In-situ Tests On Long-short-pile Composite Foundation Of High-rise Building In Collapsible Loess Region

As a new type of foundation treatment, long-short-pile composite foundation has got close attention in civil engineering realm, which combines long and short piles work together with soil to bear upper load, meanwhile, rapid development has been made in this world. This kind of composite foundation is not only good at reducing settlement, improving bearing capacity of foundation, but also expert in reducing construction cost and shortening project time. Since the application of it, good social and economic benefits have been achieved. The formers have done some useful practical and theoretical explorations, however, documents on long-short-pile composite foundation of high-rise building in collapsible loess region are rare. The theoretical research on this realm is far behind with engineering application at present.In this paper, construction quality tests of long-short-pile composite foundation have been carried out, and a series of in-situ tests which use observation instruments embed in particular part have been launched to explore the effect of combination between CFG-pile and lime-soil pile on the treatment of collapsible loess ground, and to analyze the law of pile-soil interaction, pile-body axial force and subsoil deformation with the change of upper load.Conclusions as follow:1. Under the engineering geological condition in this paper, the P-S curve which obtained from static load tests is a slow-varying model. It is suitable to use long-short-pile composite foundation to match up with high-rise building in collapsible loess region, because bearing capacity meets the requirement, the settlement is in control, loess’collapsibility has been eliminated.2. The stress of long-pile increases linearly, but stress of short-pile and soil increases slowly. Correspondingly, long-pile and soil stress ratio increases linearly, short-pile and soil stress ratio grows slowly. The load sharing ratio of soil is decreasing while the load sharing ratio of long-pile is increasing, but the short-pile’s is relatively stable. At the end of main structure construction, exerting degree of foundation bearing capacity is less than50%.3. The axial force of long-pile presents a characteristic:bigger on the top and smaller on the bottom of the pile, furthermore, the biggest axial force appears in about a quarter of pile length from the top of pile. As to short-pile, the biggest axial force appears in about one third of pile from the top of pile, the smallest axial force of short-pile presents at the bottom.4. Along with the load increases, S1accounts for the proportion of compression reinforced area decreases, while the proportion of S2is growing. The proportion of reinforced area deformation accounts for the total settlement of ground is decreasing while the compression of underlying stratum is increasing. At the end of the main structure construction, the proportion of underlying stratum compression accounts to the total settlement of ground is about70%, which reveals the key to reduce the total settlement of ground is to reduce the deformation of underlying stratum.