Countermeasures to Protect Bridge Abutments from Scour (2007)

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24 4.1 Introduction To assess existing practices with regard to the use of abut- ment scour countermeasures, a survey form was sent to the department of transportation office of each state in the United States. Additionally, general information and experi- ence were obtained from certain other entities, such as county engineer offices, and overall experience in New Zealand, the home country of one of the co-investigators. The information obtained from the survey responses was used in determining the program of experiments to be car- ried out for this project. The survey sought the following information regarding abutment scour countermeasures: • The extent to which bridges are fitted with abutment scour countermeasures • Countermeasures that have been successful in terms of the ensuing considerations:  Technical effectiveness (including no substantial adverse effects)  Constructability  Durability and maintainability  Aesthetics and environmental issues  Cost • Method of countermeasure selection • Design method for selected countermeasures • Procedure for evaluation of countermeasure effectiveness • Countermeasure construction • Pertinent conditions at abutment failure sites:  Bed material  Abutment design  Flow • Additional information (e.g., on aesthetics of countermea- sure design) Of the 50 states contacted, responses were received from 36 states. An overall summary of countermeasure practice in New Zealand was developed by Dr. B. Melville, New Zealand’s leading authority on scour. The responses varied from a simple statement that scour was not a problem to complete sets of design guidelines used for certain scour countermeasures. 4.2 Summary of State DOT Responses The responses provided by the state DOTs to survey Ques- tions 1 through 8 are summarized in Tables 4-1 through 4-9. C H A P T E R 4 Practitioner Survey Table 4-1. Question 1. “Is bridge abutment scour a problem for your agency?” Response # of States Responding Name of States Responding Definitely 4 AZ, DE, FL, VT Occasionally 16 AK, AR, CO, CT, GA, ID, IL, KS, KY, MD, MA, MI, MO, OH, OK, VA No, due to successful countermeasures 13 AL, HI, IA, MN, MT, NV, NM, NY, NC, OR, PA, TN, TX No, due to favorable site conditions 3 LA, MS, RI

25 Countermeasure (none is also an option) Bridge Name Year Bridge Constructed Year Countermeasure Constructed # successful # unsuccessful MD - grout bags 48 5 MD - riprap 10 0 MD - sheeting 2 0 MI - riprap Most (65) MI - guidebanks 2 0 MI - articulating blocks 0 3 MT - riprap All but 1 1 Table 4-2. Question 2. “Please give information on the number of abut- ments where countermeasures (i.e., riprap, cable-tied blocks, guide- banks, spur dikes, vanes, or any method to reduce abutment scour) have been used (add additional pages as necessary).” Consideration Counter- Measure, Bridge Name, Year Constructed Environ- mental Effects (0=high negative effects; 10=high positive effects) Debris Problems (0=many problems; 10=no problems) Ice Problems (0=many problems; 10=no problems) Scour Problems (0=deep scour; 10=no scour) Construction Cost (0=high cost; 10=low cost) Maintenance Cost (0=high cost; 10=low cost) AL, riprap 10 10 na 9 9 AK, riprap 5 9 8 8 5 7 AR, riprap 5 9 na 9 8 8.5 CT, riprap 5 9 9 8 5 8 FL, riprap 5 10 10 4 4 GA, riprap 10 10 na 8 4 8 GA, spur dike 10 10 na 8 6 8 IL, riprap 10 10 10 8 10 7 IL, Reno mat 10 9 9 9 9 9 IL, gabions 10 9 9 9 9 9 IL, Fabri-form 10 10 10 7 8 8 KS, riprap 9 10 10 9 9 7 KS, guidebanks 9 10 10 9 4 10 KS, gabion 9 10 10 10 5 10 KS, sheet-pile 9 8 10 8 4 10 MA, pavement 0 10 10 10 0 10 MA, sheeting 10 10 10 2 8 9 MA, new construction 10 6 10 9 8 9 MD, riprap 5 10 10 5 8 10 MD, grout bags 10 10 10 5 10 10 MN, riprap 5 10 9 4 5 8 MT, ripap 6 10 9 8 na 10 NM, guidebank 9 9 10 8 6 9 NM, spur dike 8 7 10 7 7 9 NM, riprap 5 10 10 3 10 8 Table 4-3. Question 3. “Please give information on the extent of benefit achieved by provid- ing each countermeasure on the below listed six parameters (see rating scale for each effect).”

