Module 4 (Lecture 15) SHALLOW FOUNDATIONS: ALLOWABLE BEARING CAPACITY AND SETTLEMENT

Size: px

Start display at page:

Download "Module 4 (Lecture 15) SHALLOW FOUNDATIONS: ALLOWABLE BEARING CAPACITY AND SETTLEMENT"

Ophelia Higgins
6 years ago
Views:

1 Topics Module 4 (Lecture 15) SHALLOW FOUNDATIONS: ALLOWABLE BEARING CAPACITY AND SETTLEMENT 1.1 ALLOWABLE BEARING CAPACITY 1.2 ALLOWABLE BEARING PRESSURE IN SAND BASED ON SETTLEMENT CONSIDERATION 1.3 FIELD LOAD TEST 1.4 PRESUMPTIVE BEARING CAPACITY 1.5 TOLERABLE SETTLEMENT OF BUILDINGS 1.6 FOUNDATION WITH SOIL REINFORCEMENT 1.7 SHALLOW FOUNDATION ON SOIL WITH REINFORCEMENT

2 ALLOWABLE BEARING CAPACITY ALLOWABLE BEARING PRESSURE IN SAND BASED ON SETTLEMENT CONSIDERATION Meyerhof (1956) proposed a correlation for the net allowable bearing pressure for foundations with the corrected standard penetration resistance, NN cccccc. The net pressure has been defined as qq net (all ) = qq aaaaaa γγdd ff According to Meyerhof s theory, for 1 in. (25.4 mm) of estimated maximum settlement qq net (all ) (kn/m 2 ) = 11.98NN cccccc (for BB 1.22 m [4.48] qq net (all ) (kn/m 2 ) = 7.99NN cccccc 3.28BB BB 2 (for BB > 1.22 m) [4.49] Where NN cccccc = corrected standard penetration number qq net (all ) (kip/ft 2 ) = NN cccccc 4 (for BB 4 ft [4.50] And qq net (all ) (kip/ft 2 ) = NN cccccc 6 B+1 B 2 (for BB 4 ft [4.51] Since Meyerhof proposed original correlation, researchers have observed that its results are rather conservative. Later, Meyerhof (1965) suggested that the net allowable bearing pressure should be increased by about 50%. Bowles (1977) proposed that the modified form of the bearing pressure equations be expressed as qq net (all ) (kn/m 2 ) = 19.16NN cccccc FF dd SS cc (for BB 1.22 m) [4.52] 25.4 qq net (all ) (kn/m 2 ) = 19.98NN cccccc 3.28BB BB 2 FF dd SS cc (for BB > 1.22 m) [4.53] 25.4 Where FF dd = depth factor = (D f /B) 1.33 [4.54] SS cc = torelable settlement, in mm Again, the unit of B is meters.

3 In English units qq net (all ) (kip/ft 2 ) = NN cccccc 2.5 F ds c (for BB 4 ft) [4.55] qq net (all ) (kip/ft 2 ) = NN cccccc 4 B+1 B 2 F d S c (for BB > 4 ft) [4.56] Where FF dd is given by equation (50) SS cc = torebale settlement, in in. Based on equation (55 and 56), the variation of qq net (all ) /(F d S c ) with B and NN cccccc are given in figure Figure 4.30 Plot of qq net (all ) /FF dd SS ee vs B equations (55 and 56) The empirical relations just presented may raise some questions. For example which value of the standard penetration number should be used, and what is the effect of the water table on the net allowable bearing capacity? The design value of NN cccccc should be determined by taking into account the NN cccccc values for a depth of 2B to 3B, measured from the bottom of the foundation. Many engineers are also of the opinion that the NN cccccc value

