Eng_tn_21_gravityblockwall_082011.xls

TECHNICAL NOTES U.S. Department of Agriculture

Natural Resources Conservation Service

ENGINEERING #21

SPOKANE, WASHINGTON August, 2011

Gravity Block Wall Design Tool for Ecology Block and Ultra Block size material

Workbook prepared by:

Sally Bredeweg, State Design Engineer NRCS, Spokane, WA

Purpose The purpose of this workbook is to provide the technician/engineer with a tool to assess the structural stability of a gravity block wall used in agricultural production. Application These spreadsheets allow the user to check the stability analysis of overturning and sliding of a planned gravity block wall. Note: The bearing capacity and buoyancy of the site need to be evaluated during the inventory and evaluation process. Recommendation Print out a hard copy of the final gravity block wall scenario as evaluated for the customer file.

Overview

NRCS Washington State Technical Note - Design Tool For a Gravity Block Waste Storage Facility Retaining Wall Using Precast Interlocking Concrete Blocks Two worksheets are attached (Ecology block and Ultrablock) with design analysis equations. To begin, select the tab for the type of gravity block to be used. 1 Select desired number of ecology blocks stacked in vertical wall 2 Select a number of ecology blocks to be backfilled (if any) 3 Define the maximum depth of waste to be stored against the retaining wall Four types of stability analysis should be considered for every structure: ► Bearing Capacity Inventory evaluation ► Buoyancy Site evaluation ► Overturning Calculated ► Sliding Calculated ► Bearing Capacity Analysis Bearing capacity is the load carrying capability of the soil to resist loading without shearing. The maximum pressure is the weight of the structure and the weight of all material permanently or temporarily attached to or over the structure. Blocks will be stacked on a concrete slab base for waste management purposes. The slab design must meet the waste storage and working loads bearing capacity. Thus the design of a waste storage facility dry stack slab needs to include an analysis of the bearing capacity of the site foundation soils that will support the gravity block wall. The site bearing capacity will not be addressed in this technical note but must be part of the site inventory and engineering design evaluation. ►

Buoyancy Analysis Buoyancy is the relationship of the hydrostatic uplift forces acting to float the structure compared to the downward gravity forces due to the weight of the structure and weight of all material permanently attached to or over the structure. Gravity block retaining wall structures for waste storage facilities will not be designed for situations with seasonal high or normal groundwater within the backfill or foundation. Thus buoyancy design will not be calculated as ground water shall not be present to create uplift forces. See criteria, Waste Storage Facility, 313 Practice Standard, for Fabricated Structures, Foundation. "Foundations consisting of bedrock with joints,

fractures, or solution channels shall be treated, or a separation distance (from groundwater) provided consisting of a minimum of 1 foot of impermeable soil between the floor slab and the bedrock or an alternative that will achieve equal (groundwater) protection."

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Overview

REFERENCES: 1 NRCS TR 74, LATERAL EARTH PRESSURES A. IX. STRUCTURAL STABILITY CONCEPTS, PAGE 79 - A. OVERTURNING Retaining walls should have a minimum factor of safety of 1.5 against overturning; higher safety factors may be justified in some cases, depending on the uncertainties of the soils and site conditions. B. IX. STRUCTURAL STABILITY CONCEPTS, PAGE 82 - B. SLIDING Retaining walls should have a minimum factor of safety of 1.5 against sliding. (Cohesion will be neglected in the resistance to sliding with this configuration and this factor of safety.) 2 IBC 2006, SECTION 1806, RETAINING WALLS 1806.1, General Retaining walls shall be designed to ensure stability against overturning, sliding, excessive foundation pressure (bearing capacity) and water uplift (buoyancy). Retaining walls shall be designed for a safety factor of 1.5 against lateral sliding and overturning 3 NRCS, PRACTICE STANDARD - CODE 313, WASTE STORAGE FACILITY A. General criteria applicable to all waste storage facilities i. Location. Facilities should be located outside of floodplains. However, if site restrictions require location within a floodplain, they shall be protected from inundation or damage from a 25-year flood event, or larger if required by laws, rules, and regulations. ii. Additional criteria for fabricated structures, Structural Loadings. Internal lateral pressure used for design shall be 65lb/ft3 where the stored waste is not protected from precipitation. A value of 60 lb/ft3 may be used where the stored waste is protected from precipitation and will not become saturated. Lesser values may be used if supported by measurement of actual pressures of the waste to be stored.