26 State Response AL Used to use pavement on slopes. Had failure. Now use riprap with good results. AK Mostly use riprap. Recently trying cable-tied blocks. Too early to comment on results. AZ Case by case consensus of DOT divisions. AR Structural needs or maintenance requirement. AR, CT Own classification system. DE FHWA “Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges.” Fix worst 6 bridges. FL Historical scour evidence, existing conditions, hydraulic and geotechnical software, experience and judgment. GA FHWA recommendation for riprap, HEC 20 for spur dike. IL, IA Use adequate waterway openings, spur dikes and guidebanks when significant overbank flow. 2.5:1 slope. Only riprap if unstable. KS Scour analysis, determine depths, rock riprap with toe. Guidebanks if high return flow. LA Use spill-through shape with flexible revetment and riprap. MD Hydraulic, geotechnical analysis to determine vulnerability. Get flow velocities. Determine construction method. Consider grout bags as an alternative to riprap. Get cost. MA Use existing foundation to maintain waterway opening and minimize environmental impact. Build new abutments behind existing ones. MI HEC 18. MN Screening and evaluation: field review, historical info, cal velocity. MS Experience. MT Use HEC RAS and determine abutment scour for Q-design, Q-100, and Q-overtopping. NV Past history, determine erodibility of existing abutment protection, estimate scour of abutment using pier scour equations assuming river migrates through the foundation and the abutment acts as a pier. NM HEC 18, 20, 23. Inspection reports, field inspection. NY HEC 18, 20, 23. OR Always use riprap. TN Place riprap on all abutment fill slopes at stream crossings. We take revetment up to 1 foot above the design flood. Sometimes use gabions on steep slopes. Use filter fabric under gabions and riprap. Use H47 and H49 for hydraulic analysis. VT Evaluation by team consisting of Structure Design/Inspection, Hydraulics, and Materials. VA HEC 18. Table 4-4. Question 4. “What method do you use to select if a counter- measure is needed?”

27 State Response AK Velocity-based analysis. AZ HEC 23 and team consensus. AR HEC 18 and 20, site history. CO HEC 18 and 23. CT HEC 23. DE USGS Report # 95-4153, HEC 18, WSPRO. FL Incipient motion analysis. GA FHWA recommendation for riprap, HEC 20 for spur dike. IL HEC 23. KS HEC 23 for riprap, guidebanks, and gabions. Classic methods for sheet pile. LA HEC 18. MA HEC 18. MD Has own standard guidelines. MI HEC 18 for riprap. MN Use Class III Random Riprap or greater. Check velocity. Use spur dike if necessary. Use spill through abutment. Set abutment back from main channel. Use HEC 18 and 23. MS Experience. MT Typical section with 2:1 slopes, a riprap key, and the thickness will depend on the size rock used. Construction fabric added underneath the riprap to minimize loss of fine material. NC Spill-through slope. Old bridges on unstable rock have been replaced. NM HEC 18, 20, 23. NV HEC 23, riprap HEC 11. Abutments coincide with banks. Lots of river migration. Solved by river training with riprap or spur dikes. Design abutment as if it were a pier using HEC 18. Banks protected with riprap (HEC 11). NY HEC 23, standard policy or state policy. OH Size riprap according to velocity using HEC publications. OK HEC 11, 14, and 23. OR HEC 18 and 11. PA State DOT design manual. TN HEC 18. VT Place riprap until area stabilizes. VA HEC 23. Table 4-5. Question 5. “What design method do you use for each countermeasure?” State Response AK Biennial inspection. AR Regular bridge inspection and maintenance district. AZ Regular bridge inspection and maintenance district. CO HEC 18 and 23. CT Biennial inspection. DE Always use riprap. FL Choose two most appropriate alternatives, give to DOT reviewing engineering departments. GA Riprap: two times the depth of the 100-year storm adjacent to the abutment. Spur dike: length is based on natural overbank flow adjacent to abutment. Length must be greater than 150 ft. IL Visual observation. KS 2-year inspections. MD Biennial inspections. MI Biennial inspections. MS Experience. MT HEC 18. NM Biennial inspection. NV Biennial inspection. NY Field inspection. PA Procedures for Scour Assessments at Bridges in Pennsylvania. VT Inspection team. Table 4-6. Question 6. “What evaluation procedure do you use for each countermeasure?”

