. + ( )+ )+) = Allowable load on the pile will be determined as follows: Where is the ultimate pile capacity, is the shaft resistance and is ultimate toe resistance. They are calculated as follows: Where is the area of the pile surface area which is pile perimeter length, is undrained shear strength of clay soil and is adhesion factor, is the area of the toe and is the pile adhesion or force. In our case we have 3 Qs because the clay has different adhesion that is 4 strata therefore it will be calculated as follows representing stratum 1, 2 and 3. + ( )+ )+) +( )+ ) Qs = 128 + 168 + 240 = 536kN = Qb = 2520 Qu = 2520 + 536 = 3056 This is ultimate load on the pile. Allowable load on the pile will be determined as follows: Since skin friction is different the toe will have its calculation different from the others, thus allowable load on the pile will be Qa = = 1108kN The allowable load on the pile is 1108kN Footing size The trial design axial load and moment will be determined for load combination of Equation and then checked for the other combinations. Earthquakes loads are in both directions, but the positive values are used in this calculation to create the largest bearing pressures. Ma = D+L+ = MD+ML+ Footing area should be effective to help in preventing sinking of the building. Sinking of a building can be very devastating to both human beings and property. This made possible by having proper loading capacity which includes dead loads, live loads and lateral loads. These are the loads that determine the strength of a foundation. Lateral loads are associated with soil of the ground where the house is going to be constructed. The minimum standards will depend on type of in the area where the house is being constructed. The rock soil will have a different settlement as compared to clay soil. To select the foot size the base and length are used to calculate area ( A=BL). This leads to calculation of Soil pressures due to axial load and moment using the largest values of and as This represents the total soil pressure and which is part of it is footing and soil pressure. Normally the values of seismic pressure should be equal to the force of the footing in order for the positive values of PE and ME TO BE EQUAL. This means during calculation there is no way we will have a positive figure for ME and a negative for PE because they show the direction the forces are applied. Eccentricity, e = . This is possible at point where the magnitude of is more than , where length of the designed footing and is the limit of the kern area. Example The formulae Square Footing is Where No is the bearing capacity factor (cohesion), B is the width or diameter of the foundation, D is the effective vertical strength of the soil, Nq is the bearing factor and surcharge friction, is the bearing capacity factor self weight and friction, C is the cohesion strength of the soil. In this case we are given unit weight as 19kN/m3, we have friction as 350, and we have depth base as 3m and a square footing of 3m by 3m Where is the ultimate bearing capacity, in this case we will use the formula Where Qu = In this case Nc=45, Nq=35.4, Ny=50 Qu =1.3x0x(45) + 19kN/m3 (3m) x 35.4) + 0.4 (19kN/m3 x 3m x 50 = 2,017.8 + 1,140 =3,157.8kN/m2 Allowable Bearing Capacity It is Qa = Qu Qa = kN/m2 ii) Shear failure Nc = 16, Nq = 4, N = 1.7 = 273.81 It is Qa = Qu Qa = kN/m2 Allowable total gross load Q= Qa(B)2 = 91.27 X (7)2 = 4,472.16kN The load of 1900 kN thus it is acceptable since the allowance load on the clay is 4,472.16kN In determining the consolidation settlement for the soil profile in the diagram with a load of 1900kN which is applied at two metres below the ground by a square footing with a size of 3m by 3m will be determined as follows. Where Sc is the consolidation settlement under the square footing, =wGs =0.35x2.75= 0.9625, Co is 0.009, H is height and is 6m po = (4x19) + 3(21-9.81)+3(22-9.81) = 146.14 kN Δp= [1900*(3*3)] / [(9+1)*(9+1)] = 171kN , K= 1-sin = 0.426 = 39.425125 ,, e0=0.987, = 0.00285m or 0.285cm or 2.85mm = 0.0001days Example 2 Active lateral earth pressure on a footing Active lateral earth pressure on a footing is earth pressure on footing cause slight movement from the backfill. In this case the footing rotates about the bottom to take a new position which is tilted to the point of plastic equilibrium. Effective Ranking active Pressure at z=0, z=4 and z=8 Here one begins with determining surcharge which is calculated as follows In case q is the surcharge load is given as 30kN/m and Ka is or Ka = and we given as 35o and given c’ as 0 this is because the footing is no tilted in any way and we are not given for surcharge. Thus, surcharge coefficient of ranking active pressure is determined as follows Ka = = 0.271 Therefore at Z=0 Effective Ranking active Pressure will be consist of only surcharge which is – 2c 0.271 – (2 = At z= 4, the effective ranking active pressure is We are given c’ as 0 and 18kN/m3 is therefore, the upper layer of sand its coefficient of ranking active pressure is determined as follows, Ka = = 0.271 The effective ranking active pressure is = 0.271[30 + (4m x18kN/m3] = 27.642/m2 At Z=4 but in the lower layer the force will be 0.271[30 + (4m x18kN/m3] = 27.642/m2 . This is because is 35o as in the first layer At Z=8metres the effective ranking active pressure is = (27.642/m2 x 4 + 4(20kN/m3- 9.81)) x 0.271 =151.328kN/m Rankine active force per unit length of the footing Rankine’s active force per unit length =( ½)H Therefore at Z=0, Rankine’s active force per unit length is =( ½) x 27.642/m2 x 0 =0kN/m Therefore at Z=4, Rankine’s active force per unit length is =( ½) x 27.642/m2 x (4m) = 55.284kN/m At Z=8 Rankine’s active force per unit length =( ½) x 151.328kN/m2 x (8m) = 605.312kN/m When we have a factor of safety of 1.4 and we have calculated disturbing force. The formula is FOS against overturning = The footing represents the resisting moment Disturbing moment = 151.328kN/m +27.642/m2 += 187.07kN/m 1.4 = = 1.4 187.07kN/m = 240X X= 1.09m 2X=2.18m References Coduto, Donald, Man-chu Ronald Yeung and William Kitch. Geotechnical Engineering: Principles & Practices. Boston: Prentice Hall, 2010. Print Lommler, John. Geotechnical Problem Solving. NEW York: John Wiley & Sons, 2012. Print. Sivakugan, Nagaratnam & Braja Das.Geotechnical Engineering: A Practical Problem Solving Approach. New York: J. Ross Publishing, 2009. Print Wyllie, D., 1999. Foundations on Rock: Engineering Practice. CRC Press, 1999. Read More