Cyclic Pressuremeter Tests for Cyclic Lateral Loads

ABSTRACT The results of thirteen cyclic pressuremeter tests performed in hand augured holes on a stiff clay are presented. These tests show that the slope of the unload-reload loop stabilizes after only a few cycles but that the strain at the peak of the cycle continues to cumulate long after the slope of the loop has stabilized. It is also shown that the ratio of the peak pressure of the cycle to the net limit pressure is a major factor influencing the rate at which the cyclic strain cumulates with increasing number of cycles, while the ratio of the pressure range of the cycle to the net limit pressure is only a minor factor. Finally, recommendations are made to obtain the degraded cyclic pressure meter curves from the static pressure meter curve, and potential applications are discussed. INTRODUCTION In situ testing is becoming more and more useful in offshore geotechnical engineering (4). The remote vane was the first in situ tool to be used in the early 1970's, then the cone penetrometer followed, and more recently the pressuremeter has emerged as the third generation of in situ testing devices. Already two pressuremeters have been developed and used offshore. They are the PIP from Stress probe (16) and the PAM from the French Petroleum Institute (11). Others are-in the phase of development by companies such as Fugro and OYO. The pressuremeter has advantages and drawbacks. Used in the proper situations, the pressuremeter can be extremely useful and save money. One of the most obvious applications of the pressuremeter test is the design of laterally loaded piles. Several methods have been developed (6). The purpose of this research was to study in details the soil response to cyclic pressuremeter testing; this represents the first step in the extension of an existing pressuremeter method for piles subjected to static lateral loads (7,8,10) to one for piles subjected to cyclic lateral loads. SOIL CONDITION The site was the Texas A&M University Research Annex. The soil at the site is a stiff clay with the following soil characteristics: liquid limit 50%, plastic limit 20%, natural water content 25%, total unit weight 128 lb/ft3 The undrained shear strength from unconfined compression tests varies from 1000 to 4000 lb/ft2 and averages 2000 lb/ft2. The water table is approximately 15 ft deep. The average pressure meter limit pressure was 5 tsf and the average pressure meter modulus 100 tsf for a modulus over limit pressure ratio of 20 indicative of an over consolidated clay. PRESSUREMETERS USED AND BOREHOLE PREPARATION Two pressure meters were used at the site: The OYO Elastmeter 100 and the TEXAM Pressure meter. The OYO Elastmeter 100 (14) is a gas inflated mono cell Pressure meter which is used in a prebored hole.