1、Lesson 13 concrete gravity dam on rock foundatio 巖基上的混 凝土重力壩 The designer of any dam must make basci assumptions regarding site conditions and their effects on the proposed structure. 設(shè)計(jì)人員在設(shè)計(jì)任何壩時(shí)， 都必須對(duì)有關(guān)壩址的情況及其對(duì)結(jié) 構(gòu)物的影響做出一些基本假定。 Site investigations provide the engineer with much of the information to

2、 evaluate these assumptions, the bases for safe dam design. 壩址的查勘為工程師擬定這些假定提供了許多資 料。這些假定是安全設(shè)計(jì)的基礎(chǔ)。 Some important assumptions for small dam design involve uplift pressure, seepage control measures, channel degradation and downstream toe erosion, foundation conditions, and quality of construction. 有關(guān)小

3、壩設(shè)計(jì)的一些主要假定包括： 揚(yáng)壓力， 滲流控制措施， 河槽沖刷深度以及 下游壩趾的沖蝕，壩基條件和施工質(zhì)量。 Additional assumptions should involve silt loads, ice pressures,earthquake accelerations, and wave forces.其它一些附加假定應(yīng)包括泥沙荷載、冰壓力、地震加速度和波浪力。 1 safety factors 安全系數(shù) Safety factors should be considered in the light of economic conditions. Large safety

4、factors result in a more costly structure; however, low safety factors may result in failure, which could also lead to high cost.安全系數(shù)應(yīng)根據(jù)經(jīng)濟(jì)情況來考慮。安全系數(shù)大，導(dǎo)致結(jié)構(gòu)費(fèi)用昂貴；而安全系 數(shù)小則可能引起失事，也會(huì)導(dǎo)致很大的經(jīng)濟(jì)損失。 Proper safety factors result only from an adequate determination of sliding, overturning, and overstressing force

5、s within and acting on the dam.只有在適當(dāng)?shù)卮_定壩內(nèi)和作用在壩體上的滑 動(dòng)、傾覆以及應(yīng)力超限的作用力之后，才能得到合適的安全系數(shù)。 2 overturning 傾覆 Ordinarily, the safety factor against overturning is between 2 and 3. 一般來說（通常） ，抗傾 覆安全系數(shù)在23之間。 In small dams it is ofte n larger.較小的壩通常取較大的值 If the computed safety factor falls below 2, the section of t

6、he dam should be modified to increase the safety margin. 如果算出的安全系數(shù)小于2（在 2 以下），應(yīng)修改壩的斷面以提高安 全度。 A gravity dam rarely fails from overturning since any tendency to overturn provides greater opportunity for a sliding force to create the failure. 重力壩很少因傾覆而破壞。 因?yàn)槿魏我环N傾覆 的趨勢(shì)，都更可能引起滑動(dòng)而導(dǎo)致破壞。 The safety factor

7、against overturning is the ration of the righting moment to the overturning moment about the toe of the dam. This can be expressed as for Which ： is force due to weight of concrete; is force due to weight or water on inclined surfaces; is force of water acting to displace dam downstream; is uplift f

8、orce; is length of moment arm respective forces. 抗傾覆安全系數(shù)是繞壩址的復(fù)位力矩（扶正力矩restoring coupie）和傾覆力矩的比值, 以用公式表示為： 式中： 為混凝土的重量； 為斜面上的水重； 為水對(duì)壩體向下游的推力； 為揚(yáng)壓力； 各力的力臂。 Also, if the uplift pressure at the upstream face exceeds the vertical stress at any horizontal section without uplift, the uplift forces greatly

9、increase the tendency for overturning about the downstream toe at that assumed horizontal plane此外，如果上游面的揚(yáng)壓力超過任一水平斷面上 不計(jì)揚(yáng)壓力算得的垂直應(yīng)力， 則揚(yáng)壓力將使繞壩所假定的水平斷面下游點(diǎn)傾覆趨勢(shì)大大增加。 The dam may still be considered safe if the tension stresses developed are less than the allowable stresses in the concrete and in the found

10、ation material. This assumption is based on good workmanship and development of a tensile strength within the structure on all horizontal planes. 在 這種情況下，如果引起的拉應(yīng)力小于混凝土內(nèi)和地基材料的允許應(yīng)力，則該壩仍可認(rèn)為是安 全的。這個(gè)假定是基于施工工藝良好和結(jié)構(gòu)內(nèi)所有平面上都具有抗拉強(qiáng)度做出的。 3 sliding 滑動(dòng) Three approaches are used by engineers in evaluating the safe

11、ty of a dam from being displaced downstream. 工程師們衡量向下游移動(dòng)的安全性有三種方法。 Each has merit and generally involves the same relationship of forces. 每一種都有優(yōu)點(diǎn)，并且 所考慮作用力之間的關(guān)系大體上都相同。 Although the computed values are safe, they are considerably different. 盡管這幾種方法算得 的結(jié)果都安全，但他們又很大的差別。 The three approaches are: a safe

12、 sliding factor; a safety factor ; a shear- friction safety factor.這三種方法是：滑動(dòng)安全系數(shù)；安全系數(shù)；剪摩安全系數(shù)。 Clear dist inction must be made among these three app roaches.必須將這三種方法區(qū)另 U清楚。 The primary purpose of each is to obtain a safe coefficient that when exceeded would put the dam in jeop ardy of bei ng pushed d

