Module 4 (Lecture 16) SHALLOW FOUNDATIONS: ALLOWABLE BEARING CAPACITY AND SETTLEMENT

Transcription

1 Topics Module 4 (Lecture 16) SHALLOW FOUNDATIONS: ALLOWABLE BEARING CAPACITY AND SETTLEMENT 1.1 STRIP FOUNDATION ON GRANULAR SOIL REINFORCED BY METALLIC STRIPS Mode of Failure Location of Failure Surface Force Induced in Reinforcement Ties 1.2 FACTOR OF SAFETY OF TIES AGAINST BREAKING AND PULLOUT 1.3 DESIGN PROCEDURE FOR STRIP FOUNDATION ON REINFORCED EARTH Determination of qq oo Determination of QQ RR Calculation of Tie Force Calculation of tie Resistance Due to Friction, FF BB Calculation of Tie Thickness to Resist Tie Breaking Calculation of Minimum Length of Ties STRIP FOUNDATION ON GRANULAR SOIL REINFORCED BY METALLIC STRIPS Mode of Failure

2 The nature of bearing capacity failure of a shallow strip foundation resting on a compact and homogeneous soil mass was presented in figure 4.1a (from chapter 3). In contrast, if layers of reinforcing strips, or ties, are placed in the soil under a shallow strip foundation, the nature of failure in the soil mass will be like that shown in figure 4.36a, b, and c. The type of failure in the soil mass shown in figure 4.36a generally occurs when the first layer of reinforcement is placed at a depth, d, greater than about 2 BB (B = width of the 3 foundation). If the reinforcements in the first layer are strong and they are sufficiently concentrated, they may act as a rigid base at a limited depth. The bearing capacity of foundations in such cases can be evaluated by the theory presented by Mandel and Salencon (1972). Experimental laboratory results for the bearing capacity of shallow foundations resting on a sand layer with a rigid rough base at a limited depth have also been provided by Meyerhof (1974), Pfeifle, and Das (1979), and Das (1981). The type of failure shown in figure 4.36b could occur if dd/bb is less than about 2 and the 3 numbers of layers of reinforcement, N, is less than about 2-3. In this type of failure, reinforcement tie pullout occurs. The most beneficial effect of reinforced earth is obtained when dd/bbless than about is 2 3 and the number of reinforcement layers is greater than 4 but no more than 6-7. In this case, the soil mass fails when the upper ties break (see figure 4.36c).

3 Figure 4.36 Three modes of bearing capacity failure in reinforced earth (redrawn after Binquet and Lee, 1975b) Location of Failure Surface Figure 4.37 shows an idealized condition for development of the failure surface in soil for the condition shown in figure 4.36c. It consists of a central zone-zone I-immediately below the foundation that settles along with the foundation with the application of load. On each side of zone I, the soil is pushed outward and upward-this zone II. The points A, A, A, and B;, B, B, which define the limiting lines between zones I and II, can be obtained by considering the shear stress distribution, ττ xxxx, in the soil caused by the foundation load. The term ττ xxxx refer to the shear stress developed at a depth z below the foundation at a distance x measured from the center line of the foundation. If integration of Boussinesq s equation is performed, ττ xxxx is given by the relation Figure 4.37 Failure mechanism under a foundation supported by reinforced earth [part (b) after Binquet and Lee, 1975b] ττ xxxx = 4bbqq RR xxxx 2 ππ[(xx 2 +zz 2 bb 2 ) 2 +4bb 2 zz 2 ] [4.68] Where q