4 should be reduced somewhat if the water table is close to the foundation. However, the author believes that this reduction is not required because the penetration resistance reflects the location of the water table. Meyerhof (1956) also prepared empirical relations for the net allowable bearing capacity of foundations based on the cone penetration resistance, qq cc : qq net (all ) = qq cc 15 (for BB 1.22m and settlement of 25.4 mm) [4.57] And qq net (all ) = qq cc BB BB 2 (for BB > 1.22m and settlement of 25.4 mm) [4.58] Note that in equations (57 and 58) the unit of B is meters, and the units of qq net (all ) and qq cc are kn/m 2. qq net (all ) (lb/ft 2 ) = q c (lb /ft 2 ) 15 (for BB 4ft and settlement of 1 in. ) [4.59] And qq net (all ) (lb/ft 2 ) = q c (lb /ft 2 ) BB (for BB > 4ft and settlement of 1 in. ) [4.60] Note that in equations (59 and 60), the unit of B. The basic philosophy behind the development of these correlations is that, if the maximum settlement is not more than 1 in. (25.4 mm) for any foundation, the differential settlement would be no more than 0.75 in. (19 mm). These are probably the allowable limits for most building foundation designs. FIELD LOAD TEST The ultimate load-bearing capacity of a foundation, as well as the allowable bearing capacity based on tolerable settlement considerations, can be effectively determined from the field load test. It is generally referred to as the plate load test (ASTM, 1982; Test Designation D ). The plates that are used for tests in the field are usually made of steel and are 25 mm (1 in.) thick and 150 mm to 762 mm (6 in. to 30 in.) in diameter. Occasionally, square plates that are 305 mm 305 mm (12 in. 12 in.) are also used. To conduct a plate load test, a hole is excavated with a minimum diameter 4B (B = diameter of the test plate) to a depth of DD ff (DD ff = depth of the proposed foundation). The plate is placed at the center of the hole. Load is applied to the plate in steps-about onefourth to one-fifth of the estimated ultimate load-by means of jack. A schematic diagram of the test arrangement is shown in figure 4.31a. During each step load application, the settlement of the plate is observed on step. The test should be conducted until failure, or at least until the plate has gone through 25 mm (1 in.) of settlement. Figure 4.32 shows

5 the nature f the load-settlement curve obtained from such tests, from which the ultimate load per unit area can be determined. Figure 4.31 Plate load test: (a) test arrangement; (b) nature of load-settlement curve For tests in clay, qq uu(ff) = qq uu(pp) [4.61] W, is Here qq uu(ff) = ultimate bearing capacity of the proposed foundation qq uu(pp) = ultimate bearing capacity of the test plane Equation (61) implies that the ultimate bearing capacity in clay is virtually independent of the size of the plate.

6 For tests in sandy soils, qq uu(ff) = qq uu(pp) BB FF BB PP [4.62] Where BB FF = width of the foundaiton BB PP = width of the test plate The allowable bearing capacity of a foundation, based on settlement considerations and for a given intensity of load, qq oo, is SS FF = SS PP BB FF BB PP (for clayey soil) [4.63] And SS FF = SS PP 2BB FF BB FF +BB PP 2 (for sandy soil) [4.64] The preceding relationship is based on the work of Terzaghi and Peck (1967). Figure 4.32 shows a comparison of several large-scale field test results in with equation (64). Based on this comparison, it can be said that equation (64) is fairly approximate. Figure 4.32 Comparison of field test results with equation (64) (after D Appolonia et al., 1970) Housel (1929) proposed a different technique for determining the load-bearing capacity of shallow foundations based on settlement considerations: 1. Requirement is to find the dimensions of a foundation that will carry a load of QQ cc with an allowable settlement of SS cc(aaaaaa ).