28 State Response AR Spurs use earth-moving equipment, riprap from barge, temporary work platforms, or from shore. CT Dump riprap from barges (large rivers), pump area dry (small rivers). DE Excavate, cofferdam, geotextile, place stone, key riprap into channel. FL Floating platform barges, filter fabric. GA Riprap: filter fabric first, then dump riprap. Spur dike: earth-moving equipment. Hand place filter fabric, then dump stone. KS Kansas Standard Specs for State Road and Bridge Construction. MD Riprap: earth-moving equipment. Grout bag: fill bags on site. MT Backhoe or crane to place rock. Placed during low flow. NV River diversion, dewatering. OH Place rock by land or barge. TN Dozer or track excavator. Rock is countersunk. VA Riprap dumped directly on fill slope and excavate as necessary around toe to preclude riprap reducing the flow across the sectional area. Table 4-7. Question 7. “What are the construction methods used for each countermeasure?” 5 Worst Scour Sites Abutment Characteristics ARKANSAS 1 2 3 4 5 Name (Bridge Number) AB5109 01432 02549 02819 Location (Nearest Town) Mulberry Malvern Texarkana Marble Scour depth (ft) 10 7 6 18 % gravel 40 (w/ boulders) 0 Below road fill is 100% gravel & boulders % sand 80 % silt % clay 60 20 100 d90 d50 Est. 0.25” Est. 0.02” 0.00004” Est. Est. 6” d30 Bed Material Bedload (high, med., low) Med. Low Low Low Setback from riverbed (ft) 200 30 0 0 On floodplain? Yes Yes No No Abutment slope (V:H) 1:2 Vertical 1:1.5 1:1.5 Abutment height (ft) 25 16 13 17.5 Abutment length (ft) 1,000 1,300 500 36 Span between abutment and nearest pier or opposite abutment (ft) 33 from toe of slope 35 6 from toe of slope 18 Abutment plan shape See drawing See drawing See drawing See drawing Straight approach? No Yes No Yes On a river bend? Yes No Yes No Abutment Properties Near tributary? No No No No Velocity range (ft/s) Unknown 4.1 13.4 12 avg. Discharge range (ft3/s) Unknown 7,000 7,480 3,600 approx. Flow depth range (ft) 15 13.1 14 17 Max. scour depth (ft) 12 7 6 18 Upstream channel width (ft) 70 20 30 45 Channel width under bridge (ft) 410 68 72 20 Downstream channel width (ft) 70 20 30 45 Ice problems? NA NA NA NA Debris problems? Yes No No Yes, major Flow Parameters Environmental problems? No No No No Table 4-8. Question 8. “Please summarize the conditions at the 5 worst abutment scour sites (please send drawings, if possible).”

29 5 Worst Scour Sites Abutment Characteristics CONNECTICUT 1 2 3 4 5 Name BN02781 BN 01048 BN01383 BN05419 BN08014R Location Stinington Oxford Haddam Sherman Stamford Scour depth (ft) 6 4 0-3 2 4-5 % gravel 50 10 10 % sand 50 75 % silt 15 % clay d90 58 mm 10 mm d50 38 mm 3 mm 1.6 mm d30 0.7 mm 0.75 mm Bed Material Bedload (high, med., low) Med. Med. High Med. Med. Setback from riverbed (ft) 1 0 0 0 0 On floodplain? No Yes No No No Abutment slope Vertical Vertical Vertical Vertical Vertical Abutment height (ft) 8.5 5-6 21 10 9.5 Abutment length (ft) 43 32 87 50 98 Span between abutment and nearest pier or opposite abutment (ft) 22 21 71.5 20 26 Abutment plan shape Skewed flared wings Flared wings Flared wings Wings ext. of abut. Flared wings Straight approach? No No No No No On a river bend? Yes Yes Yes Yes Yes Abutment Properties Near tributary? No No Yes Yes No Velocity range (ft/s) 4.9-7.7 4.5-5.1 9.8-15.9 10.3-15.0 Discharge range (ft3/s) 1,086 (10 yr) 3,631 (500 yr) 950 (10 yr) 3,800 (500 yr) 6,924 (10 yr) 2,587 (500 yr) 390 (10 yr) 1,720 (500 yr) 1,180 (10 yr) 3,200 (500 yr) Flow depth range (ft) 6.7-14.4 4.9 9.4-19.8 2.4-8.0 10-22 Max. scour depth (ft) 19.8 17 34.7 10 Upstream channel width (ft) 20 10-20 80-90 18-20 28-30 Channel width under bridge (ft) 22 40 150 13.7 26 Downstream channel width (ft) 20 30 130 22 28-30 Ice problems? No No Yes No No Debris problems? No No No No No Flow Parameters Environmental problems? Yes Yes Yes Yes Yes Table 4-8. (Continued) (continued on next page)

