Types of Inspections

Transcription

1 BIA Summer Water Resources Training Dam Safety Inspections Part 4 - Inspections July 28, 2012 Presented by Michael Johnson, Ph.D., P.E. Types of Inspections Periodic Inspections A comprehensive visual onsite inspection is conducted. Not all of the mechanical equipment may not be tested. Routine Inspection Typically conducted by field or operating personnel. Focus is on the current conditions of the dam and its features. Data may not be reviewed and evaluated. Special Inspection Conducted when only a particular feature of a dam is to be inspected.

2 Types of Inspections Dam Safety Oversight Inspections Frequency contingent on hazard class, annual, triennial and once every five years. Following Unusual or Stressful Events Conducted immediately following severe storm events, overtopping, earthquakes, etc. Typical Inspection Equipment

3 Prior to Inspection Review As-built Drawings Correspondence Files Associated Data and Reports Operations & Maintenance Plans Inspection Plans Contact Owner to Schedule Visit Embankment Dams

4 Crest Settlements, slides, depressions Misalignment Longitudinal/Transverse Cracking Animal Burrows Adverse Vegetation Erosion Sighting the Crest

5 Transverse Cracking Indicates differential settlement within the embankment or underlying foundation. Longitudinal Cracking Indication of uneven settlement between adjacent embankment zones of differing compressibility.

6 Desiccation Cracking Caused by the drying out and shrinking of certain types of embankment soils. Develop in a random, honeycomb pattern. Typically occurs in the crest and the downstream slope. Cracking and Jugholes

7 Inspection Tips - Cracks Photograph and record the location, depth, length, width, and offset of each crack observed. Monitor the crack for changes. A useful monitoring tool is to establish a reference line using stakes. Side Slopes Inspection Methods Zigzag Back and Forth

8 Upstream & Downstream Slopes Erosion Inadequate ground cover Adverse vegetation Longitudinal/Transverse Cracking Inadequate Riprap Stone Deterioration Settlements, Slides, Depressions, Bulges Animal Burrows Soft Spots or Boggy Areas (Downstream) Instability can indicate: Instability Overly steep slope aggravated by a rapid lowering of the reservoir. Can indicate an overly steep slope.

9 Instability Deep Seated Slides: Well Defined Scarp Toe Bulge Arc Shaped Cracks Inspection Tips - Slides Photograph and record the location of the slide. Measure and record the extent and displacement of the slide. Look for any surrounding cracks, especially uphill from the slide. Probe the entire area to determine the depth and extent of the slide. Determine if there are seepage areas near or in the slide. Monitor the area to determine if the condition is becoming worse. A useful monitoring tool is to establish a reference line using stakes.

10 Inspection Tip - Slides In most instances, deep-seated slides will require the lowering or draining of the reservoir to prevent the possible breaching of the dam. The owner must be notified. Slope Instability and Damaged Outlet Works

11 Depressions Wave action against the upstream slope that removes embankment fines or bedding from beneath riprap may form a depression as the riprap settles into the vacated space. Localized settlement in the embankment due to poor compaction or foundation due to compressible materials. Loss of sub-surface material through the decay of vegetative matter, or through internal erosion or piping. Inspection Tip - Depressions Although depressions, in most cases, do not represent an immediate danger to the dam, they may be early indicators of more serious problems. If you observe a depression: Photograph and record the location, size, and depth of the depression. Probe the floor of the depression to determine whether or not there is an underlying void. An underlying void is indicative of a sinkhole. Frequently observe the depression to ensure it has stopped developing.

12 Erosion Inspection Tips - Erosion Make sure that the slope protection is adequate to prevent erosion. Look for signs of wave action erosion and degradation of the slope protection. Make sure you check the low points along the upstream and downstream shoulders and groins since surface runoff can concentrate in these areas. Check for any unique problems, such as people, animal, or recreational vehicle traffic, that may be contributing to erosion.

13 Deep-rooted Vegetation Large trees could be blown over and uprooted during a storm. Root systems could penetrate into the dam's cross section. Root systems can pose a threat by providing seepage paths. Animal Burrows The following animals can cause destruction to embankment dams: Muskrats Beavers Groundhogs Ground Squirrels Prairie Dogs Badgers Gophers

14 Abutments Erosion Differential Movement Cracks Settlement, Slides, Depressions, Bulges Seepage Animal Burrows Seepage/Piping Control Design Features Internal Drains Flowing Seepage at or Beyond toe Does it contain fines? Evidence of sand boils at or beyond toe

15 What is Seepage? Passage of water through the embankment and foundation materials. Problems due to excessive seepage: Instability Piping Internal Erosion Internal Drains Designed to intercept and discharge it safely

16 What is Piping and Sand Boils? Sand Boil and Sand Bags

17 Inspection Tip Piping and Sand Boils Record the reservoir's water surface elevation, and approximate elevation. Photograph and record the size of any deposition cone. Measure or estimate the flow rate. Make sure that it is evaluated by a qualified engineer so that appropriate remedial action, if necessary, can be taken. Internal Erosion

18 Internal Erosion Inspection Tips - Seepage Drop a dirt clod in a container of water. If the water rapidly becomes cloudy, the soil is dispersive. Look for evidence of seepage along the toe, around appurtenant structures, near embankment-abutment contacts, and at transverse cracks. Look for subtle vegetation changes.