7 2. Conduct two plate load tests with plates of diameters BB 1 and BB From the load-settlement curves obtained in step 2, determine the total loads on the plates (QQ 1 and QQ 2 ) that correspond to the settlement of SS cc(aaaaaa ). For plate no. 1, the total load can be expressed as QQ 1 = AA 1 mm + PP 1 nn [4.65] Similarly, for plate no. 2 QQ 2 = AA 2 mm + PP 2 nn [4.66] Where AA 1, AA 2 = areas of the plate no. 1 and no. 2, respectively PP 1, PP 2 = perimeter of the plates no. 1 and no. 2, respectively mm, nn = two constants that corresponds to the bearing presure and perimeter shear, respectively The values of mm and nn can be determined by solving equations (65 and 66). 4. For the foundation to be designed, QQ oo = AAAA + PPPP [4.67] Where AA = area of the foundation PP = perimeter of the foundation Because QQ oo, mm and nn are known, equation (67) can be solved determine foundations width. Example 7 The results of a plate load test in a sandy soil are shown in figure The size of the plate is 0.305m 0.305m. Determine the size of a square column foundation that should carry a load of 2500 kn with a maximum of 25 mm.

8 Figure 4.33 Solution The problem has to be solved by trial and error. Use the following table and equation (64): QQ oo (kn)(1) Assume width BB FF (m)(2) qq oo = QQ oo BB FF 2 (kn /m 2 )(3) SS PP corresponding to qq oo in col. 3 (mm) (4) SS FF from equation (64) (mm) (5) So, a column footing with dimensions of 3.2 m 3.2 m will be appropriate. Example 8 The results of two plate load tests are given in the following table: Plate diameter, B (m) Total load, QQ (kn) Settlement (mm)

9 A square column foundation has to be constructed to carry a total load of 715 kn. The tolerable settlement is 20 m. determine the size of the foundation. Solution Use equations (65 and 66): 32.2 = ππ 4 (0.305)2 mm + ππ(0.305)nn [a] 71.8 = ππ 4 (0.610)2 mm + ππ(0.610)nn [b] From (a) and (b), mm = kn/m 2 nn = kn/m For the foundation to be designed [equation (67)], QQ oo = AAAA + PPPP Or QQ oo = BB FF 2 mm + 4BB FF nn For QQ oo = 715 kn, 715 = BB FF 2 (50.68) + 4BB FF (29.75) Or 50.68BB FF BB FF 715 = 0 BB ff 2.8 m Example 9 A shallow square foundation for a column is to be constructed. It must carry a net vertical load of 1000 kn. The foundation soil is sand. The standard penetration numbers obtained from field exploration are given in figure Assume that the depth of the foundation will be 1.5 m and the tolerable settlement is 25.4 mm. determine the size of the foundation.

10 Figure 4.34 Solution The field standard penetration numbers need to be corrected by using the Liao and Whitman relationship (table 4 from chapter 2). This is done in the following table Depth (m) Field value of NN FF σσ vv(kn/m 2 ) Corrected NN cccccc ) Rounded off

11 From the table, it appears that a corrected average NN cccccc value of about 10 would be appropriate. Using equation (53) qq nnnnnn (aaaaaa ) = 11.98NN cccccc 3.28BB BB 2 FF dd SS cc 25.4 Allowable SS cc = 25.4 mm and NN cccccc = 10, so qq nnnnnn (aaaaaa ) = BB BB 2 FF dd The following table can now be prepared for trial calculations: B (m) FF dd qq nnnnnn (aaaaaa ) (kn/m 2 ) qq oo = qq nnnnnn (aaaaaa ) BB 2 (kn) Because QQ oo required is 1000 kn, B will be approximately equal to 2.4 m. PRESUMPTIVE BEARING CAPACITY Several building codes (for example, Uniform Building Code, Chicago Building Code, New York City Building Code) specify the allowable bearing capacity of foundations on various types of soil. For minor construction, they often provide fairly acceptable guidelines. However, these bearing capacity values are based primarily on the visual classification of near-surface soils. They generally do not take into consideration factors such as the stress history of the soil, water table location, depth of the foundation, and tolerable settlement. So, for large construction projects, the codes presumptive values should be used only as guides. TOLERABLE SETTLEMENT OF BUILDINGS As has been emphasized in this chapter, settlement analysis is an important part of the design and construction of foundations. Large settlements of various components of a structure may lead to considerable damage and/or may interfere with the proper functioning of the structure. Limited studies have been made to evaluate the conditions for tolerable settlement of various types of structures (for example, Bjerrum, 1963;