4 bb = half width of the foundation = BB/2 BB = width of foundation qq RR = load per unit area on the foundation The variation of ττ xxxx at any depth, z, is shown by the broken lines in figure 4.37a. Points A and B refer to the points at which the value of ττ xxxx is maximum at zz = zz 1. Similarly, A and B refer to the points at which ττ xxxx is maximum at zz = zz 2. The distances xx = XX oo at which the maximum value of ττ xxxx occurs take a nondimensional form and are shown in figure 4.37b. Force Induced in Reinforcement Ties Assumptions needed to obtain the tie force at any given depth are as follows: 1. Under the application of bearing pressure by the foundation, the reinforcing ties at points A, A, A, and B;, B, B, take the shape shown in figure 4.37c. That is, the ties take two right-angle turns on each side of zone I around two frictionless rollers. 2. For N reinforcing layers, the ratio of the load per unit area on the foundation supported by reinforced earth, qq RR, to the load per unit area on the foundation supported by unreinforced earth, qq oo, is constant irrespective of the settlement level, S (see figure 4.38). Benquet and Lee (1975a) proved this relation in laboratory experiments. Figure 4.39a shows a continuous foundation supported by unreinforced soil and subjected to a load of qq oo per unit area. Similarly, figure 4.39b shows a continuous foundation supported by a reinforced soil layer (one layer of reinforcement, or NN = 1) and subjected to a load of qq RR per unit area. (Due to symmetry only one-half of the foundation is shown in figure 4.39). In both cases-that is, in figure 4.39a and 4.39b-let the settlement equal SS cc. For one-half of each foundation under consideration, the following are the forces per unit length on a soil element of thickness HH located at a depth z.

5 Figure 4.38 Relationship between load per unit area and settlement for foundations resting on reinforced and unreinforced soil

6 Figure 4.39 Derivation of equation (87)

7 Figure 4.39a continued Unreinforced Case FF 1 and FF 2 are the vertical forces and SS 1 is the shear force. Hence, for equilibrium, FF 1 FF 2 SS 1 = 0 [4.69] Reinforced Case Here, FF 3 and FF 4 are the vertical forces, SS 2 is the shear force, and TT (NN=1) is the tensile force developed in the reinforcement. The force TT (NN=1) is vertical because of the assumption made for the deformation of reinforcement as shown in figure 4.37c. So FF 3 FF 4 SS 2 TT (NN=1) = 0 [4.70] If the foundation settlement, SS cc, is the same in both cases, FF 2 = FF 4 [4.71] Subtracting equation (69) from equation (70) and using the relationship given in equation (71), we obtain TT (NN=1) = FF 3 FF 1 SS 2 + SS 1 [4.72] Note that the force FF 1 is caused by the vertical stress, σσ, on the soil element under consideration as a result of the load qq oo on the foundation. Similarly, FF 3 is caused by the vertical stress imposed on the soil element as a result of the load qq RR. Hence

8 FF 1 = FF 3 = xx oo 0 xx oo 0 σσ(qq oo )dddd σσ(qq RR )dddd [4.73] [4.74] SS 1 = ττ xxxx (qq oo ) HH [4.75] SS 2 = ττ xxxx (qq RR ) HH [4.76] Where σσ(qq oo ) and σσ(qq RR ) are the vertical stresses at a depth z caused by the loads qq oo and qq RR on the foundation ττ xxxx (qq oo ) and ττ xxxx (qq RR ) are the shear stresses at a depth z and at a distance XX oo from the center line caused by the loads qq oo and qq RR Integrating Boussinesq s solution yields σσ(qq oo ) = qq oo ππ tttttt 1 σσ(qq RR ) = qq oo ππ tttttt 1 zz xx bb tttttt 1 zz xx bb tttttt 1 zz 2bbbb (xx 2 zz 2 bb 2 ) xx+bb (xx 2 +zz 2 bb 2 ) 2 +4bb 2 zz 2 [4.77] zz 2bbbb (xx 2 zz 2 bb 2 ) xx+bb (xx 2 +zz 2 bb 2 ) 2 +4bb 2 zz 2 [4.78] ττ xxxx (qq oo ) = ττ xxxx (qq RR ) = 4bbqq oo XX oo zz 2 ππ[(xx oo 2 +zz 2 bb 2 ) 2 +4bb 2 zz 2 ] 4bbqq RR XX oo zz 2 ππ[(xx oo 2 +zz 2 bb 2 ) 2 +4bb 2 zz 2 ] [4.79] [4.80] Where bb = BB/2 The procedure for derivation of equations (77 to 80) is not presented here; for this information see a soil mechanics textbook (for example, Das, Proper substitution of equations (77 to 80) into equations (73 to 76) and simplification yields FF 1 = AA 1 qq oo BB [4.81] FF 3 = AA 1 qq RR BB [4.82] SS 1 = AA 2 qq oo HH [4.83] SS 2 = AA 2 qq RR HH [4.84] Where AA 1 and AA 2 = ff(zz/bb)

