Sinkhole presentation

transcript

INTRODUCTIONSTODAY’S PRESENTERS

CHRIS KOTCH, P.G. ALEX ULMER, P.G.

A SINKHOLE

A SINKHOLE?

A SINKHOLE??

YEAH, A SINKHOLE.

Limestone

A SINKHOLE!

A SINKHOLE!!

A SINKHOLE!!!

THE ROOT CAUSE OF SINKHOLES

CARBONATE BEDROCK

THE FORMATION OF SINKHOLES

INVESTIGATION

What is the Purpose of an Investigation ?• Due diligence prior to Site Acquisition• Due diligence / satisfy Ordinance prior to

Construction• Determination of Causation• To obtain data to be used in Repair Design• Investigation of sinkhole Development Trends

Due DiligenceWould you want to know the condition of this

detention basin before purchase ?

Due Diligence

Would You Want to Know the Cause ?

Assessing causation and obtaining data to be used in repair design

Investigation of Karst Hazards prior to Site Development

Do You See A Trend ?

Do You See A Trend

INVESTIGATIONWhat are the Components of an Investigation ?• Review of site geology• Conduct site walk site to observe physical

conditions• Review of site history• Conduct intrusive investigations

Geophysical Survey Soil Borings / Probes Excavation Drilling

GEOLOGY

• Review geologic maps• Review historic sinkhole mapping• Conduct a geophysical survey• Conduct soil probes/borings

Geologic Map and Topographic Map

Open File Historic Sinkhole Map

3-D ERI Survey

Microgravity Survey

Comparison of Geophysical Surveys

3-D ERI Survey Microgravity Survey

Investigation of Karst Hazards prior to Site Development

Investigation of Karst Hazards via Geophysical Survey

Investigate - Take A Look Around !!!

What’s behind the bush ? Sinkhole ?

Investigate

Look for Sources of Water During Investigation

• Storm water runoff in natural features• Storm water runoff over/into man-made

features• Utilities• Grading

Investigate

History ReviewWhat Would Be Important About Site History ?• Has there been sinkhole activity history?• Were prior sinkhole repairs completed ?• Has any construction/site work been

completed recently ?• What was the chronology of events ?• Was there historic mining in the area ?

Surface Subsidence Repair or Deeper Soils Issue ?

How long has subsidence been active ?

Was there a history of settlement or any repairs ?

Surface Subsidence Repair or Deeper Soils Issue ?

What did geology, site history, and site conditions tell you ?

Any Prior Sinkhole History/Repair ?

What was the Chronology of Events ?Did sinkhole occur after site work?

Recent site work / repair ?

Storm water influences ?

Is all settlement sinkhole related ?

Where does the Storm Water Drain?

Where Does the Storm Water Drain?

Where Does Storm Water Drain ?

• Mitigation practices are retrofits of proper design

• BMP’s should be used during both mitigation & construction

• Showing the proper design on the plans leads the contractor to better results

Mitigation, BMPs & Design

Excavation is typically one of the final steps in the investigation of a sinkhole

and the first step in the mitigation.

Excavation:• Removes loose soils•Provides a 1st hand view of the collapse•Permits channels to be traces•May expose the throat or bedrock

Water can aid in identifying cracks and channels

Once excavation is completed backfill can begin.

• Native soils typically have enough clay content to be used in the repair.

• Use of bentonite clay will decrease the permeability of the soil limiting water movement.

• Flowable fill may be appropriate, too.

Pressure Grouting (Compaction, Permeation, Consolidation, and Penetration Grouting)

• Improves bearing capacity of soil• Fills voids• Stabilized and supports foundations• Seals channels within the soil or rock

Mitigation, BMPs & DesignUtilities are frequently the source of water that can cause sinkholes. Proper design and construction can minimize those impacts.

Bedding

Compaction

Clay Dikes

Piping

Storm Water

Drinking Water

Sewers

Irrigation

Electrical

•Following BMPs during construction limits the amount of water encountering carbonate bedrock.

•The BMPs are mostly just plain common sense.

Mitigation, BMPs & Design• Provide positive drainage away from building areas

and exposed rock at all times during construction.

• Avoid water ponding or the concentration of surface flow, which could result in surface erosion and allow runoff to enter the sub-grade.

• Excavations, such as those for footings and utilities, should be covered as soon as possible after

exposure. No more trenching should be opened than can be backfilled that same day.

• Backfill should be placed in layers and compacted and should be no more permeable than the adjacent soils.

• Where rock is encountered, excavation methods other

than blasting are favored. Blasting should be performed by a contractor experienced with localized, controlled blasting to minimize rock disturbance away from the blast area.

• Exterior finish grades should provide for positive drainage of run-off away from the building areas.

• Avoid deep cuts, especially in the building areas. Where possible, keep ground floor levels at or above existing grades. Where basements are considered, poured

concrete walls should be considered.

• The area immediately adjacent to building walls should be impermeable to reduce or prevent infiltration of runoff into foundation areas.

• Underground piping should be leak-proof (gasketed joints, etc.). Use somewhat conservative design to allow flexibility, particularly at building entrances. Leakage can be a prime source of origin of post-construction sinkhole activity.

• Roof drainage should be piped and discharged into the storm system.

• Underground piping should be properly bedded. Where non-uniform rock conditions, e.g. pinnacles, boulders, etc., are encountered at the bedding elevation, the rock should be over excavated at least 1 ft. and replaced with compacted soil.

Resources• Geologic Mapping:

http://www.dcnr.state.pa.us/topogeo/gismaps/geomaps.aspx

• Historic Sinkhole and Karst Related Features:

http://www.dcnr.state.pa.us/topogeo/pub/openfile.aspx

• Aerial Photography:

http://www.pennpilot.psu.edu/

Resources• Sinkhole Index:

http://www.dcnr.state.pa.us/topogeo/hazards/sinkhole/default.asp

• Interviews:• Property Owner• Neighbors• Municipal Officials/Workers

• Barry Isett & Associates—610-398-0904

QUESTIONS?

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