Ground Vibrations: Effects of masses placed on or in the ground

Abstract

Denne rapport omhandler et reelt og aktuelt problem for bygninger påvirket af vibrationer genereret af forskellige årsager. Projektet fokuserer på formering af vibrationer gennem jorden og undersøger løsninger for at mindske størrelsen af disse vibrationer. En relativ ny teori er analyseret for at forhindre vibrationerne ved at placere en masse mellem vibrationens kilde og modtageren, som enten kan være placeret på jorden eller indlejret i jorden. Problemet er analyseret gennem to metoder; laboratorie tests foretaget på en skaleret model og numerisk analyse. Forskningsarbejdet er startet med at skalere vibrationernes karakteristika ved brug af tilgængelige materialer til anvendelige laboratorie dimensioner. Derudover er materialernes og set uppets adfærd trin-for-trin undersøgt ved at bruge specialiseret udstyr (accelerometer, impulshammer og Pulse software til at bearbejde data) i format af frekvensresponsfunktioner (FRF) analyse. Undersøgelsen leder op til to endelige forsøgsopstillinger, og hver af dem med mulighed for at blive tilpasset med en række masse konfigurationer. Den første model simulerer et enkelt jordlag hvor massen er placeret på overfladen mellem exciter og en modtager. Den anden model forestiller et tykkere jordlag med massen indlejret i selve jorden. Efterfølgende er en række numeriske modeller simuleret med software programmet Abaqus CAE med de samme randbetingelser og materiale data, som blev brugt i forsøgene. Resultaterne fra laboratorie forsøgene og de numeriske modeller er derefter sammenlignet, hvorefter de numeriske modeller er valideret. Det er observeret, at den numeriske model matcher de relative simple modeller bedre, hvor sandsynligheden for usikkerheder er mindre. Effekten af at tilføje masse kunne observeres ved alle test set ups både eksperimentelt og numerisk. Den respons registreres dæmpning af de accelerationer på nogle frekvensområder, men på samme tid øget respons kunne observeres ved andre frekvenser. Den væsentligste positive indflydelse observeres ved at tilføje flere masser mellem exciter og modtager der simulerer en periodisk konfiguration af masserne.

This report concerns a real and actual problem of buildings affected by vibrations generated by various urban activities. The project focuses on the vibrations propagation through ground and investigates solutions to reduce the magnitude of these vibrations. In order to mitigate vibrations a relative new idea is studied by using masses placed in between the vibration source and the receiver which can be either placed on the ground surface or embedded in the ground. The problem is studied from two different approaches, a small scale laboratory experiment and by numerical models. The research work starts with the task of scaling the vibrations characteristics to convenient laboratory dimensions while using accessible materials. Furthermore the behavior of the materials and of the test setup is investigated step by step using specialized equipment (accelerometers , impact hammer and a data processing software PULSE) in the format of a frequency response function (FRF) analysis. The investigation lead to a number of two final experimental test setups, each of them having the possibility to be adapted to a series of added masses configurations. The first model simulates a single soil layer with masses positioned on the surface between an exciter and a receiver. The second model resembles a thicker layer of soil with the masses embedded in the ground volume. Subsequently, using the commercial software Abaqus CAE a series of numerical models are modeled with the same boundary conditions and the same materials used in the experimental analysis. The results from the experimental and numerical analysis are then compared and the ability of the numerical models of simulating this phenomenon is tested. It was discovered that the numerical model matches better relative simple models where less uncertainties are likely to be introduced. The effect of added masses could be observed in all test setups configurations for both experimental and numerical analyses. The response recorded attenuation of the accelerations at some frequency ranges but in the same time increased response could be observed at other frequencies. The most significant positive influence was observed when multiple masses were placed in between the exciter and receiver simulating a periodic configuration of the masses.

A master thesis from Aalborg University

Ground Vibrations: Effects of masses placed on or in the ground