12 Burland and Worth, 1974; Grant et al., 1974; Polshin and Tokar, 1957; and Wahls, 1981). Wahls (1981) has provided an excellent review of these studies. Figure 4.35 gives the parameters for definition of tolerable settlement. Figure 4.35a is for a structure that has undergone settlement without tilt; Figure 4.35b is for a structure that has undergone settlement with tilt. Figure 4.35 Parameters for definition of tolerable settlement (redrawn after Wahls, 1981) The parameters are ρρ ii = total vertical displacement at point ii δδ uu = differential settlement between point ii and jj = relative deflection ωω = tilt ηη uu = δδ uu II uu Δ LL ωω = angular distortion = deflection ratio LL = lateral dimension of the structure Bjerum (1963) provided the conditions of limiting angular distortion, ηη, for various structures (see table 6).

13 Polshin and Tokar (1957) presented the settlement criteria of the 1955 U.S.S.R. Building Code. These criteria were based on experience gained from observations of foundation settlement over 25 years. Tables 7 and 8 present the criteria. FOUNDATION WITH SOIL REINFORCEMENT SHALLOW FOUNDATION ON SOIL WITH REINFORCEMENT It was discussed in chapter 3 that the ultimate bearing capacity of shallow foundations can be improved by including tensile reinforcement such as metallic strips, geotextiles, and geogrids in the soil supporting the foundation. The procedure for designing shallow foundations for limiting settlement condition (that is, allowable bearing capacity) with layers of geogrid as reinforcement is still n the research and development stages. However, the problem of allowable bearing capacity of shallow foundations resting on granular soil reinforced with metallic strips was studied in detail by Binquet and Lee (1975a, b), who proposed the rational design method presented in the following sections. Table 6 Limiting Angular Distortion as Recommended by Bjerrum Category of potential damage ηη Danger to machinery sensitive to settlement 1/750 Danger to frames with diagonals 1/600 Safe limit for no cracking of buildings b 1/500 First cracking of panel walls 1/300 Difficulties with overhead cranes 1/300 Tilting of high rigid buildings becomes visible Considerable cracking of panel and brick walls Danger of structural damage to general buildings 1/250 1/150 1/150 Safe limit for flexible brick walls, LL/HH > 1/150 4 b a After Wahls (1981) b Safe limits include a factor of safety. HH = height of building

14 Table 7 Allowable Settlement Criteria: 1995 U.S.S.R.Building Code Type of structure Sand and hard clay Plastic clay Civil-and industrial-building column foundations: (a)ηη For steel and reinforced concrete structure For end rows of columns with brick cladding For structures where auxillary strain does not arise during nonuniform settlement of foundations Tilt of smokestacks, towers, and so on Craneways Plain brick walls: For multistory dwelling and civil buildings at LL/HH 3 at LL/HH 5 for one-story mills (b)δ/ll a After Wahls (1981). HH = height of building Table 8 Allowable Average Settlement for Different Building Types Type of building Allowable average settlement, in (mm) Building with plain brick walls

15 LL/HH 2.5 LL/HH 1.5 Building with brick walls, reinforced with reinforced concrete or reinforced brick Framed building 3(80) 4(100) 6(150) 4(100) Solid reinforced concrete foundations of smokestacks, soils, towers, and so on 12(300) After Wahls (1981). HH = height of building

The Bearing Capacity of Soils. Dr Omar Al Hattamleh

The Bearing Capacity of Soils. Dr Omar Al Hattamleh The Bearing Capacity of Soils Dr Omar Al Hattamleh Example of Bearing Capacity Failure Omar Play the move of bearing Capacity failure The Philippine one Transcona Grain Silos Failure - Canada The Bearing