30 5 Worst Scour Sites Abutment Characteristics FLORIDA 1 2 3 4 5 Name 070026 Location Scour depth (ft) % gravel % sand % silt % clay d90 d50 d30 Bed Material Bedload (high, med., low) Abutment scour was not calculated due to low projected velocities, nor were geotechnical data obtained. Setback from riverbed (ft) 10 On floodplain? Yes Abutment slope (V:H) 2:1 Abutment height (ft) 20 Abutment length (ft) 50 Span between abutment and nearest pier or opposite abutment (ft) 15 Abutment plan shape Spill- through Straight approach? No On a river bend? Yes Abutment Properties Near tributary? Yes Velocity range (ft/s) 3-5 Discharge range (ft3/s) 1,000 Flow depth range (ft) 5-7 Max. scour depth (ft) 7-8 Upstream channel width (ft) 60 Channel width under bridge (ft) 50 Downstream channel width (ft) 60 Ice problems? No Debris problems? Yes Flow Parameters Environmental problems? No Table 4-8. (Continued)

31 5 Worst Scour Sites Abutment Characteristics GEORGIA 1 2 3 4 5 Name SR 38 SR 38 Location Long. Co Lowndes Co. Scour depth (ft) 30 25 % gravel 0 0 % sand 80 80 % silt 10 10 % clay 10 10 d90 d50 d30 Bed Material Bedload (high, med., low) Low Low Setback from riverbed (ft) Yes Yes On floodplain? Yes Yes Abutment slope (V:H) 2:1 2:1 Abutment height (ft) 20 15 Abutment length (ft) 40 30 Span between abutment and nearest pier or opposite abutment (ft) 33 38 Abutment plan shape Spill- through Spill- through Straight approach? Yes Yes On a river bend? No Yes Abutment Properties Near tributary? No No Velocity range (ft/s) 3.5-3.75 Discharge range (ft3/s) 40,000- 50,000 Flow depth range (ft) 40 30 Max. scour depth (ft) 25 20 Upstream channel width (ft) Channel width under bridge (ft) 1,089 920 Downstream channel width (ft) Ice problems? No No Debris problems? No No Flow Parameters Environmental problems? No No Table 4-8. (Continued) (continued on next page)

32 5 Worst Scour Sites Abutment Characteristics MICHIGAN 1 2 3 4 5 Name B01 B03/04 Location 81,032 61,075 Scour depth (ft) 4 % gravel % sand 90 100 % silt % clay d90 d50 0.0005 d30 Bed Material Bedload (high, med., low) Med. Low Setback from riverbed (ft) 0 On floodplain? Yes Abutment slope (V:H) Arch Vertical Abutment height (ft) 2.5 Abutment length (ft) 85 70 Span between abutment and nearest pier or opposite abutment (ft) 117 72 Abutment plan shape Rectangular Straight approach? No No On a river bend? Yes Yes Abutment Properties Near tributary? No Yes Velocity range (ft/s) 6.9-9.2 3.5-5.8 Discharge range (ft3/s) 7,100- 11,000 14,525- 29,208 Flow depth range (ft) 17.5-20 22.6-25.5 Max. scour depth (ft) 26 26 Upstream channel width (ft) 130 300 Channel width under bridge (ft) 117 324 Downstream channel width (ft) 110 300 Ice problems? No No Debris problems? No No Flow Parameters Environmental problems? No No Table 4-8. (Continued)