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Ecology block

Ecology Block Wall

► Overturning Analysis

Legend

► Sliding Analysis

Calculated-locked

Lock-Block Cross-Section Dimensions Gravity lock block weight Type Width, ft Height, ft Standard Block Ecology Block 2 2 3550 lbs for 6 ft block Definition of Point 'A' for reference:

Select input-unlocked

Weight (lbs) per foot of standard block length 592 lb/ft of block

Point 'A' is the unsupported, outside base of the block wall, about which overturning can occur.

► Overturning Analysis Overturning moments will act on the structure in the direction of potential overturning. Resisting moments will act on a structure opposite the direction of potential overturning. The factor of safety is the relationship between the overturning and resisting moments.

FS 

 ResistingMom ents  OverturnMo ments

NRCS TR 74 and IBC 2006, OVERTURNING factor of safety ≥ 1.5 Given:

Density of the stored waste material

65

pcf

Impact Load This represents equipment impact loads from a fork lift or a front loader acting on the wall, moving stored waste material in and out of the bins. For design purposes, an equipment impact point load on the gravity block retaining wall face will be assumed at 30 inches (2.5 feet) below the top of the maximum stack height. This input is a variable that can range from 20 lbs., for hand held equipment, to 1000 lbs. for larger mechanical equipment. If this value is unknown, a fork lift impact load of 200 lbs acting horizontally on the wall section, is a reasonable assumption. Surcharge load It will be assumed that equipment operating on a stack higher than 5 ft. will exert a surcharge load in the vertical direction. This input variable can be based on the plan of operations. For design purposes, this surcharge load can range from 0 lbs., for no mechanical equipment, to 1000 lbs for large equipment. If this value is unknown, a reasonable surcharge load of 200 lbs. (equivalent of 2 ft. of compacted soil) is recommended. For this design analysis, values for the following loads must be selected: Impact load The block wall will experience a horizontal impact load from equipment = X-direction Minimum value = 20 lbs Range: 20 lbs/ft to 1000 lbs/ft of wall Equipment impact load on wall 200 lbs/ft of block wall With point of impact on wall located at 2.5 ft from top of waste pile or at top of wall.

Surcharge load There will be equipment surcharge vertical load on waste pile = Y-direction Used when waste is stacked higher than 5 feet and equipment must up on the pile. Surcharge Load = 2 ft. * soil weight lb. = 2*soil wt., lb. Common value used = soil weight = 100 lbs

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Ecology block Equipment surcharge load

100

lbs. Range: 0 lbs. to 1000 lbs.

Figure 1 Overturning

Overturning Analysis about Point A Point A is located at the outside toe of the wall. If backfill is placed level behind the wall, Point A is at the top of the last block completely backfilled. Number of ecology blocks stacked in vertical wall

2

# blocks in wall

(above point A)

(H + F) in diagram Maximum depth of waste to be stored against retaining wall 4 FT (above point A)

If heavy equipment is operating on pile, Surcharge Moment for Overturning (assumed in effect - only when pile is greater than 5 feet high)

0

FT-LB

Resisting Moment = weight of wall * 0.5 * width 1184 FT-LB Overturning Moment = (1/6*density*(H+F)^3) + (Impact force*(H+F-2.5)) + Surcharge*(1/2(H+F)^2, if H+F>5) 993 FT-LB ►

Overturning Safety Analysis

FS =

1.192

Overturning Factor of Safety (FS) Check FS≥1.5, OK or Not OK

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Ecology block Not OK

► Sliding Analysis Relationship of the driving forces act on a structure in the direction of potential sliding compared to the resisting forces act on a structure opposite the reaction of potential sliding. Given: Sliding analysis considered for two cases: Case A (full bin) and Case B (empty bin) The factor of safety is the relationship between the resisting forces and the driving forces. NRCS TR 74 and IBC 2006, SLIDING factor of safety ≥ 1.5

FS 

∑ Re sistingForces ∑ DrivingForces

CASE A The bin area against the wall is full. Resisting and driving forces are calculated from the elevation of the last block fully backfilled on the slab surface. 1) Density of stored waste material retained by the wall in this analysis is based on if the manure solids are protected from precipitation, Value from Overturning Analysis Density of stored waste 65 pcf Assume Friction factor for concrete-to-concrete (f), in Portland Cement Association (PCA) and Precast/Prestressed Concrete Institute (PCI) Design Handbook for dry conditions is 0.80. PCA's Concrete Masonry Handbook gives a precast concrete-toconcrete masonry friction coefficient of 0.4 based on a safety factor of two. However, the typical design conditions for this analysis are assumed to be wet. Given the wet conditions for this analysis half of the dry condition friction factor will be used, f = 0.4. Figure 2 Sliding - Case A Bin Full

