Construction Monitoring For Earth Dams Reasons for Construction

Construction Monitoring For Earth Dams

Reasons for Construction Monitoring Ensure proper materials are used p Ensure proper construction and design is followed p Quickly modify design and construction practices based on encountered site conditions p IT IS IMPERATIVE TO HAVE FULL TIME SITE INSPECTION

Inspection Requirements Must not hinder or slow down contractor p Must work with contractor p Must consider contractor construction practices p

Construction Monitoring MATERIALS p Grain size distribution analysis of materials n n n Core, filters, drains Make sure material installed meets specifications Make sure that the borrow materials do not change…

Construction Monitoring MATERIAL tests p Triaxial extension/shear – filter and core p Consolidation – core mv p Hydraulic conductivity n Lab tests: filters - Constant or falling head p core – triaxial p n Field clay: Double ring infiltrometer p Centrifuge permeanometer p

Construction Monitoring Proctor Tests n n p Source materials in borrow pit Materials hauled to site Field Compaction n Uncompacted layer thickness (300 mm max) Compaction equipment is suitable Moisture content and Maximum dry density Nuclear Density, sand cone, rubber balloon p Make sure Nuclear density is calibrated p

Goal of Compaction p Place loose soil in the field and compact it to make soil strong as possible n Maximum shear strength Very little settlement Low hydraulic conductivity n Find soil lowest emin ……highest dry unit weight n n

Knead Clay Chunks Sheepsfoot roller

p p p Soil compacted wet optimum will be ductile and self healing Soils compacted dry of optimum will be brittle and suspectible to cracking Specify optimum plus 2% for clay cores

Soil Compaction Measurement

Soil Compaction Measurement p p p Use dry sand with known dry density and specific gravity Use dry sand to get volume of hole Quick and reliable method

Soil Compaction Measurement p p Use radioactive material to get moisture content and soil density Quick method Reliable if calibrated Radioactive device therefore special transportation and rules must be followed

Compaction Specification Standard Proctor Specification p 95 to 100 percent of MDUW Modified Proctor Specification p 92 to 98 percent of MDUW

Compaction Specification p p 95% Field Specification Make sure compacted soil same as Proctor material (grain size distribution analysis) Add water to soil if too dry

Field Instrumentation Measure performance of structure during construction p Long-term monitoring of structure behaviour and health p Must not impact structure performance p Geotechnical instrumentation can reduce undesirable consequences from construction. These consequences may be the results of adverse performances, damage to the adjacent facility and/or delays.

Justification for Instrumentation Engineers should developed justifications for geotechnical instrumentation program on their projects p In practice such programs are used to save lives, save money and/ or reduce risk of failure p In concept, these are simple and easy to understand benefits but in practice it is difficult to quantify

Reasons to Install Instrumentation p p p p Indicate impending failures Provide a warning Reveal unknowns Evaluate critical design assumptions Assess contractor's means and methods Minimize damage to the adjacent structures Control construction Control operation Provide data to help select remedial methods to fix problems Documents performance for assessing damages Inform stakeholders Satisfy regulators Reduce litigation Advanced state- of – knowledge

Field Instrumentation p Piezometers n n p Excess pwp in core during compaction Uplift pressures Foundation head loss Core pheatic surface Inclinometers n Stability of slopes and foundations Settlement gauges p Extensometers p Total earth pressures (soil arching) p

Suggested Piezometer locations Earth fill dam: 1 - Control placement of fill, monitor pwp to find shear strength and measure uplift pressure 2 - Control placement of fill, monitor pwp to find shear strength and measure uplift pressure and monitor seepage 3 - Control placement of fill and monitor seepage.

Piezometer

Inclinometers: Monitor lateral earth movements in embankment e. g. detect movement of D/S of earth fill dam, particularly during impounding. Determine type of shear and zone in foundation. Monitor stability of U/S slope during and after impounding. Determine depth, direction, magnitude and rate of movement

Inclinometer system (Courtesy of N. Sivakugan, James Cook University, Australia)

Inclinometer: Embankment: • Locate shear zone and help identify whether shear is planner or circular • Measure the movement at the shear zone. Determine whether the movement is constant, accelerating or slowing.

Inclinometer must be founded into solid foundation

5 - Tilt meter: Monitor changes in the tilt of the structure. Activities such as dewatering, tunnelling, excavation causes settlement or lateral deformation. Placement of surcharge and pressure may cause heaves. Dam impounding, excavation beyond diaphragm wall etc. Monitor differential settlement Dewatering

6 - Settlement cell: Pneumatic settlement provide a single point measurement of settlement. They can be read from central location and arte particularly useful where asses is difficult. Monitor consolidation during construction and long term settlement in the foundation of the fill. Earth fill dam

Settlement cell Monitor long term settlement and consolidation in the foundation of embankments

Settlement cell

7 - Bore hole extensometer: Monitor settlement heaves, convergence, and lateral deformation in the soil and rock For vertical settlement profile

Borehole extensometer: Earth fill dam: Monitor vertical settlement in the toe of the dam ( Magnetic extensometer)

Borehole extensometer Embankment: Monitor settlement to determine when construction can continue. ( Magnetic extensometer used inside the inclinometer)

8 - Total pressure cell: Measured combined pressure of effective stress and pwp Embankment dam Verify assumptions and warn of the soil pressures in excess of those a structure is designed to withstand. It determine distribution, magnitude and direction of the total stress.

Earth pressure cell (Courtesy of N. Sivakugan, James Cook University, Australia)

Total pressure cell

Rock fill dam SM: Strong motion accelerograph ( for monitoring earth tremors) TS: temperature sensor EX: Extensometer( Identify movement of dam base and ground at base) PZ: piezometers PC: Pressure cell W: V-Notch weir SC: settlement cell WL: water level meter

Earthfill dam SM: Strong motion accelerograph ( for monitoring earth tremors) TS: temperature sensor EX: Extensometer( Identify movement of dam base and ground at base) PZ: piezometers PC: Pressure cell W: V-Notch weir SC: settlement cell WL: water level meter

Key Components for Design of Instrumentation p Put in redundancy n n p Protect equipment from contractors n n n p p Instruments will get lost due to construction activities Equipment will stop working Put in safe areas Mark equipment Protect it during installation and post installation Spend money so can remotely monitor and collect data Consider data analysis cost

Key References Geotechnical Instrumentation for Monitoring Field Performance by John Dunnicliff 1993 Wiley & Sons Rock Slope Engineering by Hoek & Bray 3 rd Edition – Can be downloaded from web. By searching Evert Hoek US Corps of Engineers- Instrumentation of Embankments Dams and Levees (posted on course website)