2016 International Conference on Power Engineering & Energy, Environment (PEEE 2016) ISBN: 978-1-60595-376-2

A Deep Excavation Supporting Design and Application Technology Research of Zhengzhou

Yong-wei Wang1, Yan-qin Guo1, Kun Zhang1

1Huanghe Science and Technology College, Zhengzhou 450063，China

Keywords: Deep foundation excavation; pile anchor system; support design; foundation monitoring; application

Abstract. A deep foundation pit is located in Zhengzhou city, is located in the Yellow River alluvial plain, excavation depth of 12.9 m, engineering site surrounding environment is complex, the for and silty layer, embedded depth of in the shallow. In the foundation pit supporting design, combining with the field information and physical and mechanical parameters of soil, the elastic foundation beam method considering interaction between supporting pile, anchor and soil and the deformation coordination, selection of CU cohesive force, internal Angle parameters of the experiment. "M" method is used to reflect the interaction of piles and soil, anchor cable using common pull type high pressure jet grouting anchor cable, anchor cable and the high pressure jet grouting anchor cable selection, increase the anchor stiffness, inhibit deformation. Through anchor stress, deep horizontal displacement of foundation pit monitoring process monitoring data, it is concluded that the rationality of the design of foundation and feasibility. Through engineering practice, under the premise that ensure the security of foundation pit side wall, has obtained the good economic benefits. Through the system summary and analysis for the pile anchor supporting system similar to the soil and surrounding environment in Zhengzhou region to provide experience for the design and application of reference.

Introduction Deep foundation pit engineering is generally temporary works, has large quantities, many influence factors, high technical difficulty, risk characteristics of unpredictable factors etc.[1] According to the statistical data of Wang Shuguang et al., nationwide foundation pit accident is caused by improper design and construction, which accounted for over 80% of the total number of pit accident.[2] In the main city zone, foundation pit excavation of a greater impact on the surrounding buildings, and underground pipelines, how to ensure that the supporting structure of the safety level of surrounding deformation meet the requirements as the most important condition of deep foundation pit design and Application.[3] Pile anchor supporting system as the anchor cable and pile body jointly bear the soil pressure, work together, for the active force system, inhibition of deformation of foundation pit in pit significantly, the city's main urban support has been widely used. In this paper, combined with the practical engineering experience, this paper introduces the design and application of pile anchor retaining structure in the main urban areas of Zhengzhou in a foundation pit.

1. Engineering Survey Engineering field area is located in Henan province Zhengzhou city Jinshui District Agricultural and the three road in the vicinity, planning for commercial and high-rise residential, high-rise hotel. Site excavation things about 262m long, north-south length of about 182m, the depth of 10.8 m ~ 12.9m excavation, two storey basement, super high-rise hotel as the core tube structure, the excavation depth is 16.6m. Northeast and northwest side of the site has been built two story steel frame sales department and a layer of model of the housing area, based on an independent basis, base bearing layer of compacted soil, the top edge distance based outside line about 3.4m excavation, foundation depth is about 2.0m. The field area around for municipal roads and underground pipelines, pipeline is buried under the ground for 3.5m.

2. Geological Condition According to the drilling, static , combined with indoor analysis, site formation have influence on the calculation of support structure for the Holocene (Q4): according to the difference of lithology strata can be divided into the upper, middle and lower three sections: the upper section (Q4-3), surface for the 50cm soil, the alluvial form for the Yellow River brown yellow, yellow silt and silty clay layer thickness of about 9.0m The middle section (Q4-2) buried below 9.0m, consists of a light gray ~ Black silt, silty clay, because sediments into hydrostatic the Yellow River River floodplain facies formed or slow phase, the average thickness of 6.3m. The next section (Q4-1) buried in the 15.3m ~ 35.2m, composed of gray to brown yellow fine silty , silty soil, because the Yellow River sediments. According to the regional geological conditions and near field experience, the site is divided into phreatic aquifer and lower upper quaternary confined water of two layers, the upper diving occurs mainly silty soil, more than 16.0m of silty clay layer, is a low permeability layer, dynamic change the effect of seasonal precipitation. Confined water mainly occurs in the 16.0m following powder sand, fine sand, micro pressure of. Regional data show that: in the past 3 to 5 years the highest water level of groundwater is about 7.0m, affected by the precipitation of subway construction during the exploration, the water depth is about 10.0m, and the construction of the MTR is coming to an end, not affected by nearby foundation pit , ground upper phreatic water level at the present ground 4.5m, water pressure under the Department with micro bearing, confined water in the depth of about 6.5m.

