Insitu-ch7.ppt
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1、Chapter 7 In-Situ Tests 第七章 原位测试,绪 论,Definition: In-situ test is to obtain the engineering mechanical properties of subsoil under undisturbed (in-situ) conditions.,Advantages: 1. Can obtain the engineering mechanical properties of subsoil under undisturbed (in-situ) conditions . 2. Can avoid the neg
2、ative effects of sampling disturbance. 3. Can minimum the negative effects of boundary conditions. 4. Saving time for geological investigation.,Disadvantages: 1. Special limitations for the application of different in-situ techniques. 2. All the empirical relations between measuring parameters and m
3、echanical properties of subsoil are based on statistical results. 3. Influencing factors are complicate.,Techniques 7.1 Loading Test (静载荷试验) 7.2 Cone Penetration Test (静力触探) 7.3 Standard Penetration Test(标准贯入) 7.4 Shear Vane Test (十字板剪切),7.1 Loading Test,Plate loading test provides valuable informat
4、ion by which the bearing capacity and settlement characteristics of a foundation or pile can be assessed.,Such test is carried out in a trial pit, usually at excavation base level. Plates can be round or square. The size of the plate varies from 0.5 m2 to the size you want. The plate should be prope
5、rly bedded and the test carried out on undisturbed materials so that reliable results may be obtained.,Loading Test,The load is applied by a jack, in increments of one tenth of the proposed bearing pressure.,a jack,Technical Requirements 1.承压板:Plate should with high rigidity 2.试坑宽度:Width of the test
6、ing pit(3d) 3.加荷方法:loading way(快速法与慢速法) 4.稳定标准:Standards(0.1mm/h) 5.荷载分级:loading increments(1/10-1/8) 6.破坏标准:Failure limits(土体侧向挤出或裂纹;沉降等速增加within 24h;P增加很小,但s急剧增加,P-s出现陡降),Three phases: I: Linear II:Non-linear III:Failure,Purposes: To determine critical pressure(临塑荷载),ultimate load(极限荷载) To Estimat
7、e the modulus of deformation (变形模量E0) To determine un-drained shear strength Coefficient of sub-grade reaction (基床反力系数) Results: Ps curve,slogt curve.,Testing results,Determination of bearing capacity,Pu,Determination of bearing capacity,Disadvantages 1. Limited plate size controls the influencing d
8、epth. Loading process does not last for enough time. 2. Stiff shell of subsoil pays a negative effects on the tests.,Loading Test,Bulb of pressure beneath a foundation as compared with one beneath a plate load test,Penetrometer,7.2 CPT Test,Cone head,Cone head,Cone sleeve,Cone rod,The ratio of sleev
9、e resistance to that of cone resistance is higher in cohesive than in cohesion-less soils. The ratio of penetration resistance: Ps=P/A P: net penetration resistance; A cross-sectional area of cone.,Cone penetrometer traces,Suitable conditions: 1. Soft soil: Clay, silts and sands 2. Penetration capab
10、ility can cover soil properties, depth 3. Anchoring resistance not less than net penetration pressure,Applications 1. To Classify soil strata 2. To estimate physical and mechanical properties of subsoil 3. To evaluate bearing capacity of subsoil 4. To choose bearing stratum for pile foundation, to e
11、stimate ultimate bearing capacity of signal pile and to evaluate the possibility of pile driving 5. To evaluate possibility of soil liquefaction under earthquake,7.3 Standard Penetration Test SPT test (标准贯入),标准贯入试验,A kind of dynamical penetration test, this empirical test consist of driving a split-
12、spoon sampler, with an outside diameter of 50mm, into the soil at the base of a borehole. Driving is accomplished by a trip hammer, weighing 63.5kg, falling freely through a distance of 750mm onto the drive head, which is fitted at the top of the rods.,标准贯入试验,During the test, the split-spoon is driv
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