33 5 Worst Scour Sites Abutment Characteristics MINNESOTA 1 2 3 4 5 Name 5236 87007 87015 Location TH 212 - Lac Qui Parle R. TH 23 - Minnesota R. Overflow TH 212 - Minnesota R. Overflow Scour depth (ft) 4 observed 7 observed 7 observed % gravel 10 NA NA % sand 70 NA NA % silt 20 NA NA % clay - NA NA d90 2 mm NA NA d50 300 um NA NA d30 150 um NA NA Bed Material Bedload (high, med., low) NA NA NA Setback from riverbed (ft) 0 0/8 0 On floodplain? No No No Abutment slope (V:H) 1:2 1:2 1:2 Abutment height (ft) 21 9 13 Abutment length (ft) 54 107 52 Span between abutment and nearest pier or opposite abutment (ft) 31 40 117 Abutment plan shape Spill-through Spill-through Spill-through Straight approach? No Yes No On a river bend? Yes No Yes Abutment Properties Near tributary? No No No Velocity range (ft/s) 6 9 16 Discharge range (ft3/s) 5,5002 15,0003 15,0003 Flow depth range (ft) 13.3 16 10 Max. scour depth (ft) 17 pier 25 abut NA Upstream channel width (ft) 80 65 70 Channel width under bridge (ft) 90 55 100 Downstream channel width (ft) 95 80 180 Ice problems? No No No Debris problems? No No No Flow Parameters 2 design or approximate 100 yr event 3 500 yr Environmental problems? No No No Table 4-8. (Continued) (continued on next page)

34 5 Worst Scour Sites Abutment Characteristics NEW MEXICO 1 2 3 4 5 Name 6479 8996 8979 5714 Location Cuba Clayton US 285 Pecos River Scour depth (ft) None so far None so far Aggreda- tion, 1’ None so far % gravel % sand % silt % clay d90 d50 d30 Bed Material Bedload (high, med., low) Low Low Low Low Setback from riverbed (ft) No No No 100 On floodplain? No Yes Yes Yes Abutment slope (V:H) 1:1.5 1:1.5 1.5:1 1.5:1 Abutment height (ft) 9 9.75 20 20 Abutment length (ft) 45 43 200 100 Span between abutment and nearest pier or opposite abutment (ft) 35 99 80 40 Abutment plan shape Straight approach? No No Yes Yes On a river bend? Yes Yes Yes No Abutment Properties Near tributary? No No Yes No Velocity range (ft/s) 12 10 8.7 5-10 Discharge range (ft3/s) 5,000 13,500 77,200 54,000 Flow depth range (ft) 8 16 16.1 24 Max. scour depth (ft) 9 calc. 9.1 calc. Aggreda- tion 30’ calc. Upstream channel width (ft) 150 82 400 80 Channel width under bridge (ft) 120 150 400 80 Downstream channel width (ft) 150 78 200 80 Ice problems? No No No No Debris problems? Silt Silt Silt and Debris No Flow Parameters Environmental problems? No No No No Table 4-8. (Continued)

35 5 Worst Scour Sites Abutment Characteristics OKLAHOMA 1 2 3 4 5 Name East Clay Creek/US 64 Location Alfalfa Co. Scour depth (ft) 82 % gravel % sand % silt % clay d90 d50 0.002 d30 Bed Material Bedload (high, med., low) Setback from riverbed (ft) 10 On floodplain? Abutment slope (V:H) Vertical Abutment height (ft) 3 Abutment length (ft) 24 Span between abutment and nearest pier or opposite abutment (ft) 25 Abutment plan shape 7 concrete piles 25’ long Straight approach? Yes On a river bend? Close Abutment Properties Near tributary? No Velocity range (ft/s) 10-24 Discharge range (ft3/s) 1,060- 17,370 Flow depth range (ft) 2-25 Max. scour depth (ft) 82 Upstream channel width (ft) 100 Channel width under bridge (ft) 26 Downstream channel width (ft) 80 Ice problems? No Debris problems? No Flow Parameters Environmental problems? No Table 4-8. (Continued)