9 The variations of AA 1 and AA 2 with nondimensional depth z are given in figure Substituting equations (81)-(84) into equation (72) gives Figure 4.40 Variation of AA 1, AA 2, and AA 3 with zz/bb (after Binquet and Lee, 1975b) TT (NN=1) = AA 1 qq RR BB AA 1 qq oo BB AA 2 qq RR HH +AA 2 qq oo HH = AA 1 BB(qq RR qq oo ) AA 2 HH(qq RR qq oo ) = qq oo qq RR qq oo 1 (AA 1 BB AA 2 HH [4.85] Note that the derivation of equation (85) was based on the assumption that there is only one layer of reinforcement under the foundation shown in figure 4.39b. However, if there are N layers of reinforcement under the foundation with center-to-center spacing of HH, as shown in figure 4.39c, the assumption can be made that TT (NN) = TT (NN =1) NN [4.86] Combining equations (85 and 86) gives TT (NN) = 1 NN qq oo qq RR qq oo 1 (AA 1 BB AA 2 HH [4.87] The unit of TT (NN) in equation (87) is lb/ft (or kn) per unit length of foundation. FACTOR OF SAFETY OF TIES AGAINST BREAKING AND PULLOUT Once the tie forces that develop in each layer as the result of the foundation load are determined from equation (87), an engineer must determine whether the ties at any depth z will fail either by breaking or by pullout. The factor of safety against tie breaking at any depth z below the foundation can be calculated as

10 FFFF (BB) = wwwwww ff yy TT (NN ) [4.88] Where FFFF (BB) = factor of safety against the breakin ww = width of a single tie tt = thickness of each tie nn = number of ties per unit length of the foundation ff yy = yield or breaking strength of the tie material The term wwww may be defined as the linear density ratio, LDR, so FFFF (BB) = ttff yy TT (NN ) (LLLLLL) [4.89] The resistance against the tie being pulled out derives from the frictional resistance between the soil and the ties at any depth. From the fundamental principles of statics, we know that the frictional force per unit length of the foundation resisting tie pullout at any depth z (figure 4.41). Figure 4.41 Variation of equation (91) FF BB = 2 tan uu [normal force] [4.90] = 2 tan uu two sides of tie (i. e., top and bottom) LL oo (LLLLLL) σσ(qqrr )dddd + (LLLLLL)(γγ)(LL oo XX oo )(zz + DD ff ) xx oo

11 Where γγ = unit weight of soil DD ff = depth of foundation φφ uu = tie soil friction angle The relation for σσ(qq RR ) was defined in equation (78). The value of xx = LL oo is generally assumed to be the distance at which σσ(qq RR ) equals to 0.1qq RR. The value of LL oo as a function of depth z is given in figure Equation (90) may be simplified as FF BB = 2 tan φφ yy (LLLLLL) AA 3 BBqq oo qq RR qq oo + γγ(ll oo XX oo )(zz + DD ff ) [4.91]