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Ecology block Resisting Forces = weight of wall * f 473.6 LB. Case A Sliding (Surcharge, S - only when pile is greater than 5 feet high) F.S. = 0.658 Driving Forces = (1/2*density*(H+F)^2)+S+Impact= 720.0 LB Not OK * If NOT OK Consider installing a 12" high x 8" wide curb or Backfill half way up the back side of the block. Sliding, Case B NRCS TR 74 and IBC 2006, SLIDING factor of safety ≥ 1.5

FS 

∑ Re sistingForces ∑ DrivingForces

Case B The bin area against the wall is empty. Backfill is placed behind the gravity block retaining wall. The backfill material is filled level to the height identified above. For this design analysis, values for the following elements must be confirmed: Backfill density Density characteristics for backfill can be specified, however a commonly acceptable backfill density is 120 lb/cu. Ft. 120 pcf Backfill surcharge A surcharge load is added to the backfill to account for equipment operating near the wall. A typical equipment surcharge is equal to an additional weight of 2 feet of earthfill material. Range: 200 lbs. to 1000 lbs. Surcharge load 200 lbs. Assume Friction factor for concrete-to-concrete, f = 0.4; (same as Case A, item 2 discussion above) Figure 3 Sliding - Case B Bin Empty

1) Total number of ecology blocks stacked vertically in the lock block wall, as selected above. 2 plus fully backfilled blocks:

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1

Ecology block 2) Select the depth to which the ecology block wall will be backfilled. Backfilled above toe of wall 1.25 Resisting Forces = Driving Forces=

weight of wall * f

(0.39*density*depth^2*1/2) + (0.39*S*depth^2*1/2) =

Portion of blocks backfilled

710.4

LB.

390.0

LB.

Case B Sliding F.S. = 1.822 OK

Summary Ecology Block Wall Design ►

Factor of Safety 1.192

Overturning

Not OK

► Sliding - Bin Full, Case A 0.658 Not OK If overturning FS is OK and FS for sliding from point A < 1.5, can install a 12 inch high x 8 inch wide curb or backfill half way up the back side of the block. ► Sliding - Bin Empty, Case B 1.822 OK

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Ultrablock

Ultrablock Wall

► Overturning Analysis

Legend

► Sliding Analysis Lock-Block Type Ultrablock

Calculated-locked

Cross-Section Dimensions Gravity lock block weight

Width, ft 2.5

Height, ft 2.5

Definition of Point 'A' for reference:

Select input-unlocked

Standard Block 4320 lbs for 5 ft block

Weight (lbs) per foot of standard block length 864 lb/ft of block

Point 'A' is the unsupported, outside base of the block wall, about which overturning can occur.

► Overturning Analysis Overturning moments will act on the structure in the direction of potential overturning. Resisting moments will act on a structure opposite the direction of potential overturning. The factor of safety is the relationship between the overturning and resisting moments.

FS 

 ResistingMom ents  OverturnMo ments

NRCS TR 74 and IBC 2006, OVERTURNING factor of safety ≥ 1.5 Given: Impact Load

Surcharge load

Density of the stored waste material

65

pcf

Equipment impact loads from a fork lift or a front loader will act on the wall moving stored waste material in and out of the bins. For design purposes, an equipment impact point load on the gravity block retaining wall face is assumed at 30 inches (2.5 feet) below the top of the maximum stack height. This input is a variable that can range from 20 lbs., for hand held equipment, to 1000 lbs. for larger mechanical equipment. If this is value is unknown, a fork lift impact load of 200 lbs acting horizontally on the wall section, is a reasonable assumption. It will be assumed that equipment operating on a stack higher than 5 ft. will exert a surcharge load in the vertical direction. This input variable can be based on the plan of operations. For design purposes, this surcharge load can range from 0 lbs., for no mechanical equipment, to 1000 lbs for large equipment. If this value is unknown, a reasonable surcharge load of 200 lbs. (equivalent of 2 ft. of compacted soil) is recommended.

For this design analysis, values for the following loads must be selected: Impact load The block wall will experience a horizontal impact load from equipment = X-direction Minimum value = 20 lbs Range: 20 lbs/ft to 1000 lbs/ft of wall Equipment impact load on wall 50 lbs/ft of block wall With point of impact on wall located at 2.5 ft from top of waste pile or at top of wall.

Surcharge load There will be equipment surcharge vertical point load on waste pile = Y-direction Used when waste is stacked higher than 5 feet and equipment must up on the pile. Surcharge Load = 2 ft. * soil weight lb. = 2*soil wt., lb. Common value used = soil weight = 100 lbs Equipment surcharge load 200 lbs

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Ultrablock Range: 0 lbs. to 1000 lbs.

