USING POINT CLOUDS TO EVALUATE SLOPE PERFORMANCE WITH RESPECT TO ROCKFALL GENERATION

Analyzing slope performance is key to determining a rock slope’s risk of generating rockfalls. US Departments of Transportation use rockfall hazard rating systems to evaluate highway cut slope’s potential to generate rockfalls and maintain inventory. Our proposed method of comparing time lapse digital point clouds, sets of data points defined by X, Y, and Z coordinates to represent three dimensional surfaces, provides a way of quantifying volume of rockfalls generated from cut slopes along highways. Sets of point clouds are built in Agisoft PhotoScan software using structure from motion (SFM) photogrammetry from overlapping photographs taken from different times. Using the software Cloud Compare, the rate and quantity of rockfall generation is found by calculating the numerical difference between point clouds of the same slope collected at two different times. The numerical value recognizes either a positive, negative, or null value based on what has changed from the slope at one time to another. For example, a single rock falling from the top to the bottom of the slope is shown by a negative value at the top and a positive value at the bottom. Anchoring points that do not move over the period of time in question are required to accurately ‘lay’ the second cloud over the first. As proof of concept, point clouds of a rock pile were built using SFM collected by a Phantom 4 Pro unmanned aerial vehicle (UAV). One point cloud was of the unaltered rock pile to represent time interval-one, and the second point cloud was of the same pile but slightly altered to represent time interval-two. In the altered pile, rocks of three different size groups by length a) greater than one foot, b) between six inches and one foot, c) less than six inches were rotated, displaced, or removed completely. Upon comparing the two clouds, results showed that the method was able to pick up rock removal, rock rotation, and slight displacement (as fine as a few inches). It was found that the proposed method of evaluating rockfall generation from cut slopes using photographs from UAVs is an efficient method of evaluating cut slope performance and its potential hazard to roadways.