4.8 SURCHARGE LOADS Minimum Construction Surcharge Load CT TRENCHING AND SHORING MANUAL

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Marvin Norris
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1 CT TRENCING AND SORING MANUAL 48 SURCARGE LOADS A surcarge load is any load wic is imposed upon te surface of te soil close enoug to te excavation to cause a lateral pressure to act on te system in addition to te basic eart pressure Groundwater will also cause an additional pressure, but it is not a surcarge load Examples of surcarge loads are spoil embankments adjacent to te trenc, streets or igways, construction macinery or material stockpiles, adjacent buildings or structures, and railroads 481 Minimum Construction Surcarge Load Te minimum lateral construction surcarge of 7 psf ( ) sall be applied to te soring system to a dept of 10 feet (s) below te soring system or to te excavation line wicever is less See Figure 4-47 Tis is te minimum surcarge loading tat sall be applied to any soring system regardless of weter or not te system is actually subjected to a surcarge loading Surcarge loads wic produce lateral pressures greater tan 7 psf would be used in lieu of tis prescribed minimum Tis surcarge is intended to provide for te normal construction loads imposed by small veicles, equipment, or materials, and workmen on te area adjacent to te trenc or excavation It sould be added to all basic eart pressure diagrams Tis minimum surcarge can be compared to a soil aving parameters of = 109 pcf and Ka = 033 for a dept of feet [(033)(109)() = 7 psf] Q s Figure 4-47 Minimum Lateral Surcarge Load 4-70

2 EART PRESSURE TEORY AND APPLICATION 48 Uniform Surcarge Loads Were a uniform surcarge is present, a constant orizontal eart pressure must be added to te basic lateral eart pressure Tis constant eart pressure may be taken as: K Q Eq 4-66 Were: = constant orizontal eart pressure due to uniform surcarge K = coefficient of lateral eart pressure due to surcarge for te following conditions: Use Ka for active eart pressure Use Ko for at-rest eart pressure Q = uniform surcarge applied to te wall backfill surface witin te limits of te active failure wedge 4-71

3 CT TRENCING AND SORING MANUAL 483 Boussinesq Loads Typically, tere are tree (3) types of Boussinesq Loads Tey are as follows: 4831 Strip Load Strip loads are loads suc as igways and railroads tat are generally parallel to te wall Te general equation for determining te pressure at distance below te ground line is (See Figure 4-48): Q R sin cos Eq 4-67 Were R is in radians L 1 Q (psf) L a L 1 = Distance from wall to left edge of strip load L = Distance from wall to rigt edge of strip load / a = widt of surcarge strip R = ( /180) Figure 4-48 Boussinesq Type Strip Load 4-7

4 EART PRESSURE TEORY AND APPLICATION 483 Line Load A line load is a load suc as a continuous wall footing of narrow widt or similar load generally parallel to te wall K-Railing could be considered to be a line load Te general equation for determining te pressure at distance below te ground line is: (See Figure 4-49) For m 04: For m > 04 Q l 0n Eq n Q l m n 18 Eq 4-69 m n L 1 m Ql (plf) n L 1 = Distance from wall to line load Figure 4-49 Boussinesq Type Line Load 4-73

5 CT TRENCING AND SORING MANUAL 4833 Point Load Point loads are loads suc as outrigger loads from a concrete pump or crane A weel load from a concrete truck may also be considered a point load wen te concrete truck is adjacent an excavation and in te process of te unloading Te truck could be positioned eiter parallel or perpendicular to te excavation Te general equation for determining te pressure at distance below te ground line is: (See Figure 4-50) For m 04: For m > 04 Q p n 08 Eq n 3 Q p m n 177 Eq 4-71 m n 3 L 1 m Qp (lbs) n L 1 = Distance from wall to point load Figure 4-50 Boussinesq Type Point Load 4-74

6 EART PRESSURE TEORY AND APPLICATION In addition, is furter adjusted by te following wen te point is furter away from te line closest to te point load: (see Figure 4-51) ' cos 1 1 Eq 4-7 L at =Lateral distance to desired point on wall Wall Qp (lbs) L at PLAN VIEW Figure 4-51 Boussinesq Type Point Load wit Lateral Offset 4-75

7 CT TRENCING AND SORING MANUAL 484 Traffic Loads Traffic near an excavation is one of te more commonly occurring surcarge loads Trying to analyze every possible scenario would be time consuming and not very practical For normal situations, a surcarge load of 300 psf spread over te widt of te traveled way sould be sufficient Te following example compares te pressure diagrams for a S0 truck (using point loads) centered in a 1' lane to a load of Q = 300 psf (using te Boussinesq Strip metod) Te dept of excavation is 10' x 1 m m x m m n dept Dept n For line AB, see Eq 4-71 For loads at an angle to AB, see Eq

8 EART PRESSURE TEORY AND APPLICATION Front and rear rigt weels: 668, cos Front and rear left weels: 494, cos ) Rigt rear weels: ) Left rear weels: 3) Rigt center weels: 4) Left center weels: 5) Rigt front weels: 6) Left front weels: Combine and simplify similar equations: a) 11n 036 n n 144 n b) ,00006 n n ,0001 n 10 1 n ,00006 n n ,0001 n 10 1 n ,00006 n n ,0001 n 10 1 n 3 Dept n a) b)

9 CT TRENCING AND SORING MANUAL S0 Truck 1 Strip (Point loading witout impact) (Q = 300 psf) CONCLUSION: Strip load of Q = 300 psf compares favorably to a point load evaluation for S0 truck loadings 4-78

10 EART PRESSURE TEORY AND APPLICATION 4841 Example 4-7 Sample Problem Surcarge Loads Example 4-7 Surcarge Loads Surcarge Lateral Pressures (psf) Dept Q = 100 Q = 00 Q = 300 Sum * * * * Minimum construction surcarge load Add soil pressures of surcarge loads to derive combined pressure diagram 4-79

11 CT TRENCING AND SORING MANUAL 485 Alternate Surcarge Loading (Traffic) An acceptable alternative to te Boussinesq analysis described below consists of imposing imaginary surcarges beind te soring system suc tat te resulting pressure diagram is a rectangle extending to te computed dept of te soring system and of a uniform widt of 100 psf Generally, alternative surcarge loadings are limited to traffic and ligt equipment surcarge loads Oter loadings due to structures, or stockpiles of soil, materials or eavy equipment will need to be considered separately Q D Figure 4-5 Alternate Traffic Surcarge Loading 4-80

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