13、ow nstream.每種方法的基本目的都是要求得一個(gè)安全系數(shù), 越出此安全系數(shù)范圍時(shí)，壩就有向下游滑動(dòng)的危險(xiǎn)。 The sliding factor of a gravity dam with a horizontal base equals the tangent of the anglebetween the perpendicular to the base and the resultant foundation reaction. The sliding factor for small dams is computed by taking the ratio of the sum

14、mation of horizontal forces, to the summation of vertical forces, including the uplift, or If computed in this manner is equal to or less than the static friction coefficient, the dam is considered safe. A unit width of 1 ft. is assumed for these calculations. Safe values for the sliding factor coef

15、ficient are given in table13.1 for various foundation materials. 重力壩具有水平地基面時(shí)，其滑動(dòng)系數(shù)等于基面垂線和地基反力合力之間夾角的正切。 在小壩上，滑動(dòng)系數(shù)的計(jì)算可以取水平力總 和和垂直力總和 （包括揚(yáng)壓力 ），或 如果這種方法求得的 值等于或小于靜摩擦系數(shù) ，則認(rèn)為壩是安全的。計(jì)算中假定取單 13.1 中。 位寬度為 1 英尺。各種地基材料的滑動(dòng)系數(shù)安全值列表于 The factor of safety, against sliding is defined as the ratio of the coefficient

16、of static friction, to the tangent of the angle between a perpendicular to the base and the direct foundation reaction, expressed as 抗滑安全系數(shù) 的定義是靜摩擦系數(shù) 和地基面垂線與地基反力夾角的正切之間的比值，即 This approach also assumes shear forces as added safety measure. 這個(gè)方法也假定剪力為附 加安全度。 The safety factor against sliding is usuall

17、y between 1 and 1.5 for gravity dams on rock utilizing a conservative cross-section. The inclusion of uplift and seismic forces in the calculations may reduce the safety factor to about unity. 對(duì)于巖基上斷面偏保守的重力壩，抗滑安全系數(shù)一般在 1。 11.5之間。計(jì)算中記入揚(yáng)壓力和地震力后，安全系數(shù)可降低到接近 These values are for safety against sliding on

18、a horizontal plane; if the foundation slopes downstream, the safety factors against sliding on the plane of the base are correspondingly reduced. 這些數(shù)值都是指水平面上的抗滑安全系數(shù)。如果地基傾向下游，則基面上的抗滑安全系數(shù)會(huì) 相應(yīng)降低。 Designers often use concrete placed into cutoff s or rock foundations to decreasethe sliding tendency of th

19、e dam.設(shè)計(jì)人員經(jīng)常采取在槽或基巖中澆筑混凝土的辦法來減少壩滑動(dòng)的趨勢(shì)。 Another approach, favored by many engineers, includes the evaluation of shear into the safety factor. The shear-friction relationship is: 另一個(gè)許多工程師樂于采用的方法，是在安全系數(shù)中包括剪力的計(jì)算。剪摩關(guān)系式： Which: is base length at plane of shear being studied; is allowable working shear str

20、ess of material or materials at plane of shear. 式中：為所考慮的剪切面上的底寬； 為剪切面上的一種或幾種材料的允許工作剪力。 Safety factors computed in this manner should approach values used in normal structural computations. 這種方法算得的安全系數(shù)應(yīng)該接近正常結(jié)構(gòu)計(jì)算中使用的數(shù)值。 Static friction values often assumed for concrete moving rock or concrete on concr

21、ete surfaces varies from 0.65 to 0.75. 對(duì)于混凝土和巖石或混凝土之間的滑動(dòng)，靜摩擦系數(shù)常假定為 0.65 0.75。 The working shear stress of concrete is related to the compressive strength of concrete. 混凝土 的工作剪切力 和混凝土的抗壓強(qiáng)度有關(guān)。 The unit shearing strength of concrete is about one-fifth of the compression breaking stress from standard cy

22、linders. 混凝土的單位剪切強(qiáng)度約為標(biāo)準(zhǔn)圓柱體壓縮破壞應(yīng)力的 1/5。 This indicates strength of 400 to 800 psi for concrete in dams. It also provides a safety factor of 4 if the unit working stresses used in computations are 100 to 200 psi. Greater working stresses are not recomme nded uni ess the con crete for the smaller dams

23、is actually p retestec對(duì)壩體混凝土來說, 即為 400 800 磅 /英寸 2。如果計(jì)算中取工作應(yīng)力為 100 200 磅/英寸 2，則安全系數(shù)還有 4。 除非小壩的混凝土確實(shí)事先做過試驗(yàn)，否則不宜采用更大的工作應(yīng)力。 4 internal concrete stresses 混凝土的內(nèi)部應(yīng)力 The unit stresses in the concrete and foundation materials must be kept within prescribed maximum values to avoid failures. 混凝土和地基材料內(nèi)的單位應(yīng)力必須限定在規(guī)定的最大值之 內(nèi)，以免引起失事。 Small dams normally develop stresses within the concrete that are less than the actual strength that may develop if the proper concrete mix is