Experimental Investigation Of Non-Reinforced Concrete Pier Foundation

Non-reinforced concrete pier foundation, which has been applied in the engineering for many years as a new type of foundation, has brought great economic and social efficiency. It is appropriate for the masonry structure less than seven floors. So when the site geologic conditions includes3m preferable stabilization horizon within the top burial depth, such foundation has more advantages than the shallow one and other foundations.The in site experimental work includes:the single pier’s bearing capacity for sandy and stony foundation experiments are28groups, and the single pier’s bearing capacity for silt and cohesive soil foundation experiments are23goups, the bearing capacity for deep loading bat experiments are4groups, and the differential shape of pier’s bottom contrastive experiments are4groups, the horizontal bearing capacity for single pire experiments are15groups, and the pier’s bottom stress tests are8groups; the single pier’s bearing capacity for silt and cohesive loading bat tests are9groups, and the contradistinctive experiments between the pier bottom dynamic test and loading tests are68groups. From those experiment results and the statistical anlysis are developed. The vertical and horizontal bearing capacity experiential formula are given, the correlative factors are also anlaysised, the stress distribution law for pier’s bottom are drawed. And the impact of the pier foundation transmutation on the bearing capacity are put forward to study, and the academic formula are concluded. The implementary scheme and technical illuminatation are bring forward according to the experimental results and engineering practice. The following detailed research are done.As for sandy and stony soil foundation, single pier’s bearing capacity, which is closely related to the ratio of the height in soil to the diameter of pier’s bottom (H/D), is also influenced by the compactness and lithologic character of supporting course. Moreover, it has something related with pier’s control settling volume. The experimental results shows: Bearing capacity regulation factor (β) increases as H/D rises, and bearing capacity characteristic value Ra also rises. Under the same condition, bearing capacity regulation factor of sandy soil is bigger than the one of stony soil. And the bigger the control settling volume is, the higher Ra is.And for silt and cohesive soil foundation, single pier’s bearing capacity empirical formula comprises two parts:bearing capacity offered by the soil surrounding the pier (frictional resistance), and the other offered by the soil on the end of the pier (end support force). The experimental results shows:The shape of pier’s bottom exerts great impact on pier’s bearing capacity characteristic value(Ra), while bearing capacity value of flat bottom’s is11-20%higher than that of dome-shaped bottom, and the latter is about25%higher than sphenic bottom.Single pier’s lateral bearing capacity characteristic value is concerned with pier’s diameter and the diameter of pier’s bottom, pier’s depth in soil and lithologic character of foundation soil.RHagoes up as D, d and H increases. The experimental results shows:The better the soil lithologic character is, the higher RHa is. In addition, the better the property of the soil on the end of the pier is, the more hysteretic the plastic zone’s development of the soil in the bottom’s compressive region is, and also the higher RHa is. Single pier’s lateral bearing capacity with the influence of axial force is50%higher than that without such influence.Under the effect of vertical load, the measured result of the contact stress from the bottom of the foundation indicates:Before the foundation soil is destroyed, the counterforce distribution at the bottom is in the shape of saddle. On the condition that horizontal force act on pier’s top and that Q=Ra, the bottom counterforce increases with the lateral load(Ho＜Hcr) rises, the stress distribution which still presents in shape of saddle shows that the stress at the time of deflection is the strongest on the supporting point, less stronger on the center point, the weakest on the edge. And while the load is added till the soil is destroyed(H0=Hu), the stress distribution curve turns to be an approximately tilted straight-line, which shows that the pier body is in deflection and the soil body is destroyed.The result of single pier vertical resisting compression static test reveals the settlement (s) in correspondence with pier’s bearing capacity characteristic value (Ra) equals to10-20mm when the supporting course is sandy soil or stony soil and to3-5mm when the supporting course is silt or cohesive soil. Thus it can be seen that differential settlement and deformation can both meet the demand of the norm in the case of pier foundation design under the most adverse contions. Consequently, the norm states that deformation checking calculation is not required in designing pier foundation.The detection content of pier foundation includes the detection of supporting course, pier body’s integrity and single’s pier’s bearing capacity, In general, single pier’s bearing capacity static test is not indispensable. What is the most crucial is the control for supporting course quality:for cohesive soil, silt, sandy soil and stony soil foundation, dynamic penetration test is available to define the foundation intensity, and for bedrock, sampling point load test or uniaxial compressive strength test is viable. Dynamic sounding blow count and ground bearing capacity table are established by in-situ comparison test. Besides, the diameter of pier’s bottom is the most important factor affecting bearing capacity. This requires us to guard a pass strictly during the construction to ensure that it reach the design demand.The result of the bearing capacity test for shallow foundation on cohesive soil which includes the shallow plate load test in moderately compressed silt clay foundation demonstrates that:when the width of the pressure plate b is less than0.07m, bearing capacity of pressure plates with different sizes is approximately same to each other. When0.07m≤b≤3.0m, the bearing capacity decreases in the form of decreasing power functions with b increasing. Such law of change resembles cohesive soil pier foundation, but is completely different from the current foundation design norm.As we conduct foundation design, some factors such as foundation soil’s compressibility, top bearing diversity, change of foundation’s width, mutual effect between top structure and the foundation, and some other things should be comprehensively considered to evaluate the dimension effect of foundation’s bearing capacity. Foundation intensity and deformation should also be taken into consideration to ensure the value of foundation’s bearing capacity and we can propose the conceptual design while focusing on the control of deformation.