Figure 1 Overturning

Overturning Analysis about Point A Point A is located at the outside toe of the wall. If backfill is placed level behind the wall, Point A is at the top of the last block completely backfilled. Number of ecology blocks stacked in vertical wall

2

# blocks in wall

(above point A)

(H + F) in diagram Maximum depth of waste to be stored against retaining wall 5 FT (above point A)

If heavy equipment is operating on pile, Surcharge Moment for Overturning (assumed in effect - only when pile is greater than 5 feet high)

0

FT-LB

Resisting Moment = weight of wall * 0.5 * width 2160 FT-LB Overturning Moment = (1/18*density*(H+F)^3) + (Impact force*(H+F-2.5)) + Surcharge*(1/2(H+F) if H+F>4) 576 FT-LB ►

Overturning Safety Analysis

FS =

3.750

Overturning Factor of Safety (FS) Check FS≥1.5, OK or Not OK

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Ultrablock OK

► Sliding Analysis Relationship of the driving forces act on a structure in the direction of potential sliding compared to the resisting forces act on a structure opposite the reaction of potential sliding. Given: Sliding analysis considered for two cases: Case A (full bin) and Case B (empty bin) The factor of safety is the relationship between the resisting forces and the driving forces. NRCS TR 74 and IBC 2006, SLIDING factor of safety ≥ 1.5

FS 

∑ Re sistingForces ∑ DrivingForces

CASE A The bin area against the wall is full. Resisting and driving forces are calculated from the elevation of the last block fully backfilled or the slab surface. 1) Density of stored waste material retained by the wall in this analysis is based on if it is protected from precipitation. (Value from Overturning Analysis above) Density of stored waste 65 pcf Assume

Friction factor for concrete-to-concrete (f), in Portland Cement Association (PCA) and Precast/Prestressed Concrete Institute (PCI) Design Handbook for dry conditions is 0.80. PCA's Concrete Masonry Handbook gives a precast concrete-to-concrete masonry friction coefficient of 0.4 based on a safety factor of two. However, the typical design conditions for this analysis are assumed to be wet. Given the wet conditions for this analysis half of the dry condition friction factor will be used, f = 0.4.

Figure 2 Sliding - Case A Bin Full

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Ultrablock Resisting Forces = weight of wall * f 691.2 LB. Case A Sliding (Surcharge, S - only when pile is greater than 5 feet high) F.S. = 0.801 Driving Forces = (1/2*density*(H+F)^2)+S+Impact= 862.5 LB Not OK * If NOT OK Consider installing a 12" high x 8" wide curb or Backfill half way up the back side of the block. Sliding, Case B NRCS TR 74 and IBC 2006, SLIDING factor of safety ≥ 1.5

FS 

∑ Re sistingForces ∑ DrivingForces

Case B The bin area against the wall is empty. Backfill is placed behind the gravity block retaining wall. The backfill material is filled level to the height identified above. For this design analysis, values for the following elements must be confirmed: Backfill density Density characteristics for backfill can be specified, however a commonly acceptable backfill density is 120 lb/cu. Ft. 120 pcf Backfill surcharge A surcharge load is added to the backfill to account for equipment operating near the wall. A typical equipment surcharge is equal to an additional weight of 2 feet of earthfill material. Range: 200 lbs. to 1000 lbs. Surcharge load 200 lbs. Assume Friction factor for concrete-to-concrete, f = 0.4; (same as Case A, item 2 discussion above) Figure 3 Sliding - Case B Bin Empty

1) Total number of Ultrablocks stacked vertically in the lock block wall, as selected above. 2 plus fully backfilled blocks:

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0

Ultrablock 2) Select the depth to which the ecology block wall will be backfilled. Backfilled above toe of wall 1 Resisting Forces = Driving Forces=

weight of wall * f

(0.39*density*depth^2*1/2) + (0.39*S*depth^2*1/2) =

Portion of blocks backfilled

691.2

LB.

390.0

LB.

Case B Sliding F.S. = 1.772 OK

Summary Ultrablock Wall Design ►

Overturning

Factor of Safety 3.750

OK

► Sliding - Bin Full, Case A 0.801 Not OK If overturning FS is OK and FS for sliding from point A < 1.5, can install a 12 inch high x 8 inch wide curb or backfill half way up the back side of the block. ► Sliding - Bin Empty, Case B 1.772 OK

Note: Check to make sure that the final design scenarios for overturning and sliding (full) and sliding (empty) all represent your design conditions.

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