19 Change in vegetation pattern or type of vegetation can indicate seepage Monitoring Seepage Location and quantity or flow rate Recent precipitation that may affect the appearance and quantity of seepage. Temperature of the discharge as compared to the reservoir. Level of the reservoir at the time of the observation. Turbidity of the discharge.

20 Monitoring Devices for Seepage Weirs & Flumes Piezometers Drains River Reservoir Dam Example

21 Principal Outlets Outlets release water impounded by the dam. Typical purposes: Normal release for irrigation, power generation, municipal, and industrial Stream bypass for min. release requirements Reservoir drainage. Principal Outlet Components

22 Principal Outlet Approach Channels/Inlet Structures Seepage into the structure Debris or obstructions Concrete Spalling/Scaling Cracking Erosion Exposed reinforcement Metal Corrosion Coating deficient Misalignment Joints Displacement/offset Loss of Material Leakage Erosion or backcutting Broken or Bent Corroded or rusted Obstructed Trashracks

23 Operator, Gates, and Valves Broken or Bent Corroded or Rusted Leaking Not seated Properly Not Operational or Maintained Principal Outlet Conduits Seepage Debris Concrete Spalling/Scaling Cracking Erosion Exposed reinforcement Metal Corrosion Coating Deficient Misalignment or split seams Joints Displacement/offset Loss of material Leakage

24 Stilling Basins/Energy Dissipaters Concrete Spalling/Scaling Cracking Erosion Exposed Reinforcement Joints Displacement/offset Loss of material Leakage Energy Dissipaters Signs of deterioration Covered with debris Signs of inadequacy Principal Outlet Channels Eroding or Backcutting Sloughing Obstructions or restrictions Poorly riprapped Tailwater elevation and flow condition

25 Inspection Tip Principal Outlets Do not enter pipes Confined Space Video Inspect Use Inspection Divers who are qualified to inspect and dive safely Be mindful of critters! Ponding of water in certain reaches of the pipe may indicate settlement-related problems. Inspection Tips Principal Outlet Cavitation damage downstream from gates and valves, and at sharp bend, joints, and discontinuities Cracking or buckling is a sign of stress Deformation of conduit shape Efflorescence or gel on concrete Erosion is areas of high velocity

26 Emergency Spillways Pass water in a manner to protect the structural integrity of the dam. Designed to carry the maximum flood event the dam will withstand. Emergency Spillway Components

27 Emergency Spillway Entrance Channel Seepage into the structure Debris/obstructions Concrete Spalling/Scaling Cracking Erosion Exposed reinforcement Metal Corrosion Coating deficient Misalignment Joints Displacement/offset Loss of Material Leakage Erosion/backcutting Concrete Emergency Spillway Control Section Spalling/Scaling Cracking Erosion Exposed Reinforcement Displacement/Offset Loss of Joint Material Leakage Unlined Erosion Sloughing

28 Emergency Spillway Discharge Channel Obstructions/Restrictions Concrete Spalling/ Scaling Cracking Erosion Exposed reinforcement Joints Displacement/offset Loss of Material Leakage Unlined Erosion/backcutting Emergency Spillway Stilling Basin/Energy Dissipaters Concrete Spalling/Scaling Cracking Erosion Exposed Reinforcement Joints Displacement/offset Loss of material Leakage Energy Dissipaters Signs of deterioration Covered with debris Signs of inadequacy

29 Emergency Spillway Outlet Channel Eroding or Backcutting Sloughing Obstructions or Restrictions Reservoir High Water Marks Erosion/Slides into Pool Area Sediment Accumulation Floating Debris Present Depressions, sinkholes or vortices Low ridges/saddle allowing overflow Structures below dam crest elevation

30 Instrumentation Repair or Replacement Date of the last monitoring report Instrumentation Differences between on-site and remote data collection What to observe and document? Reservoir level, outlet/ spillway flows: If operating, are the outlets and spillway functioning as expected? Visual clarity and amount of flow from internal drains Vertical crest settlement and slope movements of embankment dams Vertical and horizontal surveys of concrete dams

31 Keeping Records Importance? Accurate records can help in the evaluation of the performance and condition of the structure over time. Keeping Records Inspection Checklist: Convenient to compile inspection observations. Field Sketch: Sketch observed conditions. A copy of the plan view of the dam, usually included as part of the construction drawings for the dam, can be a good base. Photographs: Photographs provide a pictorial history. Annotate and date photos. Monitoring Data: Collect seepage rates, spillway discharge rates, settlement, and for some dams, readings from instruments such as piezometers, crack gauges, and survey monuments. Record reservoir level.

32 How to Document S Sketch if a photograph would not capture important aspects. I Investigate the extent of the deficiency. M Measure and record the dimensions of the deficiency. P Photograph the deficiency and describe its characteristics in your notes. L Locate the deficiency in relation to some standard reference point. E Engage a qualified engineer if the deficiency could pose a threat to the safety of the dam. Questions?