C. Buckley, R. Torrez, C.W. Kohlenberger

Soil and ground structure stability risk issues are continuing to drive engineering attention toward newer, more integrated and sensitive dynamic behavior measurement and information technologies developed by MicroRadian Engineering. The true advantage yielded from MicroRadian Engineering's more than 20 years of application research and development has lead to expanded measurement and monitoring capability that offers real time acquisition and analysis capability results at reduced expense. In order to safely accept "if it's not broke Why Fix it?" a factual, data based confident knowledge is becoming essential to establishing confidence in geo-structures safety. How the use of MicroRadian Engineering electronic tiltmeter systems employing specially developed instrumentation technology helps to collect information necessary to characterize ground structure displacement behavior makes an interesting story.

Areas in which MicroRadian Engineering efforts have been advancing geo-displacement measurement technologies, include new low cost dynamic slope stability measurement and monitoring methods, as well as recursive analysis techniques applied to structural failure assessment modeling. These models are historically supported by development of a growing slope behavior information base.

TILT INSTRUMENTATION OVERVIEW
A subtle but definitive aspect of slope structural failure is the hardly perceptible acceleration and tilting dynamic behavior. Gravitationally referenced, biaxial tilt meters have evolved as key instruments, offering incredibly high tilt and acceleration amplification capability, when applied to producing continuous slope behavior history records.

In principle, a tiltmeter operates similarly to a carpenter's spirit level. The position of a bubble in a fluid is used to indicate the attitude of the container, relative to the local gravity vector. MicroRadian Engineering tilt meter sensors are configured as a cylindrical glass container partially filled with a conducting fluid, forming a bubble. As the container tilts, the bubble location shifts relative to four orthogonally located electrodes. The bubble's position relative to the electrodes enables electrical tilt sensing. Tilt is indicated as a measure of differential resistance change between opposite electrodes that form a Wheatstone bridge circuit. When the bubble is centered in its vial, the voltage output from the orthogonal (X and Y) axis is nominally zero.

Minuscule tilt changes the bubble position away from the null location. As differential resistance then changes, tilt is indicated by the resulting proportional voltage output change. The tilt response bandwidth of the electrical circuit, exceeding 100 Hz, is independent of the internal physical fluid dynamic inertial response, which dominates the tilt and acceleration-sensing characteristic.

Tiltmeter thermal instability, defined as the change in a given tilt indication due solely to temperature occurs in two ways, and requires specific engineered attention. First, small changes in tilt vs. voltage sensitivity result from internal material and design choices. For one typical sensor, scale change affects were determined to be 0.6 % of tilt offset per degree F (Kohlenberger and others, 1973). Tilt also results from gradient between material structure attach points. In practice however, net thermal affects on tilt measurement stability were able to be made negligible. Fortunately in geophysical uses, the very low thermal conductivity typical of crustal installations can conveniently provide the naturally benign environment essential for successful thermal noise isolation.

A micro-Radian, put into perspective, is an angle measuring roughly 0.00006 degrees. This small angle can be visualized as a solid beam 83 feet long raised or tilted at either end by 1/1000 of an inch, which is 1/3 the diameter of a human hair. Similarly, an angle of 1 micro-Radian occurs if one edge of a rigid crustal block 16 miles across is raised by 1 inch. The typical MicroRadian Engineering electronic tiltmeter is able to indicate tilt as small as 0.006 micro radian with a one second integration time. Thus the typical tilt resolution "floor" for the standard MicroRadian Engineering sensor is 6 nanoRadian per root Hz.

A TILTMETER APPLICATIONS CHRONOLOGY
Bi-axial electronic tilt meters have been under development since l958. Starting in the 1960's successful tilt sensors were installed as fast, robust and reliable alignment sensors on guidance system inertial platforms for intercontinental ballistic missile systems, then as monitors for gyro alignment tables and tohelp calibrate gimbal attitude deflections in precision radar antennae and optical tracking systems.

By the early 1970's, the tilt meter's role had expanded from military use, to such civilian applications as astronomical telescopes, geophysical measurement and industrial alignment. A number of investigators studied the device, and found it's long-term stability and reliability to be excellent. Wood and Allen (1971) showed that long-term drift was in comparable agreement with mercury tube tilt meters to within 2 micro- radians.

In 1973, the U. S. Geological Survey established the first documented civilian network of tilt meters in Central California. The objective of the tiltmeter network was to monitor crustal deformation associated with earthquakes. Allen and others (1973) pioneered the first methods for installing shallow (4 feet deep) borehole tilt meters for the nationwide earthquake prediction research program. Similar systems were also used in volcano inflation monitoring programs of Hawaii and Central America. These studies and others like them documented and measured components of the solid earth tide, and measured rates of secular drift of 0.4 micro radians per month. Bacon, with the California Division of Mines and Geology installed surface mount (platform tilt meters) in Oroville at about the same time. Wood (1974), in an early commercial venture, used tiltmeter arrays to detect and monitor crustal uplift associated with chamber enhancement from hydro-fracturing in oil fields. Wood, operating the tilt meters at even higher sensitivities, found that useful data at sub-earth-tide sensitivities was obtainable by hallow borehole tiltmeter installations.