12 Where Figure 4.42 Variation of LL oo /BB with zz/bb (after Binquet and Lee, 1975b) AA 3 is a nondimensionla quantity that may be expressed as a function of depth (zz/bb) (see figure 4.40) The factor of safety against tie pullout, FFFF (PP), is FFFF (PP) = FF BB TT (NN ) [4.92] DESIGN PROCEDURE FOR STRIP FOUNDATION ON REINFORCED EARTH Following is a step-by-step procedure for the design of a strip foundation supported by granular soil reinforced by metallic strips: 1. Obtain the total load to be supported per unit length of the foundation. Also obtain the quantities a. Soil-friction angle, φφ b. Soil-tie friction angle, φφ μμ c. Factor of safety against bearing capacity failure d. Factor of safety against tie breaking, FFFF (BB) e. Factor of safety against tie pullout, FFFF (PP)

13 f. Breaking strength of reinforcement ties, ff yy g. Unit weight of soil, γγ h. Modulus of elasticity of soil, EE ss i. Poisson s ratio of soil, μμ ss j. Allowable settlement of foundation, SS cc k. Depth of foundation, DD ff 2. Assume a width of foundation, B, and also d and N. the value of d should be less than 2 BB. Also, the distance from the bottom of the foundation to the lowest layer 3 of the reinforcement should be about 2B or less. Calculate ΔHH. 3. Assume a value of LLLLLL 4. For width Bi (step 2) determine the ultimate bearing capacity, qq uu, for unreinforced soil [equation (3 from chapter 3); note: cc = 0]. Determine qq aaaaaa (1) : qq aaaaaa (1) = qq uu FFFF against bearing capacity failure [4.93] 5. Calculate the allowable load, qq aaaaaa (2) based on the tolerable settlement, SS cc, assuming that the soil is not reinforced [equation (32a)]: SS cc = BBBB aaaaaa (2) (1 μμ 2 EE ss )αα rr ss For LL/BB =, the value of αα rr may be taken as 2, or qq aaaaaa (2) = EE ssss cc BB(1 μμ ss 2 )αα rr [4.94] (The allowable load for a given settlement, SS cc, could have also been determined from equations that relate to standard penetration resistances). 6. Determine the lower of the two value of qq aaaaaa obtained from steps 4 and 5. The lower value of qq aaaaaa equals qq oo. 7. Calculate the magnitude of qq RR for the foundation supported by reinforced earth: qq RR = load on foundation per unit length BB [4.95] 8. Calculate the tie force, TT (NN), in each layer of reinforcement by using equation (87) (note: unit of TT (NN) as kn/m of foundation). 9. Calculate the frictional resistance of ties for each layer per unit length of foundation, FF BB, by using equation (91). For each layer, determine whether FF BB /TT (NN) FFFF (PP). If FF BB /TT (NN) < FFFF (PP), the length of the reinforcing strips for a layer may be increased. That will increase the value of FF BB and thus FFFF (PP), and so equation (91) must be rewritten as FF BB = 2 tan φφ μμ (LLLLLL) AA 3 BBqq oo qq RR qq oo + γγ(ll oo XX oo )(zz + DD ff ) [4.96]

14 Where LL = the required length to obtain the desired value of F B 10. Use equation (89) to obtain the tie thickness for each layer. Some allowance should be made for the corrosion effect of the reinforcement during the life of the structure. 11. If he design is unsatisfactory, repeat steps The following example demonstrates the application of these steps. Example 10 Design a strip foundation that will carry a load of 1.8 MN/m. Use the following parameters: SSSSSSSS: γγ = 17.3 kn/m 3 ; φφ = 35 ; EE ss = kn/m 2 ; μμ ss = 0.35 RRRRRRRRRRRRRRRRRRRRRRRRRR tttttttt: ff yy kn/m 2 ; ; φφ μμ = 28 ; FF ss(rr) = 3; FFFF (PP) = 2.5 FFFFFFFFFFFFFFFFFFFF: DD f = 1m; Factor of safety against baring capacity failure = 3. Tolerable settlement= SS cc = 25 mm; desired life of structure = 50 years Solution Let BB = 1 m dd = depth from the bottom of the foundation to the first reinforcing layer = 0.5 ΔHH = 0.5 m NN = 5 LLLLLL = 65% If the reinforcing strips used are 75 mm wide, then wwww = LLLLLL Or nn = LLLLLL ww = m = 8.67/m