3． Foundation Pit Supporting Scheme Selection The overall design scheme of foundation pit engineering is a field of engineering and hydrogeological conditions, environmental conditions, construction conditions, foundation pit excavation based on service requirements and scale, through technical and economic comparison to determine the base pit safety level, the above factors, the pile anchor supporting system, the diameter of pile bored pile 600mm, cable can be used in the general tension type anchor cable or high pressure jet grouting anchor cable two. 3.1 Calculation theory Pile anchor bracing design method with fixed end limit equilibrium method, i.e., equivalent beam method. The supporting pile as beam supporting three hinge, earth pressure is zero, the anchorage point, fixed end moment is zero, by the soil pressure of column moment balance equation of anchoring force as zero, the point of zero moment. By adjusting the position of the anchoring force, positive and negative bending moment inflection point of roughly the same. The pile body is calculated with the elastic foundation beam "m" method and increment method to calculate the internal force changes in the process of the excavation retaining pile of situation. Settlement and deformation is calculated using the delta method, index method and cutting wire method. Earth pressure model by the method of soil pressure of the classical earth pressure model and elastic model modified by two, calculated using the Rankine earth pressure model of active earth pressure. Calculation of bolts using the assumed level of spring element, stress elongation was free segment and anchorage segment elongation elongation of both, meet the deep sliding stability requirements. Sweden slice method for stability checking using circular slip surface finishing. 3.2 Comparison of scheme selection Parameter calculation scheme selection: the depth of foundation pit H=10.8m, overload of slope design value Q1=20KPa, two storey steel frame based sales department uniform load Q2=36KPa, depth of action h=1.50m. Cable using the strength level of 1860Mpa, nominal diameter of steel wire 15.20mm. Bolt using ordinary tensile type anchor cable and high pressure jet grouting anchor cable, because the basement soil is silt and silty clay, and the soil is soft plastic to plastic shape design considering the strong upper anchor pile, strong, weak design principle of soft soil layer anchor, the following specific design parameters. Scheme 1: ordinary tensile type anchor cable pile diameter phi; =600mm, pile length L=19.0m; the first anchor, under natural ground 3.5m. Anchor with 3 beam S15.2mmsteel strand, a total length of Lm=25.0m, the length of the anchored segment LD=18.0m, free length Ls=7.0m, the free section and anchoring section diameter D=180mm; second cable, 7.0m cable under natural ground, using 4 beam S15.2mmsteel strand, cable length Lm=22.0m, length of anchoring section of LD =16.0m, the free length of free section Ls=6.0m, and the anchorage section diameter D=180mm. Scheme 2: high pressure jet grouting anchor; pile diameter phi =600mm, pile length L=17.0m; the first anchor, under natural ground 3.5m. Anchor with 3 beam S15.2mm steel strand, a total length of Lm=16.0m, the length of the anchored segment of LD=8.0m, the free length of anchoring section Ls=8.0m, diameter D1=400mm, the free section diameter D2=200mm; second cable, 7.0m cable under natural ground, using 4 beam S15.2mm steel strand, a total length of Lm=16.0m, the length of the anchored segment LD=10.0m, free length Ls=6.0m, diameter of anchoring section D1=400mm, the free section diameter D2=200mm.The results are shown in Table 1.

Table 1. The calculation results of two kinds of scheme. Parameter Scheme 1 Scheme 2 First line (MN·m) 7.05 15.51 Anchor stiffness Second (MN·m) 9.65 18.65 The maximum Moment (KN·m) 413.6 384.5 internal force of Shear (KN) 407.2 368.2 pile body The ground surface settlement (mm) 35.2 20.6 The safety factor of stability 1.335 1.583 Factor of safety against overturning 1.697 1.837 The factor of safety against basal heave 2.233 2.501

From the table, the calculation results, using high pressure jet grouting anchor cable pile anchor supporting system than ordinary tensile type anchor cable, high safety, strong ability to restrain the deformation of foundation pit. The specific reasons are: increasing the contact area with the soil, increase the adhesive force of anchoring section and the lateral friction resistance, anchor stiffness increased, improved the shear capacity of soil. Through the analysis of economy, plan a pile anchor supporting system should be combined with the high pressure jet grouting pile water stop curtain wall design, combination, can guarantee encountered soft plastic powder soil, silty clay layer in the excavation to the base, to ensure the pile soil will not occur between flake, sliding failure leads to soil erosion between piles, which led to the the large deformations. Scheme two pile anchor supporting system, high pressure jet grouting anchor can improve the shear capacity between piles and soil, higher than the traditional tensile type anchor pullout force of 2 to 3 times, compared with ordinary cable project total cost can save 20% ~ 30%. Finally the design scheme of high pressure jet grouting pile plus anchor cable. 3.3．The design scheme of foundation pit According to the above analysis and soil, the surrounding environment and load, the project is divided into 6 support area. 1) On the east side of foundation pit, the southeast side of the top 5.0m wall, slope using 1:0.3, soil nail hole diameter D=100mm, the surface layer of shotcrete thickness is 80mm, the lower part of pile anchor supporting. 2) The rest of erect excavation section, the pile anchor supporting. The specific design parameters in Table 2, table 3, table 4 Pile anchor support by incremental method of internal force calculation, the excavation to 11.7m, draw the displacement and internal force envelope diagram shown in Figure 1.