Buckley and Torrez conceived of applying the tilt meters to geo-engineering problems in 1976, leading to development of a rapid deployment technique that minimized residual stress induced by the installation process. As a result, it became possible to effectively deploy the tilt meters under rapidly changing conditions, such as moving landslides or as monitors to measure non-elastic motions in structures.

APPLICATION CONSIDERATIONS
It was found that thermo-elastic changes, such as rainfall, and cultural effects are the main contributors to tilt data noise. Noise in this case, is defined as influence from mechanical or electronic signals not irectly due or associated with ground or structural motion related to the failure being studied.

Sites for placement of tilt meters need to be carefully chosen to minimize influences or noise affects from temperature and rainfall. Simple vault enclosures offer protection from direct sunlight, and can help avoid rainfall penetration into the electronics.

In typical low noise installations, a 43 inch long, 2 Inch diameter stainless steel tube housing having the bubble sensor at the bottom is installed in a P.V.C. cased hole 8 Inches in diameter, and loosely packed with sand. As installation proceeds, great care is taken to assure the two orthogonal tilt axes are held within a narrow "null" output tolerance range. An oscilloscope having an x-y plot ability is the easiest method, although with practice, two digital voltmeters can also be used.

The electronic tiltmeter has been available commercially for almost twenty years, but has had only a limited application in slope stability problems. MicroRadian Engineering has been researching use of tilt meters as a tool for solving geotechnical-engineering problems since 1974, and has found the tiltmeter to be a highly advantageous instrument in monitoring slope stability problems.

Results obtained from large "classic' landslides such as the Bonneville Landslide In Washington, and the Gaillard Cut Landslide in the Panama Canal, gives excellent demonstration of the instrument's capability. Such information, added to many data from smaller sites has given us a very large database with which tiltmeter response to landslide motion, hence landslide behavior itself has been well characterized.

To date, we have made more than 500 tiltmeter installations. Single-instrument surveys of single family residences and multi-instrument, computer controlled arrays on slope repair projects represent the span the tiltmeter utility and applicability range. Our research has also investigated problems associated with the prediction of failure.

Because of its ease of installation and ability to resolve very small tilt, the electronic tiltmeter gives numerical measurement at least ten times more quickly and at greater resolution than other instrumentation. Higher resolution electronic tilt measurement capability offers advantage in greater informational detail with added expense of increasing the volume of data. Due to the higher visibility of tilt phenomena, relative to other types of available instrumentation, corroboration of the small tilt change values supplied by the tiltmeter requires more analytical care. It therefore becomes necessary, at times, to interpret the results relying on the tiltmeter as a "sole source" for real time phenomenological measurement. Such measurements and their effect can later be credibly corroborated over time by more classical and less sensitive means, such as optical surveys, Inclinometer data or crack maps.

MicroRadian Engineering
The tiltmeter has, as a result of its sensitivity, been aiding development of numerical criteria for the definition numerical boundaries for the end-condition called 'stability'. Since 1973, MicroRadian Engineering has been refining evaluation methods to establish the highest permissible tilt change rate that allows distinction between stable and unstable conditions. In most cases, on near-critical slopes, this value has been close to two (2) micro radians per day (0.4 arc seconds). Rates on near-critical slopes exceeding two micro radians per day will often lead to structural distress within the design life of many structures. In the case of geotechnical applications, comparable rates frequently lead to slope failure. This rate value can be compared to motions recorded on large "stable" contiguous structures (Kohlenberger et al., 1973). Some of the currently understood processes that prevent tilt change rate numbers from being zero are, neo-tectonic motion, earth tides. and thermal inflation of the soil media.

Attempts to compare data from the in-place tiltmeter and wheeled inclinometers confront a major information deficit obstacle. The problem is analogous to measuring stock market volatility using a series of weekly price samples versus the results from a low latency, real time tracking system. The real time, short sample interval tilt-monitoring system used by MicroRadian Engineering detects small tilt changes in far less than one percent of the monitoring time required by lnclinometers. As an example, an extremely slow moving landslide (l ft/5 yr.) is approximately equivalent to a strain rate of one part in 10 billion per second. The tilt meter's high sensitivity combined with nanoRadian resolution enables reliable detection and confirmation of strain rates in less than 10,000 seconds (2.7 hours) of monitoring. This is due to the high data rate ability allowing positive avoidance of alias in data sampling when exposed to such common disturbances as classical 6 second micro-seismic noise, and the typical, variable amplitude, 3 Hz vehicular oscillation sources.