15 Hence each layer will contain 8.67 strips per meter length of the foundation. Determination of qq oo For an unreinforced foundation qq uu = γγdd ff NN qq γγbbbb γγ From table 4 (from chapter 3) for φφ = 35, NN qq = and NN γγ Thus qq uu = (17.3)(1)(33.3) (17.3)(1)(48.03) = = kn/m 2 qq aaaaaa (1) = qq uu = 992 = kn/m2 FFFF 3 From equation (94) qq aaaaaa (2) = (EE ss )(SS cc ) = (30,000 kn /m 2 )(0.025m) = kn/m 2 BB(1 μμ 2 ss )αα rr (1m)( )(2) As qq aaaaaa (1) < qq aaaaaa (2), qq oo = qq aaaaaa (1) = kn/m 2 Determination of QQ RR From equation (95), QQ RR = 1.8 MN /m BB = Calculation of Tie Force From equation (87), = kn/m 2 TT (NN) = qq oo NN QQ RR qq oo 1 (AA 1 BB AA 2 HH) The tie forces for each layer are given in the following table. Layer no qq oo NN QQ RR qq oo zz(m) zz BB AA 1 BB AA 2 HH AA 1 BB AA 2 HH TT (NN) (kn /m)

16 Note: AA 1 is from figure 4.40; BB = 1 m; HH = 0.5 m; AA 2 is from figure 4.40; qq uu /qq oo = / Calculation of tie Resistance Due to Friction, FF BB Use equation (91): FF BB = 2 tan φφ μμ (LLLLLL) AA 3 BBqq oo qq RR qq oo + γγ(ll oo XX oo )(zz + DD ff ) The following table shows the magnitude of FF BB for each layer: Layer number Quantity tan φφ μμ (LLLLLL) AA AA 3 BBqq oo (qq RR /qq oo ) zz(mm ) zz/bb LL oo (m) XX oo (m) LL oo XX oo (m) zz + DD ff (m) γγ(ll oo XX oo )(zz + DD ff (m) FF BB (kn/m) FFFF (PP) = FF BB /TT (NN) Note: AA 3 is from figure 4.40; XX oo is from figure 4.37; LL oo is from figure 4.42; TT (NN) is from

17 the preceding table. The minimum factor of safety is greater than the required values of FFFF (PP) which is 2.5. Calculation of Tie Thickness to Resist Tie Breaking From equation (89), FFFF (BB) = ttff yy TT (NN ) (LLLLLL) Here, ff yy = kn/m 2, LLLLLL = 0.65 and FFFF (BB) = 3, so 3 tt = TT ( )(0.65) (NN) = ( )TT (NN) So, for layer 1 tt = ( )(66.08) = m = 1.22 mm For layer 2 tt = ( )(73.43) = m = 1.36 mm Similarly, for layer 3 tt = = 1.49 mm For layer 4 tt = 1.52 mm For layer 5 tt = 1.52 mm Thus in each layer ties with a thickness of 1.6 mm will be sufficient. However, if galvanized steel is used, the rate of corrosion is about mm/yr, so t should be (0.025)(50) = 2.85 mm.

18 Calculation of Minimum Length of Ties The minimum length of ties in each layer should equal 2LL oo. Following is the length of ties in each layer: Layer no. Minimum length of the tie, 2LL oo (m) Figure4. 43 is a diagram of the foundation with the ties. The design could be changed by varying BB, dd, NN, and HH to determine the most economical combination Example 11 Figure 4.43 Refer to example 10. For the loading given, determine the width of the foundation that is needed for unreinforced earth. Note that the factor of safety against bearing capacity failure is 3 and that the tolerable settlement is 25 mm.

