CHAPTER 6 UNRESTRAINED SHORING SYSTEMS

1 CHAPTER 6 UNRESTRAINED SHORING SYSTEMS UNRESTRAINED SHORING SYSTEMS TYPES OF UNRESTRAINED SHORING SYSTEMS There are two types of UNRESTRAINED SHORING SYSTEMS , sheet pile walls and soldier pile walls. Continuous sheet pile retaining walls may be constructed with driven precast prestressed concrete sheet piles or steel sheet piles with interlocking edges. The sheet piles are driven side by side into the ground and form a continuous vertical wall. Because of the large deflections that may develop, cantilever sheet pile retaining walls are mainly used for temporary excavations not greater than about 18 feet. However, the use of struts and/or walers can increase the wall height.

2 Figure 6-1 shows a typical cantilever sheet pile retaining wall. Figure 6-1. Sheet Pile Wall with Cap Beam Soldier pile retaining walls may be constructed with driven piles (steel, timber or concrete) or they may be placed in drilled holes and backfilled with concrete, slurry, sand, pea-gravel or similar material. A soldier pile could also be a cast in place reinforced concrete pile. Lagging is placed between soldier pile vertical elements and could be treated timber, reinforced shotcrete, reinforced 6-1 CT TRENCHING AND SHORING MANUAL cast in place concrete, precast concrete panels or steel plates. This type of wall depends on passive resistance of the foundation material and the moment resisting capacity of the vertical structural members for stability, therefore its maximum height is limited to competence of the foundation material and the moment resisting capacity of the vertical structural members.

3 The economical height of this type of wall is generally limited to a maximum height of 18 feet. Figure 6-2 shows a typical soldier pile retaining wall. Figure 6-2. Soldier Pile Wall with Cap Beam LATERAL EARTH PRESSURES FOR UNRESTRAINED SHORING SYSTEMS Non-gravity cantilever retaining walls are analyzed by assuming that the vertical structural member rotates at Point O, at the distance, DO, below the excavation line as shown in Figure 6-3 (a). The realistic load distribution is shown in (b). As a result, the mobilized active pressure develops above Point O in the back of the wall and below Point O in the front of the wall. The mobilized passive pressure develops in front of the wall above Point O and at the back of the wall below Point O.

4 The simplified load distribution is shown in Figure 6-3 (c). Force R is assumed at 6-2 UNRESTRAINED SHORING SYSTEMS Point O to compensate the resultant net active and passive pressure below point of rotation at Point O. The calculated depth, D, is determined by increasing DO by 20% to approximate the total embedment depth of the vertical wall element. The 20% increase is not a factor of safety, it accounts for the rotation of the length of vertical wall element below Point O as shown in Figure 6-3. D Do O Active Active Passive Passive Active Passive R OO (a)- Wall Deformed (b)- Load Distributions (c)- Load Simplification Figure 6-3.

5 Cantilever Retaining Walls For UNRESTRAINED SHORING SYSTEMS , depending on the site soil profile, the simplified lateral earth pressure distribution shown in Figure 6-4 through Figure 6-8 may be used. 6-3 CT TRENCHING AND SHORING MANUAL For walls with vertical elements embedded in a single layer of granular soil and retaining granular soil, Figure 6-4 may be used to determine the lateral earth pressure distribution for a cantilever SHORING system. Figure 6-4. Loading Diagram for Single Layer 6-4 UNRESTRAINED SHORING SYSTEMS For walls with vertical elements embedded in multi-layer granular soil and retaining granular soil, Figure 6-5 may be used to determine the lateral earth pressure distribution for a cantilever SHORING system.

6 Figure 6-5. Loading Diagram for Multi-Layer Soil 6-5 CT TRENCHING AND SHORING MANUAL If walls support or are supported by cohesive soils, the walls may be designed by the total stress method of analysis and undrained shear strength parameters. For the latter, the simplified lateral earth pressure distribution shown in Figure 6-6, Figure 6-7, and Figure 6-8 may be used. Figure 6-6. Loading Diagram for Multi-Layer 6-6 UNRESTRAINED SHORING SYSTEMS Figure 6-7. Loading Diagram for Multi-Layer 6-7 CT TRENCHING AND SHORING MANUAL Figure 6-8. Loading Diagram for Multi-Layer To determine the active lateral earth pressure on the embedded wall element shown above: Treat the sloping backfill above the top of the wall within the active failure wedge as an additional surcharge ( v).

7 The portion of the negative loading at the top of the wall due to cohesion is ignored Any hydrostatic pressure in the tension crack needs to be considered. The ratio of total overburden pressure to undrained shear strength (NS) must be < 3 at the design grade in front of wall. The active lateral earth pressure acting over the wall height (H) shall not be less than times the effective overburden pressure at any depth, or KSF/FT of wall height, which ever is greater. 6-8 UNRESTRAINED SHORING SYSTEMS EFFECTIVE WIDTH The effective width (d) of a soldier pile is generally considered to be the dimension of the soldier pile taken parallel to the line of the wall for driven piles or drilled piles backfilled with material other than concrete.

8 The effective width of the soldier piles may be taken as the diameter of the drilled-hole when 4-sack or better concrete is used. Soil arching however, can greatly increase the effective width described above. See Figure 6-9. Arching of the soil between soldier piles can increase the effective width of a soldier pile up to 3 times for granular soil and 2 times for cohesive soils. Figure 6-9. Soldier Pile with Arching Numerous full-scale pile experiments have shown the passive resistance in front of an isolated pile is a three dimensional problem as shown in Figure 6-9. Two dimensional classical earth pressure theories under estimates the passive resistance in front of a soldier pile.

9 Therefore, the passive resistance in front of a pile calculated by classical earth pressure theories shall be multiplied by the 6-9 CT TRENCHING AND SHORING MANUAL adjusted pile width. The adjusted pile width is a function of the effective width of the pile and the soil friction angle ( ) as shown below. Adjusted Pile Width = Effective Width Arching Capability Factor Eq. 6-1 Table 6-1. Arching Capability Factor Pile Spacing (s) Arching Capability Factor 3 * d 3 > 3 * d ( 3) Where: Effective Width = Width of the pile as described above. d = Effective Width = Internal friction angle of the soil in degrees For granular soils, if the pile spacing is 3 times the effective width (d) or less the arching capability factor may be taken as 3.

10 The arching capability for cohesive soil ranges between 1 and 2 as shown in Table 6-2. Table 6-2. Arching Capability for Cohesive Soil Below the excavation depth the adjusted pile width is used for any active loadings (including surcharge loadings) on the back of the pile as well as for the passive resistance in front of the pile. The adjusted pile width cannot exceed the pile spacing and when the adjusted pile width equals the pile spacing, soldier pile SYSTEMS can be analyzed in the same manner as sheet pile SYSTEMS . 6-10 UNRESTRAINED SHORING SYSTEMS DEFLECTION Calculating deflections of temporary SHORING SYSTEMS can be complicated. Deflection calculations are required for any SHORING system adjacent to the Railroad or high risk structures.