Table 2. The basic design parameters of soil nailed wall. Profi Soil nailing position Under The length of Soil nail Horizontal le natural ground (m) soil nail (m) rod spacing (m) 1-1 1.7 6.0 1B18 1.3 1.4 9.0 1.4 9.0 3-3 1.5 6.0 1B18 1.4 4-4 1.4 9.0 1.4 9.0

Table 3. The basic design parameters of the retaining pile. Profi The depth of The diameter The length of The embedded le foundation pit (m) of the pile the pile (m) depth (m) (mm)

1-1 10.6 600 12.0 6.4 2-2 10.6 600 16.0 5.9 3-3 12.9 600 16.0 8.1 4-4 12.4 600 16.0 8.6 5-5 11.7 600 19.0 7.8 6-6 10.6 600 17.0 6.9

Table 4. The basic design of prestressed anchor cable parameters.

Profi Position The anchor Anchorage Free The internal force le (m) information length (m) length (m) design value (KN)

1-1 -3.5 3S15.2 12.0 6.0 480 -3.5 3S15.2 8.0 8.0 360 2-2 -7.0 3S15.2 10.0 6.0 400 -7.0 3S15.2 8.0 8.0 430 3-3 -10.5 3S15.2 10.0 6.0 520 -7.0 3S15.2 8.0 7.0 380 4-4 -10.5 3S15.2 9.0 6.0 450 -3.5 3S15.2 12.0 10.0 360 5-5 -7.0 4S15.2 14.0 8.0 600 -3.5 3S15.2 8.0 10.0 360 6-6 -7.0 3S15.2 10.0 8.0 450

Figure 1. The supporting structure internal force and displacement calculation of envelope.

3.4 Precipitation Scheme The tube dewatering the diving, arranged 87 dewatering , well diameter 400mm, depth 23.0m (from the natural ground 5.0m well), the sediment concentration is not more than 1/10000, as well to the nearby underground reserves of precipitation, precipitation or groundwater level uplift, timely to reduce the water. On the base of silt, silty clay and the permeability coefficient is small, strong water retention of aquitard, in the elevator shaft, sump and other parts, if the rainfall is difficult, can be used as auxiliary light well point precipitation.

4、Foundation Pit Foundation pit engineering is a process of information construction, the deformation monitoring during excavation of foundation pit can verify the rationality of design scheme. This design scheme of foundation pit monitoring mainly for foundation pit slope horizontal displacement, vertical displacement, surrounding buildings and roads, anchor stress, underground water level, deep horizontal displacement of dynamic recording and feedback. The horizontal displacement of foundation pit slope 26 point and the vertical displacement monitoring, the maximum horizontal displacement of slope variation of cumulative < 25.0mm, the largest settlement changes the cumulative amount of <12.0mm, diurnal changes of the maximum rate < 1.0mm/d. Monitoring data show that, the maximum horizontal displacement in the excavation completed tends to be stable, changes in the range of 6 ~ 23.5mm. The maximum vertical displacement variation in the range of 5.5 ~ 12.5mm. Near the foundation pit side section of the building the largest settlement in 23 ~ 25.5m. Deep horizontal displacement in the natural ground level at 4.5 ~ 10.0m, the maximum change in volume was 34.87mm, the change range of 30.5~ 34.87mm. Underground water level variation range is 0.2 ~ 2.5m. The internal force of anchor cable is stable in 164.0KN, the internal force variation about 0.5KN. Changes of the reference point as shown in Figures 2-5.

Figure 2. The changes of D07 ~ D11 at the top of the vertical displacement.

Figure 3. C31 ~ D34 buildings settlement trend.

Figure 4. The changes of SP15 ~ SP18 horizontal displacement.

Figure 5. The change trend of deep horizontal displacement CX16.

5、Conclusion 1) In the design of this project according to the Zhengzhou silt, silty clay interbedded area with pile anchor supporting system using high pressure jet grouting anchor cable, anchor cable, according to the construction and monitoring data shows, the design scheme is reasonable, and saving cost, saving construction time. It has good popularization value. 2) High pressure jet grouting anchor because of anchoring section and contact surface soil, increase the anchor force and side resistance, compared with the ordinary tensile type anchor cable to improve the , but in the soft plastic powder soil, bearing capacity much improved in silty clay, need to combine the basic experimental field region, determine the characteristic value of the bearing capacity of anchor limit, and in specific engineering calculation, adjustment of anchoring force adjustment coefficient, the adjustment coefficient of anchoring force according to the experience of the project value is 1.20, the actual should be determined according to the basic experiment. 3) The engineering calculation is based on limit balance valve, based on the experience of earth pressure theory and the theory of elastic foundation beam internal force calculation in accordance with the incremental method. In accordance with the "JGJ120-2012" of the new rules, the fixed end is calculated by the method of the embedded depth of anchor cable are considered too long anchor stiffness, the anchor is assumed as a spring element, the oblique bolt split into level and vertical anchor bolt, not considering the vertical anchor role, resulting in longer length calculation of anchor cable, in practical engineering according to the engineering geology, determine the basic means of experiment, the need for further research and analysis. 4) In the main city zone, the underground network and complex surrounding environment area, the design scheme should be as the main index to control the deformation of foundation pit support design scheme, according to the deformation control measures, minimize the disturbance to the surrounding environment, and strengthen the monitoring, timely feedback and communication

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