19 Solution Bearing Capacity Consideration For a continuous foundation, qq uu = γγdd ff NN qq γγbbnn γγ For φφ = 35, NN qq = 33.3 and NN γγ = 48.03, so qq aaaaaa = qq uu FFFF = 1 FFFF γγdd ffnn qq γγbbbb γγ Or qq aaaaaa = 1 (17.3)(1)(33.3) + 1 (17.3)(BB)(48.03) = BB [a] 3 2 However, qq aaaaaa = kn (BB)(1) [b] Equating the right-hand sides of equations (a) and (b) yields 1800 (BB)(1) = BB Solving the preceding equation gives BB 3 m, so with BB = 3 m, qq all = 600 kn/m 2. Settlement Consideration For a friction angle of φφ = 35, the corrected average standard penetration number is about (equation 11 from chapter 2). From equation (53) for the higher value, NN cccccc = 15, qq aaaaaa = 11.98NN cccccc 3.28BB DD ff BB for a settlement of about 25 mm. now, we can make a few trials: Assumed BB(m)(1) qq aaaaaa = 11.98NN cccccc 3.28BB DD ff (kn/m 2 (2) BB QQ = (BB)(qq aaaaaa ) = Col. 1 Col. 2(kN/m) Note: DD ff = 1 m

20 Required 1800 kn/m For NN cccccc = 15, the width of the foundation should be 9m or more. Based on the consideration of bearing capacity failure and tolerable settlement, the latter criteria will control, so B is about 9 m. Note: At first, the results of this calculation may show the use of reinforced earth for foundation construction to be desirable. However, several factors must be considered before a final decision is made. For example, reinforced earth needs overexcavation and backfilling. Hence, under many circumstances, proper material selection and compaction may make the construction of foundations on unreinforced soils more economical. PROBLEMS 1. A flexible circular area is subjected to a uniformly distributed load of 3000 lb/ ft 2. The diameter of the loaded area is 9.5 ft. determine the stress increase in a soil mass at a point located 7.5 ft below the center of the loaded area. 2. Refer to figure 5, which shows a flexible rectangular area. Given: BB 1 = 1.2 m, BB 2 = 3 m, LL 1 = 3 m, and LL 2 = 6 m. If the area is subjected to a uniform load of 110 kn/m 2, determine the stress increase at a depth of 8 m located immediately below point O. 3. Repeat problem 2 with the following BB 1 = 5 ft LL 1 = 7 ft BB 2 = 10 ft LL 2 = 12 ft Uniform load on the flexible area = 2500 lb/ft 2 4. Refer to figure P-1. Using the procedure outlined in section 5, determine the average stress increase in the clay layer below the center of the foundation due to the net foundation load of 900 kn.

21 Figure P-1 5. Figure P-2 shows an embankment load on a silty clay soil layer. Determine the stress increase at points AA, BB, and CC whch are located at a depth of 15 ft below the ground surface. Figure P-2 6. A flexible load area (figure P-3) is 2m 3m in plan and carries a uniformly distributed load of 210 kn/m 2. Estimate the elastic settlement below the center of the loaded area. Assume DD ff = 0 and HH =. Figure P-3

22 7. A continuous foundation on a deposit of sand layer is shown in figure P-4 along with the variation of the modulus of elasticity of the soil (EE ss ). Assuming γγ = 115 lb/ft 3 and CC 2 = 10 yr, calculate the elastic settlement of the foundation using the strain influence factor. Figure P-4 8. Tow plate load tests with square plates were conducted in the field. At 1-in. settlement, the results were Width of plane (in.) Load (lb) 8,070 25,800 What size of square footing is required to carry a net load of 150,000 lb at a settlement of 1 in.? 9. The tie forces under a continuous foundation are given by equation (87). For the foundation described in problem 23, qq oo = 200 kn/m 2 and qq RR /qq oo = 4.5. Determine the tie forces, TT (NN), in kn/m for each layer of reinforcement.