36 State Response AK Failure consists of embankment loss due to toe loss. AZ Abutment failure can occur in nonwaterway bridges due to deck drainage, nuisance water, or roadway drainage. 246/850 waterway bridges have protection: 128 concrete slope, 23 soil cement, 190 rail bank protection, 9 grouted rock, 68 riprap, 30 gabions, 124 combination of countermeasures, 51 other types. DE There already exist sufficient design guidelines for riprap. FL Abutment scour only a problem when no abutment protection. HI Abutment scour not a problem; use riprap if it is. IL Abutment scour not a problem; use riprap if it is. KS Meandering of streams is the biggest problem. Need lab testing on cohesive soil and develop more reliable scour equations. MA Channel relocation a problem. Bends cause problems. MD Grout bags good for gradual slopes, silt, or small stones. They save money and are better for environment. Less construction time and equipment. Can install in low-access areas. Need study to determine how to reduce grout leaching out of bags and optimal design patterns and sizes of grout bags for various conditions. MN Spur dikes and riprap work well. We do not have significant flooding. Need guidance on riprap on bends, contraction scour. Difficult to distinguish between various types of scour in the field. MO Most damage to roadways with inadequate overflow capacity. Riprap used primarily. Less paperwork involved in getting permission from the COE [U.S. Army Corps of Engineers] and DNR [Department of Natural Resources] . Some use of gabions and dumping old concrete slabs as toe protection. Grouting or concreting the rock blanket has NOT been successful. MT Only scour problems occurred during the 64 floods (500 yr). Need better method for determining abutment scour using HEC RAS. OH Riprap protection is environmentally unfriendly initially until vegetation is established. Extent can be large. Use riprap for river training. Need study on riprap armoring. OR Have no confidence in scour-predicting equations in HEC 18. PA Use riprap mostly. TX No fixed strategy due to wide variation of site conditions. Hydraulics branch has been encouraging use of flexible riprap. Use TxDOT Hydraulics Manual. Table 4-9. Question 9. “Please give any additional information that you feel the NCHRP 24-18 research team should know when deciding which countermeasures to further study and develop design guidelines for.” 4.3 Summary of Responses The responses obtained from the survey of the state depart- ment of transportation offices can be summarized as follows: • Most states either have a moderate scour problem or would have one if they did not use scour countermeasures. • There are relatively few states that have severe scour prob- lems and an approximately equal number of states that have site conditions that are not conducive to scour. • Riprap seemed to be used often and only failed in one reported case. • Grout bags were used extensively by one state and worked most of the time, but there were some noted failures. • Sheeting and pavement were reported to cause scour and environmental problems, but were not used very often. • Riprap seemed to perform well in general, but was reported to have environmental problems. • A wide variety of countermeasure selection methods were reported, but HEC 18 and 20 were the most cited. • Countermeasure design was performed using the HEC reports primarily. • Visual inspection was the method most often used for eval- uation of existing countermeasures. • Construction methods include heavy equipment, includ- ing barges sometimes. In addition, the main points arising from county engineers consulted can be summarized as follows: • County bridges are inspected usually once every 2 years. The inspection teams vary greatly in their hydraulic engi- neering expertise. • The county engineers are concerned primarily with pro- tecting comparatively small bridges against scour. Such bridges typically have one to three spans and are sited at channels draining watersheds smaller than about 100 square miles. • Wing-wall abutments are usual for small bridges over channels that do not have flood plains. • Spill-through abutments are used for larger bridges over channels that have substantial floodplains. • The countermeasure concepts of greatest relevance for small bridges are simple armoring countermeasures. In this respect, riprap or comparable armor elements are of inter- est. In some locations, suitable rock for armor unit use is not available, and so there is interest in alternative armor concepts.

37 • Countermeasure structures placed in the waterway or that rely on approach-flow alignment are generally not desired. The main points arising from bridge scour experience in New Zealand are as follows: • Bridge waterways are inspected regularly. • The monograph “Bridge Scour” by Melville and Coleman (2000) is a good example of standard practice in New Zealand. • Several bridge authorities in New Zealand use a national code of practice for estimating abutment scour and scour countermeasure needs: “Code of Practice for the Design of Bridge Waterways,” M.W.D. Civil Division Publication 705/C, 1979. More recently, several agencies have updated their design methods in line with the work of Dr. S.T. Maynord et al. (1989). • Riprap is used extensively because it is generally readily available and very effective. A majority of new bridges fea- ture riprap protection to foundations and approaches. Riprap is also used to protect “problem” bridges. Other types of armor protection (e.g., gabions) have been used successfully, but are much less common than riprap. • Riprap rock usually is placed by machine. End dumping is not normally permitted. Handwork, where necessary, is required to obtain a satisfactory nesting, with each rock having a three-point bearing. Filters are used under riprap, where the natural ground contains a high proportion of fine sediments.